Trade‐offs between supportive and provisioning ecosystem services of forage species in marine food webs.

Abstract

Ecosystem-based management of natural resources involves an explicit consideration of trade-offs among ecosystem services. In marine fisheries, there is the potential for a trade-off between the supporting role of small pelagic fish and cephalopods in food webs, and the provisioning service they play as a major target of fisheries. Because these species play central roles in food webs by providing a conduit of energy from small prey to upper trophic level predators, we hypothesized that trade-offs between these two ecosystem services could be predicted based on energetic properties of predator–prey linkages and food-web structure. We compiled information from 27 marine food-web models (all within the Ecopath framework) that included either small pelagic fish or cephalopods, described predator–prey linkages involving these species, and developed a novel analytical framework to estimate how changes in yields of forage species would propagate through food webs and other fisheries. Consistent with expectations, diet overlap between predators and prey was generally low, and predator–prey linkages tended to be asymmetric; contribution of these species to predator diets was, on average, larger than the contribution of individual predator stocks to prey mortality. The estimated trade-offs between yields of forage fish and predator species were highly variable when we assumed joint bottom-up and top-down control on predation. Roughly one-third of this variance was related to an interactive effect of fishing and predation intensity; strong trade-offs were predicted when fishing intensity on forage species is high and when predators account for a high proportion of total forage mortality. When trophic connections were presumed to be driven by bottom-up processes, trade-offs were more predictable, but generally very small. Contrary to our expectations, trade-offs were not easily predicted from energetic properties, largely because predators of forage species exhibited a high degree of intra-guild predation, and also consumed many of the same prey as forage species. Given the limited ability to a priori predict the food-web implications of forage fisheries, we suggest that a precautionary risk-based approach be applied to decisions about acceptable biological removals of forage fish and biological targets used for their management.