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Food Web Interactions and Modeling Food Web Interactions and Modeling Introduction changes in the productive capacity of The Chesapeake Bay food web has the environment, the abundance of experienced significant historical planktonic and benthic prey, and the alterations due to overfishing, structure of food webs that support anthropogenic stress, and natural fisheries. disturbances. Although conventional Researchers have a good understand- single-species management 2 approaches do not typically address ing of some food web relationships in predator–prey dynamics, these the Chesapeake Bay. Atlantic menha- dynamics form the heart of den Brevoortia tyrannus, Atlantic interactions among species affecting croaker Micropogonias undulatus, bay abundance and production. Such anchovy Anchoa mitchilli, blue crab interactions have dramatic and Callinectes sapidus, and spot substantial effects on community Leiostomus xanthurus are integral structure, ultimately affecting fisheries links between and within benthic and yields in the Bay, and must be planktonic components of the Bay considered when developing or food web. Heavily exploited, predatory amending ecosystem-based fishery fish consume forage species, such as management plans (FMPs). menhaden and bay anchovy; these predators may also rely on juvenile Fishing mortality—an important blue crabs as part of their diet and may fraction of total mortality for most ultimately affect the abundance of exploited species—represents human recruiting crabs. predation on fishery resources. Multispecies fisheries management We must expand our understanding of incorporates not only fishing mortal- food web interactions, quantify their ity information but also key predator– effects, develop new food web models, prey linkages and their contributions and implement existing models to to natural mortality. Understanding provide the requisite information that such food web dynamics allows quanti- will permit managers to define sus- fication of the energy and biomass tainable catch levels and estimate transfers in the food web that dictate fishing mortality rates of species in the sustainable levels of fishery exploita- webs. In this fisheries ecosystem plan tion. Food web relationships are not (FEP), we have included preliminary independent of habitat and water diagrammed food webs of managed quality issues; they may vary with species, indicating strong and weak element Food Web Interactions and Modeling 103 interactions between predator and Food Web Dynamics prey. These webs can guide managers as they explore policy options to Importance develop ecosystem-based regula- Sustainable use of exploited species tions—allowing high yields of pis- will depend, at least in part, upon civorous fish, for example, while inclusion of multispecies fisheries conserving forage fish resources and management approaches based largely important predator–prey relation- on food web dynamics (Christensen ships. Managers can now use funda- 1996; Daan 1997; Christensen and mental knowledge of food web Pauly 1998; Pauly et al. 1998). Manag- structure and relationships in a ers have not yet applied a multispecies precautionary manner, but major approach to fisheries management in research is needed to ensure effective the Chesapeake, despite its potential utility (Houde et al. 1998) as well as multispecies fisheries management in the availability of a food web model the Bay. for the mesohaline (middle) portion of This FEP element addresses the the Bay (Baird and Ulanowicz 1989), importance and limitations of which could provide a framework for developing food web models for the additional modeling focused on man- Chesapeake, considers the degree of agement needs. connectivity between particular Fishing affects ecosystems by remov- species (or trophic groups) and their ing biomass from the complex of predators and prey, and describes species that feed upon each other in subwebs of the Bay’s economically the web (Pauly et al. 2000). It also valuable species. In addition, the shifts the relative abundance of element describes and discusses the exploited species at different trophic utility of several recognized levels. Such changes—from fishing, multispecies and ecosystem models other anthropogenic stresses (e.g., that managers could adopt for habitat alteration and pollution), or ecosystem-based fisheries environmental change—may lead to management in the Bay. shifts in the productivity and sustain- able yields of species. These shifts, in turn, may affect the value of fisheries, species biodiversity, or the structural integrity of the ecosystem (Winemiller and Polis 1996; Pauly et A food web is defined as a “network of consumer- al. 2000; Jackson et al. 2001; Link resource interactions among a group of organisms, 2002a). For instance, researchers have populations, or aggregate trophic units” (Winemiller postulated that changes in the abun- and Polis 1996). dance of key fishery species, such as the oyster and blue crab, may have altered community structure and pathways of production in the Bay (Jackson et al. 2001; Silliman and 104 Fisheries Ecosystem Planning for Chesapeake Bay Bertness 2002). Massive fishery- management. induced reductions in the abundance of eastern oyster Crassostrea virginica, Predation is key in determining the a suspension feeder on phytoplank- abundance and size structure of populations, as well as the ton, have contributed to abnormally organization and functioning of high phytoplankton production, communities in the Chesapeake and eutrophication, and seasonal hypoxia other ecosystems (Lipcius and Hines that reduce secondary production and 1986; Hines et al. 1990; Seitz et al. species diversity (Jackson et al. 2001). 2001). Predation affects all life stages In coastal salt marshes, declines in of marine organisms and constitutes blue crab abundance (Lipcius and the primary source of natural Stockhausen 2002), due partly to mortality for fish in well-studied heavy fishing pressure, may have marine ecosystems (Bax 1991, 1998), allowed marsh periwinkle Littoraria even for those species with high irrorata to become more abundant fishing mortality during their and overconsume salt marsh grasses— exploitable life stages. a process which ultimately could lead to the destruction of salt marshes The relative importance of predation important for blue crab production and fishing mortality varies among (Silliman and Bertness 2002). species, but is typically skewed towards predation for younger (and smaller) An ecosystem’s carrying capacity, individuals and towards fishing production potential, and total mortality for older individuals. For sustainable yield to fisheries cannot instance, predation largely accounts for simply be calculated as the sums of the mortality of young juvenile blue yields for individual component crabs whereas fishing becomes species (Link 2002a) using traditional, responsible for 80% of the mortality of single-species stock assessment older juveniles and adults. Predation techniques. Rather, fisheries may also play a major role in production of an ecosystem depends controlling food web dynamics in significantly on food web dynamics marine ecosystems, altering the effects (Pauly et al. 2000; Link 2002a). To of reductions or increases in fishing evaluate the impact of a species’ mortality of species (Andersen and fishing mortality upon food web Ursin 1977; Laevastu and Favorite interactions and ecosystem processes, 1988; Bax 1991, 1998; Christensen therefore, the ecosystem’s chief food 1996; Trites et al. 1999; Pauly et al. web interactions must be defined and 2000; Link 2002a). quantified (Pauly et al. 2000). Similarly, researchers must consider With a heavily fished population at the effects of other controlling low abundance, predation may limit factors, such as habitat quality and population recovery despite potentially environmental conditions (see high recruitment of incoming year Habitat Requirements and classes (Sissenwine 1984; Bax 1991, Externalities elements), within the 1998; Christensen 1996; Link 2002a). context of ecosystem-based In such cases, the predator may have Food Web Interactions and Modeling 105 remained at high population levels or or by blue crab predation; either of it may be a fished species that has these forces could prevent oyster recovered after management-induced recovery given that overfishing, reductions in fishing mortality. For habitat degradation, and disease have example, Lipcius and Stockhausen driven the population to extremely (2002) hypothesized that predation low levels (Rothschild et al. 1994). pressure by Atlantic croaker (at high abundance in Chesapeake Bay for For some species, natural mortality nearly a decade) or by striped bass through predation—especially on Morone saxatilis (which dramatically young stages—may prove more signifi- resurged during the last decade cant in controlling population abun- following rigorous management dance than fishing mortality on measures) may be responsible for the recruited stages. Such species may be lack of recovery of the depressed blue subject to little or no fishing pressure, crab population in the Bay. Similarly, but serve as forage species for a spec- restoration of the Bay’s native oyster trum of natural predators (Overholtz population may be hampered by et al. 2000). Historically, watermen disease in older juveniles and adults have fished some forage species (e.g. Atlantic menhaden), which form a major component of the Chesapeake a fisheries ecosystem. During the past 50 years, when overfishing has caused Fishery Top Predator declines
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