Lake Superior’s food web has been altered in recent decades by changes in fish populations as well as species invasions. In addition, warming surface waters may also effect food web interactions. Cisco (Coregonus artedi) is an abundant and important planktivorous fish in Lake Superior. Cisco link secondary production to top level predators and support a viable commercial fishery. However, it is unknown how these changes to the Lake Superior ecosystem have effected cisco growth rates and resource use in Lake Superior. I reconstructed growth histories of cisco from western Lake Superior from 1984-2013 and used bioenergetics modeling to explore how cisco have responded to changes in their density, the invasion of spiny water flea (Bythotrephes longimanus), and climate change. I also used bioenergetics modeling and concurrent estimates of calanoid copepod standing stock and production to estimate the current supply-demand relationship for this important prey resource of cisco in Lake Superior. Cisco growth rates have been relatively stable over the 25-year period analyzed in this study with the exception of the 1998 cohort. The 1998 cohort had reduced growth rates in comparison to other cohorts analyzed in this study which may be the result of a density dependent reduction in prey resources. Climate change and invasion by spiny water flea may not have effected cisco growth rates to date, however, bioenergetics modeling suggests spiny water flea are a poor prey item for cisco relative to native prey and could reduce growth rates of cisco that consume them. Cisco were the primary consumers of calanoid copepods in the offshore waters of Lake Superior in 2014-2015 and consumption of calanoid copepods by fish was approximately 15% of calanoid production from May 2015-October 2015. Cisco may have exerted top-down control on calanoid copepods from October 2014-May 2015 which could have increased intraspecific competition for prey resources during this time. This work helps to clarify how cisco have responded to perturbations to the Lake Superior food web. In addition, this work suggests cisco in western Lake Superior are not currently prey limited during the growing season (May-October). They may however, exert top-down control on their winter prey resource. Future studies concerning the winter ecology of cisco and calanoid copepods may improve our understanding of resource use by this important planktivore in Lake Superior.
University of Minnesota M.S. thesis. January 2017. Major: Integrated Biosciences. Advisors: Tom Hrabik, Donn Branstrator. 1 computer file (PDF); v, 80 pages.
Growth and Predatory Demand of Cisco (Coregonus artedi) in Western Lake Superior.
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