Michael D. Rennie

Changes in Benthic Invertebrate Communities of South Bay, Lake Huron Following Invasion by Zebra Mussels (Dreissena polymorpha), and Potential Effects on Lake Whitefish (Coregonus clupeaformis) Diet and Growth

ABSTRACT In this study we evaluated changes in benthic invertebrate communities of South Bay, Lake Huron following the invasion of zebra mussels (Dreissena polymorpha) and considered the implications for diets and growth of whitefish (Coregonus clupeaformis), a commercially important fish in the Great Lakes. Of the ten benthic invertebrate groups identified prior to invasion (1980–81), only densities of Diporeia and Oligochaeta have changed since the appearance of the zebra mussel, and only Diporeia and Chironomidae changed in relative abundance. These changes are similar to those observed in other areas of the Great Lakes, with the exception of an increase in Oligochaeta density. Post-invasion (2002–03) shallow-water communities appear to be more homogeneous, dominated by zebra mussels and Isopoda, whereas deep-water sites are more heterogeneous due to the loss of Diporeia. Additional data on Diporeia density for several years between 1959 and 2004 indicated that current low densities are not typical of South Bay. Based on changes in the benthic communities and published literature on whitefish diets, we predict that unless whitefish are able to switch to Mysis as an alternative to Diporeia, post-invasion whitefish diets will only contain a maximum of 57 to 84% of their former energy content. These predictions are likely underestimates, as they do not take into account increased energy costs associated with reductions in total invertebrate density at historical foraging depths.

Lazy males? Bioenergetic differences in energy acquisition and metabolism help to explain sexual size dimorphism in percids

1. Differences in energy use between genders is a probable mechanism underlying sexual size dimorphism (SSD), but testing this hypothesis in the field has proven difficult. We evaluated this mechanism as an explanation for SSD in two North American percid species--walleye Sander vitreus and yellow perch Perca flavescens. 2. Data from 47 walleye and 67 yellow perch populations indicated that SSD is associated with the onset of maturation: typically, males of both species matured smaller and earlier and attained a smaller asymptotic size than females. Males also demonstrated equal (perch) or longer (walleye) reproductive life spans compared with females. 3. To examine whether reduced post-maturation growth in males was due to lower energy acquisition or higher reproductive costs we applied a contaminant mass-balance model combined with a bioenergetics model to estimate metabolic costs and food consumption of each sex. Mature males exhibited lower food consumption, metabolic costs and food conversion efficiencies compared with females. 4. We propose that slower growth in males at the onset of maturity is a result of decreased feeding activity to reduce predation risk. Our finding that SSD in percids is associated with the onset of maturity is supported by laboratory-based observations reported elsewhere, showing that changes in growth rate, consumption and food conversion efficiency were elicited by oestrogen (positive effects) or androgen (negative effects) exposure in P. flavescens and P. fluviatilis. 5. Researchers applying bioenergetic models for comparative studies across populations should use caution in applying bioenergetic models in the absence of information on population sex ratio and potential differences between the sexes in energetic parameters.

Development and Evaluation of Condition Indices for the Lake Whitefish

Abstract Despite frequent use of length-based condition indices by fisheries managers and scientists to describe the overall well-being of fish, these indices are rarely evaluated to determine how well they correlate with more direct measures of physiological or ecological condition. We evaluated common condition indices (Fultons condition factor KF, Le Crens condition index KLC, and two methods of estimating relative weight Wr) against more direct measures of physiological condition (energy density, percent lipid content, and percent dry mass) and ecological condition (prey availability) for lake whitefish Coregonus clupeaformis in Lake Huron. We developed four standard weight (Ws) equations using the regression length percentile (RLP) method: one for the species as a whole, and three separate equations describing immature, mature male, and mature female lake whitefish from 385 populations in North America. Species RLP-Ws showed less length-related bias and more closely matched empirical quartiles of l...

Prey life-history and bioenergetic responses across a predation gradient.

To evaluate the importance of non-consumptive effects of predators on prey life histories under natural conditions, an index of predator abundance was developed for naturally occurring populations of a common prey fish, the yellow perch Perca flavescens, and compared to life-history variables and rates of prey energy acquisition and allocation as estimated from mass balance models. The predation index was positively related to maximum size and size at maturity in both male and female P. flavescens, but not with life span or reproductive investment. The predation index was positively related to size-adjusted specific growth rates and growth efficiencies but negatively related to model estimates of size-adjusted specific consumption and activity rates in both vulnerable (small) and invulnerable (large) size classes of P. flavescens. These observations suggest a trade-off between growth and activity rates, mediated by reduced activity in response to increasing predator densities. Lower growth rates and growth efficiencies in populations with fewer predators, despite increased consumption suggests either 1) a reduction in prey resources at lower predator densities or 2) an intrinsic cost of rapid prey growth that makes it unfavourable unless offset by a perceived threat of predation. This study provides evidence of trade-offs between growth and activity rates induced by predation risk in natural prey fish populations and illustrates how behavioural modification induced through predation can shape the life histories of prey fish species.