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To inform lake trout (Salvelinus namaycush) fishery management in Lake Huron that has undergone rapid ecosystem changes, we quantified lake trout production dynamics by coupling age-structured stock assessment and fish bioenergetics models. Our approach revealed the connection between piscivore production and prey consumption, included growth compensation to reproduction losses, and allowed comparisons between long-term dynamics of fishery harvests and fish production. We found that, despite the collapse of alewives, a major non-native pelagic prey fish, lake trout production appeared to be sustainable. To a certain degree, the effect of recent recruitment declines on lake trout production was offset by the release of harvest pressure from subadult lake trout and reduction in fishing and sea lamprey-induced mortality on adult lake trout. Evidence for sustainability also included the finding that no changes in average ratios of annual production to beginning-of-the-year biomass. Juvenile P:B ratio remained as high as 2.1. The effect of growth declines on adult and subadult production was offset by reduction in population mortality. Body growth and condition did not continue to decline when lake trout became more and more reliant on round goby as food, and the dynamics of total consumption of prey fish continued to be recipient controlled. [ABSTRACT FROM AUTHOR]

Since the 1950s, fishery agencies on Lake Michigan have pursued Lake Trout Salvelinus namaycush rehabilitation through Sea Lamprey Petromyzon marinus control, harvest regulations, and stocking millions of fish annually. Stocking was prioritized at four historically important spawning locations beginning in 1985, and coded wire tags (CWTs) were used to help evaluate performance. We used data from coded‐wire‐tagged fish captured in fishery‐independent surveys from 1998 to 2014 to evaluate relative postrelease survival of Lake Trout, estimated by CPUE and corrected for the number of fish stocked, across 173 CWT lots of the 1994–2003 year‐classes stocked at the four locations. Boosted regression tree models were used to assess the relative influence of four variables on Lake Trout CPUE in two age‐groups (ages 4–5 and ages 6–10) and were paired with ANOVAs to test for statistical significance. Genetic strain (29.1%), stocking location (27.8%), mortality at release (23.1%), and predator density (19.9%) had similar influences on the relative survival of younger fish, whereas the relative survival of older fish was heavily influenced by stocking location (79.8%). Survival of both age‐groups was lowest for fish stocked in the Northern Refuge, where the age structure was truncated due to fishery harvest and Sea Lamprey predation. Survival of stocked fish was higher at the Southern Refuge, Clay Banks, and Julian's Reef, where mortality from Sea Lamprey and harvest was lower and where increases in wild Lake Trout have been observed in recent years. Stocked Lake Michigan remnant genetic strains also appeared to survive better than strains from other lakes at these three locations, but strain effects could not be fully disentangled from the effects of stocking location, and the continued stocking of multiple genetic strains may provide resiliency to future selection pressures. Continued progress toward rehabilitation will require reducing fishing and Sea Lamprey‐induced mortality in northern Lake Michigan to build parental stocks of advanced ages as well as balancing efforts among competing management goals. [ABSTRACT FROM AUTHOR]

Using laboratory experiments, we assessed the effect of temperature and predator density on consumption of trout eggs by crayfish. We quantified the effect of four temperature ranges (2-3, 4-5, 7-8, and 10-12°C) on consumption of lake trout (Salvelinus namaycush) and rainbow trout (Oncorhynchus mykiss) eggs by a native (Orconectes propinquus) and an exotic (Orconectes rusticus) crayfish found in the Great Lakes. Mean individual consumption ranged from 0.1 to 1.5 lake trout eggs per day and from 1.0 to 5.5 rainbow trout eggs per day, respectively. Temperature influenced consumption for O. rusticus feeding on rainbow trout eggs but not O. propinquus. The effect of conspecific predator competition on rainbow trout egg consumption was evaluated using three densities of crayfish (1, 5, 10 per tank). For O. rusticus, mean individual consumption rate was higher at the lowest crayfish density (1 crayfish/tank) than at medium (5 crayfish/tank) or high (10 crayfish/tank) densities. A similar conspecific effect was not evident in O. propinquus and total consumption per unit area increased with crayfish density. The effect of competition among crayfish species on consumption of rainbow trout eggs was examined by comparing feeding rates at high crayfish abundance levels in single and mixed species treatments. Mixed species interactions did not significantly alter crayfish feeding rates. Results from these experiments provide empirical data to aid in modeling how temperature and predator density affect lake trout egg predation rates in the field. [ABSTRACT FROM AUTHOR]

We compared the relative abundance of lake trout Salvelinus namaycush spawners in gill nets during fall 1999-2001 in Lake Michigan at 19 stocked spawning sites with that at 25 unstocked sites to evaluate how effective site-specific stocking was in recolonizing historically important spawning reefs. The abundance of adult fish was higher at stocked onshore and offshore sites than at unstocked sites. This suggests that site-specific stocking is more effective at establishing spawning aggregations than relying on the ability of hatchery-reared lake trout to find spawning reefs, especially those offshore. Spawner densities were generally too low and too young at most sites to expect significant natural reproduction. However, densities were sufficiently high at some sites for reproduction to occur and therefore the lack of recruitment was attributable to other factors. Less than 3% of all spawners could have been wild fish, which indicates that little natural reproduction occurred in past years. Wounding by sea lamprey Petromyzon marinus was generally lower for Seneca Lake strain fish and highest for strains from Lake Superior. Fish captured at offshore sites in southern Lake Michigan had the lowest probability of wounding, while fish at onshore sites in northern Lake Michigan had the highest probability. The relative survival of the Seneca Lake strain was higher than that of the Lewis Lake or the Marquette strains for the older year-classes examined. Survival differences among strains were less evident for younger year-classes. Recaptures of coded-wire-tagged fish of five strains indicated that most fish returned to their stocking site or to a nearby site and that dispersal from stocking sites during spawning was about 100 km. Restoration strategies should rely on site-specific stocking of lake trout strains with good survival at selected historically important offshore spawning sites to increase egg deposition and the probability of natural reproduction in Lake Michigan. [ABSTRACT FROM AUTHOR]

The continued lack of natural reproduction by lake trout (Salvelinus namaycush) in the Great Lakes has prompted development of models focused on the potential negative effects of interstitial predators. To aid in parameterization of such models we investigated the effect of temperature (1-2°C, 4-5°C, 7-8°C, and 10-11°C), predator group size, interspecific competition, egg density (60-6000 eggs m-2), and an alternate food source on egg consumption by slimy (Cottus cognatus) and mottled (Cottus bairdii) sculpins, round gobies (Neogobius melanostomus), and the native crayfish (Orconectes propin- quus) in the laboratory. Egg consumption by the sculpins and round goby all increased over the range of temperatures investigated (ca 0.5 eggs·day-1 at 1-2°C to 1.5 eggs·day-1 at 10-11°C). Predator group size affected consumption; fewer eggs were eaten per individual slimy sculpin and round goby at densities of 5 or 10 per tank than I individual per tank. There was no effect of interspecific competition on egg con- sumption by slimy sculpins or round gobies at a density of 10 individuals per tank for various species combinations (10:0, 9:1, 5:5, 1.9, 0:10). A type 11 functional response to egg density was observed for sculpins, gobies, and crayfish although at extreme densities per capita consumption by crayfish and gobies declined. The presence of zebra mussels (Dreissena polymorpha) did not affect the number of rainbow trout (Oncorhynchus mykiss) eggs eaten by round gobies whose mussel consumption averaged 75% BW·day-1. Of the four species examined, round gobies appeared to be the most effective egg predator while crayfish were the least. [ABSTRACT FROM AUTHOR]

Restoration of self-sustaining populations of lake trout is underway in all of the Great Lakes and Lake Champlain, but restoration has only been achieved in Lake Superior and in Parry Sound, Lake Huron. We evaluated progress toward restoration by comparing spawning habitat availability, spawner abundance, egg and fry density, and egg survival in Parry Sound in Lake Huron, in Lake Michigan, and in Lake Champlain in 2000-2003. Divers surveyed and assessed abundance of spawners at 5 to 15 sites in each lake. Spawning adults were sampled using standardized gill nets, eggs were sampled using egg bags, and fry were sampled using emergent fry traps and egg bags left on spawning reefs over-winter. Spawning habitat was abundant in each lake. Adult lake trout abundance was low in Lake Michigan and Parry Sound, and very high at one site in Lake Champlain. Egg deposition was lowest in Lake Michigan (0.4-154.5 eggs·m-2, median = 1.7), intermediate in Parry Sound (39-1,027 eggs·m-2, median = 278), and highest in Lake Champlain (0.001-9,623 eggs·m-2, median = 652). Fry collections in fry traps followed the same trend: no fry in Lake Michigan, 0.005-0.06 fry·trap-1 day-1 in Parry Sound, and 0.08-3.6 fry·trap-1 in Lake Champlain. Egg survival to hatch in overwinter egg bags was similar in Lake Michigan (7.6%) and Parry Sound (2.3-8.9%) in 2001-02, and varied in Lake Champlain (0.4-1.1% in 2001-02, and 1.8-18.2 in 2002-03). Lake trout restoration appears unlikely in northern Lake Michigan at current adult densities, and failure of restoration in Lake Champlain suggests that there are sources of high mortality that occur after fry emergence. [ABSTRACT FROM AUTHOR]

Excessive mortality from spawning through the first year of life is likely responsible for recruitment failures in many lake trout (Salvelinus namaycush) populations. We evaluated relationships between interstitial predator species and lake trout egg deposition in three regions of the Great Lakes. The introduced crayfish Orconectes rusticus dominated the nearshore zone in northern Lake Michigan, whereas other sites in Lake Michigan and Parry Sound were dominated by native Orconectes propinquus. In Lake Champlain, sculpin (Cottus spp.) were the most common interstitial predator, yet densities were comparable with those in Lake Michigan. Predator densities in Lake Michigan (mean ± standard error = 15.5 ± 0.4 predators·m–2) were twice those in Lake Champlain or Parry Sound (6 ± 1 predators·m–2). Densities of eggs were higher in Lake Champlain (2994.1 ± 398.3 eggs·m–2) and Parry Sound (454.7 ± 36.3) than in Lake Michigan (7.5 ± 1.3; Kruskal-Wallis, P ≤ 0.05). Ratios of eggs to predator provide simple measures of the effectiveness of lake trout rehabilitation efforts. The average ratio calculated from 10 successful spawning locations is 190 eggs·predator–1·m–2. Ratios may help establish target egg densities for restoration efforts and the concentration of spawning adults required to allow survival to the fry stage. [ABSTRACT FROM AUTHOR]