Fishing effects on energy use by North Sea fishes (original) (raw)
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An estimate of the total biomass of fish in the North Sea
ICES Journal of Marine Science, 1990
Biomass estimates are made by combining ICES working group VPA biomass estimates of 11 commercial fish species with catch rates in the ICES International Young Fish Surveys (1YFS) in the first quarter of the year and in the English Groundfish Surveys (EGFS) in the third quarter of the year. Data from 1983, 1984, and 1985 are used. The total biomass of fish is calculated as the mean of the three years. In the first three months of the year the mean total biomass was 8.6 million tonnes, of which the 11 commercial species constituted 5.9 million tonnes. In the third quarter it was 13.1 million tonnes, of which the 11 commercial species constituted 8.5 million tonnes. The difference between the first and third quarters was mainly caused by the migration of 0.5 million tonnes of Western stock mackerel and 1.6 million tonnes of horse mackerel into the North Sea during the third quarter. Comparisons are made between the productivity of the North Sea and other shelf regions in temperate latitudes.
Marine Biology, 2002
Fishing has wide-ranging impacts on marine ecosystems. One of the most pervasive signs of intensive fishing is "fishing down the food web", with landings increasingly dominated by smaller species from lower trophic levels. Decreases in the trophic level of landings are assumed to reflect those in fish communities, because size-selective mortality causes decreases in the relative abundance of larger species and in mean body size within species. However, existing analyses of fishing impacts on the trophic level of fish communities have focused on the role of changes in species composition rather than size composition. This will provide a biased assessment of the magnitude of fishing impacts, because fishes feed at different trophic levels as they grow. Here, we combine body size versus trophic level relationships for North Sea fishes (trophic level assessed using nitrogen stable-isotope analysis) with species–size–abundance data from two time-series of trawl-survey data (whole North Sea 1982–2000, central and northern North Sea 1925–1996) to predict long-term trends in the trophic structure of the North Sea fish community. Analyses of the 1982–2000 time-series showed that there was a slow but progressive decline in the trophic level of the demersal community, while there was no trend in the trophic level of the combined pelagic and demersal community. Analyses of the longer time-series suggested that there was no trend in the trophic level of the demersal community. We related temporal changes in trophic level to temporal changes in the slopes of normalised biomass size-spectra (which theoretically represent the trophic structure of the community), mean log2 body mass and mean log2 maximum body mass. While the size-based metrics of community structure showed long-term trends that were consistent with the effects of increased fishery exploitation, these trends were only correlated with trophic level for the demersal community. Our analysis suggests that the effects of fishing on the trophic structure of fish communities can be much more complex than previously assumed. This is a consequence of sampled communities not reflecting all the pathways of energy transfer in a marine ecosystem and of the absence of historical data on temporal and spatial changes in the trophic level of individuals. For the North Sea fish community, changes in size structure due to the differential effects of fishing on species and populations with different life histories are a stronger and more universal indicator of fishing effects than changes in mean trophic level.
Changes in the North Sea fish community: evidence of indirect effects of fishing?
ICES Journal of Marine Science, 2005
We investigate changes in the North Sea fish community with particular reference to possible indirect effects of fishing, mediated through the ecosystem. In the past, long-term changes in the slope of size spectra of research vessel catches have been related to changes in fishing effort, but such changes may simply reflect the cumulative, direct effects of fishing through selective removal of large individuals. If there is resilience in a fish community towards fishing, we may expect increases in specific components, for instance as a consequence of an associated reduction in predation and/or competition. We show on the basis of three long-term trawl surveys that abundance of small fish (all species) as well as abundance of demersal species with a low maximum length (Lmax) have steadily and significantly increased in absolute numbers over large parts of the North Sea during the last 30 years. Taking average fishing mortality of assessed commercial species as an index of exploitation...
Modelling the effects of fishing on the North Sea fish community size composition
Ecological Modelling, 2016
Ecosystem-based management of the North Sea demersal fish community uses the large fish indicator (LFI), defined as the proportion by weight of fish caught in the International Bottom Trawl Survey (IBTS) exceeding a length of 40 cm. Current values of the LFI are ∼0.15, but the European Union (EU) Marine Strategy Framework Directive (MSFD) requires a value of 0.3 be reached by 2020. An LFI calculated from an eight-species subset correlated closely with the full community LFI, thereby permitting an exploration of the effects of various fishing scenarios on projected values of the LFI using an extension of a previously published multi-species length-structured model that included these key species. The model replicated historical changes in biomass and size composition of individual species, and generated an LFI that was significantly correlated with observations. A community-wide reduction in fishing mortality of ∼60% from 2008 values was necessary to meet the LFI target, driven mainly by changes in cod and saithe. A 70% reduction in cod fishing mortality alone, or a 75% reduction in otter trawl effort, was also sufficient to achieve the target. Reductions in fishing mortality necessary to achieve maximum sustainable harvesting rates are projected to result in the LFI overshooting its target.
Marine Ecology Progress Series, 2010
Coastal upwelling ecosystems provide the bulk of the world's fishery yields, but the biochemical ecology of the species that make up these fisheries has, surprisingly, been ignored. Biochemical indicators can provide a mechanistic, ecosystem-based link between population and ecosystem dynamics. Here we investigated long-term, inter-annual changes in the proximate composition and energetic condition of European sardine Sardina pilchardus and its relationship with oceanographic conditions in the Western Iberian Upwelling Ecosystem. Energy density (ED) ranged between 4.0 and 14.2 kJ g -1 , and the seasonal cycle largely determined temporal variability, explaining > 80% of the observed variation. ED variations were also closely linked with water (total R 2 = 99.0% in whole body; total R 2 = 95.0% in muscle) and lipid dynamics (total R 2 = 99.6% in whole body; total R 2 = 92.5% in muscle). After adjusting for seasonality (rED) and restricting the temporal analysis to the end of the feeding period (August to October), spring/early-summer oceanographic conditions explained 67% of the late-summer energetic peak. Interestingly, the sardine rED peak in year (t) explained > 54.4% of the variation in the annual catches of year (t + 1), indicating that adult energetic condition during spawning is partially translated into the fishery through parental effects in recruitment strength. Our results support earlier findings indicating that sardine population dynamics seem to be controlled by bottom-up effects, but the linkages between population dynamics and patterns in environmental variability via physiological condition seem to have previously been overlooked. We also provide empirical evidence that biochemical assessments during critical periods of the life-cycle of fish are essential in understanding the population dynamics of coastal upwelling ecosystems and in developing a more solid basis for stock management and conservation.
The Response of North Sea Ecosystem Functional Groups to Warming and Changes in Fishing
Frontiers in Marine Science
Achieving Good Environmental Status (GES) requires managing ecosystems subject to a variety of pressures such as climate change, eutrophication, and fishing. However, ecosystem models are generally much better at representing top-down impacts from fishing than bottom-up impacts due to warming or changes in nutrient loading. Bottom-up processes often have to be parameterised with little data or worse still taken as a system input rather than being represented explicitly. In this study we use an end-to-end ecosystem model (StrathE2E2) for the North Sea with 18 broad functional groups, five resource pools, and representations of feeding, metabolism, reproduction, active migrations, advection, and mixing. Environmental driving data include temperature, irradiance, hydrodynamics, and nutrient inputs from rivers, atmosphere, and ocean boundaries, so the model is designed to evaluate rigorously top-down and bottom-up impacts and is ideal for looking at possible changes in energy flows and ...
Bulletin of Marine Science, 2005
Small pelagic fish contribute up to 50% of the total landing of marine species. They are most abundant in upwelling areas and contribute to food security. Exploited stocks of these species are prone to large interannual and interdecadal variation of abundance as well as to collapse. We discuss why small pelagic fish and fisheries are so "special" with regard to their biology, ecology, and behavior. Two adjectives can sum up the characteristics of pelagic species: variability and instability. Analyses of the relationships between small pelagic fish and their physical environment at different timescales illustrate the complexity of the interplay between exploitation and environmental impacts. How small pelagic fish species are positioned and related within the trophic web suggests that these species play a central role in the functioning and dynamics of upwelling ecosystems. Finally, we discuss the sustainable exploitation of small pelagic fisheries through appropriate management, focusing on the resilience to exploitation, a comparison of different management options and regulatory mechanisms. We recommend that statistical, socio-economical, and political merits of a proposed two-level (short-and long-term) management strategy be undertaken. Despite constant progress in understanding the complex processes involved in the variability of pelagic stock abundance, especially at short and medium time scales, our ability to predict abundance and catches is limited, which in turn limits our capacity to properly manage the fisheries and ensure sustainable exploitation. Substantial progress can be expected from an integrated modeling approach of spatialized models coupling hydrodynamics, biogeochemical, and ecological processes. Although there are many definitions of "small pelagic fish," this expression most commonly refers to shoaling epipelagic fish characterized by high horizontal and vertical mobility in coastal areas and which, as adults, are usually 10-30 cm in length. The upper limit is often the most debated, since some authors use the term "medium-sized pelagic fish" to designate larger fish ranging from about 20-60 cm (Bas et al., 1995). This distinction between the two size groups simultaneously allows distinctions to be made with regard to the position of the species within the food web: the conventional "small pelagic fish" includes typical forage species like sardine and anchovy preying on phytoplankton and/or micro-meso-zooplankton, while the group of "medium-sized pelagic fish" includes mostly species from intermediate trophic levels like horse-mackerel, mackerels, and coastal tunas. When adult, the latter species prey mainly on macro-zooplankton, ichthyoplankton, and small fish or mollusks. In this work, we will focus mainly on the conventional "small pelagic fish," but will also often refer to, or contrast findings with, medium-sized pelagic fish. When referring to both small and medium-sized pelagic fish, we will use the terms "pelagic fish" or "coastal pelagic fish" to separate them from the large oceanic tunas, although not all medium-sized pelagic fish remain on the continental shelf for the duration of their whole life cycle.
Small pelagic fish dynamics: A review of mechanisms in the Gulf of Lions
Deep Sea Research Part II: Topical Studies in Oceanography
Around 2008, an ecosystem shift occurred in the Gulf of Lions, highlighted by considerable changes in biomass and fish mean weight of its two main small pelagic fish stocks (European anchovy, Engraulis encrasicolus; European sardine, Sardina pilchardus). Surprisingly these changes did not appear to be mediated by a decrease in fish recruitment rates (which remained high) or by a high fishing pressure (exploitation rates being extremely low). Here, we review the current knowledge on the population's dynamics and its potential causes. We used an integrative ecosystem approach exploring alternative hypotheses, ranging from bottom-up to top-down control, not forgetting epizootic diseases. First, the study of multiple population characteristics highlighted a decrease in body condition for both species as well as an important decrease in size resulting both from a slower growth and a progressive disappearance of older sardines. Interestingly, older sardines were more affected by the decrease in condition than younger ones, another sign of an unbalanced population structure. While top-down control by bluefin tuna or dolphins, emigration and disease were mostly discarded as important drivers, bottom-up control mediated by potential changes in the plankton community appeared to play an important role via a decrease in fish energy income and hence growth, condition and size. Isotopic and stomach content analyses indicated a dietary shift pre-and post-2008 and modeled mesozooplankton abundance was directly linked to fish condition. Despite low energy reserves from 2008 onwards, sardines and anchovies maintained if not increased their reproductive investment, likely altering the lifehistory trade-off between reproduction and survival and resulting in higher natural mortality. The current worrying situation might thus have resulted from changes in plankton availability/diversity, which remains to be thoroughly investigated together with fish phenotypic plasticity.
Comparative metabolic rates of common western North Atlantic Ocean sciaenid fishes
Journal of Fish Biology, 2011
The resting metabolic rates (R R ) of western North Atlantic Ocean sciaenids, such as Atlantic croaker Micropogonias undulatus, spot Leiostomus xanthurus and kingfishes Menticirrhus spp., as well as the active metabolic rates (R A ) of M. undulatus and L. xanthurus were investigated to facilitate inter and intraspecific comparisons of their energetic ecology. The R R of M. undulatus and L. xanthurus were typical for fishes with similar lifestyles. The R R of Menticirrhus spp. were elevated relative to those of M. undulatus and L. xanthurus, but below those of high-energy-demand species such as tunas Thunnus spp. and dolphinfish Coryphaena hippurus. Repeated-measures non-linear mixed-effects models were applied to account for within-individual autocorrelation and corrected for non-constant variance typical of noisy R A data sets. Repeated-measures models incorporating autoregressive first-order [AR(1)] and autoregressive moving average (ARMA) covariances provided significantly superior fits, more precise parameter estimates (i.e. reduced s.e.) and yintercept estimates that more closely approximated measured R R for M. undulatus and L. xanthurus than standard least-squares regression procedures.