Yule D.L.,U.S. Geological Survey |
Adams J.V.,U.S. Geological Survey |
Hrabik T.R.,University of Minnesota |
Vinson M.R.,U.S. Geological Survey |
And 3 more authors.
Fisheries Research | Year: 2013
Acoustic methods are used to estimate the density of pelagic fish in large lakes with results of midwater trawling used to assign species composition. Apportionment in lakes having mixed species can be challenging because only a small fraction of the water sampled acoustically is sampled with trawl gear. Here we describe a new method where single echo detections (SEDs) are assigned to species based on classification tree models developed from catch data that separate species based on fish size and the spatial habitats they occupy. During the summer of 2011, we conducted a spatially-balanced lake-wide acoustic and midwater trawl survey of Lake Superior. A total of 51 sites in four bathymetric depth strata (0-30. m, 30-100. m, 100-200. m, and >200. m) were sampled. We developed classification tree models for each stratum and found fish length was the most important variable for separating species. To apply these trees to the acoustic data, we needed to identify a target strength to length (TS-to-L) relationship appropriate for all abundant Lake Superior pelagic species. We tested performance of 7 general (i.e., multi-species) relationships derived from three published studies. The best-performing relationship was identified by comparing predicted and observed catch compositions using a second independent Lake Superior data set. Once identified, the relationship was used to predict lengths of SEDs from the lake-wide survey, and the classification tree models were used to assign each SED to a species. Exotic rainbow smelt (Osmerus mordax) were the most common species at bathymetric depths <100. m with their population estimated at 755 million (3.4. kt). Kiyi (Coregonus kiyi) were the most abundant species at depths >100. m (384 million; 6.0. kt). Cisco (Coregonus artedi) were widely distributed over all strata with their population estimated at 182 million (44. kt). The apportionment method we describe should be transferable to other large lakes provided fish are not tightly aggregated, and an appropriate TS-to-L relationship for abundant pelagic fish species can be determined. © 2013.
Vandehey J.A.,South Dakota State University |
Vandehey J.A.,Wisconsin Cooperative Fishery Research Unit |
Kaemingk M.A.,South Dakota State University |
Jansen A.C.,Parks and Tourism |
And 3 more authors.
Journal of Applied Ichthyology | Year: 2013
Acute reductions in water temperature (i.e. cold fronts) may influence larval fish survival directly via limits on physiological tolerance or indirectly by acting as a sublethal stressor. The primary objective was to quantify survivorship of yellow perch yolk-sac fry exposed to two different temperature declines (4 and 8°C) and compare survivorship to that of perch fry under ambient temperatures representative of natural conditions. Behaviour of yolk-sac fry following temperature declines was also qualitatively assessed. Mean survival in the control, -4, and -8 treatment tanks was 90, 91 and 97%, respectively, and no significant differences in percent survival were observed between the control and the -4 treatment (ts = -0.10; df = 7; P = 0.93), the control and -8 treatment (ts = -1.85; df = 7; P = 0.11) or the -4 and -8 treatments (ts = -1.33; df = 7; P = 0.22). Observations of yellow perch eggs and fry behaviour following temperature declines differed among treatments. Any remaining eggs in the control treatment and -4 treatments continued to hatch during the experiment, and fry were documented swimming throughout the water column in all tanks. However, in the -8 treatment, any eggs that had not hatched remained inactive and all fry within all -8 treatment tanks ceased swimming activity and settled to the bottom of the tanks once the temperature reached 3.9°C. Fry remained at the bottom of the tanks for the entire 48 h simulated cold-front. Fry resumed swimming activity once water temperatures began to increase (by approximately 6°C). Results indicated that drops in temperature (i.e. cold fronts) similar to or greater than those found in small impoundments did not cause direct mortality of yellow perch during the yolk-sac fry (post-hatch larvae) stage. Although an acute drop in temperature may not induce sudden high mortality, it may be a sub-lethal stressor, leading to increased starvation or predation risk. © 2012 Blackwell Verlag GmbH.