Long-term decline in bluegill size structure and growth rates in Wisconsin: evidence of genetic parental effects?

Background/Question/Methods

The bluegill is a widespread fish species throughout North America, and has major ecological, economic and cultural significance. However, heavy exploitation has long-occurred on many bluegill populations via angling, and the effects of this exploitation are rarely studied.  A large historical fisheries database in Wisconsin, USA, was evaluated to examine potential shifts in bluegill size structure and growth rates over the last 68 years.

Results/Conclusions

A significant decline has occurred in both average and maximum size of bluegills throughout the region (mixed effects model regressions, P’s<0.0001). These changes have occurred concurrently with elevated exploitation rates by anglers and liberal allowable harvest limits.  Regionally-collapsed growth rates (i.e., size-at-age regressions) were significantly reduced post-1960 compared to pre-1960, and in 8 of 9 populations where historical and new growth assessments were available >40 y apart, significant growth reductions were also apparent. However, the nature of growth differences was such that regression elevations (not slopes) differed, thereby suggesting actual rate of growth has not changed over time, but initial sizes have. Results are most consistent with a genetic parental effects hypothesis (i.e., phenotype is determined by parental genotype) that could be maternal and/or paternal based, but would originate from removal of large, fast-growing individuals via angling. The possibility of a systematic ageing bias is being explored by re-examining archived scale samples using modern age determination equipment and other alternative hypotheses like a temperature, density or predator effect would be expected to have produced an opposite growth pattern.  Additional research is needed as these patterns potentially pose a major problem with broad ecological and management implications.