Geology topics

M.J. Hansen

Publications and source records attributed to M.J. Hansen.

3 recordsLinked to original sources

USGS2020

Gut contents from multiple morphs of lake trout (Salvelinus namaycush) at two offshore shoals in Lake Superior

Four lake trout, Salvelinus namaycush , Walbaum 1792 morphs occur in Lake Superior: lean, siscowet, humper, and redfin. Diets of lean and siscowet have been relatively well described. However, less is known about diets of humper and redfin, and overall few studies have been conducted at offshore shoals. We compared gut content data among mature (357–867 mm) sympatric lake trout morphs caught at two offshore shoals in Lake Superior, Stannard Rock and Superior Shoal, in 2013 and 2014 (total n = 416). All morphs were caught in shallow (<50 m), mid (50–100 m), and deep (>100 m) strata. Invertebrates made up a greater portion of the stomach contents than did fish for all morphs by both percent occurrence and proportional biomass, and Mysis was the primary invertebrate consumed by all morphs at both sites. Coregonus spp. and deepwater sculpin, Myoxocephalus thompsonii were the most commonly consumed fish. Humper had the highest average proportional biomass of deepwater sculpin and had no other identifiable species of fish in their guts. Biomass of fish in redfin guts was highest for Coregonus spp., followed by similar amounts of deepwater sculpin and burbot, Lota lota . Diet overlap among morphs was high, and differences in prey consumption between sites are likely related to prey availability. Additional study is needed to determine if differences in trophic ecology between humper and other morphs are sufficient to support concurrent stocking of multiple morphs, particularly in light of recent declines in native prey fishes, especially Coregonus spp., in the Laurentian Great Lakes.

Lake Superior↗

USGS2020

Dietary versus nondietary fatty acid profiles of lake trout ecotypes from Lake Superior and Great Bear Lake: Are fish really what they eat?

Fatty acids are well-established biomarkers used to characterize trophic ecology, food-web linkages, and the ecological niche of many different taxa. Most often, fatty acids that are examined include only those previously identified as “dietary” or “extended dietary” biomarkers. Fatty acids considered as nondietary biomarkers, however, represent numerous fatty acids that can be extracted. Some studies may include nondietary fatty acids (i.e., combined with dietary fatty acids), but do not specifically assess them, whereas in other studies, these data are discarded. In this study, we explored whether nondietary biomarker fatty acids can provide worthwhile information by assessing their ability to discriminate intraspecific diversity within and between lakes. Nondietary fatty acids used as biomarkers delineated variation among regions, among locations within a lake, and among ecotypes within a species. Physiological differences that arise from differences in energy processing can be adaptive and linked to habitat use by a species’ ecotype and likely explains why nondietary fatty acid biomarkers can be a relevant tool to delineate intraspecific diversity. Little is known about the nondietary-mediated differences in fatty acid composition, but our results showed that nondietary fatty acid biomarkers can be useful tool in identifying variation.

Lake Superior↗

USGS2005

Relationships between recapture rates from different gears for estimating walleye abundance in northern Wisconsin lakes

Maximizing sampling efficiency and reducing sampling costs are desirable goals for fisheries management agencies. Expensive and labor-intensive methods (such as mark-recapture) are commonly used to estimate the population abundance of walleye Sander vitreus, but more efficient methods may be available. We compared recapture rates from surveys and harvests to evaluate the efficiency of currently used recapture gears and the potential for using gears that require less effort. To evaluate the usefulness of walleye harvest as mark-recapture samples, we used errors-in-variables models to determine whether recapture rates differed between fyke-netting and spearing, electrofishing and spearing, and electrofishing and angling. We found no significant differences between fyke-netting and adult walleye electrofishing recapture rates or between spearing and adult walleye electrofishing recapture rates. In contrast, we found that recapture rates from angling and electrofishing differed significantly in lakes with and without minimum length limits. We concluded that the lack of significant differences between the slopes of some harvest and survey recapture rates may allow the use of harvest recapture rates to estimate walleye abundance, but the biases associated with each gear should be considered. We also concluded that more attention should be given to understanding the biases of recapture gears. ?? Copyright by the American Fisheries Society 2005.

North American Journal of Fisheries Management↗