A comparison of age-0 alewife energy content between two Lake Michigan habitats and year classes

The energy content and diet mass of juvenile alewife Alosa pseudoharengus was determined in Muskegon Lake, a drowned river mouth lake, and in the adjacent nearshore zone of Lake Michigan during summer and fall 2021 and 2022 and spring 2022 and 2023. Larger age-0 alewife had disproportionately higher total energy as a function of length than small alewife at both sites. Because age-0 alewife from Muskegon Lake grew to a larger average size, individuals had 43 and 204 % greater total energy content than in Lake Michigan in fall 2021 and 2022, respectively. Contrary to expectations, length-specific energy content declined from summer through fall each year, possibly related to declines in diet mass in the fall. Although diet mass was higher in 2021 than 2022, there were no differences in length-adjusted energy content between years during the fall in either lake. However, in Lake Michigan, the average length was much larger in fall 2021 (79 mm) than 2022 (66 mm) so that the average predicted total energy content in 2021 (20,889 J) was over twice that in fall 2022 (9852 J). Energy content declined over winter, but in spring 2023, small yearlings had surprisingly lost less size-adjusted energy than larger fish. Age-0 alewife had lower length-adjusted energy content in 2021 and 2022 than in 2001 and/or 2002 in both lakes. Nursery habitats such as the drowned river mouths where age-0 fish have higher diet mass and grow larger provide an energetic advantage over the main basin habitat of Lake Michigan.