Lipid concentrations in Lake Michigan fishes: Seasonal, spatial, ontogenetic, and long-term trends

Lipid concentrations were measured in seven species of fish from several lo cations in Lake Michigan during spring, summer, and fall in 1994 to 1995. A dult alewife (Alosa pseudoharengus) and age-a coho salmon (Oncorhynchus kis utch) exhibited pronounced seasonal changes in lipid content. Adult alewive s averaged 7.4% lipid, on a wet weight basis, during spring (May), 2.6% in slimmer (July), and 12.2% in fall (late September through October). Spring lipid concentration was low in age-2 coho salmon, averaging only 1.9%, then increased to 7.8% during summer and decreased to 4.5% by fall. In contrast , lipid content in adult bloater (Coregonus hoyi) was relatively constant w ith respect to season, ranging between 10.6% and 12.4% during the year. Lip id concentration increased with fish size for all species except rainbow sm elt (Osmerus mordax). Although deepwater sculpin (Myoxocephalus thompsoni) were considerably larger than slimy sculpin (Cottus cognatus) (mean total l ength of 117 mm vs 68 mm), mean lipid content of deepwater sculpin (7.6%) w as only slightly higher than that for slimy sculpin (6.6%). Comparison of l ipid concentrations from this study with previous studies indicated that li pid concentration in lake trout (Salvelinus namaycush) and alewives in Lake Michigan did not change significantly from 1969-1971 to 1994-1995. Lipid c oncentration in large (about 250 mm total length) adult bloaters near Sauga tuck (along the southeastern shore of the lake) decreased from 23.3% in 198 0 to 11.9% in 1986, but showed no significant change between 1986 and 1994- 1995.