Habitat-mediated production and recruitment of young alewives in Lake Michigan

The identification and subsequent protection of essential fish habitats (areas with high densities, growth, survival, and/or production rates) can help sustain fish populations. Alewives (Alosa pseudoharengus) in Lake Michigan spawn in a variety of habitats, including near-shore areas, drowned river mouth lakes, embayments, and tributaries. However, the relative contributions of these different nursery habitats to the adult alewife population have not been evaluated. I used a suite of methods to identify critical habitats for young alewives. I integrated bioenergetics models with GIS (Geographic Information Systems) to generate spatially-explicit estimates of potential young-of-year (YOY) alewife population production in near-shore areas of Lake Michigan during August--September 1994 and 1995. Analyses indicated that YOY alewife habitat quality and yield varied widely throughout Lake Michigan during both years, and alewife yield was not spatially correlated with predictions of habitat quality. During 2001 and 2002, I sampled age-0 alewives in a near-shore area of Lake Michigan and three drowned river mouth lakes, characterized physical and biotic habitat factors and related these factors to alewife densities, hatch dates, growth, mortality, and production rates. Alewives in drowned river mouth lakes emerged earlier, grew faster, and had higher survival rates than in near-shore Lake Michigan. In addition, alewives in Muskegon Lake (a drowned river mouth lake) tended to grow to larger size by late fall, and were therefore less likely to experience size-dependent over-winter mortality than alewives in Lake Michigan. Despite lower habitat quality, near-shore Lake Michigan may have contributed the majority of alewife recruits due to the relatively large volume of this habitat type. Finally, I used an individual-based model to estimate inter-annual variation in the ultimate recruitment success (survival through the winter) of alewife cohorts emerging in different habitats. Model simulations suggest that relative contributions of young alewives from different habitat types vary annually, depending upon summer temperatures. During average or warm years, the relative contribution of drowned river mouth lakes to lake-wide alewife recruitment is relatively small. However, during cold years of low recruitment, drowned river mouth lakes may produce the majority of alewife recruits.