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Use of Equilibrium Yield Models to Evaluate Length Limits for Crappies in Weiss Lake, Alabama
Author(s) -
Maceina Michael J.,
Ozen Ozcan,
Allen Micheal S.,
Smith Stephen M.
Publication year - 1998
Publication title -
north american journal of fisheries management
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.587
H-Index - 72
eISSN - 1548-8675
pISSN - 0275-5947
DOI - 10.1577/1548-8675(1998)018<0854:uoeymt>2.0.co;2
Subject(s) - fishing , range (aeronautics) , fish <actinopterygii> , statistics , population , mathematics , limit (mathematics) , biology , fishery , zoology , ecology , demography , mathematical analysis , materials science , sociology , composite material
We used a Beverton–Holt equilibrium yield model to predict the effects of four different length limits (203, 229, 254, and 279 mm) on harvest of black crappies Pomoxis nigromaculatus and white crappies P. annularis in Weiss Lake, Alabama. The current 254‐mm length limit took effect in 1990, and we wanted to assess length limits with more recent data. We documented angler harvest and catch rates before and after the initiation of the size limit. Size structure, growth, and total mortality were estimated for fish collected with trap nets and by electrofishing. Growth was above average compared with other reservoirs in the state with fish reaching 254 mm in about 2.4 years. Estimates of total annual mortality ranged from 51% to 64% and annual exploitation was 33%, but because of uncertainty, a wide range of fishing and natural mortality rates were incorporated into the simulations. Three years after the length limit took effect, angler harvest and catch of crappies increased two–four fold and the size structure of the population was skewed towards smaller fish. However, recruitment indices showed production of strong year‐classes in the 1990s, compared with weak year‐classes in the 1980s, which confounded interpretation of creel and size structure data and the effects of the length limit. Modeling indicated higher harvest in weight would be achieved with a 254‐mm size limit only if conditional natural mortality was less than 35%. Yield benefits were comparable or decreased at higher length limits when conditional natural mortality rates were higher than 35%. We predicted substantially reduced numbers harvested at progressively higher natural mortality rates and increased length limits. The 254‐mm minimum length limit for crappies in Weiss Lake appeared to provide potential benefits to the fishery. Yield (in weight) would increase because conditional natural mortality was probably less than 35%; however, the actual numbers of fish that anglers could keep would be reduced.