Migrating salmon: linking oceans to forests

Studies in the US have shown that salmon migrating upstream bring nutrients from the ocean into rivers and catchments. A recent study of Japanese streams has told a similar story. It has also shown how human activity can reduce this boon to streams and forests.

In ecology we are often reminded how connected things are. Change what is growing in a catchment and you change how much water runs off during storms and what it carries with it. This affects the stream and may affect whatever ecosystem is at the end of the stream – often estuaries.

A connection that is less intuitive is how oceans may affect streams and forests hundreds of kilometres away. Oceans and forests connect, however, when fish such as salmon travel up rivers in large numbers. These fish have impacts far beyond those of reproduction.

By analyzing the building blocks of life (nitrogen and carbon) scientists can tell whether the nitrogen and carbon that makes up any given organism had its source in the sea. These analyses have found marine-derived nutrients throughout the aquatic food chains of rivers in which salmon migrate*. From algae and macroinvertebrates (insects, worms, etc) living in streams, to trees living near streams. How does this happen?

Migrating salmon help feed streams and forests like these in Japan. Photo by Takayoshi Kamaka

Migrating salmon help feed streams and forests like these in Japan. Photo by Takayoshi Kamaka

There are several pathways for carbon and nitrogen from the ocean to become incorporated in other organisms via salmon.  When salmon die after spawning their carcasses and the eggs they have laid are eaten by a variety of both aquatic and terrestrial organisms, as seasonal food resources.

In the Japanese study* the main pathways by which salmon carcasses were deposited in adjacent forests were by floods and by brown bears (Ursus arctos). Deposition of half eaten carcasses plus bear urine and faeces provide a nutrient source to the surrounding vegetation. Blowflies and faeces of birds who have feasted on salmon are also incorporated into soil and vegetation enhancing the nutrient status of the forest.

By feeding on salmon, Brown Bears help transport nutrients form the oceans to forests. Photo by Carl Chapman.

By feeding on salmon, Brown Bears help transport nutrients form the oceans to forests. Photo by Carl Chapman.

The Japanese study differed from North American studies in that the level of enrichment from the oceans was not as high. One possible explanation is that artificial structures such as dams hinder the migration of salmon. Another is that forest clearing in the Japanese catchments has lead to higher velocity water flows which may flush salmon carcasses away before they can be eaten or decompose.

Human changes could prevent nutrients from oceans being incorporated into freshwater biota and forests, even if there is a large number of migratory salmon. It is believed this has happened across Pacific Rim ecosystems. Another example of interconnectedness in ecology and the sometimes unforeseen results of breaking connections.

The full article on which this post is based is:

Stable isotope evidence indicates the incorporation into Japanese catchments of marine-derived nutrients transported by spawning Pacific Salmon. By Y. Koshino, H. Kudo and M. Kaeriyama, Freshwater Biology 2013, 58, 1864–1877.

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