Sustainability of the Lake Superior fish community: Interactions in a foodweb context

The restoration and rehabilitation of the native fish communities is a long -term goal for the Laurentian Great Lakes. In Lake Superior, the ongoing re storation of the native lake trout populations is now regarded as one of th e major success stories in fisheries management. However, populations of th e deepwater morphotype (siscowet rake trout) have increased much more subst antially than those of the nearshore morphotype (lean lake trout), and the ecosystem now contains an assemblage of exotic species such as sea lamprey, rainbow smelt, and Pacific salmon (chinook, coho, and steelhead). Those sp ecies play an important role in defining the constraints and opportunities for ecosystem management. We combined an equilibrium mass balance model (Ec opath) with a dynamic food web model (Ecosim) to evaluate the ecological co nsequences of future alternative management strategies and the interaction of two different sets of life history characteristics for fishes at the top of the food web. Relatively rapid turnover rates occur among the exotic fo rage fish, rainbow smelt, and its primary predators, exotic Pacific salmoni ds. Slower turnover rates occur among the native lake trout and burbot and their primary prey-lake herring, smelt, deepwater cisco, and sculpins. The abundance of forage fish is a key constraint for all salmonids in Lake Supe rior. Smelt and Mysis play a prominent role in sustaining the current troph ic structure. Competition between the native lake trout and the exotic salm onids is asymmetric. Reductions in the salmon population yield only a modes t benefit for the stocks of lake trout, whereas increased fishing of lake t rout produces substantial potential increases in the yields of Pacific salm on to recreational fisheries. The deepwater or siscowet morphotype of lake trout has become very abundant. Although it plays a major role in the struc ture of the food web it offers little potential for the restoration of a va luable commercial or recreational fishery. Even if a combination of strong management actions is implemented, the populations of lean (nearshore) lake trout cannot be restored to pre-fishery and pre-lamprey levels. Thus, mana gement strategy must accept the ecological constraints due in part to the p resence of exotics and choose alternatives that sustain public interest in the resources while continuing the gradual progress toward restoration.