Research Document - 2002/010

Indices of Fish Distribution as Indicators of Population Status

By K. Zwanenburg, J. Black, and R. Mohn

Abstract

We examined measures of concentration, prevalence and local density for seven fishes commonly caught during the annual summer groundfish trawl survey conducted by the Department of Fisheries and Oceans. We also examined the relationship of these distributional indices to estimates of overall species abundance. Concentration was defined as the proportion of the total survey area occupied by the top nth percent of the total annual population estimate. Prevalence was defined as the proportion of the total number of standardized trawl hauls (sets) completed in year i containing > 0 individuals of the species of interest and indicates how widely the species is distributed in the survey area without reference to density. Local density or CPUE where present is the average number (or weight) of the species of interest only in those sets where at least one individual of the species of interest was caught. This measure gives an indication of the average density of the species in those areas where it actually occurs. We conclude that these measures are difficult to interpret individually but in concert provide indicators of various attributes of overall species abundance. They should not be interpreted individually. All should be interpreted over time and with reference to longer-term data and should be interpreted with reference to a map of overall species abundance and in the area of interest.

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