Ecosystem Approaches for Management 571 Alaska Sea Grant College Program • AK-SG-99-01, 1999

(Re)Constructing Food Webs and Managing Fisheries

Jason Link National Marine Fisheries Service, Northeast Center, Woods Hole, Massachusetts

Abstract The construction and analysis of a food web is an ecosystem approach that augments classical . Food web analysis delin- eates important ecosystem linkages representing species interactions such as predator-prey relationships; from these linkages emerges a better un- derstanding of ecosystem resiliency, resistance, connectivity, energy trans- fer efficiency, mass flux, and energy partitioning. In turn, with an understanding of food web dynamics, critical fisheries issues, including the relative importance of and natural mortality, identification of critical life stages, production surplus and partitioning, multispecies yield dynamics, and forecasting, the impact of fishery management scenarios can be more effectively examined. Unfortunately, elucidating a food web is not a trivial task. One of the more parsimonious, cost-effective, and fisheries-amenable methods of reconstructing (at least portions of) a food web is diet analysis. Incorporated as part of standard resource surveys, analysis of stomach composition can not only qualify the linkages of a food web, but can also quantify the magnitude and rate of energy and mass exchange. Examples of statistical analyses from northwest Atlantic diet data demonstrate the utility of this approach in constructing a food web that produces information useful in addressing key fisheries issues.

Introduction Food webs are the skeletal and circulatory analogues of ecosystem “anatomy and physiology.” That is, the predator-prey interactions that comprise food webs can ultimately determine the fate and flux of every population in an ecosystem, particularly upper level consumers of fiscal importance (Hairston et al. 1960, Oksanen et al. 1981, Carpenter et al. 1985, Carpenter and Kitchell 1993, Christensen 1996). Numerous examples demonstrate the utility and insight that examining multispecies interactions have had 572 Link — (Re)Constructing Food Webs and Managing Fisheries on fisheries management around the world (e.g., Andersen and Ursin 1977, Mercer 1982, Daan and Sissenwine 1991, Payne et al. 1992, Christensen and Pauly 1993, Walters et al. 1997, Pauly et al. 1998). While assessing populations with disregard to other species is known to be unrealistic, intellectually unsatisfying, and potentially misleading (sensu May et al. 1979, Murawski 1996), there persist challenges, be they scientific, institu- tional, philosophical or otherwise, that impede the implementation of multispecies and ecosystem approaches to fisheries management. Per- haps this is best expressed in a quote from Hilborn and Walters (1992: p. 448), which captures a pervasive attitude among fisheries scientists re- garding food webs: We believe the food web modelling approach is hopeless as an aid to formulating management advice; the number of parameters and assumptions required are enormous. Such large model-build- ing exercises are perhaps of some utility in designing research programs, but will not help manage fisheries. The use of trophic relationships to estimate general yield potentials also seems un- likely to produce anything that fishery managers can use in the near future, although such relationships may become clearer in the long term as data are accumulated from a wide variety of fish- eries. I hope to demonstrate the contrary in this work, and will attempt to elucidate the utility that a classical food web approach can provide for fisheries management. What Is a Food Web? Food webs have been defined literally as road maps of species interac- tions, or as the relati