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Where's the Fat? Fish Lipids Gone Missing

Photo: Fish looking at scale

Do you have a theory for why Siscowets are leaner or what this slimmer version of a fat fish might mean to the Lake Superior food web? If so, share your thoughts with us at seagr@d.umn.edu or by phone at (218) 726-8106.

The extent of the Siscowet population took fishery managers by surprise when they began looking for them in 1997 and conducted extensive surveys in 2006.3 The deeper they looked for these deep-dwelling fish, the more they found: some say over 47 million pounds of them. Without question, Siscowet Lake Trout are the most abundant predator in Lake Superior.

Before European settlement, before smelt and sea lamprey, the Ojibwa people noticed that some Lake Trout were so fat that they could burst into flames while roasting over an open flame. Hence the name "Siscowet," which translates to "fish that cooks itself." Many people still think incorrectly that these "fat Lake Trout," or simply "fats," are much fattier cousins of lean Lake Trout. In actuality, leans and fats are subspecies with marginally different genetic coding and somewhat divergent habitat preferences.

Despite being 10-15 times more abundant than leans, fats are rarely targeted by anglers and never become featured on restaurant menus. They are too unpalatable, both in texture and taste. Even more off-putting, they tend to harbor higher concentrations of pollutants like PCBs and chlordane, which accumulate in fat.

Entrepreneurs, however, are interested in capitalizing on Lake Superior's abundance of fish fat, which is high in healthy Omega 3 fatty acids. The idea is that the Siscowets' fats and oils could supply the nutraceutical supplement market, which can remove organic contaminants during processing.

In response to this interest, Sea Grant and others gathered in Baraga, Michigan.4 There participants got an inkling that Siscowets had fundamentally changed. Subramaniam Sathivel, Assistant Professor of Food Engineering at Louisiana State University's Agriculture Center, showed evidence that modern Siscowet fillets from 5 pound fish were less than 14% fat (wet weight). Sathivel said Siscowets had a fat content similar to sockeye salmon, which shocked those who were expecting an estimate at least three times that high.

With more probing, it became clear that other studies corroborated Sathivel's estimates. Additionally, the Wisconsin Department of Natural Resources4 reports an apparent flip-flop between fats and leans in the amount of two important components of Omega 3 fatty acids. They found that modern Siscowets have less than half of the eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) than lean Lake Trout now carry.5 The opposite was true in the late 1980s.6

If millions of fish have shed million pounds of fat, where did it go and why? Experts don't really know. However, there are theories about how the much leaner "fats" came to be:

Theory 1: There are so many Siscowets that they are on starvation rations. However, the Siscowet population seems to have declined since 1996 and their prey deepwater ciscoes (Kiyi) are apparently doing well. Deepwater Sculpin may be feeling a bit of the bite but they, too, are still relatively abundant.

Theory 2: Leans and fats are hybridizing. Researchers found that crosses between lean and Siscowet Lake Trout resulted in offspring with intermediate lipid levels.7 They concluded that their respective lipid levels were genetically controlled.8 However, if hybridization is the cause of the observed loss of fat, pockets of the old, high-lipid Siscowet genotype should be found; this does not appear to be the case.

Theory 3: Mixed methods make for inconsistent results. Variances in the analytical methods on the apparent slimming of Siscowets probably aren't at play, but they are worth noting. Some documented differences might be an artifact of changing analysis protocols, wet versus dry weight comparisons, portion of the fish sampled, size of fish sampled, time of year sampled, reproductive state, and misidentification of Lake Trout strains.

Theory 4: The increased metabolic cost to living a leaner life in the deep and migrating up for a meal compounds energy loss and the result has been ever-leaner Siscowets. Siscowets' high lipid levels are thought to make life possible in the deepest waters of Lake Superior.

Theory 5: The age structure of the population is different than it was 50 years ago. Younger fish tend to be less fatty. Old Siscowet are not in short supply but there is an abundance of younger Siscowet, which are eating and growing at relatively faster rates compared to the old-timers.

Theory 6: When the Lake Trout populations of both leans and fats flirted with extirpation around 1970, the gene pool might have irrevocably changed. Sea Lamprey and overfishing sent commercial harvest rates from millions of pounds to thousands. Today's Siscowets are descendants of the rogue individuals that dodged the proverbial bullet(s).

Crazy Theory 7: Lake Superior demonstrates a clear trend in increasing nitrate levels.9 This 100-year trend might be affecting the lipids in the Lake Superior food chain. Correlation doesn't necessarily mean causation, but maybe in this case the additional nitrate is resulting in leaner Siscowets with fewer essential fatty acids. Sea Grant studies and other research have shown that:

  • The species composition of diatom and zooplankton communities has changed in Lake Superior over 100 years;

  • Different species of plankton produce different levels of lipids and essential fatty acids;

  • In cultured algae, as nitrate levels increase, lipid levels decline;

  • Fish and other consumers in the aquatic food chain show dramatic differences in lipid content depending on their diet.

More than "why," maybe the bigger question is "so what?" Now that they are nearly as lean as lean Lake Trout, are Siscowets less adapted to life in the deep?

Siscowets and leans differ genetically in the way they metabolize energy reserves, a difference that provides adaptive approaches to living in their respective habitats.7 These Lake Trout might not have the necessary lipid levels to conduct their usual extensive vertical migrations to feed.10 Modern Siscowets could be in metabolic distress and, if they are not, the assumption that the Siscowet lifestyle is tied to high lipid levels11 might need to be reexamined. Furthermore, if slimmer Siscowets are more suited to life in shallower water, they might be in increasing competition with Lake Superior's lean Lake Trout.

When a fish with a genetic propensity toward Rubenesque becomes slim, there is certainly something going on. Lipid trends in other Great Lakes fish have also been noted. Jason Stockwell, Director of the Rubenstein Ecosystem Science Laboratory and Associate Professor of Aquatic Ecology at the University of Vermont, found that the lipid levels of Mysis, a shrimp-like creature important within the Siscowet's food chain, show striking patterns with depth in Lake Champlain.

He looks at lipids as a sort of currency in Lake Superior, saying, "All calories are not equal. DHA is worth $100, EPA is worth $80 and a lipid like ARA is worth $10. The answer to the Siscowet mystery has got to be within the food chain."

1 Eschmeyer, P.H., Phillips, A.M.J., 1965. Fat content of the flesh of siscowets and lake trout from Lake Superior. Trans. Am. Fish. Soc. 94: 6274.

2 Thurston, C.E., 1962. Physical characteristics and chemical composition of two subspecies of Lake Trout. J. Fish. Res. Bd. Can. 19(1): 3945.

3 Sitar S.P., Morales H.M., Mata M.T. et al., 2008. Survey of siscowet lake trout at their maximum depth in Lake Superior. J. Great Lakes Res. 34: 276286.

4 Exploring the Potential for a Siscowet Lake Trout Fishery, 2009: led by Michigan Sea Grant, with cosponsors from the Minnesota and Wisconsin Sea Grant programs, the Great Lakes Indian Fish and Wildlife Commission, the Keweenaw Bay Indian Community, and Michigan State University Extension.

5 Williams, M.C.W., Schrank, C., and Anderson, H.A., 2014. Fatty acids in thirteen Wisconsin sport fish species. J. Great Lakes Res. 40(3): 771-777.

6 Wang, Y.J., Miller, L.A., Perren, M., Addis, P.B., 1990. Omega-3 fatty acids in Lake Superior fish. J. Food Sci. 55: 7173.

7 Goetz, F., Jasonowicz, A., Johnson, R., Biga, P., Fischer, G., and Sitar, S., 2013. Physiological differences between lean and siscowet lake trout morphotypes: Are these metabolotypes? Can. J. Fish. Aquat. Sci. 71: 427-435.

8 Goetz, F., Rosauer, D., Sitar, S., Goetz, G., Simchick, C., Roberts, S., Johnson, R., Murphy, C., Bronte, C.R., MacKenzie, S., 2010. A genetic basis for the phenotypic
differentiation between siscowet and lean lake trout (Salvelinus namaycush). Mol. Ecol. 19, 176196.

9 Sterner, R.W., Anagnoston, E., Brovold, S. Bullerjahn, G.S., Finlay, J.C., Kumar, S., McKay, M.L., and Sherrell, R.M. 2007. Geophisical Research Letters, Vol. 34, L10406

10 Hrabik, T.R., Jensen, O.P., Martell, S.J.D., Walters, C.J., Kitchell, J.F., 2006. Diel vertical migration in the Lake Superior pelagic community. I. Changes in vertical migration of coregonids in response to varying predation risk. Can. J. Fish. Aquat. Sci. 63, 2286-2295.

11 Henderson, B.A. and Anderson, D.M. 2002. Phenotypic differences in buoyancy and energetics of lean and siscowet lake char in Lake Superior. Environmental Biology of Fishes 64:203-209.

By Jeff Gunderson
June 2015

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