Predation Pressure on Emergent Lake Trout Fry in Lake Champlain and Techniques for Assessing Lake Trout Reproduction in Deep-Water Habitats

摘要

Lake trout (Salvelinus namaycush) were extirpated from Lake Champlain around 1900 and from the lower four Great Lakes by 1960. Their ecological, commercial and recreational importance has prompted extensive restoration efforts. Despite widespread evidence of natural reproduction by stocked lake trout, there is minimal evidence of survival of wild progeny beyond age-0. Various abiotic and biotic impediments may be preventing self-sustaining lake trout populations from becoming established. Unsuccessful restoration in shallow areas has recently prompted a shift to restoration efforts to offshore, deep reefs in the Great Lakes. The first objective of this study was to develop, test, and implement methods for evaluating lake trout reproduction in deep water, where previously established techniques were ineffective. The second objective addressed the recruitment bottleneck between the emergent fry and juvenile life stages in Lake Champlain, by assessing the severity of predation on lake trout fry by epi-benthic fish. In order to quantify egg density on deep-water habitats (u3e18 m), we paired a deep-water egg trap with egg bags to establish a relationship between the two types of gear in Lake Champlain. There was no significant difference between densities in the egg bags and deep-water traps, but there was a positive correlation of their ranks (correlation coefficient = 0.514, pu3c0.0001). The deep-water traps were then used in Lake Michigan to successfully acquire the first egg density data from two sites on the deep Mid Lake Reef Complex. A drop electroshocker was developed to detect fry presence and tested in Lake Champlain in conjunction with emergent fry traps. Both types of gear exhibited similar patterns of fry relative abundance. To assess fry predation in Lake Champlain, two-hour gillnet sets during the period of fry emergence to identify fry predators and to describe how predation patterns changed diurnally and temporally. Seven species of epi-benthic fry predators were identified, including five species that had not been previously identified as fry predators. Yellow perch and rock bass dominated the predator community at two study sites (83% of total catch, N=1179, 77% of all fry predators, N=57). Predator presence and fry consumption was almost entirely nocturnal. There was a linear aggregational response in predator CPUE (fish/hr) to increasing fry relative abundance (pu3c0.033) but confirmed predators did not exhibit a functional response. There was evidence of a threshold of fry relative abundance at 1 fry/trap/day for the onset and conclusion of fry predation. Temperature was a driving factor in the timing of fry emergence and predator abundance, allowing us to predict the relative impact of predators based on temperature scenarios. Only 5% of the potential predators consumed fry. We used empirical probabilities of consumption to model loss of fry due to predation. This consumption model revealed that predator abundances would have to be extremely high for predation to significantly reduce the population of fry. However, given the relatively high species richness of predators observed at the shallow water study sites, lake trout fry survival is likely to be higher at deep, offshore reefs. These results support the recent shift in restoration efforts to focus on deep reefs.