No abstract

No abstract

Effective conservation and management of mixed-stock fisheries depends upon accurate stock identification of individual fish. We evaluated the utility of otolith chemical analysis as an approach to classify juvenile, premigratory steelhead Oncorhynchus mykiss collected in tributaries of Lake Michigan as either wild or hatchery origin. Two primary hypotheses proposed were that otolith chemistry can be used (1) to classify juvenile, hatchery-origin steelhead to the correct hatchery of origin and (2) to correctly classify stream-collected juvenile steelhead as wild or hatchery origin. Using laser-ablation inductively coupled plasma mass spectrometry, we analyzed the concentrations of seven elements (using 25 Mg, 55 Mn, 65 Cu, 66 Zn, 88 Sr, 137 Ba, 208 Pb) in the otoliths of juvenile steelhead collected from five hatcheries and 25 streams in the Lake Michigan basin. When discriminating among hatcheries, only 4.41% of fish misclassified to an alternate hatchery when subjected to the best random forest classification algorithm that included all elements as predictor variables. Distinct chemical signatures between fish of wild and hatchery origin supported 100% classification accuracies of known-wild, age-0 steelhead as wild origin for 19 of the 25 streams sampled. Misclassification of wild, age-0, and hatchery-origin fish, which tended to occur for streams that were located in close proximity to the hatchery, never exceeded 3.4% for a given stream. Our findings demonstrate highly successful discrimination of hatchery-origin and wild juvenile steelhead across a broad geographic range. Applying the classification algorithms developed herein to unknown-origin steelhead provides the ability to infer survival of year-classes from specific hatcheries. Further, the ability to differentiate hatchery-and wild-origin fish will assist in stock assessment efforts allowing for increased effectiveness of conservation and management of the species.

Watershed development may alter tributaries in ways that influence the growth and development of juvenile fish. For Rainbow Trout Oncorhynchus mykiss, the extent of land cover influence on the diet composition of stream‐residing juveniles is still an open but important question, particularly given their broad global distribution. Our study evaluated the effect of land cover on diet composition and condition metrics of juvenile Rainbow Trout in Lake Michigan tributaries. Juveniles were collected in the fall of 2014 and 2015 from 18 sampling locations across the Lake Michigan basin and included a wide variety of land cover types. Multivariate statistical procedures were used to quantitatively score sites along axes that maximized variation in land cover and diet composition. Subsequently, Spearman's rank correlations were used to test for significance of correlations between (a) land cover and diet composition, (b) land cover and fish condition, and (c) diet composition and fish condition. Our results indicated that land cover had an influence on the diet compositions of juvenile Rainbow Trout, but neither land cover nor diet composition had a significant influence on Rainbow Trout condition. These findings contrast with previously published work focusing on the growth responses of other fish species and provide insight into the plastic nature of Rainbow Trout. Further, we discuss potential ways in which fish may mitigate the effect of land cover changes as they are transmitted through stream food webs; however, the magnitude of the mitigation likely varies among fish species.

In the Laurentian Great Lakes, zooplankters are often sampled using standard ≤ 153 µm mesh nets without regard to the time of day they are collected. We sampled Cercopagidae during 2013-2014 in northern Lake Huron during day, dusk, and night using two different nets (a 0.5 m wide 153 µm mesh "standard" net and a 0.75 m wide 285 µm mesh "Bythotrephes" net) to determine if there were any differences in their sampled densities. Bythotrephes densities with the standard net were approximately 2.07-fold greater when captured at night than during the day. No time of day bias occurred with the Bythotrephes net. Nighttime Bythotrephes densities did not differ between the two net types. Cercopagis densities did not vary with net type or the time of day in this study, but future work should revisit this result given our low sample size and the low occurrence of Cercopagis in Lake Huron. To reduce bias and calculate accurate density estimates, Cercopagidae should be sampled at night if using a standard net or any time of day with the Bythotrephes net. Given the large impact of invasive predatory cladocerans Bythotrephes longimanus and Cercopagis pengoi on food webs since their invasion in the Laurentian Great Lakes in the 1980s, proper estimation of their densities is essential.