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.

Successful management of natural resources requires local action that adapts to larger‐scale environmental changes in order to maintain populations within the safe operating space (SOS) of acceptable conditions. Here, we identify the boundaries of the SOS for a managed freshwater fishery in the first empirical test of the SOS concept applied to management of harvested resources. Walleye (Sander vitreus) are popular sport fish with declining populations in many North American lakes, and understanding the causes of and responding to these changes is a high priority for fisheries management. We evaluated the role of changing water clarity and temperature in the decline of a high‐profile walleye population in Mille Lacs, Minnesota, USA, and estimated safe harvest under changing conditions from 1987 to 2017. Thermal–optical habitat area (TOHA)—the proportion of lake area in which the optimal thermal and optical conditions for walleye overlap—was estimated using a thermodynamic simulation model of daily water temperatures and light conditions. We then used a SOS model to analyze how walleye carrying capacity and safe harvest relate to walleye thermal–optical habitat. Thermal–optical habitat area varied annually and declined over time due to increased water clarity, and maximum safe harvest estimated by the SOS model varied by nearly an order of magnitude. Maximum safe harvest levels of walleye declined with declining TOHA. Walleye harvest exceeded safe harvest estimated by the SOS model in 16 out of the 30 yr of our dataset, and walleye abundance declined following 14 of those years, suggesting that walleye harvest should be managed to accommodate changing habitat conditions. By quantifying harvest trade‐offs associated with loss of walleye habitat, this study provides a framework for managing walleye in the context of ecosystem change.

Estimates of size-and sex-specific gear selectivity are important for making informed management decisions. Sexspecific selectivity curves may be needed for two-sex statistical catch-at-age models when information about sex ratios in the catch is unavailable. We used data from three tagging programs in Minnesota and Wisconsin to estimate the size-and sex-specific selectivity of angling and spearing for Walleyes Sander vitreus. We estimated capture selectivity (the relative catchability of each component of the population) and harvest selectivity (the combined effect of capture selectivity and the decision to retain or release a fish from a given component). These components are of interest because (1) the hooking mortality of released fish contributes substantially to total mortality, so that it is important to know how harvest and release vary by size; and (2) capture selectivity is likely similar across lakes, such that data from other lakes may provide information on capture selectivity for the lake of interest, while harvest selectivity is lake specific. Estimates were obtained using generalized linear models to determine the significance of the individual and interactive effects of length and sex on selectivity. Angling capture and harvest selectivity were both greater for females than males of every length. In contrast, spearing harvest selectivity was greater for males. For both sexes, harvest selectivity for angling and spearing peaked at around 400-450 mm. The capture selectivity of anglers peaked at 350-375 mm. The interaction between sex and size was significant for capture selectivity for angling, with the sex effect for small fish being less than that for large fish. Above 400 mm, spearing selectivity did not appear to vary with length for either sex, but at lengths below that it was lower for males.

We evaluated a modified daily bag limit for walleyes Sander vitreus to determine whether management goals were achieved. The modified bag limit consisted of no minimum length limit but allowed a daily harvest of only one walleye longer than 356 mm. The Wisconsin Department of Natural Resources implemented the single‐fish, 356‐mm regulation in 1997 on a number of lakes in northern Wisconsin that were previously subject to a 381‐mm minimum length limit. The intent of the regulation was to increase the harvest of smaller walleyes in order to increase growth, spawning stock abundance, and harvest rate in lakes characterized by high densities of slow‐growing walleyes. We examined the effects of the regulation in light of variables associated with walleye populations, the walleye fishery, and tribal harvest. All lakes included in our evaluation are located within the ceded territory of northern Wisconsin, where tribal harvest of walleyes, which is regulated differently than angler harvest, occurs annually. The daily bag limit for walleyes ranged between two and five, depending on the level of tribal harvest. Walleye fishery characteristics measured before 1997 (381‐mm limit) and after 1999 (single‐fish, 356‐mm limit) demonstrated an increased angler harvest rate and a decreased mean length of harvested walleyes. However, we found none of the desired changes in walleye population characteristics. In addition, we found no evidence that lakes that were changed to the single‐fish, 356‐mm regulation were characterized by slower growth than those that retained the 381‐mm regulation. The single‐fish, 356‐mm regulation requires increased harvest to achieve the desired changes to walleye population characteristics. Our results suggest that a significant decline in angler effort over time, in conjunction with the possibility that the lakes selected to receive the single‐fish, 356‐mm regulation did not exhibit particularly slow walleye growth, acted to offset increases in harvest rate and played a role in the general failure of this regulation to meet its goals.

No abstract