1. Effects of Environmental Factors on Crayfish Abundance in the Boundary Water Canoe Area Wilderness
Nathan Servey, Brennan Dow, Brittany Burant and Mason Loden
Department of Biology  University of Wisconsin-Eau Claire
Background
Results
Results Cont.
The objective of this study was to investigate how crayfish in the Boundary Water Canoe Area Wilderness (BWCA) responded to lake productivity and water depth. Both are important aspects of crayfish habitat and may determine food availability, predation risk and other factors (Weinlaender & Fuereder 2012, Ficetola et al. 2012). We estimated lake productivity using a Secchi disk, a simple but reliable measure of lake trophic state, and surveyed crayfish populations in five BWCA lakes using two methods (Dorn 2005). Five commercial crayfish traps were set at depths ranging 1-5 m, baited and left overnight. Trapped crayfish were then identified, counted, sexed and weighed, and their carapace length was measured. Active sampling involved doing a visual count of crayfish along lake shorelines for 5-min intervals to obtain an average number crayfish observed per minute.
Hypothesis:
We predict that lakes showing higher levels of productivity will correlate with increased crayfish abundance and biomass.
r2 = 0.55
p = 0.046
Figure 3. Orconectus obscurus crayfish weight as a function of carapace length. O. obscurus was the most abundant crayfish species in our traps, comprising 89% of all individuals captured. We found that the length-weight relationship did not vary across lakes, despite differences in O. obscurus weight among lakes (Fig. 1). The data suggest these corresponding traits show little plasticity.
Methods
Lake selection based on:
1.) Accessibility
2.) Rocky substrate
Discussion
The data did not support our hypothesis; in fact, we observed the opposite of what had been predicted. Rather than crayfish being more abundant in productive lakes, less productive lakes had more and larger crayfish (Fig. 1).
This raises the possibility that, rather than responding to lake productivity, non-native Orconectus obscurus crayfish may be negatively affecting lake productivity. Invasive species abundance often corresponds to declines in native crayfish and primary producers (Weinlaender & Fuereder, 2012), with detrimental implications for aquatic ecosystems (Ficetola et al., 2012).
Most crayfish occurred in depths ranging 2-4 m (Fig. 2), suggesting that shallow and deep water present less favorable habitat. We speculate that shallow water presents greater risk of predation from shore predators (e.g., herons, raccoons and otters) whereas deep water has less abundant food (i.e., fewer aquatic plants and invertebrates).
Little is known about how non-native O. obscurus affects native crayfish populations and aquatic ecosystems, much less those in the Boundary Waters. Further studies examining the spread, competitiveness, and abundance of O. obscurus are warranted.
This study will guide future BWCA crayfish research in Summer 2013.
Environmental factors measured:
Productivity (with Secchi disk)
Lake perimeter and maximum depth
Trap sampling depth
Figure 1. Mean crayfish weight (± 1 SE) and total crayfish captured in each lake as a function of average Secchi depth. Five Secchi readings were taken per lake. Low lake productivity correspond to higher Secchi readings.
Active sampling
Each lake visually surveyed 5 times using successive 5-min sampling periods.
If no crayfish sighted, sampling interval continued until crayfish sighted or 15 min.
Passive sampling
Lakes sampled using 5 baited minnow traps set at 1 m intervals along 1-5 m depth gradient.
Traps set in the afternoon and recovered the following day. Captured crayfish were identified, sexed, weighed and had carapace length measured.
Acknowledgements: Funding and support for this research project was provided by the UWEC Biology Department and individual course fees. We would also like to thank Todd Wellnitz, our mentor and professor, and our BWCA outfitter, Williams and Hall.
Literature Cited:
Crocker, D.W. & D.W. Barr Handbook of the crayfishes of Ontario. Royal Ontario Museum of Life Sciences, miscellaneous publications.
Dorn, N.J., R. Urgelles & J.C. Trexler Evaluating active and passive sampling methods to quantify crayfish density in a freshwater wetland. Journal of the North American Benthological Society 24:
Ficetola, G.F., M E. Siesa, F. De Bernardi, & E. Padoa-Schioppa Complex impact of an invasive crayfish on freshwater food webs. Biodiversity and Conservation 21:
Minnesota DNR, Lake Information Report, (accessed November, 11, 2012).
Weinlaender, M. & L. Fuereder Associations between stream habitat characteristics and native and alien crayfish occurrence. Hydrobiologia 693:
Figure 2. The proportion of crayfish trapped at different depths were calculated as a proportion of the crayfish captured for a given lake, and then the mean (± 1 SE) for each depth was calculated across the 5 lakes sampled. Crayfish were proportionally more abundant at intermediate depths.