Trophic Economics of Lake Trout Management in Reservoirs of Differing Productivity

Abstract

Because of their potential to reach large size, lake trout Salvelinus namaycush are managed as trophy fish in many reservoirs in the western United States. In Colorado, restrictive harvest regulations for lake trout were enacted in reservoirs exhibiting a range of productivities. Annual stockings of kokanee (lacustrine sockeye salmon Oncorhynchus nerka) and rainbow trout O. mykiss sustained sport fisheries, but stocked fish also dominated the prey fish assemblages in these systems. Hydroacoustic surveys suggested that lake trout management allowed an imbalance to develop between prey fish biomass and the biomass of piscivorous lake trout. Piscivorous fish biomass was, on average, 60% of total pelagic fish biomass. Bioenergetics modeling confirmed the imbalance: annual lake trout consumption was near or exceeded annual pelagic prey fish supply (standing stock plus production); the degree of imbalance was greater in less productive reservoirs. Annual subsidies to the food web, in the form of stocked sport fish, were necessary to allow the imbalance between predator and prey populations to persist, especially in the least-productive systems. Though highly sensitive to the size at which hatchery fish are consumed, the per capita costs to sustain lake trout growth at observed levels would total about US$200 per lake trout in the more productive reservoirs and $300-600 per lake trout in the less productive reservoirs. The cost of maintaining high lake trout biomass in these stocked systems may be a difficult management strategy to justify and sustain, either economically or socially. Trophic economics is a term coined by Ney (1990) to describe the relationship between prey resource supply and consumer demand. Ney's the- ses were that (1) sport fish production is often limited by food supply, (2) a variety of methods exist to quantify predator-prey relationships in fisheries, and (3) fishery managers' actions should maintain some balance between predator con- sumption demand and prey supply. Failure to man- age within the constraints dictated by ecosystem productivity and sport fish food supply threatens the ecological integrity of the ecosystem and re- sults in expensive and ultimately unsustainable management programs. In practice, trophic economics has been an un- derlying tenet of fishery management for decades. Swingle (1950) was perhaps the first fishery sci- entist to attempt to balance piscivorous sport fish and their prey in ponds for the purpose of manip- ulating growth rates and size structure of large- mouth bass Micropterus salmoides and bluegills Lepomis macrochirus.More recently, trophic eco-