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Dietary sources of thiamine (vitamin B 1 ) and thiamine-degrading enzymes (thiaminases) are thought to be primary factors in the development of thiamine deficiency among Great Lakes salmonines. We surveyed major forage fish species in Lake Michigan for their content of thiamine, thiamine vitamers, and thiaminase activity. Concentrations of total thiamine were similar (P Յ 0.05) among most forage fishes (alewife Alosa pseudoharengus, bloater Coregonus hoyi, spottail shiner Notropis hudsonius, deepwater sculpin Myoxocephalus thompsonii, yellow perch Perca flavescens, ninespine stickleback Pungitius pungitius, and round goby Neogobius melanostomus) and slightly lower in rainbow smelt Osmerus mordax. Concentrations of total thiamine were all above the dietary requirements of coldwater fishes, suggesting the thiamine content of forage fish is not the critical factor in the development of thiamine deficiency in Lake Michigan salmonines. Thiamine pyrophosphate was the predominant form of thiamine in most species of forage fish, followed by free thiamine and thiamine monophosphate. Total thiamine was slightly greater in summer collections of alewife and rainbow smelt than in spring and fall collections, but the same was not true for bloater. Thiaminase activity varied among species and was greatest in gizzard shad Dorosoma cepedianum, spottail shiner, alewife, and rainbow smelt. Thiaminase activity in alewife varied among collection locations, season (greatest in spring), and size of the fish. Size and condition factors were positively correlated with both total thiamine and thiaminase activity in alewife. Thus, thiamine and thiaminase activity in forage fishes collected in Lake Michigan varied among species, seasons, year caught, and size (or condition). Therefore, multiple factors must be considered in the development of predictive models for the onset of thiamine deficiency in Great Lakes salmonines. Most importantly, thiaminase activity was great in alewives and rainbow smelt, suggesting that these prey fish are key causative factors of the thiamine deficiency in Great Lakes salmonines.

Abstract.-We compared the relative abundance of lake trout Salvelinus namaycush spawners in gill nets during fall [1999][2000][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 * Corresponding author: charles_bronte@fws.gov 1 Retired.

Stability of the Lake Michigan fishery for chinook salmon Oncorhynchus tshawytscha at high levels became questionable after stocks declined dramatically following spring epizootics in which bacterial kidney disease (BKD) was a major factor. Initially stocked in 1967, favorable survival and growth of chinook salmon through the 1970s led to increases in abundance and in popularity with anglers. Returns of chinook salmon improved annually until the late-1980s, when, with little warning, spring epizootics reduced the abundance of adult salmon by 50% or more. Reduced abundance of alewives (Alosa pseudoharengus), coupled with an increase in chinook salmon density and heavy parasite infection rates were hypothesized to have reduced chinook salmon growth and fitness and to have increased their susceptibility to BKD. Evidence of slower growth exists and low food availability may be the stressor that triggered the epizootics. Chinook salmon were a major component of the economic development and subsequent hardship of the sportfishing industry on Lake Michigan. Sustaining the chinook salmon fishery at previous levels may require managing for high abundance of alewives, which may be inconsistent with overall fish community management goals. The future sustainability and role of chinook salmon needs to be reevaluated in the context of the entire Lake Michigan fish community. * Corresponding author: markholey@mail.fws.gov so abruptly. The crash of the chinook salmon population also resulted in a significant decline in angling effort.The dynamics of BKD within Lake Michigan are poorly understood. Coho salmon Oncorhynchus kisutch, steelhead Oncorhynchus mykiss, and chinook salmon have shown clinical signs of BKD, but chinook salmon was the only Lake Michigan salmonine to experience epizootics and high mortalities. This paper describes the rise and fall of the chinook salmon population and fishery in Lake Michigan. We review available data, much of it previously unpublished, before and after the population crash and discuss the possible factors contributing to the epizootics. How the future management of chinook salmon in Lake Michigan may change because of the species susceptibility to BKD is also discussed.

We quantified the distance that lake trout Salvelinus namaycush moved in northwestern Lake Michigan and examined (1) the directional preference and (2) the effect of population density on movement. Lake trout were captured in spring and fall 1983–1996, tagged with Floy anchor tags, and recaptured during subsequent agency sampling and by commercial fishers and anglers during 1983–1997. Angler recaptures were used to quantify movements; these recaptures were standardized to 10,000 salmonid angler‐hours (giving recaptures per effort (RPE)) to account for the spatial and temporal variation in recapture effort. Movement was inferred from the spatial and temporal differences in the distribution of RPE. The dispersal radius, an index of the area occupied by tagged lake trout, was defined as the area containing 90% of the total RPE. It was estimated by fitting the cumulative proportion of RPE versus distance from the tagging location to an exponential sigmoid model and using inverse prediction. We used linear regression to test for density dependence in movement. Lake trout tagged in spring occupied a larger area than those tagged in fall and increased their range as population density increased. Directional movement differed for spring‐ and fall‐tagged fish. Spring‐tagged fish showed no directional tendencies in movement, but fall‐tagged fish tended to move more to the south than to the north. There was no significant difference in directional movement between recapture seasons. Our results will be useful for the management and restoration of lake trout in Lake Michigan by providing information that can be incorporated into population models and management decisions about refuges and management zones within the lake.

Most of the polychlorinated biphenyl (PCB) body burden accumulated by coho salmon (Oncorhynchus kisutch) from the Laurentian Great Lakes is from their food. We used diet information, PCB determinations in both coho salmon and their prey, and bioenergetics modeling to estimate the efficiency with which Lake Michigan coho salmon retain PCBs from their food. Our estimate was the most reliable estimate to date because (a) the coho salmon and prey fish sampled during our study were sampled in spring, summer, and fall from various locations throughout the lake, (b) detailed measurements were made on the PCB concentrations of both coho salmon and prey fish over wide ranges in fish size, and (c) coho salmon diet was analyzed in detail from April through November over a wide range of salmon size from numerous locations throughout the lake. We estimated that coho salmon from Lake Michigan retain 50% of the PCBs that are contained within their food.

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