Case Studies in Fisheries Conservation and Management: Applied Critical Thinking and Problem Solving
Case 9: Misapplication of a Minimum Length Limit for Crappie Populations: Could the Mistake Have Been Avoided?
Minimum length limits for crappie populations were not implemented by the South Dakota Department of Game, Fish and Parks (SDGFP) prior to 1996. Generally, minimum length limits should be applied to low to moderate density crappie populations that have moderate to fast growth by the standards of that region and in which population size structure has been negatively affected by angler harvest. Minimum length limits should not be applied to high-density, slow-growing crappie populations (Colvin 1991; Allen and Miranda 1995). The use of such a regulation on a high-density population would compound the problem of overpopulation and slow growth by further increasing crappie density and intraspecific competition.
The size structure of black crappie and white crappie populations in Lake Alvin, South Dakota had apparently been negatively affected by excessive angler harvest. Anglers complained about the small average size of crappies harvested from the lake. Relative abundance of these populations was lower than other South Dakota crappie populations that exhibited high density and slow growth, based on catch per unit effort (CPUE) in trap (i.e., modified fyke) nets. Age and growth data indicated that the Lake Alvin crappies grew at a moderate rate near the statewide average. However, size structure of both black and white crappies collected with trap nets in Lake Alvin during the early 1990s typically were below objective ranges for a balanced population. The age structure of both species showed few fish older than age 2 in trap net samples, indicating that either the fish were harvested at a young age or natural mortality limited survival of fish in these populations. The combination of low to moderate density, moderate growth, small size, and young age structure indicated that Lake Alvin crappies may have been overharvested.