Variation between Electrofishing and Otter Trawling for Sampling Black Crappies in Two Florida Lakes

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Abstract
We compared trawling with electrofishing for assessing populations of black crappie Pomoxis nigromaculatus in two shallow central Florida lakes. Lakes Jackson and Weohyakapka were sampled with both gears from November 2005 through March 2006. Size-selective biases were apparent, the trawl nets consistently collecting small individuals less than 100 mm in total length (TL), whereas electrofishing collected significantly larger, more adult fish exceeding 200 mm TL. The Kolmogorov–Smirnov two-sample test indicated that the size structure of the fish captured in the two types of gear differed significantly. The total annual mortality estimates were 28% and 72% for electrofishing on both lakes, 55% for otter trawls on Lake Jackson. The age ranges for catch curves derived from the two gears also displayed large differences in mortality estimates or gave no mortality estimate at all. Trawls tended to collect younger age-classes, whereas electrofishing tended to collect larger age-classes; thus, data collected with the two gears may represent different aspects of mortality in the population. When aquatic macrophyte cover is low so that both gears can be used, we recommend that otter trawl sampling be used to assess the recruitment of juvenile crappies and that simultaneous electrofishing be conducted to collect larger fish for the determination of age and growth. Fisheries biologists should be careful when using just one gear to evaluate crappie populations given that the size-selective biases associated with both trawling and electrofishing can influence basic mortality and length-frequency assessments.

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