Trophic Disruption Effects on the Diet and Condition of Lake Whitefish

Grant: # 0836

Grant Amount: $81,000.00

Board Decision Year: 2006

Fisheries and Oceans Canada - Great Lakes Laboratory for Fisheries and Aquatic Sciences (Burlington, ON, Canada)

Great Lakes Laboratory for Fisheries and Aquatic Sciences

GLFT - Ecosystem Health and Sustainable Fish Populations-H - Ecological and biological fisheries research to inform management

Project Details

Commercial fisheries for lake whitefish (Coregonus clupeaformis) represent some of the largest and most valuable fisheries in the Great Lakes. There have been lake whitefish fisheries on all of the Great Lakes, and many of these fisheries continue to be commercially important. Lake whitefish populations have gone through declines associated with over-fishing, sea lamprey (Petromyzon marinus) invasion and parasitism, and the effects of cultural eutrophication. All of these threats have been addressed by management agencies, and the recovery and prosperity of lake whitefish fisheries are viewed as a resounding success.

In the 1980s, the Great Lakes were invaded by first zebra mussels (Dreissena polymorpha) and then quagga mussels (D. bugensis). The resulting impacts from dreissenids included increased water clarity, changes in nutrient cycling, changes in the depth distributions of some fishes, and changes in the benthic invertebrate community structure.

Diporeia are benthic coldwater holarctic amphipods that, at one time, comprised a major component of the benthic invertebrate biomass of the Great Lakes. Following the invasion and colonization of the Great Lakes by dreissenids, Diporeia declined in abundance. Declines in Diporeia abundance have been temporally linked to the colonization of the Great Lakes by dreissenids.

In recent years, the growth and condition of lake whitefish in lakes Michigan and Ontario have declined. As benthic feeders, Diporeia have historically comprised a major component (up to 70%) of lake whitefish diets in the Great Lakes. As Diporeia abundance declines in an area, lake whitefish food habits have shifted toward chironomids, mysids, and dreissenids. Diporeia are rich in essential fatty acids and are high in energetic value.

The linkages between the dreissenid invasion, declines in Diporeia, and lake whitefish dietary shifts are hypothesized to have resulted from trophic disruption. A number of hypotheses have been proposed regarding the mechanism by which dreissenids have impacted Diporeia, but the connection has been made between the loss of Diporeia and declines in the growth and condition of lake whitefish. We tested the hypothesized link between lake whitefish condition and diet using stable isotope analysis (SIA).

Lake whitefish samples had been previously collected from six sites around Lake Michigan in 2004 and 2005. These samples were originally analysed for lipid and fatty acid composition, fecundity, egg size, egg lipid and fatty acid content, sperm morphology and behaviour, and adult growth history. We further analysed archived dorsal muscle tissue from these same fish and samples of potential prey sources for stable isotope composition. Due to the uniquely low carbon isotope values for Diporeia, the data allowed us to test the hypothesis that the decline in lipid- rich, isotopically-depleted, Diporeia since the Great Lakes invasion of dreissenid mussels has

resulted in energetically poorer diets for lake whitefish and is associated with the production of poorer condition individuals.

Our results suggest that lake whitefish may be using a variety of prey items, especially the Naubinway population where fish occupy the largest isotopic niche space. However, trophic niche width inferred from stable isotope measures did not vary among regions of Lake Michigan. Relative weight was significantly higher in the southeast and lower for all northern regions. The mean measured lipid contents were highest for northwest fish. Docosahexaenoic acid (DHA, an omega-3 essential fatty acid) was significantly different among regions, with high mean values in Elk Rapids and the northwest. No linear relationships between stable isotope measures and condition metrics were found. The results suggest that lake whitefish are coping with declining Diporeia abundances by feeding on alternate prey. Overall, our results do not substantiate the hypothesis of a relationship between condition and prey use, though lake whitefish from Elk Rapids and the northwest did have high quality prey and good condition.

While these data do not support the hypothesized direct link between the diet and condition of lake whitefish, there are other possible mechanisms by which diet and condition may be linked. There are a number of life history trade-offs between reproduction (number and size of eggs), growth, and energy reserves. The data from this project can now be used to test some of these hypotheses.

By combining measures of carbon (C) and nitrogen (N) from SIA with measured lipid content, we were also able to test published models that predict lipid content from C:N ratios. If they work, these models can provide surrogate measures of condition and have become a key element in mathematically-based corrective measures designed to remove the influence of lipids in SIA. We used the large data set from this project to examine the utility of C:N ratios for predicting lipid content within and among populations and to validate published lipid-C:N ratio models. No common lipid-C:N ratio model was found to apply; instead population-specific influences on lipid content were observed. Published lipid prediction models significantly underestimated lake whitefish lipid content and often had worse prediction error than the error obtained by using the observed mean as the prediction for all samples. Maximum prediction error ranged from 50.7% to 65.0%. Our results provide no support for the idea that there is a predictable relationship between bulk C:N ratios and lipid content. We recommend that sample-specific relationships be developed in situations where lipid prediction is needed.

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