On the bioeconomics of predator and prey fishing

Abstract

A brief review of biological, bioeconomic and non-use objectives in fisheries management is given. It is shown that management strategies, with respect to optimal long run effort levels and stock sizes, might differ considerably with the choice of objective, and of a single-species or multispecies framework. Within a deterministic two-species predator–prey biomass model, equilibrium catch and resource rent of the predator are positively affected by an increased prey stock level. The effects on the equilibrium harvest and resource rent of the prey of a change in the predator stock level are ambiguous, although a negative relationship is most likely. In a simplified model with a linear equilibrium relationship between the two stocks this negative effect is always present. Such a simplified approach is applied to the case of northeast Arctic cod's consumption of its commercially most important prey species: capelin, herring, shrimp and small cod, i.e. cannibalism. It is shown that on average for the years 1984–92 capelin constituted about 75% of the feed, on the basis of weight, of age-specific cod of age 2 and older. Shrimp is the second most important prey, constituting about 10–20% of the feed; the higher figure occurs for 2-year old cod and the lower for the 7+ age group. Using Norwegian data on prices of fish and cost of effort for the years 1991–92, and data on stomach content 1984–92, it is shown that shrimp constitute 40–80% of the feed costs for all age groups of 2 years and older, whereas capelin constitutes only 15–20%. For the 7+ age group cannibalism is just above half the feed costs. This, despite the fact that capelin is by far the major feed in weight. The assumption of the specific functional forms in biological multispecies models are of importance for the management implications derived. It might be that the inclusion of economic factors will weaken the relative importance of some biological factors for management strategies. The partial bioeconomic analyses in this paper strongly indicate that biological and economic factors should be considered simultaneously in management analyses.