Steve Pothoven - NOAA GLERL
Energy content is a useful metric of physiological status of fishes and may help elucidate spatial and temporal variation in vital rates (growth and mortality). Similarly, energy density (energy per unit wet weight) is an important input variable for the application of bioenergetics models (Hanson et al. 1997), and predictions emanating from such models are highly sensitive to input data on energy densities of both predators and prey. Detailed information on the energy content of young fish is particularly critical. Energy content can change rapidly during early ontogeny (e.g., Wuenschel et al. 2006), and due to the numerical abundance and high mass-specific consumption rates of young fish, early life stages can play important roles as both predator and prey within ecosystems. For young temperate-zone fish, there are season-specific trade-offs between energy allocation to different tissue-types (low energy density, structural tissue versus high energy density, storage tissue) which have clear consequences for survival.
In Lake Michigan, the invasive Alewife Alosa pseudoharengus is a key component of the ecosystem: 1) constituting the primary prey of salmon and trout (which in turn support economically important recreational fisheries; Madenjian et al. 2002), 2) capable of restructuring zooplankton communities (Brooks and Dodson 1965; Wells 1970; Hewett and Stewart 1989), and 3) through competition and predation, influencing the dynamics of several native species (e.g., Yellow Perch Perca flavescens; Shroyer and McComish 2000). Studies on the energy dynamics of young alewives are few in number and include measures of alewives collected prior to some dramatic changes to the Lake Michigan ecosystem. In fact, there is only one published energy density measure which definitively represents an age-0 Alewife: 5,020 J g wet weight-1, for individuals with mean length of 48.5-mm collected during October 1979-1981 (Stewart and Binkowski 1986 from Flath and Diana 1985). In this study, we documented energy dynamics of young alewives in southern Lake Michigan during 1998-2003 in relation to individual size, season, year and habitat. We also evaluated whether energy content of young alewives has changed dramatically over the past 25 years.
Our analysis demonstrates that energy content of young alewives in Lake Michigan and the drowned river mouth Muskegon Lake varies across years, seasons, habitats and ontogeny. Consistent with previous observations of high overwinter mortality, young alewives deplete a dramatic amount of body energy stores between late fall and late spring (i.e., declines in length-specific energy content and energy density overwinter) (Figure 1). Interestingly, unlike several past studies of young fish in their native range, size-specific energy content of young alewives does not appear to increase over the summer and fall (i.e., no evidence of increased energy storage in preparation for the resource-scarce winter period). Finally, our measured values of energy density are roughly consistent with (or slightly lower than) the few previously-published values for young Alewife (Figure 1).
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Hook, T.O. and S. A. Pothoven. 2009. Energy Content of Young Alewives in Eastern Lake Michigan and Muskegon Lake, a Connected Drowned River Mouth Lake. North American Journal of Fisheries Management. 29: 378-387.