A thirty year diet record of largemouth bass from a small north temperate lake

Background/Question/Methods

Much has been published on diet and foraging behavior of largemouth bass (LMB), Micropterus salmoides.  We report on the diet dynamics of an adult (>150mm) population of LMB over a 30 year period from a small (1.5ha), unexploited, north temperate lake in Michigan’s Upper Peninsula, USA. LMB dominated the fish community comprising >95% of the fish biomass.  Paul Lake supports a very high population of LMB which varied from a low of 87 (95% CI 71, 114) to a high of 457 (95% CI 380, 574).  To assess interannual diet fluctuation we condensed 22 diet categories into seven functional diet groupings: zooplankton, Chaoborus spp., benthic invertebrates, pelagic invertebrates, terrestrial insects, fish and terrestrial vertebrates.  To meter these diet changes we used an annual composite Index of Relative Importance, IRI (based on Ʃ of % number, % mass and frequency of occurrence of each diet category).  Because of high conspecific LMB density relative to prey abundance, LMB in Paul Lake are foraging generalists.  We address the basic question to what affect will interannual density fluctuations have on LMB diet?  We hypothesize that diet will vary significantly in respective years, but independent of largemouth bass interannual density fluctuation.

Results/Conclusions

We chronicled diets of 5098 adult LMB from Paul Lake over three decades. Significant variation (Z ± 1.96, p<0.05) occurred in each of the seven functional diet categories, but these were not related to interannual cycles in LMB population density. Those diet categories which had the fewest significant differences in annual IRIs as compared with the 30 year composite IRI value were zooplankton and terrestrial vertebrates at 24% and 34%, respectively.  Fish prey and Chaoborus spp. had the most significant interannual variations at 55% and 59%, respectively. The variance of the remaining diet functional groups ranged from 24-48%.  Zooplankton, because of their small size and terrestrial vertebrates for their uncommonness in the diet, have low energetic cost-benefits and subsequently are least variable in the diet. The higher variance of fish prey (mainly LMB YOYs) is probably aligned with interannual variance in recruitment and the dietary fluctuation seen in Chaoborus is related to a foraging specialization demonstrated by this LMB population.