FISH DISTRIBUTION AND CATCHABILITY - WHAT IS THE APPROPRIATE MEASURE OF DISTRIBUTION

Catchability to commercial fisheries has been predicted to be density dependent due to density-dependent variation in stock area. Previous studies have used indices of stock area based on thresholds of absolute density. These indices will increase with abundance even if density increases uniformly over all areas. We show that spatially uniform changes in abundance can affect catchability given certain models for the distribution of fishing effort, but that this effect is slight compared with the effect of changes in the spatial spread of fish distribution. We describe an index of distribution that depends only on spatial spread: the minimum area over which a specified percentage of the population is spread. We tested the density dependence of this index using data on Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence. Results depended on the percentage of the population for which the index was evaluated. The area containing most (90 or 95%) of the population was density dependent, expanding as population size increased. The area of highest cod concentration (i.e., the area containing 50% of the population) did not expand significantly as population size increased.