Salmon, Isotopes, and Fertile Lakes

Spawned-out salmon carcasses aren't a pretty sight. The tattered bodies littering Alaska's watersides every year seem not only a sad reminder of mortality but a waste; all that food, too much for even the greediest gulls, quickly becoming too rank even for rodents or ravens.

Or so I used to think, until Tom Kline set me straight. I should have known that nature doesn't waste anything, but graduate student Kline has proved it--at least in the case of salmon.

His Ph.D. work, as reported in a recent newsletter from the Alaska Sea Grant College Program at the University of Alaska Fairbanks, involves figuring out the sources of nitrogen in several Alaska lakes. Nitrogen is vital stuff for all life--it's virtually synonymous with "fertilizer," and plants grab it as fast as they can. Kline tackled the problem by studying the natural abundance of nitrogen isotopes in the food webs, the what-eats-what connections, in the lakes.

This was a useful approach because nitrogen has a heavier isotope, with an atomic mass number of 15; normally, nitrogen has an atomic mass number of 14. The heavy form of nitrogen is more common in sea water than it is in fresh water and on land. (The ocean has only about one percent more, but that's quite enough for the purposes of science.) Nitrogen₁₅ appears in measurably higher quantities in creatures living in the sea, and in anything that eats something that once lived in the sea. The ratio of heavy nitrogen to normal nitrogen, N₁₅ to N₁₄, found in lake organisms is thus a very good indicator of how much the sea contributes to life in the lake.

Since 1985, Kline has been collecting creatures--plankton, insect larvae, resident fish and plants- -from Alaska lakes. He works not just by wading about near shore and towing nets from boats, but also by prowling about in diving gear at deeper levels. (In fact, an incidental byproduct of his research is a collection of fine photos of salmon taken at fish-eye level.) Snared in the shallows or depths, the sample organisms are freeze-dried, then combusted. The gases from the combustion are analyzed by a mass spectrometer, which can provide a highly accurate assessment of how much of each kind of nitrogen the sample contains.

Kline has measured the nitrogen isotope levels in the animals and plants of Karluk and Iliamna lakes, where salmon returned from the sea come to spawn and die, and in organisms from Kakhonak and other small lakes with no salmon runs. He's found high ratios of N₁₅ to N₁₄ in samples from lakes with salmon; in fact, most of the nitrogen available to organisms in Karluk Lake seems to come upstream from the sea with the salmon. Lakes with no salmon show low ratios, again hinting that the migrant fish are the source for the heavier nitrogen.

Kline's research results may have quick practical implications. The Alaska Department of Fish and Game has a program in which some lakes serving as salmon nursery areas are being artificially fertilized to enhance their carrying capacity. The kind of exact information about nutrients that this multi-year student project provides can improve management efforts with the added fertilizer. Kline's research also relates to the number of salmon a lake can support, which in turn affects decisions about catch limits.

His research certainly affected how I see the forlorn aftermath of salmon home from the sea. Now I understand that those abandoned bodies are an important gift from the distant ocean to the inland waters of Alaska.