Adriaan Rijnsdorp | Wageningen University and Research Centre (original) (raw)

Papers by Adriaan Rijnsdorp

Research paper thumbnail of Investigating the effects of mobile bottom fishing on benthic biota: a systematic review protocol

Environmental Evidence, 2014

Background: Mobile bottom fishing, such as trawling and dredging, is the most widespread direct h... more Background: Mobile bottom fishing, such as trawling and dredging, is the most widespread direct human impact on marine benthic systems. Knowledge of the impacts of different gear types on different habitats, the species most sensitive to impacts and the potential for habitats to recover are often needed to inform implementation of an ecosystem approach to fisheries and strategies for biodiversity conservation. This knowledge helps to identify management options that maximise fisheries yield whilst minimising negative impacts on benthic systems. Methods/design: The methods are designed to identify and collate evidence from experimental studies (e.g. before/after, control/impact) and comparative studies (spanning a gradient of fishing intensity) to identify changes in state (numbers, biomass, diversity etc.) of benthic biota (flora and fauna), resulting from a variety of mobile bottom fishing scenarios. The primary research question that the outputs will be used to address is: "to what extent does a given intensity of bottom fishing affect the abundance and/or diversity of benthic biota?" Due to the variety of gear and habitat types studied, the primary question will be closely linked with secondary questions. These include: "how does the effect of bottom fishing on various benthic biota metrics (species, faunal type, trait, taxon etc.) vary with (1) gear type and (2) habitat, and (3) gear type-habitat interactions?" and (4) "how might properties of the community and environment affect the resilience (and recovery potential) of a community to bottom fishing?"

Research paper thumbnail of Importance of fish biodiversity for the management of fisheries and ecosystems

Fisheries Research, 2008

A group of fisheries scientists participating in a European Union Network of Excellence (MARBEF) ... more A group of fisheries scientists participating in a European Union Network of Excellence (MARBEF) summarizes risks to the biodiversity of fish in European seas and recommends ways how existing fish diversity can be conserved, restored and managed.

Research paper thumbnail of How climate warming impacts the distribution and abundance of two small flatfish species in the North Sea

Journal of Sea Research, 2010

Climate change, specifically temperature, affects the distribution and densities of species in ma... more Climate change, specifically temperature, affects the distribution and densities of species in marine and terrestrial ecosystems. Here, we looked at the effect of temperature during winter and spawning period on latitudinal range shifts and changes in abundance of two non-commercial North Sea fish species, solenette (Buglossidium luteum) and scaldfish (Arnoglossus laterna). Both species have increased in abundance and moved to the north since the late 1980s, coinciding with a series of mild winters. In 1996, following a very cold winter, the abundance of both species temporarily decreased as they retracted to the south. The shift in temperature affected adult habitat conditions, allowing them to immigrate into new areas where they subsequently reproduced successfully. We can conclude this because recruitment improved following the increase in abundance. The recruitment relates significantly to the higher adult stock and higher temperatures. The predictions of higher average temperatures and milder winters in the North Sea make it likely that these species will increase further in abundance and move northward. The observed increase in abundance of these small flatfish species will affect the North Sea food web and therefore commercial species, e.g. plaice, by predation on juveniles and competition for benthic food resources.

Research paper thumbnail of Fisheries-induced evolution in growth, maturation and reproductive investment of the sexually dimorphic North Sea plaice (Pleuronectes platessa L.)

Journal of Sea Research, 2010

Changes in the onset of sexual maturation, reproductive investment and growth of North Sea plaice... more Changes in the onset of sexual maturation, reproductive investment and growth of North Sea plaice are studied between three periods: 1900s, 1980s and 2000s. Probabilistic maturation reaction norms of both males and females, describing the probability of becoming mature conditional on age and size, shifted towards smaller sizes and younger ages, indicating a fisheries-induced evolutionary change. A higher rate of change was observed during the past 20 years, which may be related to higher temperature conditions. Reproductive investment was estimated from the decrease in lipid, protein, dry weight content and condition factor of the whole body between pre-and post-spawning adults. Reproductive investment expressed as the energy loss over the spawning period increased with body size from 19% at 20 cm to 30% at 40 cm in males and from 35% at 30 cm to 48% at 50 cm in females. No change in reproductive investment could be detected between the 1980s and the 2000s. Von Bertalanffy (VB) growth parameters showed a decrease in L ∞ the asymptotic size and an increase in K, the velocity to reach L ∞ , in both males and females. The changes in VB growth are consistent with an increase in energy acquisition and reproductive investment. The observed changes in maturation, reproductive investment and growth are consistent with fisheries-induced evolution, but the changes in reproductive investment and growth need further investigation to disentangle the role of phenotypic plasticity.

Research paper thumbnail of Bolle, L. et al (2004). Growth changes in plaice, cod, haddock and saithe in the North Sea: a comparison of (post-)medieval and present-day growth rates based on otolith measurements. Journal of sea research, 51(3-4), 313-328.

Fishing effort has strongly increased in the North Sea since the mid-19th century, causing a subs... more Fishing effort has strongly increased in the North Sea since the mid-19th century, causing a substantial reduction in the population size of exploited fish stocks. As fisheries research has developed simultaneously with the industrialisation of the fisheries, our knowledge of population dynamics at low levels of exploitations is limited. Otoliths retrieved from archaeological excavations offer a unique opportunity to study growth rates in the past. This study compares historical and present-day growth rates for four commercially important demersal fish species. A total of 2532 modern otoliths (AD 1984 and 1286 historical otoliths (AD 1200 -1925) obtained from archaeological excavations in Belgium and Scotland were analysed. Comparison of the growth patterns between eras revealed a major increase in growth rate of haddock, whereas growth changes were not observed in saithe and only in the smaller size classes of plaice and cod. Comparison of our results with literature data indicates that the observed growth rate changes in plaice and cod occurred within the 20th century. Apparently the onset of industrialised fisheries has not greatly affected the growth of plaice, cod and saithe populations in the North Sea. This result contradicts the expectation of density-dependent limitation of growth during the era of pre-industrialised fishing, but is in agreement with the concentration hypothesis of Beverton (Neth. J. Sea Res. 34 (1995) 1) stating that species which concentrate spatially into nursery grounds during their early life-history may 'saturate' the carrying capacity of the juvenile habitat even though the adult part of the population is not limited by the adult habitat. D

Research paper thumbnail of Dietary overlap between the potential competitors herring, sprat and anchovy in the North Sea

Marine Ecology Progress Series

European anchovy Engraulis encrasicolus increased its abundance and distribution in the North Sea... more European anchovy Engraulis encrasicolus increased its abundance and distribution in the North Sea during the mid-1990s and may consume similar zooplankton to and/or compete with other occupants of the North Sea like herring Clupea harengus and sprat Sprattus sprattus. The diets of adult anchovy, sprat and juvenile herring of comparable sizes, sampled close in time and space, were compared to understand how the 3 species prey on zooplankton and establish whether their diets overlap or not. Anchovy was found to be more generalist, consuming a higher diversity of prey items. Herring was more specialized, with low diversity of food items. Sprat was intermediate between anchovy and herring. The dietary overlap between anchovy and sprat was highest, followed by herring and sprat before anchovy and herring. The mean weight of stomach contents did not differ between species. We conclude that of the 3 species, anchovy is likely to be the least affected by changing plankton communities.

Research paper thumbnail of Influence of temperature and food availability on juvenile European anchovy Engraulis encrasicolus at its northern boundary

Marine Ecology Progress Series

The European anchovy Engraulis encrasicolus population of the North Sea has increased and spread ... more The European anchovy Engraulis encrasicolus population of the North Sea has increased and spread in recent decades, probably in response to the relaxation of limiting factors in its life history. We use models and empirical data to explore the effects of temperature and food availability during the first growing season on the adult anchovy population across the North Sea. First, we compare simulated growth during summer and autumn, from a dynamic energy budget model, with trends in the time series of anchovy survey catch per unit effort. The proportion of the area of the North Sea in which anchovy can grow to 10 cm (the potential growth habitat) correlates with the abundance of anchovy caught in surveys the following year. Second, spatio-temporal statistical modeling is used to show that anchovy abundance in surveys is related to environmental variables (temperature and food availability). Temperature explains the distribution and abundance of anchovy in the North Sea better than fo...

Research paper thumbnail of Anchovy Engraulis encrasicolus diet in the North and Baltic Seas

Journal of Sea Research

The diet of anchovy (Engraulis encrasicolus) in the North and Baltic Seas was studied using stoma... more The diet of anchovy (Engraulis encrasicolus) in the North and Baltic Seas was studied using stomach analysis from four sampling events in different areas. Zooplanktivory was confirmed; the most frequent prey items (in over 40% of stomachs) were copepods, malacostracan larvae and fish larvae. In the Baltic Sea, Paracalanus spp. and Pseudocalanus spp. were important in relative terms; in the German Bight, Temora spp. dominated the stomach contents. Relative abundances of prey items varied with area more than absolute abundance or presence absence of items. Moreover, the level of resolution of prey categories influenced which prey categories were considered to be most important in driving variability in stomach content. Anchovy diet is broad across the seasons, years and areas sampled, suggesting that it is not a specialist feeder in the North Sea. The similarity of diet between anchovy and other clupeids, as well as anchovy consumption of larval fish, makes the new increased anchovy p...

Research paper thumbnail of Spatial variation in growth, maturation schedules and reproductive investment of female sole Solea solea in the Northeast Atlantic

Journal of Sea Research

Latitudinal variation in life-history traits is often explained by phenotypically plastic respons... more Latitudinal variation in life-history traits is often explained by phenotypically plastic responses or local adap-tations to different thermal regimes. We compared growth, maturation schedules and reproductive invest-ment of female sole Solea solea between 8 populations, covering much of the species' distribution in northern Europe, with respect to thermal gradients. An energy allocation model was fitted to size–age data, and probabi-listic maturation reaction norms were estimated from size–age–maturity data. We found that northern pop-ulations from colder environments had higher rates of energy acquisition and reproductive investment, an intrinsic tendency to mature earlier, and had smaller asymptotic sizes than southern populations from warmer environments. Consequently, growth rate was higher before maturation but lower after maturation in the north compared to the south. This is opposite to Bergmann's rule according to which slower growth, delayed maturation and larger a...

Research paper thumbnail of The result of the 1989 egg surveys for cod and plaice. ICES CM 1990/G: 27

Research paper thumbnail of Adaptive response of beam trawl fishers to rising fuel cost

In this paper, we develop models to test different hypotheses on the optimal towing speed at whic... more In this paper, we develop models to test different hypotheses on the optimal towing speed at which fuel savings are traded off against the reduction in catch due to the decrease in swept area. The model predicts that optimal towing speed is a decreasing function of fuel price and an increasing function of fish abundance and price. The model was fitted to vessel monitoring system (VMS) data. By means of mixture analysis, these VMS data were attributed to one of three behavioural modes: floating, towing, or navigating. Data attributed to the towing mode were used to determine the model that best fit the data. The preferred model includes a maximum towing speed and a component describing the decline in catch efficiency with decreasing towing speed. Towing speed is reduced by up to 14%. The savings obtained by reducing towing speed were estimated for each month and showed that vessels reduced their fuel consumption by between 0 and 40%.

Research paper thumbnail of Mixed fisheries management: protecting the weakest link

Research paper thumbnail of The result of the 1989 egg surveys for cod and plaice

Research paper thumbnail of Reconstructing age distribution, season of capture and growth rate of fish from archaeological sites based on otoliths and vertebrae

International Journal of Osteoarchaeology, 1999

The growth increments of otoliths and vertebrae of plaice (Pleuronectes platessa) derived from a ... more The growth increments of otoliths and vertebrae of plaice (Pleuronectes platessa) derived from a 15th century single depositional event at Raversijde (Belgium) are analysed with the aim of reconstructing (a) the age distribution of the population, (b) the season of capture, and (c) the growth rate. Otoliths and vertebrae give slightly different age distributions but it is possible to arrive at similar seasonality estimations in both structures when information from the literature and our own data from monthly captures of plaice from the North Sea are taken into account. These modern data show that the timing of annulus formation in otoliths and vertebrae is more or less similar. Back-calculations on vertebrae and otoliths yield similar growth curves. The age distribution, the edge condition of both vertebrae and otoliths, and the growth rate obtained on the material from Raversijde all show that the plaice from the studied assemblage were captured during spring in the southern part of the North Sea. Vertebrae are commonly preserved in archaeological sites whereas otoliths rarely survive. Although they are more difficult to read than otoliths, vertebrae of plaice can be used for growth increment analyses, and the growth rates obtained from vertebrae from archaeological sites can, therefore, be compared in the future to growth data from modern otoliths studied in sea fisheries research. Archaeozoological material predating industrialized fishing since the 19th century can hence serve as a reference in the study of the compensatory response of commercially important species to heavy exploitation.

Research paper thumbnail of The dawn of Darwinian fishery management

Applications of Evolutionary Theory, 2011

Research paper thumbnail of Predation by North Sea Herring Clupea harengus on Eggs of Plaice Pleuronectes platessa and Cod Gadus morhua

Transactions of the American Fisheries Society, 1985

The stomach contents of 5,408 herring caught during North Sea trawl surveys in February 1980Febru... more The stomach contents of 5,408 herring caught during North Sea trawl surveys in February 1980February , 1982February , and 1983 were analysed with a view of estimating predation mortalities exerted by the herring stock on plaice and cod eggs, based on the spatial distribution of size categories of herring and the numbers at age estimated from virtual population analysis. Predation on plaice and cod eggs was generally confined to the southern North Sea and only the younger age groups of herring (ages 2 and 3) consumed substantial numbers of fish eggs. Occasionally fish larvae (herring and plaice) were encountered. The estimated fraction consumed of the initial numbers of eggs produced varied from 0.7 to 1.9% for plaice and from 0.04 to 0.19% for cod. These estimates are necessarily crude and can only give a rough indication of the true predation mortalities. Although some effect of herring stock size on the level of recruitment of plaice might be expected, it is unlikely that the generally observed increase in recruitment to various demersal stocks in the 1970s could be entirely accounted for by a reduced predation on eggs by herring.

Research paper thumbnail of Low effective population size and evidence for inbreeding in an overexploited flatfish, plaice (Pleuronectes platessa L.)

Proceedings of the Royal Society B: Biological Sciences, 2005

Overexploitation and subsequent collapse of major worldwide fisheries has made it clear that mari... more Overexploitation and subsequent collapse of major worldwide fisheries has made it clear that marine stocks are not inexhaustible. Unfortunately, the perception remains that marine fishes are resilient to large population reductions, as even a commercially 'collapsed' stock will still consist of millions of individuals. Coupled with this notion is the idea that fisheries can, therefore, have little effect on the genetic diversity of stocks. We used DNA from archived otoliths collected between 1924 and 1972 together with 2002 juvenile's tissue to estimate effective population size (Ne) in plaice (Pleuronectes platessa). Ne was estimated at 20 000 in the North Sea and 2000 in Iceland. These values are five orders of magnitude smaller than the estimated census size for the two locations. Populations examined between 1924 and 1960 were in Hardy-Weinberg equilibrium, whereas populations examined after approximately 1970 were not. Extensive testing was performed to rule out genotyping artefacts and Wahlund effects. The significant heterozygote deficiencies found from 1970 onward were attributed to inbreeding. The emergence of inbreeding between 1950 and 1970 coincides with the increase in fishing mortality after World War II. Although the biological mechanisms remain speculative, our demonstration of inbreeding signals the need for understanding the social and mating behaviour in commercially important fishes.

Research paper thumbnail of Multiple growth-correlated life history traits estimated simultaneously in individuals

Oikos, 2010

We present a new methodology to estimate rates of energy acquisition, maintenance, reproductive i... more We present a new methodology to estimate rates of energy acquisition, maintenance, reproductive investment and the onset of maturation (four-trait estimation) by fitting an energy allocation model to individual growth trajectories. The accuracy and precision of the method is evaluated on simulated growth trajectories. In the deterministic case, all life history parameters are well estimated with negligible bias over realistic parameter ranges. Adding environmental variability reduces precision, causes the maintenance and reproductive investment to be confounded with a negative error correlation, and tends, if strong, to result in an underestimation of the energy acquisition and maintenance and an overestimation of the age and size at the onset of maturation. Assuming a priori incorrect allometric scaling exponents also leads to a general but fairly predictable bias. To avoid confounding in applications we propose to assume a constant maintenance (three-trait estimation), which can be obtained by fitting reproductive investment simultaneously to size at age on population data. The results become qualitatively more robust but the improvement of the estimate of the onset of maturation is not significant. When applied to growth curves back-calculated from otoliths of female North Sea plaice Pleuronectes platessa, the four-trait and three-trait estimation produced estimates for the onset of maturation very similar to those obtained by direct observation. The correlations between life-history traits match expectations. We discuss the potential of the methodology in studies of the ecology and evolution of life history parameters in wild populations. West, et al. 2001). They differ mostly in terms of priorities of energy flows to the 52 different functions. Allocation schedules typically comprise four traits, namely energy 53 acquisition, maintenance, onset of maturation, and thereafter reproductive investment, 54 whereas somatic growth arises as a by-product: the energy that remains after accounting 55 for the primary energy flows to maintenance and reproductive investment is available for 56 somatic growth. The study of energy allocation schedules in individual organisms is 57 difficult because of a lack of data at the individual level as this would require monitoring 58 separate individual organisms throughout their life time. Studies therefore have focused 59 on the population level as well as on single traits (Stevenson and Woods Jr. 2006). 60 Studying the four traits together (acquisition, maintenance, onset of maturation and 61 4 reproductive investment) at the individual level would offer several advantages over the 62 widely used single trait estimation at the population level: (1) phenotypic correlations 63 between traits could be estimated; (2) changes in one trait could be interpreted 64 conditionally on changes in other traits, precisely because of the previous correlations; 65 (3) it would be more consistent with the fact that physiological trade-offs apply at the 66 individual and not at the population level. 67 Organisms in which the individual growth history is recorded in hard structures offer a 68 unique opportunity to study energy allocation schedules at the individual level. Fish for 69 instance show indeterminate growth and the growth history of individuals can be 70 reconstructed from the width of the seasonal structures imprinted in hard structures such 71

Research paper thumbnail of Hunting in the kestrel, Falco tinnunculus, and the adaptive significance of daily habits

Oecologia, 1981

1. In an attempt to evaluate the importance of individual daily habits to a freeliving animal, fo... more 1. In an attempt to evaluate the importance of individual daily habits to a freeliving animal, foraging behaviour of kestrels was observed continuously for days in sequence in open country. Data obtained in 2,942 observation hours were used. Flight-hunting was the prominent foraging technique yielding 76% of all prey obtained.

Research paper thumbnail of Estimating age at maturation and energy-based life-history traits from individual growth trajectories with nonlinear mixed-effects models

Oecologia, 2013

A new method is presented to estimate individuals' (1) age at maturation, (2) energy acquisition ... more A new method is presented to estimate individuals' (1) age at maturation, (2) energy acquisition rate, (3) energy expenditure for body maintenance, and (4) reproductive investment, and the multivariate distribution of these traits in a population. The method relies on adjusting a conceptual energy allocation model to individual growth curves using nonlinear mixed-effects modelling. The method's performance was tested using simulated growth curves for a range of life-history types. Individual age at maturation, energy acquisition rate and the sum of maintenance and reproductive investment rates, and their multivariate distribution, were accurately estimated. For the estimation of maintenance and reproductive investment rates separately, biases were observed for lifehistories with a large imbalance between these traits. For low reproductive investment rates and high maintenance rates, reproductive investment rate estimates were strongly biased whereas maintenance rate estimates were not, the reverse holding in the opposite situation. The method was applied to individual growth curves back-calculated from otoliths of North Sea plaice (Pleuronectes platessa) and from scales of Norwegian spring spawning herring (Clupea harengus). For plaice, maturity ogives derived from our individual estimates of age at maturation were almost identical to the maturity ogives based on gonad observation in catch samples. For herring, we observed 51.5 % of agreement between our individual estimates and those directly obtained from scale reading, with a difference lower than 1 year in 97 % of cases. We conclude that the method is a powerful tool to estimate the distribution of correlated life-history traits for any species for which individual growth curves are available.

Research paper thumbnail of Investigating the effects of mobile bottom fishing on benthic biota: a systematic review protocol

Environmental Evidence, 2014

Background: Mobile bottom fishing, such as trawling and dredging, is the most widespread direct h... more Background: Mobile bottom fishing, such as trawling and dredging, is the most widespread direct human impact on marine benthic systems. Knowledge of the impacts of different gear types on different habitats, the species most sensitive to impacts and the potential for habitats to recover are often needed to inform implementation of an ecosystem approach to fisheries and strategies for biodiversity conservation. This knowledge helps to identify management options that maximise fisheries yield whilst minimising negative impacts on benthic systems. Methods/design: The methods are designed to identify and collate evidence from experimental studies (e.g. before/after, control/impact) and comparative studies (spanning a gradient of fishing intensity) to identify changes in state (numbers, biomass, diversity etc.) of benthic biota (flora and fauna), resulting from a variety of mobile bottom fishing scenarios. The primary research question that the outputs will be used to address is: "to what extent does a given intensity of bottom fishing affect the abundance and/or diversity of benthic biota?" Due to the variety of gear and habitat types studied, the primary question will be closely linked with secondary questions. These include: "how does the effect of bottom fishing on various benthic biota metrics (species, faunal type, trait, taxon etc.) vary with (1) gear type and (2) habitat, and (3) gear type-habitat interactions?" and (4) "how might properties of the community and environment affect the resilience (and recovery potential) of a community to bottom fishing?"

Research paper thumbnail of Importance of fish biodiversity for the management of fisheries and ecosystems

Fisheries Research, 2008

A group of fisheries scientists participating in a European Union Network of Excellence (MARBEF) ... more A group of fisheries scientists participating in a European Union Network of Excellence (MARBEF) summarizes risks to the biodiversity of fish in European seas and recommends ways how existing fish diversity can be conserved, restored and managed.

Research paper thumbnail of How climate warming impacts the distribution and abundance of two small flatfish species in the North Sea

Journal of Sea Research, 2010

Climate change, specifically temperature, affects the distribution and densities of species in ma... more Climate change, specifically temperature, affects the distribution and densities of species in marine and terrestrial ecosystems. Here, we looked at the effect of temperature during winter and spawning period on latitudinal range shifts and changes in abundance of two non-commercial North Sea fish species, solenette (Buglossidium luteum) and scaldfish (Arnoglossus laterna). Both species have increased in abundance and moved to the north since the late 1980s, coinciding with a series of mild winters. In 1996, following a very cold winter, the abundance of both species temporarily decreased as they retracted to the south. The shift in temperature affected adult habitat conditions, allowing them to immigrate into new areas where they subsequently reproduced successfully. We can conclude this because recruitment improved following the increase in abundance. The recruitment relates significantly to the higher adult stock and higher temperatures. The predictions of higher average temperatures and milder winters in the North Sea make it likely that these species will increase further in abundance and move northward. The observed increase in abundance of these small flatfish species will affect the North Sea food web and therefore commercial species, e.g. plaice, by predation on juveniles and competition for benthic food resources.

Research paper thumbnail of Fisheries-induced evolution in growth, maturation and reproductive investment of the sexually dimorphic North Sea plaice (Pleuronectes platessa L.)

Journal of Sea Research, 2010

Changes in the onset of sexual maturation, reproductive investment and growth of North Sea plaice... more Changes in the onset of sexual maturation, reproductive investment and growth of North Sea plaice are studied between three periods: 1900s, 1980s and 2000s. Probabilistic maturation reaction norms of both males and females, describing the probability of becoming mature conditional on age and size, shifted towards smaller sizes and younger ages, indicating a fisheries-induced evolutionary change. A higher rate of change was observed during the past 20 years, which may be related to higher temperature conditions. Reproductive investment was estimated from the decrease in lipid, protein, dry weight content and condition factor of the whole body between pre-and post-spawning adults. Reproductive investment expressed as the energy loss over the spawning period increased with body size from 19% at 20 cm to 30% at 40 cm in males and from 35% at 30 cm to 48% at 50 cm in females. No change in reproductive investment could be detected between the 1980s and the 2000s. Von Bertalanffy (VB) growth parameters showed a decrease in L ∞ the asymptotic size and an increase in K, the velocity to reach L ∞ , in both males and females. The changes in VB growth are consistent with an increase in energy acquisition and reproductive investment. The observed changes in maturation, reproductive investment and growth are consistent with fisheries-induced evolution, but the changes in reproductive investment and growth need further investigation to disentangle the role of phenotypic plasticity.

Research paper thumbnail of Bolle, L. et al (2004). Growth changes in plaice, cod, haddock and saithe in the North Sea: a comparison of (post-)medieval and present-day growth rates based on otolith measurements. Journal of sea research, 51(3-4), 313-328.

Fishing effort has strongly increased in the North Sea since the mid-19th century, causing a subs... more Fishing effort has strongly increased in the North Sea since the mid-19th century, causing a substantial reduction in the population size of exploited fish stocks. As fisheries research has developed simultaneously with the industrialisation of the fisheries, our knowledge of population dynamics at low levels of exploitations is limited. Otoliths retrieved from archaeological excavations offer a unique opportunity to study growth rates in the past. This study compares historical and present-day growth rates for four commercially important demersal fish species. A total of 2532 modern otoliths (AD 1984 and 1286 historical otoliths (AD 1200 -1925) obtained from archaeological excavations in Belgium and Scotland were analysed. Comparison of the growth patterns between eras revealed a major increase in growth rate of haddock, whereas growth changes were not observed in saithe and only in the smaller size classes of plaice and cod. Comparison of our results with literature data indicates that the observed growth rate changes in plaice and cod occurred within the 20th century. Apparently the onset of industrialised fisheries has not greatly affected the growth of plaice, cod and saithe populations in the North Sea. This result contradicts the expectation of density-dependent limitation of growth during the era of pre-industrialised fishing, but is in agreement with the concentration hypothesis of Beverton (Neth. J. Sea Res. 34 (1995) 1) stating that species which concentrate spatially into nursery grounds during their early life-history may 'saturate' the carrying capacity of the juvenile habitat even though the adult part of the population is not limited by the adult habitat. D

Research paper thumbnail of Dietary overlap between the potential competitors herring, sprat and anchovy in the North Sea

Marine Ecology Progress Series

European anchovy Engraulis encrasicolus increased its abundance and distribution in the North Sea... more European anchovy Engraulis encrasicolus increased its abundance and distribution in the North Sea during the mid-1990s and may consume similar zooplankton to and/or compete with other occupants of the North Sea like herring Clupea harengus and sprat Sprattus sprattus. The diets of adult anchovy, sprat and juvenile herring of comparable sizes, sampled close in time and space, were compared to understand how the 3 species prey on zooplankton and establish whether their diets overlap or not. Anchovy was found to be more generalist, consuming a higher diversity of prey items. Herring was more specialized, with low diversity of food items. Sprat was intermediate between anchovy and herring. The dietary overlap between anchovy and sprat was highest, followed by herring and sprat before anchovy and herring. The mean weight of stomach contents did not differ between species. We conclude that of the 3 species, anchovy is likely to be the least affected by changing plankton communities.

Research paper thumbnail of Influence of temperature and food availability on juvenile European anchovy Engraulis encrasicolus at its northern boundary

Marine Ecology Progress Series

The European anchovy Engraulis encrasicolus population of the North Sea has increased and spread ... more The European anchovy Engraulis encrasicolus population of the North Sea has increased and spread in recent decades, probably in response to the relaxation of limiting factors in its life history. We use models and empirical data to explore the effects of temperature and food availability during the first growing season on the adult anchovy population across the North Sea. First, we compare simulated growth during summer and autumn, from a dynamic energy budget model, with trends in the time series of anchovy survey catch per unit effort. The proportion of the area of the North Sea in which anchovy can grow to 10 cm (the potential growth habitat) correlates with the abundance of anchovy caught in surveys the following year. Second, spatio-temporal statistical modeling is used to show that anchovy abundance in surveys is related to environmental variables (temperature and food availability). Temperature explains the distribution and abundance of anchovy in the North Sea better than fo...

Research paper thumbnail of Anchovy Engraulis encrasicolus diet in the North and Baltic Seas

Journal of Sea Research

The diet of anchovy (Engraulis encrasicolus) in the North and Baltic Seas was studied using stoma... more The diet of anchovy (Engraulis encrasicolus) in the North and Baltic Seas was studied using stomach analysis from four sampling events in different areas. Zooplanktivory was confirmed; the most frequent prey items (in over 40% of stomachs) were copepods, malacostracan larvae and fish larvae. In the Baltic Sea, Paracalanus spp. and Pseudocalanus spp. were important in relative terms; in the German Bight, Temora spp. dominated the stomach contents. Relative abundances of prey items varied with area more than absolute abundance or presence absence of items. Moreover, the level of resolution of prey categories influenced which prey categories were considered to be most important in driving variability in stomach content. Anchovy diet is broad across the seasons, years and areas sampled, suggesting that it is not a specialist feeder in the North Sea. The similarity of diet between anchovy and other clupeids, as well as anchovy consumption of larval fish, makes the new increased anchovy p...

Research paper thumbnail of Spatial variation in growth, maturation schedules and reproductive investment of female sole Solea solea in the Northeast Atlantic

Journal of Sea Research

Latitudinal variation in life-history traits is often explained by phenotypically plastic respons... more Latitudinal variation in life-history traits is often explained by phenotypically plastic responses or local adap-tations to different thermal regimes. We compared growth, maturation schedules and reproductive invest-ment of female sole Solea solea between 8 populations, covering much of the species' distribution in northern Europe, with respect to thermal gradients. An energy allocation model was fitted to size–age data, and probabi-listic maturation reaction norms were estimated from size–age–maturity data. We found that northern pop-ulations from colder environments had higher rates of energy acquisition and reproductive investment, an intrinsic tendency to mature earlier, and had smaller asymptotic sizes than southern populations from warmer environments. Consequently, growth rate was higher before maturation but lower after maturation in the north compared to the south. This is opposite to Bergmann's rule according to which slower growth, delayed maturation and larger a...

Research paper thumbnail of The result of the 1989 egg surveys for cod and plaice. ICES CM 1990/G: 27

Research paper thumbnail of Adaptive response of beam trawl fishers to rising fuel cost

In this paper, we develop models to test different hypotheses on the optimal towing speed at whic... more In this paper, we develop models to test different hypotheses on the optimal towing speed at which fuel savings are traded off against the reduction in catch due to the decrease in swept area. The model predicts that optimal towing speed is a decreasing function of fuel price and an increasing function of fish abundance and price. The model was fitted to vessel monitoring system (VMS) data. By means of mixture analysis, these VMS data were attributed to one of three behavioural modes: floating, towing, or navigating. Data attributed to the towing mode were used to determine the model that best fit the data. The preferred model includes a maximum towing speed and a component describing the decline in catch efficiency with decreasing towing speed. Towing speed is reduced by up to 14%. The savings obtained by reducing towing speed were estimated for each month and showed that vessels reduced their fuel consumption by between 0 and 40%.

Research paper thumbnail of Mixed fisheries management: protecting the weakest link

Research paper thumbnail of The result of the 1989 egg surveys for cod and plaice

Research paper thumbnail of Reconstructing age distribution, season of capture and growth rate of fish from archaeological sites based on otoliths and vertebrae

International Journal of Osteoarchaeology, 1999

The growth increments of otoliths and vertebrae of plaice (Pleuronectes platessa) derived from a ... more The growth increments of otoliths and vertebrae of plaice (Pleuronectes platessa) derived from a 15th century single depositional event at Raversijde (Belgium) are analysed with the aim of reconstructing (a) the age distribution of the population, (b) the season of capture, and (c) the growth rate. Otoliths and vertebrae give slightly different age distributions but it is possible to arrive at similar seasonality estimations in both structures when information from the literature and our own data from monthly captures of plaice from the North Sea are taken into account. These modern data show that the timing of annulus formation in otoliths and vertebrae is more or less similar. Back-calculations on vertebrae and otoliths yield similar growth curves. The age distribution, the edge condition of both vertebrae and otoliths, and the growth rate obtained on the material from Raversijde all show that the plaice from the studied assemblage were captured during spring in the southern part of the North Sea. Vertebrae are commonly preserved in archaeological sites whereas otoliths rarely survive. Although they are more difficult to read than otoliths, vertebrae of plaice can be used for growth increment analyses, and the growth rates obtained from vertebrae from archaeological sites can, therefore, be compared in the future to growth data from modern otoliths studied in sea fisheries research. Archaeozoological material predating industrialized fishing since the 19th century can hence serve as a reference in the study of the compensatory response of commercially important species to heavy exploitation.

Research paper thumbnail of The dawn of Darwinian fishery management

Applications of Evolutionary Theory, 2011

Research paper thumbnail of Predation by North Sea Herring Clupea harengus on Eggs of Plaice Pleuronectes platessa and Cod Gadus morhua

Transactions of the American Fisheries Society, 1985

The stomach contents of 5,408 herring caught during North Sea trawl surveys in February 1980Febru... more The stomach contents of 5,408 herring caught during North Sea trawl surveys in February 1980February , 1982February , and 1983 were analysed with a view of estimating predation mortalities exerted by the herring stock on plaice and cod eggs, based on the spatial distribution of size categories of herring and the numbers at age estimated from virtual population analysis. Predation on plaice and cod eggs was generally confined to the southern North Sea and only the younger age groups of herring (ages 2 and 3) consumed substantial numbers of fish eggs. Occasionally fish larvae (herring and plaice) were encountered. The estimated fraction consumed of the initial numbers of eggs produced varied from 0.7 to 1.9% for plaice and from 0.04 to 0.19% for cod. These estimates are necessarily crude and can only give a rough indication of the true predation mortalities. Although some effect of herring stock size on the level of recruitment of plaice might be expected, it is unlikely that the generally observed increase in recruitment to various demersal stocks in the 1970s could be entirely accounted for by a reduced predation on eggs by herring.

Research paper thumbnail of Low effective population size and evidence for inbreeding in an overexploited flatfish, plaice (Pleuronectes platessa L.)

Proceedings of the Royal Society B: Biological Sciences, 2005

Overexploitation and subsequent collapse of major worldwide fisheries has made it clear that mari... more Overexploitation and subsequent collapse of major worldwide fisheries has made it clear that marine stocks are not inexhaustible. Unfortunately, the perception remains that marine fishes are resilient to large population reductions, as even a commercially 'collapsed' stock will still consist of millions of individuals. Coupled with this notion is the idea that fisheries can, therefore, have little effect on the genetic diversity of stocks. We used DNA from archived otoliths collected between 1924 and 1972 together with 2002 juvenile's tissue to estimate effective population size (Ne) in plaice (Pleuronectes platessa). Ne was estimated at 20 000 in the North Sea and 2000 in Iceland. These values are five orders of magnitude smaller than the estimated census size for the two locations. Populations examined between 1924 and 1960 were in Hardy-Weinberg equilibrium, whereas populations examined after approximately 1970 were not. Extensive testing was performed to rule out genotyping artefacts and Wahlund effects. The significant heterozygote deficiencies found from 1970 onward were attributed to inbreeding. The emergence of inbreeding between 1950 and 1970 coincides with the increase in fishing mortality after World War II. Although the biological mechanisms remain speculative, our demonstration of inbreeding signals the need for understanding the social and mating behaviour in commercially important fishes.

Research paper thumbnail of Multiple growth-correlated life history traits estimated simultaneously in individuals

Oikos, 2010

We present a new methodology to estimate rates of energy acquisition, maintenance, reproductive i... more We present a new methodology to estimate rates of energy acquisition, maintenance, reproductive investment and the onset of maturation (four-trait estimation) by fitting an energy allocation model to individual growth trajectories. The accuracy and precision of the method is evaluated on simulated growth trajectories. In the deterministic case, all life history parameters are well estimated with negligible bias over realistic parameter ranges. Adding environmental variability reduces precision, causes the maintenance and reproductive investment to be confounded with a negative error correlation, and tends, if strong, to result in an underestimation of the energy acquisition and maintenance and an overestimation of the age and size at the onset of maturation. Assuming a priori incorrect allometric scaling exponents also leads to a general but fairly predictable bias. To avoid confounding in applications we propose to assume a constant maintenance (three-trait estimation), which can be obtained by fitting reproductive investment simultaneously to size at age on population data. The results become qualitatively more robust but the improvement of the estimate of the onset of maturation is not significant. When applied to growth curves back-calculated from otoliths of female North Sea plaice Pleuronectes platessa, the four-trait and three-trait estimation produced estimates for the onset of maturation very similar to those obtained by direct observation. The correlations between life-history traits match expectations. We discuss the potential of the methodology in studies of the ecology and evolution of life history parameters in wild populations. West, et al. 2001). They differ mostly in terms of priorities of energy flows to the 52 different functions. Allocation schedules typically comprise four traits, namely energy 53 acquisition, maintenance, onset of maturation, and thereafter reproductive investment, 54 whereas somatic growth arises as a by-product: the energy that remains after accounting 55 for the primary energy flows to maintenance and reproductive investment is available for 56 somatic growth. The study of energy allocation schedules in individual organisms is 57 difficult because of a lack of data at the individual level as this would require monitoring 58 separate individual organisms throughout their life time. Studies therefore have focused 59 on the population level as well as on single traits (Stevenson and Woods Jr. 2006). 60 Studying the four traits together (acquisition, maintenance, onset of maturation and 61 4 reproductive investment) at the individual level would offer several advantages over the 62 widely used single trait estimation at the population level: (1) phenotypic correlations 63 between traits could be estimated; (2) changes in one trait could be interpreted 64 conditionally on changes in other traits, precisely because of the previous correlations; 65 (3) it would be more consistent with the fact that physiological trade-offs apply at the 66 individual and not at the population level. 67 Organisms in which the individual growth history is recorded in hard structures offer a 68 unique opportunity to study energy allocation schedules at the individual level. Fish for 69 instance show indeterminate growth and the growth history of individuals can be 70 reconstructed from the width of the seasonal structures imprinted in hard structures such 71

Research paper thumbnail of Hunting in the kestrel, Falco tinnunculus, and the adaptive significance of daily habits

Oecologia, 1981

1. In an attempt to evaluate the importance of individual daily habits to a freeliving animal, fo... more 1. In an attempt to evaluate the importance of individual daily habits to a freeliving animal, foraging behaviour of kestrels was observed continuously for days in sequence in open country. Data obtained in 2,942 observation hours were used. Flight-hunting was the prominent foraging technique yielding 76% of all prey obtained.

Research paper thumbnail of Estimating age at maturation and energy-based life-history traits from individual growth trajectories with nonlinear mixed-effects models

Oecologia, 2013

A new method is presented to estimate individuals' (1) age at maturation, (2) energy acquisition ... more A new method is presented to estimate individuals' (1) age at maturation, (2) energy acquisition rate, (3) energy expenditure for body maintenance, and (4) reproductive investment, and the multivariate distribution of these traits in a population. The method relies on adjusting a conceptual energy allocation model to individual growth curves using nonlinear mixed-effects modelling. The method's performance was tested using simulated growth curves for a range of life-history types. Individual age at maturation, energy acquisition rate and the sum of maintenance and reproductive investment rates, and their multivariate distribution, were accurately estimated. For the estimation of maintenance and reproductive investment rates separately, biases were observed for lifehistories with a large imbalance between these traits. For low reproductive investment rates and high maintenance rates, reproductive investment rate estimates were strongly biased whereas maintenance rate estimates were not, the reverse holding in the opposite situation. The method was applied to individual growth curves back-calculated from otoliths of North Sea plaice (Pleuronectes platessa) and from scales of Norwegian spring spawning herring (Clupea harengus). For plaice, maturity ogives derived from our individual estimates of age at maturation were almost identical to the maturity ogives based on gonad observation in catch samples. For herring, we observed 51.5 % of agreement between our individual estimates and those directly obtained from scale reading, with a difference lower than 1 year in 97 % of cases. We conclude that the method is a powerful tool to estimate the distribution of correlated life-history traits for any species for which individual growth curves are available.