Evaluating the negative effect of benthic egg predators on bloater recruitment in northern Lake Michigan

Abstract

As the only extant deepwater cisco in Lake Michigan, bloater is currently at record low levels of abundance. Several mechanisms to regulate their recruitment have been proposed, including skewed sex ratios, predation on their larvae by adult alewife, and climatic factors during early life history stages, but none has unequivocal support. In this research, we evaluated an alternative mechanism of egg predation that was supported by an inverse relationship between bloater recruitment and biomass of slimy sculpin , which are known to be effective egg predators. To that end, we used a combination of field sampling, laboratory experiments, and modeling to estimate the proportion of bloater eggs consumed by sculpins each year between 1973 and 2008. Monthly field sampling between January through May 2009-2010 (when bloater eggs were incubating) offshore of Frankfort (Michigan), Sturgeon Bay (Wisconsin), Two Rivers (Wisconsin), and Muskegon (Michigan) provided benthivore diets for subsequent laboratory processing. Identification and enumeration of stomach contents and subsequent genetic analyses of eggs revealed that the mean proportion of bloater eggs in slimy sculpin diets (N = 1016) equaled 0.04. Bloater eggs also were consumed by deepwater sculpins (N = 699) at a slightly lower mean proportion (0.02), and only one round goby diet among 552 enumerated revealed a bloater egg. Based on the diet results, we developed daily ration models to estimate consumption for both deepwater and slimy sculpins . We conducted feeding experiments to estimate gastric evacuation (GEVAC) for water temperatures ranging 2-5 °C, similar to those observed during egg incubation. GEVAC rates equaled 0.0115/ h for slimy sculpin and 0.0147/h for deepwater sculpin , and did not vary between 2.7 and 5.1 °C for either species or between prey types ( Mysis relicta and fish eggs) for slimy sculpin . Index of fullness [(g prey/g fish weight)100%] was estimated from sculpins sampled in bottom trawls in the same seasons and years as the diets, and varied with fish size (averaging 1.93% and 1.85% for slimy and deepwater sculpins , respectively). Estimates of daily consumption ranged from 0.2-0.8% of sculpin body weight. Annual estimates of bloater egg consumption predicted higher values for deepwater sculpin than slimy sculpin between 1973 and 2005. This pattern was reversed in 2006, 2008, 2009, 2010 as slimy sculpin abundance increased while that of deepwater sculpin declined. The sum of sculpin consumption of bloater eggs exceeded 25% of bloater population egg production early (1975-1980) and late (2008-2010) in the time series. Despite the strong field pattern implicating egg predation by slimy sculpin , our consumption models failed to fully support this hypothesis. In particular, our results were unable to explain why bloater recruitment was relatively poor during 1995-2005 when the proportion of bloater eggs consumed was very low ( < 0.06). The results did, however, demonstrate that bloater recruitment was consistently poor when the proportion of eggs consumed was relatively high. In conclusion, consumption by native benthivores can be a contributing factor to poor recruitment of bloater, especially when slimy sculpin reach high levels of abundance. This result exemplifies the importance of ecosystem-based fishery management, given that the maintenance of healthy lake trout populations in the Great Lakes should control the abundance of slimy sculpin egg predators. In addition, future research will be required to fully understand the primary bottleneck to bloater recruitment in Lake Michigan so that efforts to stock and restore bloater in Lake Ontario have a greater probability of resulting in naturalized and sustainable populations.