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Harvesting Forage Fish Can Prevent Fishing-Induced Population

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Harvesting Forage Fish Can Prevent Fishing-Induced Population Harvesting forage fish can prevent fishing-induced population collapses of large piscivorous fish Floor H. Soudijna,b,c,1 , P. Daniel¨ van Denderend , Mikko Heinoc,e,f , Ulf Dieckmannc,g , and Andre´ M. de Roosb,h aEcological Dynamics Group, Wageningen Marine Research, 1976 CP IJmuiden, The Netherlands; bInstitute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, 1098 XH Amsterdam, The Netherlands; cEvolution and Ecology Program, International Institute for Applied Systems Analysis, A-2361 Laxenburg, Austria; dCentre for Ocean Life, Technical University of Denmark Aqua, Technical University of Denmark, 2800 Kongens Lyngby, Denmark; eDepartment of Biological Sciences, University of Bergen, 5006 Bergen, Norway; fInstitute of Marine Research, 5005 Bergen, Norway; gDepartment of Evolutionary Studies of Biosystems, The Graduate University for Advanced Studies (Sokendai), Hayama, Kanagawa 240-0193, Japan; and hSanta Fe Institute, Santa Fe, NM 87501 Edited by Alan Hastings, University of California, Davis, CA, and approved December 16, 2020 (received for review October 1, 2019) Fisheries have reduced the abundances of large piscivores—such that are considered essential in models of fish communities are: as gadids (cod, pollock, etc.) and tunas—in ecosystems around 1) fish-population size structure, 2) consistent accounting of the the world. Fisheries also target smaller species—such as herring, bioenergetic flows through fish populations and communities, capelin, and sprat—that are important parts of the piscivores’ and 3) size-selective predation and harvesting (10, 11). diets. It has been suggested that harvesting of these so-called In this study, we investigate the effects of fishing for for- forage fish will harm piscivores. Multispecies models used for fish- age fish on their predators, the piscivorous fish. We do this eries assessments typically ignore important facets of fish commu- using a published model of the central Baltic Sea community nity dynamics, such as individual-level bioenergetics and/or size dynamics (12) that was specifically designed to investigate effects structure. We test the effects of fishing for both forage fish and of fisheries on fish communities (10, 12). The model incor- piscivores using a dynamic, multitrophic, size-structured, bioener- porates size-structured fish populations, size-dependent feed- getics model of the Baltic Sea. In addition, we analyze historical ing interactions, and individual-level energy budgets. In addi- patterns in piscivore-biomass declines and fishing mortalities of tion, consumption by fish has a direct effect on their food piscivores and forage fish using global fish-stock assessment data. sources, and the flows of energy throughout the system are Our community-dynamics model shows that piscivores benefit thus accounted for consistently. Using this model, we explore from harvesting of their forage fish when piscivore fishing mor- the effects of fishing for both forage fish and piscivores. We ECOLOGY tality is high. With substantial harvesting of forage fish, the first focus on the Baltic Sea because its food web is rela- piscivores can withstand higher fishing mortality. On the other tively simple and the exploited fish species include both pis- hand, when piscivore fishing mortality is low, piscivore biomass civores (cod) and forage fish (sprat and herring) (13). Using decreases with more fishing of the forage fish. In accordance the global Ransom A. Myers (RAM) Legacy Stock Assessment with these predictions, our statistical analysis of global fisheries Database (14), we then statistically evaluate historical patterns data shows a positive interaction between the fishing mortali- in piscivore biomasses and fishing pressures on forage fish and ties of forage-fish stocks and piscivore stocks on the strength of piscivores. Our dynamic and statistical model analyses agree piscivore-biomass declines. While overfishing of forage fish must in demonstrating that harvesting forage fish does not always be prevented, our study shows that reducing fishing pressures affect piscivore populations negatively. Instead, such fishing can on forage fish may have unwanted negative side effects on pis- civores. In some cases, decreasing forage-fish exploitation could cause declines, or even collapses, of piscivore stocks. Significance fisheries j predator–prey dynamics j multiple trophic levels j In many marine ecosystems, fisheries target predatory fish, size-structured model j bioenergetics known as piscivores, as well as their prey fish, known as forage fish. It is generally thought that harvesting of forage fish neg- atively affects piscivore population abundance and resilience. isheries target both large piscivorous fish—such as gadids Here, we show that, contrary to this widely held belief, piscivo- F(cod, pollock, etc.) and tunas—and small planktivorous fish rous fish stocks exposed to high fishing mortality benefit from or forage fish—such as herring, capelin, and sprat (1). Large harvesting of their forage fish. On the other hand, piscivorous piscivores are generally more valued for human consumption. fish stocks exposed to low fishing mortality are reduced by har- Yet, forage fish constitute a substantial 20 to 30% of global fish- vesting of their forage fish. The beneficial effect occurs when eries landings (2). Often, both forage fish and large piscivores the harvesting of forage fish releases density dependence in the are fished for in the same ecosystems (3, 4). Forage fish serve forage-fish population. Our findings have implications for pol- as a food source for large piscivores, and it is commonly under- icy advice regarding the management of forage-fish fisheries stood that harvesting of forage fish may indirectly harm the large and the protection of piscivorous fish stocks. piscivores that depend on them (3, 4). The importance of an ecosystem-based, multispecies approach Author contributions: F.H.S., P.D.v.D., M.H., U.D., and A.M.d.R. designed research; F.H.S. to fisheries management is underscored by potential indirect performed research; F.H.S. and P.D.v.D. analyzed data; F.H.S., P.D.v.D., M.H., U.D., and negative effects of forage-fish fisheries on piscivores (5, 6). A.M.d.R. wrote the paper; and F.H.S. and A.M.d.R. provided community-dynamics model Unraveling the effects of multispecies fisheries is a serious chal- construction and analysis.y lenge due to feedbacks between fisheries, fish populations, and The authors declare no competing interest.y the fishes’ food sources (e.g., ref. 7). Understanding these effects This article is a PNAS Direct Submission.y is further complicated by nonlinearities in population-level pro- Published under the PNAS license.y cesses (8). The effects of fishing on multiple species at different See online for related content such as Commentaries.y trophic levels of marine ecosystems are usually assessed by using 1 To whom correspondence may be addressed. Email: [email protected] multispecies fisheries models (4, 9). However, it has recently This article contains supporting information online at https://www.pnas.org/lookup/suppl/ been argued that such models do not contain all necessary pro- doi:10.1073/pnas.1917079118/-/DCSupplemental.y cesses to predict fish-community dynamics (10, 11). Components Published February 2, 2021. PNAS 2021 Vol. 118 No. 6 e1917079118 https://doi.org/10.1073/pnas.1917079118 j 1 of 8 Downloaded by guest on September 30, 2021 protect large-piscivore populations from fishing-induced col- Community-Dynamics Model of the Baltic Sea. To analyze the effects of mul- lapses. These results challenge the generally accepted idea that tispecies fishing, we used the stage-structured bioenergetics model of the large piscivores always benefit from less fishing of their forage central Baltic Sea introduced by van Leeuwen et al. (12). The model includes fish (6, 15). the key ecological interactions between predatory and forage fish and their resources; it is aimed to qualitatively reproduce the dynamics of this sys- Materials and Methods tem. We improve on the model by van Leeuwen et al. (12) by implementing Below, we present the models used for our analyses in general terms. reproduction as a seasonal process, following Soudijn and de Roos (16). The Details are described in SI Appendix, in Appendix A for the community- model structure and size-based, stage-specific parameterization are derived dynamics model and in Appendix B for the statistical model. Analysis of from individual-level data of Baltic cod, sprat, and herring (SI Appendix, the community-dynamics model was carried out by using publicly avail- Appendix A). Fig. 1A shows the interactions between fish populations in able C-based simulation programs. The statistical model analysis is based the model. Sprat and herring, the prey fish of cod, are assumed to have a on publicly available data. similar ecological role. Hence, they are modeled as a clupeid population that A Cod Clupeid Juvenile cod resource Adult cod resource Clupeid resource BC 0.012 ) 0.06 3 ) 3 0.009 0.04 0.006 0.02 Adult cod 0.003 Annual clupeid biomass (kg/m 0.000 reproduction (kg/m 0.00 0.35 0.04 ) ) 3 3 0.30 0.03 0.02 0.25 Adult clupeid 0.01 Juvenile clupeid biomass (kg/m biomass (kg/m 0.20 0.00 ) 0.12 -1 0.003 ) 3 0.09 0.002 0.06 0.001 0.03 Adult clupeid biomass (kg/m Clupeid resource 0.000 0.00 net-production rate (d 0 102030 Low High Time (years) Clupeid fishing mortality Fig. 1. Interactions between cod and clupeids in the community-dynamics model of the Baltic Sea. (A) Trophic interactions among fisheries (narrow gray arrows), cod and clupeid stages, and their resources (black arrows; see SI Appendix, Table S4 for the foraging preferences of the cod stages). Individual fish grow through stages from left to right (broad gray arrows). (B) Time series of adult cod biomass (Top) and adult clupeid biomass (Middle), both including −1 −1 reproductive storages; and biomass of the clupeid resource (Bottom) for low (black lines; FS = 0.2 year ) and high (red lines; FS = 0.5 year ) clupeid fishing −1 mortality.
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