Theses and Dissertations


Title: The Influence of Recruitment Variation on Minimum Length Limit Evaluations for Crappies in Four Alabama Reservoirs

Name: Martin, Allen David

Degree: MS

Chair: Michael J. Maceina

Resides:

University: Auburn University

Location: Auburn, Alabama

Date: 2001

Pages: 84

Keywords: Crappie,reservoir management,length limit,recruitment,population modeling

Abstract:

Minimum length limits were evaluated for black crappie Pomoxis nigromaculatus and white crappie P. annularis in four Alabama reservoirs using both pre- and post- minimum length limit data and simulation modeling. On three reservoirs, Jones Bluff, Aliceville, and Demopolis, a minimum length limit of 229 mm total length (TL) was imposed in October 1993, where previously no minimum length limit existed. Fish were sampled from 1990-92 to 2000 with trap nets, aged with otoliths, and population characteristics including catch rates of fish over 203 and 254 mm, stock density indices, and growth were compared between pre minimum length limit (< 1993) and post-minimum length limit (> 1995) time periods. On Weiss Lake, a minimum length limit of 254 mm TL was imposed in March 1990 where previously no minimum length limit existed, opulation characteristics between 1990 and 2000.For all four reservoirs, no consistent change in trap net catch rates of fish greater than 203 and 254 mm TL occurred over time. After the minimum length limit was imposed, growth rates declined slightly in all reservoirs for same ages, but increased slightly for age-2 fish in one reservoir. In a few instances, length-at-age was inversely related to catch for that age group and suggested density-dependent growth depression. In one of four reservoirs, stock density indices declined for both proportional stock density (PSD: N > 200 mm TL/ N > 130 mm TL) and relative stock density-preferred (RSD-P: N > 250 mm TL/ N > 130 mm TL), but did not change over time in the other reservoirs. Thus, expected responses of increased catch rates and a higher proportion of fish greater than 203 mm were not realized. My analysis was confounded by highly variable crappie recruitment as average annual age-0 and age-1 catch rates varied 10 to 20 fold over time in each reservoir with coefficients of variation (CV: standard deviation/mean) ranging from 62 to 103%. In addition, the implementation of the 229 mm TL minimum length limit in three reservoirs probably just reinforced many anglersÆ own self-imposed minimum size of fish that they would harvest.For simulation modeling, I examined and compared the effects of 203 (no minimum length limit), 229, and 254 mm minimum length limits and predicted the long term (1000 years) response of abundance of age-3 and older fish (about 254 mm long), angler catch (weight and number), and PSD and RSD-P indices. The four populations were modeled using empirically derived growth and natural mortality rates. Age-0 recruitment CV was incorporated and low (16-17%), medium (32-34%), and high (49-52%) levels of exploitation were explored. Power analysis was performed using results from the population modeling in order to estimate the number of years of

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