Effects of sunfish predators on crayfish (Procambarus spp.) recruitment and species turnover

Background/Question/Methods

Species sorting along habitat and predator transitions in freshwater systems is well documented for a variety of aquatic organisms. Previous studies in south Florida wetlands indicate that two species of crayfish (Procambarus alleni & P. fallax) turnover across the hydrologic gradient with P. alleni dominating assemblages in most temporary wetlands and P. fallax dominating more permanently flooded sloughs. We examined effects of sunfish (Lepomis spp.) on recruitment and growth of the two crayfish species in a replicated mesocosm experiment. In particular we wanted to determine whether sunfish predation on early life stages of crayfish shifted the community towards dominance by P. fallax. We established a sunfish density gradient (0-1.7/m²) in 12 mesocosms (18 m² each) simulating Everglades slough habitats and stocked them with typical Everglades prey species. The sunfish stocking densities used are similar to those commonly observed in slough habitat of the Everglades. Two common sunfish were used as predators in the study and they were stocked at equal ratios across the gradient. Small young-of-year crayfish (~3 mm carapace length) of both species were reared independently and stocked in equal numbers in all tanks after establishment of the treatments; 67% of the stocked crayfish were P. alleni. 

Results/Conclusions

After eight weeks we sampled the tanks with a series of trapping methods (throw traps, minnow traps, bar seine sweeps). Crayfish, and other prey species, declined across the sunfish density gradient and crayfish total biomass declined 99.8% from the controls (no sunfish) to the highest densities. Total density declined as well as average Procambarus size across the density gradient, suggesting that sunfish may have caused a change in crayfish growth rates. The effect of sunfish on final size was especially clear for P. alleni. The proportion of the total crayfish catch that was P. alleni declined with increasing sunfish density (weighted linear regression: P=0.007, R²=0.53). While stocking proportions favored P. alleni (2:1), all three of the tanks with the highest sunfish densities had an assemblage that was ≥50% P. fallax, suggesting that, although predators have strong overall effects on crayfish recruitment, there was a proportionally stronger effect on P. alleni. This is consistent with the hypothesis that fish predators in longer-hydroperiod wetlands are responsible for the pattern of crayfish species turnover in the Everglades.