ASSESSING PREY FISH POPULATIONS IN LAKE-MICHIGAN - COMPARISON OF SIMULTANEOUS ACOUSTIC-MIDWATER TRAWLING WITH BOTTOM TRAWLING

The Lake Michigan fish community has been monitored since the 1960s wi th bottom trawls, and since the late 1980s with acoustics and midwater trawls. These sampling tools are limited to different habitats: botto m trawls sample fish near bottom in areas with smooth substrates, and acoustic methods sample fish throughout the water column above all sub strate types. We compared estimates of fish densities and species rich ness from daytime bottom trawling with those estimated from night-time acoustic and midwater trawling at a range of depths in northeastern L ake Michigan in summer 1995. We examined estimates of total fish densi ty as well as densities of alewife Alosa pseudoharengus (Wilson), bloa ter Coregonus hoyi (Gill), and rainbow smelt Osmerus mordax: (Mitchell ) because these three species are the dominant forage of large piscivo res in Lake Michigan. In shallow water (18 m), we detected more specie s but fewer fish (in fish/ha and kg/ha) with bottom trawls than with a coustic-midwater trawling. Large aggregations of rainbow smelt were de tected by acoustic-midwater trawling at 18 m and contributed to the di fferences in total fish density estimates between gears at this depth. Numerical and biomass densities of bloaters from all depths were sign ificantly higher when based on bottom trawl samples than on acoustic-m idwater trawling, and this probably contributed to the observed signif icant difference between methods for total fish densities (kg/ha) at 5 5 m. Significantly fewer alewives per ha were estimated from bottom tr awling than from acoustics-midwater trawling at 55 m, and in deeper wa ters, no alewives were taken by bottom trawling. The differences detec ted between gears resulted from alewife, bloater, and rainbow smelt ve rtical distributions, which varied with lake depth and time of day. Be cause Lake Michigan fishes are both demersal and pelagic, a single sam pling method cannot be used to completely describe characteristics of the fish community. (C) 1997 Elsevier Science B.V.