Non-native alewives have been implicated as contributors to the inability to reestablish native lake trout in the Laurentian Great Lakes. In lakes Huron, Michigan, and Ontario, where alewives have been abundant, lake trout rehabilitation efforts have met with little success. Alewives can cause direct mortality through predation on lake trout fry (i.e., newly hatched lake trout). Predation of lake trout fry was observed based on gut contents of alewives caught at Stony Island reef, Lake Ontario during spring over several years. Young lake trout fry, after absorption of their yolk sac, swim at night from the lake bottom to the surface to gulp air, fill their swimbladders (i.e., a sac used to maintain buoyancy in the water column), and become free-swimming. Likely, emergent fry making this transition between sac fry to the free swimming stage are highly vulnerable to predation. The actual predation potential of alewives on lake trout fry remains unknown due to the logistical difficulties associated with studying these interactions in the field, such as the rapid digestion of fry in alewife stomachs and the difficulty of accurately estimating alewife and fry abundance on a spawning reef. The question remains as to whether alewives, under any circumstances, could exert enough predation to measurably reduce lake trout fry numbers. We sought to determine the potential of alewife predation to affect the abundance of lake trout fry as they transitioned from pre-emergent to the free-swimming emergent stage by using artificial reefs in a laboratory. The fry during this transition would show typical diel movement in and out of stony substrate as sac fry, a brief night-time period of swimming to the surface to fill their swim bladders, and then become free-swimming as emergent fry. We hypothesized that alewives would affect fry abundance over a 12 d time period that approximated the length of the developmental interval required to transition from sac fry to emergent life stages. The study objective was to determine whether lake trout fry showing typical swimming and avoidance behavior over artificial reefs differed in survival when alewives were present versus when alewives were absent.
Our experimental design provided typical reef habitat (i.e., stony reef structure with deep interstices), springtime photoperiod, water temperatures similar to those in Lake Ontario during lake trout emergence, and densities of predators and prey that approximated those reported from Lake Ontario. Six identical, 1.22-m-diameter, circular fiberglass tanks served as laboratory reefs; four tanks were used as experimental treatments and two as controls. The bottom of each tank was layered with stone cobbles and pebbles. The six tanks were each stocked with 153 lake trout fry (density = 131· m-2), a density comparable to that recorded at Stony Island reef, Lake Ontario. Four treatment tanks each contained ten alewives (density = 8· m-2) and two control tanks contained no alewives. At the end of the 12 d experiment, alewives and lake trout were removed from the tanks and counted. Alewives were caught using a small dip net after partially draining each tank. Lake trout fry were caught and counted by removing and washing each stone to ensure no fry were missed.
The mean number of lake trout recovered at the end of the experiment was significantly lower in treatment tanks that contained alewives (mean = 31.5 fry of 153 fry stocked per tank) than in control tanks. In one treatment tank, 82 fry were recovered, whereas the other three tanks suffered high mortality; only 11, 16, and 17 fry were recovered at the end of the experiment. Mortality in the control tanks over the 12-d experiment was about 2% in contrast to the 46, 89%, 90%, and 93% mortality experienced in tanks containing alewives. Alewives were rarely observed to attempt predation during daylight hours on those fry that had become emergent and free-swimming; predation likely occurred at night on the sac fry during their journey to the surface to fill their swim bladders, but was not directly observed. Once at the emergent stage with inflated swim bladders, fry in control tanks freely swam throughout the water column during the day. In contrast, emergent fry in treatment tanks with alewives swam only on the bottom close to the substrate during the day and not throughout the water column, choosing to hover in spaces between rocks, apparently seeking shelter to avoid alewives.
Lake trout fry mortality (46-92%) over the 12-d period spanning emergence was much greater in treatment tanks due to predation by alewives than in control tanks without alewives (<2%). Our interpretation is that alewives were capable of regularly feeding on lake trout fry that used stony substrates as cover and showed typical behavior of swim-up and free-swimming emergent fry. The results of this study support the hypothesis that predation by alewives could cause substantial lake trout fry mortality, and impede lake trout rehabilitation. The potential negative effects on fry survival caused by alewives should temper expectations for the success of population rehabilitation of lake trout in areas of high alewife abundance; and, low alewife density may explain the recent resurgence of lake trout in Lake Huron.
Full study published by Krueger, C.C., W.C. Shepherd, and A.M. Muir. 2014. Predation by alewife on lake trout fry emerging from laboratory reefs: estimation of fry survival and assessment of predation potential. Journal of Great Lakes Research 40:429-434.
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