PREDATION BY ALEWIVES ON LAKE TROUT FRY IN LAKE-ONTARIO - ROLE OF AN EXOTIC SPECIES IN PREVENTING RESTORATION OF A NATIVE SPECIES

Lake trout (Salvelinus namaycush) restoration efforts in Lake Ontario have resulted in an abundance of spawning fish of hatchery-origin, bur virtually no detectable natural recruitment. One explanation has been predation by non-native alewives (Alosa pseudoharengus) on lake trout fry. The purpose of this study was to determine if alewives could be important predators on lake trout fry. In the laboratory, fry behavior was examined to ascertain when fry would be present in the water colu mn during a 24-hour period and to determine the acceptability of fry a s food for alewives. In aquaria exposed to ambient light regimes, sac fry activity in the water column was much greater at night than during daylight hours (P < 0.001). In laboratory tanks, lake trout fry (15-3 4 mm) were aggressively eaten by alewives (118-175 mm). Field studies were conducted at Stony Island Reef Lake Ontario in 1989-1993 to deter mine whether alewives and fry were present at the same time on the ree f if alewives fed when on the reef and if alewives fed upon naturally- produced lake trout fry. Lake trout fry captured in traps indicated th at sac and emergent fry were available as prey from the middle of Apri l through the third week of May. The first capture of alewives in gill nets set adjacent to the fry traps was typically in early May and corr esponded to the peak capture of sac fry in traps. Food was present in 86% of the 1,239 alewives captured after sunset over the 5-year period . Ten lake trout fry were found in 6 of the 62 alewives captured after sunset on 20 May 1993 at Stony Island Reef; no fry were found in alew ife stomachs caught on other dates. Predation by alewives might have c aused substantial mortality of lake trout fry from spawning areas in L ake Ontario where alewives were abundant and could also be an importan t source of mortality in similar areas of Lakes Michigan and Huron. In creased stocking of predatory salmonids to suppress the alewife could enhance survival of fry and speed restoration in Lake Ontario, but sup pression seems unlikely under current strategies to manage the alewife as forage for non-native salmonids. In this context, lakewide goals s hould be re-focused on restoration in localized areas where alewives d o not congregate during the spring and predation on lake trout fry wou ld be minimal-such as at offshore shoals.