Spatial and Temporal Changes in the Offshore Larval Fish Assemblage in Southeastern Lake Michigan

Abstract

The larval fish assemblage in southeastern Lake Michigan was sampled monthly (May to September 1983) between 1.7 and 23.9 km offshore. Larval densities were calculated for the neuston, epilimnion, metalimnion, and hypolimnion strata of the water column. Larval densities were generally less than 0.4 m −3 with the exception of bloater ( Coregonus hoyi) which reached 15 m −3 in the offshore neuston in August. The abundance of larvae was estimated for a 1-km-wide (N–S) by 26.2-km-long (E–W) strip of the lake and discussed with respect to inshore (0.7–7.1 km), intermediate (7.1–11.1 km), and offshore (11.1–26.9 km) zones. The inshore was dominated by larval rainbow smelt ( Osmerus mordax) and alewife ( Alosa pseudoharengus), and the offshore zone was dominated by bloater and deepwater sculpin ( Myoxocephalus thompsoni). The changes in abundance and distribution of larvae reflected differences in spawning and hatching seasons and behavior of the different species. Species richness ( number of species) increased and subsequently decreased in a progression from inshore to offshore. Maximum species richess occurred in June and July. Abundance was always higher in offshore waters. Maximum larval abundance occurred early in the season ( June) and declined steadily through to September. Deepwater sculpin was the most abundant larval species in the areas sampled. Together, deepwater sculpin and bloater constituted 70–99% of the total abundance of larvae. In general, there was little overlap in the distributions of the various species over temporal and depth scales. Bloater and deepwater sculpin were separated from each other by their depth distributions; alewives were separated temporally from all other species.