Isaac C. Kaplan

Impacts of Fishing Low-Trophic Level Species on Marine Ecosystems

High harvest levels of low–trophic level fishes may have cascading marine ecosystem effects. Low–trophic level species account for more than 30% of global fisheries production and contribute substantially to global food security. We used a range of ecosystem models to explore the effects of fishing low–trophic level species on marine ecosystems, including marine mammals and seabirds, and on other commercially important species. In five well-studied ecosystems, we found that fishing these species at conventional maximum sustainable yield (MSY) levels can have large impacts on other parts of the ecosystem, particularly when they constitute a high proportion of the biomass in the ecosystem or are highly connected in the food web. Halving exploitation rates would result in much lower impacts on marine ecosystems while still achieving 80% of MSY.

Visualizing the Food-Web Effects of Fishing for Tunas in the Pacific Ocean

We use food-web models to develop visualizations to compare and evaluate the interactions of tuna fisheries with their supporting food webs in the eastern tropical Pacific (ETP) and the central north Pacific (CNP) Oceans. In the ETP and CNP models, individual fisheries use slightly different food webs that are defined by the assemblage of targeted tuna species. Distinct energy pathways are required to support different tuna species and, consequently, the specific fisheries that target different tuna assemblages. These simulations suggest that catches of tunas, sharks, and billfishes have lowered the biomass of the upper trophic levels in both systems, whereas increases in intermediate and lower trophic level animals have accompanied the decline of top predators. Trade- offs between fishing and predation mortality rates that occur when multiple fisheries interact with their respective food webs may lead to smaller changes in biomass than if only the effect of a single fishery is considered. Historical simulations and hypothetical management scenarios further demonstrate that the effects of longline and purse seine fisheries have been strongest in upper trophic levels, but that lower trophic levels may respond more strongly to purse-seine fisheries. The apex predator guild has responded most strongly to longlining. Simulations of alternative management strategies that attempt to rebuild shark and billfish populations in each ecosystem reveal that (1) changes in longlining more effectively recover top predator populations than do changes in purse seining and (2) restrictions on both shallow-set longline gear and shark finning may do more to recover top predators than do simple reductions in fishing effort.

Impacts of depleting forage species in the California Current

Human demands for food and fish meal are often in direct competition with forage needs of marine mammals, birds and piscivorous harvested fish. Here, two well-developed ecosystem models for the California Current on the West Coast of the USA were used to test the impacts on other parts of the ecosystem of harvesting euphausiids, forage fish, mackerel and mesopelagic fish such as myctophids. Depleting individual forage groups to levels that led to maximum sustainable yield of those groups may have both positive and negative effects on other species in the California Current. The most common impacts were on predators of forage groups, some of which showed declines of >20% under the scenarios that involved depletion of forage groups to 40% of unfished levels. Depletion of euphausiids and forage fish, which each comprise >10% of system biomass, had the largest impact on other species. Depleting euphausiids to 40% of unfished levels altered the abundance of 13–30% of the other functional groups by >20%; while depleting forage fish to 40% altered the abundance of 20–50% of the other functional groups by >20%. There are clear trade-offs between the harvest of forage groups and the ability of the California Current to sustain other trophic levels. Though higher trophic level species, such as groundfish, are often managed on the basis of reference points that can reduce biomass to below half of unfished levels, this level of forage species removal is likely to impact the abundance of other target species, protected species and the structure of the ecosystem.

A statistical approach for estimating fish diet compositions from multiple data sources: Gulf of California case study

Trophic ecosystem models are one promising tool for providing ecosystem-based management advice. Diet and interaction rate parameters are critical in defining the behavior of these models, and will greatly influence any predictions made in response to management perturbations. However, most trophic ecosystem models must rely on a patchwork of data availability and must contend with knowledge gaps and poor quantification of uncertainty. Here we present a statistical method for combining diet information from field samples and literature to describe trophic relationships at the level of functional groups. In this example, original fieldwork in the northern Gulf of California, Mexico, provides gut content data for targeted and untargeted fish species. The field data are pooled with diet composition information from FishBase, an online data repository. Diet information is averaged across stomachs to represent an average predator, and then the data are bootstrapped to generate likelihood profiles. These are fit to a Dirichlet function, and from the resulting marginal distributions, maximum-likelihood estimates are generated with confidence intervals representing the likely contribution to diet for each predator-prey combination. We characterize trophic linkages into two broad feeding guilds, pelagic and demersal feeders, and explore differentiation within those guilds. We present an abbreviated food web for the northern Gulf of California based on the results of this study. This food web will form the basis of a trophic dynamic model. Compared to the common method of averaging diet compositions across predators, this statistical approach is less influenced by the presence of long tails in the distributions, which correspond to rare feeding events, and is therefore better suited to small data sets.

An Integrated Coral Reef Ecosystem Model to Support Resource Management under a Changing Climate

Millions of people rely on the ecosystem services provided by coral reefs, but sustaining these benefits requires an understanding of how reefs and their biotic communities are affected by local human-induced disturbances and global climate change. Ecosystem-based management that explicitly considers the indirect and cumulative effects of multiple disturbances has been recommended and adopted in policies in many places around the globe. Ecosystem models give insight into complex reef dynamics and their responses to multiple disturbances and are useful tools to support planning and implementation of ecosystem-based management. We adapted the Atlantis Ecosystem Model to incorporate key dynamics for a coral reef ecosystem around Guam in the tropical western Pacific. We used this model to quantify the effects of predicted climate and ocean changes and current levels of current land-based sources of pollution (LBSP) and fishing. We used the following six ecosystem metrics as indicators of ecosystem state, resilience and harvest potential: 1) ratio of calcifying to non-calcifying benthic groups, 2) trophic level of the community, 3) biomass of apex predators, 4) biomass of herbivorous fishes, 5) total biomass of living groups and 6) the end-to-start ratio of exploited fish groups. Simulation tests of the effects of each of the three drivers separately suggest that by mid-century climate change will have the largest overall effect on this suite of ecosystem metrics due to substantial negative effects on coral cover. The effects of fishing were also important, negatively influencing five out of the six metrics. Moreover, LBSP exacerbates this effect for all metrics but not quite as badly as would be expected under additive assumptions, although the magnitude of the effects of LBSP are sensitive to uncertainty associated with primary productivity. Over longer time spans (i.e., 65 year simulations), climate change impacts have a slight positive interaction with other drivers, generally meaning that declines in ecosystem metrics are not as steep as the sum of individual effects of the drivers. These analyses offer one way to quantify impacts and interactions of particular stressors in an ecosystem context and so provide guidance to managers. For example, the model showed that improving water quality, rather than prohibiting fishing, extended the timescales over which corals can maintain high abundance by at least 5–8 years. This result, in turn, provides more scope for corals to adapt or for resilient species to become established and for local and global management efforts to reduce or reverse stressors.

Circle Hooks for Pacific Longliners: Not a Panacea for Marlin and Shark Bycatch, but Part of the Solution

Abstract Blue marlin Makaira nigricans, striped marlin Tetrapturus audax, and pelagic sharks (e.g., blue shark Prionace glauca) are commonly caught as bycatch by longline fisheries in the central North Pacific Ocean. Recently, concern has increased about depletion of these species. Modifications in longline gear may offer one solution. Here, we test the use of circle hooks, rather than the conventional tuna-style hooks, on longlines using an ecosystem model of the central North Pacific Ocean. The simulations considered span a range of reasonable circle hook catchability and survival rates for released fish. The results suggest that if circle hooks have higher catchability than the currently used tuna-style hooks, switching to circle hooks depletes marlin biomass by 25–40% and shark biomass by 15–35% over 30 years. However, these depletions do not occur if circle hook catchability is equal to or lower than that of tuna-style hooks. When the effects of catch-and-release requirements for marlins and sharks w...

Exploring trade-offs between fisheries and conservation of the vaquita porpoise (Phocoena sinus) using an Atlantis ecosystem model.

Background Minimizing fishery bycatch threats might involve trade-offs between maintaining viable populations and economic benefits. Understanding these trade-offs can help managers reconcile conflicting goals. An example is a set of bycatch reduction measures for the Critically Endangered vaquita porpoise (Phocoena sinus), in the Northern Gulf of California, Mexico. The vaquita is an endemic species threatened with extinction by artisanal net bycatch within its limited range; in this area fisheries are the chief source of economic productivity. Methodology/Principal Findings We analyze trade-offs between conservation of the vaquita and fisheries, using an end-to-end Atlantis ecosystem model for the Northern Gulf of California. Atlantis is a spatially-explicit model intended as a strategic tool to test alternative management strategies. We simulated increasingly restrictive fisheries regulations contained in the vaquita conservation plan: implementing progressively larger spatial management areas that exclude gillnets, shrimp driftnets and introduce a fishing gear that has no vaquita bycatch. We found that only the most extensive spatial management scenarios recovered the vaquita population above the threshold necessary to downlist the species from Critically Endangered. The scenario that excludes existing net gear from the 2008 area of vaquita distribution led to moderate decrease in net present value (US

Indirect Effects of Conservation Policies on the Coupled Human-Natural Ecosystem of the Upper Gulf of California

42 million) relative to the best-performing scenario and a two-fold increase in the abundance of adult vaquita over the course of 30 years. Conclusions/Significance Extended spatial management resulted in the highest recovery of the vaquita population. The economic cost of proposed management actions was unequally divided between fishing fleets; the loss of value from finfish gillnet fisheries was never recovered. Our analysis shows that managers will have to confront difficult trade-offs between management scenarios for vaquita conservation.

Trade-Offs between Species Conservation and the Size of Marine Protected Areas

High bycatch of non-target species and species of conservation concern often drives the implementation of fisheries policies. However, species- or fishery-specific policies may lead to indirect consequences, positive or negative, for other species or fisheries. We use an Atlantis ecosystem model of the Northern Gulf of California to evaluate the effects of fisheries policies directed at reducing bycatch of vaquita (Phocoena sinus) on other species of conservation concern, priority target species, and metrics of ecosystem function and structure. Vaquita, a Critically Endangered porpoise endemic to the Upper Gulf of California, are frequently entangled by finfish gillnets and shrimp driftnets. We tested five fishery management scenarios, projected over 30 years (2008 to 2038), directed at vaquita conservation. The scenarios consider progressively larger spatial restrictions for finfish gillnets and shrimp driftnets. The most restrictive scenario resulted in the highest biomass of species of conservation concern; the scenario without any conservation measures in place resulted in the lowest. Vaquita experienced the largest population increase of any functional group; their biomass increased 2.7 times relative to initial (2008) levels under the most restrictive spatial closure scenario. Bycatch of sea lions, sea turtles, and totoaba decreased > 80% in shrimp driftnets and at least 20% in finfish gillnet fleets under spatial management. We found indirect effects on species and ecosystem function and structure as a result of vaquita management actions. Biomass and catch of forage fish declined, which could affect lower-trophic level fisheries, while other species such as skates, rays, and sharks increased in both biomass and catch. When comparing across performance metrics, we found that scenarios that increased ecosystem function and structure resulted in lower economic performance indicators, underscoring the need for management actions that consider ecological and economic tradeoffs as part of the integrated management of the Upper Gulf of California.

Risks of ocean acidification in the California Current food web and fisheries: ecosystem model projections

Moving from single-species- to ecosystem-based management requires an understanding of how community-level attributes such as diversity change with area. We used survey data from bottom trawls to examine spatial patterns of species richness in U.S. Pacific coastal fishes. Specifically, we generated and compared species-area relationships (SARs) for species classified into several groups on the basis of maximum body size, trophic level, diet, maximum depth, geographic affinity, and taxonomic order. Because SARs among groups were not parallel and z values varied significantly for several groups, groups of species were under- or overrepresented (depending on the size of the area) relative to their proportions in the entire community (i.e., entire U.S. Pacific coast). In this way, differences in SARs help demonstrate trade-offs between species representation and coastal area and suggest strategies (such as targeting the protection of habitats and locations where a particular species or groups of species are maximized) that may minimize the size of marine protected areas (MPAs) but protect diversity at the level of the community and functional group.