Importance of Lake Ontario embayments and nearshore habitats as nurseries for larval fishes with emphasis on alewife (Alosa pseudoharengus)

Productive embayments, areas morphometrically sheltered from open lake physical processes, were compared to less productive, exposed shorelines of the nearshore as nursery habitats for larval alewives (Alosa pseudoharengus), the principal forage fish in Lake Ontario. Species composition and densities of pelagic larval fishes were assessed with towed plankton nets at three embayments and adjacent nearshore habitats located along the New York shore, two sites in 1997 and a third site in 1998. Peak nearshore densities of larval alewives in 1997 and 1998 were compared to 1977 and 1978, a time period prior to major phosphorus reductions and the establishment of exotic dreissenid mussels in Lake Ontario. Seasonal changes in the pelagic larval fish assemblage were generally similar in the embayments; yellow perch (Perca flavescens) were prevalent in late-May shifting to co-occurrence of alewives and Lepomis spp. in June and July. At all nearshore sites, the larval fish assemblage throughout summer consisted mainly of alewives (> 65%). Densities of larval alewives peaked earlier in embayments but generally at lower densities than observed later in the nearshore. Because the amount of nearshore habitat greatly exceeds that of embayment habitat in Lake Ontario, nearshore waters served as the principal nurseries for larval alewives. However, densities of all larval fish species other than alewives (principally yellow perch and Lepomis spp.) were usually greater in embayments than the nearshore. Compared to three nearshore sites in 1977 and 1978, timing of peak densities of alewives in 1997 and 1998 were similar and temperature dependent; highest densities occurred in late July to early August after nearshore waters were greater than or equal to 18degreesC. Peak median nearshore alewife densities (larvae/100 m(3)) in 1997 (47) and 1998 (60) were similar to those in 1977 (8 to 100) and 1978 (34 to 49). This similarity suggested a limited effect of lowered phosphorus and food web changes on peak larval alewife densities in Lake Ontario.