Villy Christensen | University of British Columbia (original) (raw)

Papers by Villy Christensen

Marine Ecology Progress Series, 2012

ABSTRACT In recent decades the eastern stock of Atlantic bluefin tuna Thunnus thynnus thynnus has... more ABSTRACT In recent decades the eastern stock of Atlantic bluefin tuna Thunnus thynnus thynnus has declined, driven by excessive catches partially mediated by the growth of the capture-based aquaculture of bluefin in the Mediterranean. This study addresses the potential direct and indirect food-web effects on trophic linkages in the ecosystem through the removal of both small pelagic fish species and wild bluefin tuna for aquaculture operations in the Mediterranean. A mass-balanced model representing the southern Catalan Sea during the 1990s was modified to include a bluefin tuna farm partially supplied from fish captured in the area modeled. Six scenarios were developed to simulate possible changes to the capture-based aquaculture operations and possible impacts to the ecosystem. The addition of one bluefin tuna farm in the Catalan Sea did not produce substantial effects in the ecosystem. Simulation scenarios that include the level of production already present in a similar ecosystem in the Murcia region resulted in large fluctuations of both biomass and yield for bluefin tuna, as well as for many species in the modeled ecosystem. Increases in biomass of lower trophic level functional groups were observed with reductions in biomass from higher trophic level predators. These outcomes demonstrate that removal of biomass at top and intermediate trophic levels can have direct and indirect outcomes on the structure of the ecosystem due to the complexity of the food web. Our results suggest, in the case of the Western Mediterranean Sea, increasing bluefin tuna farming activities will likely contribute towards further degradation of an already highly exploited ecosystem.

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Ecological Modelling, 2011

Ecosystem models play an important role in supporting ecosystem approaches to management. To impr... more Ecosystem models play an important role in supporting ecosystem approaches to management. To improve the representation of how ecosystems work, ecosystem models should be able to represent mediating effects (e.g., habitat provision) that species provide to each other as well as species (re)introductions, both common situations that can strongly influence ecosystem dynamics. We examine how such processes can be incorporated into Ecopath with Ecosim (EwE), a widely used tool for represent aquatic ecosystems with the potential to support ecosystem-based management. We used the reintroduction of sea otters (Enhydralutris) to the west coast of Vancouver Island, British Columbia, Canada as a case study. The model demonstrates how to account for benefits provided by kelp forests by contributing to primary production, increased feeding areas and food availability through prey retention. It also demonstrates how the reintroduction and range expansion of sea otters can be represented in Ecospace, and the implications of these options.

Ices Journal of Marine Science, Jan 1, 2005

Environmental Biology of Fishes, Jan 1, 1989

The influence of light and prey abundance on the vertical distribution of herring larvae was eval... more The influence of light and prey abundance on the vertical distribution of herring larvae was evaluated by three investigations made under calm weather conditions in the North Sea off the Scottish coast. The investigations took place at different time after hatching and the vertical distributions of three size groups of larvae (mean sizes 8,15 and 19 mm) were related to time of day and the vertical distribution of copepods. No migratory behaviour of copepods was observed but their vertical distribution differed between investigations. In the investigation on intermediate sized larvae, copepod density peaked at the pycnocline (40 m). Larvae concentrated at this depth at noon. At dawn and dusk larvae migrated towards the surface and the vertical distributions fluctuated semidielly. In the two other investigations, copepods were homogeneously distributed in the water column and after migration towards the surface at dawn larvae stayed in the upper water column during the day. The observations suggest that the daytime vertical distribution of larvae in calm weather is mainly determined by feeding conditions: the larvae move to depths were light is sufficient for feeding, and refinement within that zone is made according to a compromise between optimal light conditions for feeding and optimal prey densities.

Ecological Engineering, Jan 1, 1995

A tentative list of ecological attributes; diversity, cycling, stability and capacity, is propose... more A tentative list of ecological attributes; diversity, cycling, stability and capacity, is proposed for quantification and ranking of farming systems along a hypothetical scale of increasing ecological sustainability. The modelling and preliminary analysis of four rice-based systems from the Philippines, two on-station experimental systems and two smallholder farms, confirms our intuitive perception of ecological sustainability and suggests a framework for further research into the area. This approach to farming systems analysis indicates that the route to an ecologically sustainable agriculture relies more on the principles of applied agro-ecology and less on the application of external agrochemical inputs. The practice is here termed integrated resources management and shares many of the characteristics of the agro-ecological engineering approach to integrated agriculture-aquaculture farming pioneered in China.

Marine Ecology-progress Series, Jan 1, 1988

American Scientist, Jan 1, 2000

Ecological Applications, Jan 1, 1998

Using a top-down modeling approach, published mass-balance models of trophic interactions and sta... more Using a top-down modeling approach, published mass-balance models of trophic interactions and state variables in the western central Pacific Ocean and the northern Gulf of Mexico shelf were used to explore how large increases in top predator biomasses can be accommodated with given primary productions. It appears that the biomasses of top predators in these models can be increased an order of magnitude, which leads to a six-to sevenfold increase in overall consumer biomasses. This results in changes in food web structures that are in agreement with major aspects of E. P. Odum's theory of ecosystem development, particularly with regard to features associated with the retention and recycling of detritus. Based on the simulations and Odum's theory, we propose a functional definition of carrying capacity: the upper limit of biomass that can be supported by a set primary production and within a variable food web structure is reached when total system respiration equals the sum of primary production and detritus import. The theory supported by the simulations has a number of corollaries relevant to sustainable management of ecosystems.

Journal of Fish Biology, Jan 1, 1998

Two mass-balance trophic models are constructed to describe the Gulf of Thailand ecosystem (10-50... more Two mass-balance trophic models are constructed to describe the Gulf of Thailand ecosystem (10-50 m depth): one model pertains to the initial phase of fisheries development, and the other to when the resources were severely depleted. The two phases are compared, and changes brought about by fishing discussed. A dynamic simulation model, Ecosim, is then used successfully to reproduce the 1980 state of the fishery based on the 1963 model and the development in catches. In addition the 1980 model is used to predict how the ecosystem groups may bounce back following marked reduction in fishing pressure. Finally, the 1963 model is used to study alternative scenarios for how the fisheries development could take place, notably the effect of exploiting only the resources of larger species. The study validates that the Ecosim model can be used to predict ecosystem level changes following changes in fishing pressure, therefore fishing induced changes can to a large extent explain the changes in ecosystem pools and fluxes observed over time. 1998 The Fisheries Society of the British Isles

Reviews in Fish Biology and Fisheries, Jan 1, 1996

Several management strategies for ecosystems with biological interaction are discussed, including... more Several management strategies for ecosystems with biological interaction are discussed, including predator removal, predator-prey coexistence, prey exploitation, overexploitation, and introduction of sanctuaries. Some case studies related to ecosystem management are briefly presented; these describe Lakes Victoria and Tanganyika, discarding from shrimp trawl fisheries and the development in the North Sea that led to introduction of multispecies analysis. The concept of ‘fishing down the food web’ is discussed and the average trophic levels at which the fisheries operate in different ecosystem types are estimated based on quantified trophic flow models. On a global level, while on average fisheries operate around two trophic levels above the primary producers, still one third of the catch of the 70 major fish species caught in the world is of piscivorous fish. Using exploitation-predation rate indices for different ecosystem types, the amount of finfish consumed globally by finfish is roughly estimated to be three times the catches of finfish. Finally some implications for the management of ecosystems are drawn up. It makes little difference if short-term prognoses are based on single-species or multispecies considerations. Multispecies models may, however, give the better long-term advice, and adaptive management may facilitate the move towards such long-term goals.

Journal of Northwest Atlantic Fishery Science, Jan 1, 1997

The degree of competition between fisheries and marine mammals in the Pacific Ocean was estimated... more The degree of competition between fisheries and marine mammals in the Pacific Ocean was estimated for 7 statistical areas defined by the Food and Agriculture Organization of the United Nations (FAO). Catch statistics compiled from FAO sources show that the amount of fish caught in the Pacific Ocean rose from 2 million tons in the late-1940s to over 50 million tons in the early-1990s. Recent stagnation and declines occurring in some areas of the Pacific suggest that Pacific fisheries cannot continue to expand as they had previously.

Ecosystems, Jan 1, 2000

EcoSim II uses results from the Ecopath procedure for trophic mass-balance analysis to define bio... more EcoSim II uses results from the Ecopath procedure for trophic mass-balance analysis to define biomass dynamics models for predicting temporal change in exploited ecosystems. Key populations can be represented in further detail by using delay-difference models to account for both biomass and numbers dynamics. A major problem revealed by linking the population and biomass dynamics models is in representation of population responses to changes in food supply; simple proportional growth and reproductive responses lead to unrealistic predictions of changes in mean body size with changes in fishing mortality. EcoSim II allows users to specify life history mechanisms to avoid such unrealistic predictions: animals may translate changes in feeding rate into changes in reproductive rather than growth rates, or they may translate changes in food availability into changes in foraging time that in turn affects predation risk. These options, along with model relationships for limits on prey availability caused by predation avoidance tactics, tend to cause strong compensatory responses in modeled populations. It is likely that such compensatory responses are responsible for our inability to find obvious correlations between interacting trophic components in fisheries time-series data. But Ecosim II does not just predict strong compensatory responses: it also suggests that large piscivores may be vulnerable to delayed recruitment collapses caused by increases in prey species that are in turn competitors/predators of juvenile piscivores.

Ecosystems, Jan 1, 1999

Growing disillusion with the predictive capability of single species fisheries assessment methods... more Growing disillusion with the predictive capability of single species fisheries assessment methods and the realization that the management approaches they imply will always fail to protect bycatch species has led to growing interest in the potential of marine protected areas (MPAs) as a tool for protecting such species and allowing for rebuilding populations of target species and damaged habitat. Ecospace is a spatially explicit model for policy evaluation that allows for considering the impact of MPAs in an ecosystem (that is, trophic) context, and that relies on the Ecopath mass-balance approach for most of its parameterization. Additional inputs are movement rates used to compute exchanges between grid cells, estimates of the importance of trophic interactions (top-down vs bottom up control), and habitat preferences for each of the functional groups included in the model. An application example, including the effect of an MPA, and validation against trawl survey data is presented in the form of a color map illustrating Ecospace predictions of biomass patterns on the shelf of Brunei Darussalam, Southeast Asia. A key general prediction of Ecospace is spatial “cascade” effects, wherein prey densities are low where predators are abundant, for example, in protected areas or areas where fishing costs are high. Ecospace also shows that the potential benefits of local protection can be easily negated by high movement rates, and especially by concentration of fishing effort at the edge of the MPAs, where cascade effects generate prey gradients that attract predators out of the protected areas. Despite various limitations (for example, no explicit consideration of seasonal changes or directed migration), the outward simplicity of Ecospace and the information-rich graphs it generates, coupled with the increasingly global availability of the required Ecopath files, will likely ensure a wide use for this approach, both for generating hypotheses about ecosystem function and evaluating policy choices.

Ices Journal of Marine Science, Jan 1, 1998

1998. Diet composition and trophic levels of marine mammals. -ICES Journal of Marine Science, 55:... more 1998. Diet composition and trophic levels of marine mammals. -ICES Journal of Marine Science, 55: 467-481.

Ecological Modelling, Jan 1, 1995

An attempt is made to rank 41 steady-state models of aquatic ecosystems on the basis of their mat... more An attempt is made to rank 41 steady-state models of aquatic ecosystems on the basis of their maturity. Maturity is quantified using several of Odum's attributes of ecosystem maturity. It is shown that it is possible to make such a ranking, and that the ranking seems to confirm our intuitive perception of ecosystem maturity. The ranking is compared to rankings based on various ecosystem goal functions. The maturity ranking shows a strong negative correlation with relative ascendency, and thus a strong positive correlation with system overhead, a possible measure of ecosystem stability. The analyses suggest that another goal function, exergy, as calculated here is mainly a function of system biomass, and that it might be appropriate to reconsider the computational aspect of exergy estimation. Most importantly the analyses point to the feasibility of using comparisons of ecosystem models as a tool for enhancing our understanding of ecosystem characteristics, notably sustainability.

Fish and Fisheries, Jan 1, 2003

We estimate the biomass of high-trophic level ¢shes in the North Atlantic at a spatial scale of 0... more We estimate the biomass of high-trophic level ¢shes in the North Atlantic at a spatial scale of 0.58 latitude by 0.58 longitude based on 23 spatialized ecosystem models, each constructed to represent a given year or short period from1880 to1998.We extract over 7800 data points that describe the abundance of high-trophic level ¢shes as a function of year, primary production, depth, temperature, latitude, ice cover and catch composition. We then use a multiple linear regression to predict the spatial abundance for all North Atlantic spatial cells for 1900 and for each year from 1950 to 1999. The results indicate that the biomass of high-trophic level ¢shes has declined by two-thirds during the last 50-year period, and with a factor of nine over the century. Catches of hightrophic level ¢shes increased from 2.4 to 4.7 million tonnes annually in the late 1960s, and subsequently declined to below 2 million tonnes annually in the late 1990s. The ¢shing intensity for high-trophic level ¢shes tripled during the ¢rst half of the time period and remained high during the last half of the time period. Comparing the ¢shing intensity to similar measures from 35 assessments of high-trophic level ¢sh populations from the North Atlantic, we conclude that the trends in the two data series are similar. Our results raise serious concern for the future of the North Atlantic as a diverse, healthy ecosystem; we may soon be left with only low-trophic level species in the sea.

Ecological Modelling, Jan 1, 2004

The Ecopath with Ecosim (EwE) modeling approach combines software for ecosystem trophic mass bala... more The Ecopath with Ecosim (EwE) modeling approach combines software for ecosystem trophic mass balance analysis (Ecopath), with a dynamic modeling capability (Ecosim) for exploring past and future impacts of fishing and environmental disturbances as well as for exploring optimal fishing policies. Ecosim models can be replicated over a spatial map grid (Ecospace) to allow exploration of policies such as marine protected areas, while accounting for spatial dispersal/advection effects.The Ecopath approach and software has been under development for two decades, with Ecosim emerging in 1995, and Ecospace in 1998, leading to an integrated and widely applied package. We present an overview of the computational aspects of the Ecopath, Ecosim and Ecospace modules as they are implemented in the most recent software version. The paper summarizes the capabilities of the modeling system with respect to evaluating how fisheries and the environment impact ecosystems. We conclude by a warning about pitfalls in the use of the software for policy exploration.