New target fisheries lead to spatially variable food web effects in an ecosystem model of the California Current

Highlights

• We model the effects of developing fisheries that target low biomass species.

• A spatially explicit Atlantis ecosystem model shows scale-dependent effects.

• Regional effects were much larger than system-wide effects.

• Even fishing low biomass targets has indirect effects in the model.

Abstract

Growing human populations put increasing demands on marine ecosystems. Studies have demonstrated the importance of large biomass forage groups in model food webs, but small biomass contributors are often overlooked. Here, we predict the ecosystem effects of three potential future fisheries targeting functional groups that make up only a small proportion of total ecosystem biomass using the California Current Atlantis Model: deep demersal fish such as grenadier (Albatrossia pectoralis and Coryphaenoides acrolepis), nearshore fish such as white croaker (Genyonemus lineatus), and shortbelly rockfish (Sebastes jordani). Using a spatially explicit ecosystem model, we explored individual fishing scenarios for these groups that resulted in abundance levels of 75, 40, 25, and 0 percent of the status quo fishing scenario and a combined fishing scenario simultaneously targeting all three groups. We evaluated the effects on coast-wide biomass and describe variation in affected groups by region. Results indicate that developing fisheries on the proposed targets would have small coast-wide effects on other species. However, effects varied significantly within the ecosystem, with higher impacts concentrated in the central California region of the model. Effects of fishing all three groups simultaneously were additive in some cases coastwide, but were not additive at the regional scale. This work provides a framework for evaluating effects of new fisheries and suggests that regional effects should be evaluated within a larger management context.

Introduction

Anthropogenic stressors on ecosystems have never been greater. Demands for food and freshwater have led to increased fishing (Anticamara et al., 2011), changes in land use patterns (Allan, 2004), and greater regulation of flows in rivers (Caissie, 2006). As pressure to extract resources builds, managers are increasingly confronting trade-offs between the direct value of the extracted resource and the indirect value of the resource left in the ecosystem (Rodriguez et al., 2006, Lester et al., 2010). Understanding these trade-offs requires approaches that can predict the indirect effects of targeted resource extraction on the rest of the ecosystem.

In marine ecosystems, demands for fish and fishmeal have led to fishing activities targeting more species than in previous decades (Alder et al., 2008, Branch et al., 2010, Anderson et al., 2011). Often, new target species have low trophic levels, are important prey species for higher trophic level species that are of commercial importance and/or conservation concern. Therefore, a trade-off exists between the value of harvesting these forage species and the value of leaving them as prey for other species in the ecosystem. On the US West Coast, fishing limits have been put into place to protect high biomass, low trophic level species such as krill and anchovy (Pacific Fishery Management Council, 1978, Pacific Fishery Management Council, 2008). These rules seek to protect from ecosystem overfishing and maintain the prey base for species that are the targets of existing fisheries.

Recently, the effect of adding new fisheries for low trophic level species that constitute large proportions of the biomass of marine ecosystems has been the target of much research (Cury et al., 2011, Smith et al., 2011, Kaplan et al., 2013a, Kaplan et al., 2013b). These efforts often focus on species or groups with high biomass, while low biomass species or groups are more easily overlooked. Fishing on low biomass groups may indeed have few impacts on food webs if those species are functionally redundant to high-biomass prey groups (Walker, 1992). However, removals of low biomass groups may have disproportionately large impacts, depending on their role in the ecosystem and spatial distribution and overlap of predators and prey. For example, central place foragers, like many seabirds, depend on locally abundant seasonal prey resources (Ainley et al., 2009, Pichegru et al., 2010, Cury et al., 2011). Fluctuations in these resources over small spatial and temporal scales could have severe impacts on populations that rely on them, even if a prey contributes low proportions to the overall seabird diet over the course of a year when compared to other potential prey species (Hipfner, 2009).

In this study, we investigated the effects of targeted fisheries on relatively low biomass functional groups in a large marine ecosystem. Similar to previous modeling studies, we report biomass responses of species in the food web. However this work is novel in that we also explore regional variation in biomass impacts, as well as ecosystem-wide effects. We investigated target species that were broadly and narrowly distributed across the ecosystem to explore the effects of spatial variation in the food web on the impacts of fishery development.

The California Current Large Marine Ecosystem (CCLME) and its associated fisheries provide an excellent example system to investigate how future fishery development could affect existing fishery yields and food web biomass distribution. The largest fleet in the ecosystem, the US west coast groundfish fishery, uses midwater and bottom gear to target 90 species of flatfish, rockfish, and roundfish. Four of these stocks are currently overfished and subject to rebuilding plans. Coastal pelagic fisheries target sardine, mackerel, anchovy and squid. To protect the pelagic prey base, krill fishing is completely banned, and sardine catch limits are set to maintain a minimum of 150,000 mt (Pacific Fishery Management Council, 2006).

Here, we explore the potential effects of fishery development targeting new species with lower biomass than species previously investigated by others (Smith et al., 2011, Kaplan et al., 2013a, Kaplan et al., 2013b). We created ecological forecasts under new fishing scenarios using an Atlantis ecosystem model for the California Current (Horne et al., 2010, Kaplan et al., 2012). Using these forecasts we explored the impacts of new fishery development on the coast-wide and regional distribution of other functional groups in the model. We describe how variation in the distributions of target and non-target species affects impacts of fisheries in a spatially heterogeneous ecosystem.