Mark Wipfli (University of Alaska Fairbanks) with humpback salmon (pink salmon) eggs found in the stomach of salmon fry (Photo: Kathy Dunlop/Akvaplan)

Can the alien species humpback salmon have positive effects on our salmonids?

NORWAY
Monday, November 30, 2020, 01:00 (GMT + 9)

A recently published article in the Ecology of Freshwater Fish presents the results of a study of the ecological effects of the alien species humpback salmon (Oncorhynchus gorbuscha). First author is Kathy Dunlop (formerly Akvaplan-niva now HI) and co-authors are Antti Eloranta (NINA / University of Jyväskylä), Eirik Schoen and Mark Wipfli (University of Alaska Fairbanks), Jenny Jensen and Guttorm Christensen (Akvaplan-niva), Rune Muladal (Nature services in the North). The study is funded by the Framsenteret (Terrestrial flagship).

A Pacific species in Norwegian waters

In recent years, we have had an invasion of humpback salmon in Norway and the fish is placed on the Species Data Bank's list of alien species under the category «high risk». The humpback salmon is originally a Pacific species, and like many other Pacific salmon, it dies after spawning. The species has a faster life cycle than the salmonids we know from the Atlantic Ocean. It migrates out shortly after hatching in fresh water, and returns to spawn after a little over a year in the sea. Humpback salmon were cultivated and released into Russian rivers on the Kola Peninsula in the period 1956 - 1999. The species is not as native as Atlantic salmon species, and has since the 60s occurred in greater or lesser numbers in Finnmark and some in Troms. In 2017 and 2019, the species was registered as far south as the Oslo Fjord and sporadically in other European countries. We know little about how the species affects our ecosystem and salmonids and this spread has therefore aroused concern in Norway. Among other things, there is a concern that the species may compete for food and spawning grounds with native salmonids. Furthermore, the researchers fear that large quantities of fish that die after spawning can cause oxygen depletion and disturb the nutrient balance in the rivers.

Photo: Researchers fear that humpback salmon that die after spawning can cause oxygen depletion and disturb the nutrient balance in the rivers. (Photo: Kathy Dunlop)

Positive effects of humpback salmon

However, humpback salmon can also have positive effects on the ecosystem. From the humpback salmon's original areas, it is known that the marine nutrients that Pacific salmon take up in the rivers can have a positive effect on both terrestrial and freshwater ecosystems. Terrestrial animals such as bears, foxes and birds eat the spawning and dying fish, the increased supply of food leads to increased production of e.g. microalgae and benthic animals in the rivers, and fish fry feed on humpback salmon's very nutritious eggs. This last point is particularly interesting for our Norwegian salmon fishermen, as the winter in the north often involves varying degrees of hunger due to short days, low production and few prey. Being able to build up a good reserve stock in the form of fat in the autumn will be very beneficial for our salmon fish.

Photo: From left Mark Wipfli (Univ. Alaska Fairbanks), Jenny Jensen (Akvaplan-niva) and Erik Schoen (Univ. Of Alaska Fairbanks). (Photo: Kathy Dunlop)

Vesterelva in Finnmark

To study whether the Norwegian salmonids manage to utilize the humpback salmon's eggs as a source of food, a research team took the trip to the Vesterelva in Finnmark in 2019. This is a small river that empties into the innermost part of the Varangerfjord. It is known that there are large numbers of humpback salmon in this river in odd-numbered years. With regard to data collection, it is an advantage that the Vesterelva has a waterfall in the middle part that can accommodate trout and salmon but not the humpback salmon. Through this, the researchers were able to use the upper part of the river as a comparison area for an ecosystem without influence from humpback salmon. Stomach samples and samples of stable isotopes from liver and muscle from fish above and below the waterfall were collected, as well as samples taken during and after the humpback salmon spawned.

Photo: Kathy Dunlop led the study and fieldwork and is the first author of the article (Photo: Nature Services in the North)

Results

The stomach samples give a snapshot of what the fish have eaten in the last 1-2 days. 14% of the trout fry and 20% of the salmon fry had eaten humpback salmon eggs in the lower part of the river during the humpback salmon's spawning season. In addition to humpback salmon eggs, the researchers found that the fish had eaten a large proportion of aquatic and terrestrial invertebrates (insect larvae and insects) and some fish (stingrays). Based on analyzes of the stable isotopes, which thus reflect the diet in recent weeks or months, a few individuals were found who had eaten the humpback salmon's eggs.

It is demanding to conduct studies of ecological interactions in natural environments. In this study, there has been an uncertainty about 1) whether the Vesterelva is actually productive enough for the trout and salmon stocks that live in the river, 2) the high catch of humpback salmon that was carried out prior to the study, and 3) whether the researchers should actually have completed the second sampling round later in the fall or spring to more reliably detect an increase in marine stable isotopes. The study concludes that the increased nutrient supply in the form of humpback salmon eggs may not affect the trout and salmon stocks as a whole, but that this new food source can be very beneficial for individuals who specialize in exploiting this nutrient source. The effect of this connection can advantageously be studied further.

Source: Akvaplan Niva (translated from original in norwegian)

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