MOVEMENT DYNAMICS IN A FISHERY - APPLICATION OF THE IDEAL FREE DISTRIBUTION TO SPATIAL ALLOCATION OF EFFORT

Many traditional analyses of fisheries data assume that there is a neg ligible effect of alternative fish stocks on the spatial distribution of fishing effort and that the amount of local effort does not influen ce catchability. There is growing evidence that contradicts these assu mptions. Because of the potential biases that these erroneous assumpti ons may cause in the interpretation of catch-per-unit-effort (CPUE) st atistics, it is important to determine the factors governing the spati al distribution of effort in a fishery. We used data on the Hecate Str ait, British Columbia, Canada, trawl fishery to test hypotheses about spatial allocation of effort and interaction among fishing vessels. Th e ideal free distribution of Fretwell and Lucas (1970. Acta Biotheor. 19: 16-36) was the foundation for deriving these tests. We found evide nce for competition among vessels, although we could not distinguish w hether the mechanism was interference or exploitation competition. As well, CPUE was generally equalized among the areas fished, as predicte d by the ideal free distribution, because of movement of boats among a reas. Thus, area-specific CPUE would not be a reliable index of relati ve abundance of fish in different areas; relative fishing effort may b e better.