Direct and indirect estimates of black crappie size selectivity to a common sampling gear: Potential biases and limitations for assessment

Reliable stock assessments require estimates of gear selectivity to separate selection from true changes in population structure, but true measures of selectivity are rare in the literature. We estimated size selectivity of bottom trawl sampling for black crappie Pomoxis nigromaculatus using capture–recapture methods to directly measure the effects of fish size on catchability (q, the fraction of a fish stock collected with a given unit of fishing effort) at Lake Jeffords, Florida, USA. Additional indirect estimates of selectivity were obtained with a population model applied to long-term data at four Florida lakes. Direct measures of selectivity indicated catchability was highest for the 90–119 mm length-group and lowest for fish greater than or equal to 180 mm, with q declining by a factor of 2 or 3 for large fish relative to small fish. The indirect age-structured modeling approach revealed dome-shaped gear selectivity patterns with relative selectivities peaking at age-1 for three of four lakes. Overall model trends indicated greater selectivity of younger fish (age-0 and age-1) to the gear followed by decreasing relative selectivity to older age-classes (age-2+). Trawl selectivity patterns suggested that otter trawls would be best for monitoring the abundance of small black crappie and useful for indices of recruitment. Our results showed that adult black crappie were underrepresented in bottom trawl samples which would influence age structure and growth rate estimates and the effectiveness of this gear as an assessment tool for tracking adult black crappie populations.