Potential competitors to salmon in the sea
Many animals feed on the same prey items and are potential competitors to Atlantic salmon if the availability of prey is limited. These potential competitors include birds, marine mammals and other fish species. Two of the species are herring and mackerel. The biomass of Norwegian Spring spawning herring and Northeast Atlantic mackerel are several million tonnes, and these species outnumber salmon in the sea by several orders of magnitude. In addition, herring and mackerel are also often feeding close to the surface and are of similar size as salmon.
Competition in the sea can potentially explain the reduced return rates of salmon to many European rivers. However, this is complicated to estimate due to the large geographic area and the rapid changes with time. For example: salmon post-smolts along the Norwegian coast in May experience a different world, with other challenges, than salmon north of Iceland during the winter. If competition with mackerel or herring affects salmon, it is most likely during their first summer at sea (May-August). In a recent study done as part of the Seasalar project, we looked closer into the potential for competition between these species.
Does salmon eat the same as herring and mackerel?
The first part of the work was to analyze and compare the stomach content of salmon, mackerel and herring caught in the same areas. Sampling spanned from west of Ireland and northwards towards Svalbard. If salmon, mackerel and herring feed on the same prey organisms, it is more likely that they compete for food.
The results showed that salmon do feed on some of the same prey organisms as the other two species, but the diet was surprisingly different given that the fish included in the comparison were caught in the same areas, and often in the same trawl hauls. Mackerel and herring had mainly been feeding on Calanus finmarchicus and other similar sized zooplankton (1-2 mm long) while salmon post-smolts had targeted fish larvae. Mackerel can also feed on fish larvae, but this was not a large part of their diet. All three fish species preyed on Amphipoda and to some extent krill, especially later in the summer.
Does salmon aggregate in the same areas as the two other species?
The second part of the work was to test if the species aggregate in the same area; in other words, is there more salmon in the areas with a lot of herring or mackerel? These analyses were done with data sampled in the central and northern part of the Norwegian Sea. Salmon, herring and mackerel all feed in the Norwegian Sea during the summer, but it was unknown if these species actively migrate to the same areas.
The results show that salmon and herring did not aggregate in the same areas, but salmon post-smolt were found in areas where mackerel were abundant. However, some of the post-smolt smolts were found further north than mackerel during the summer.
Does the number of salmon returning to home rivers decrease when there is more mackerel or herring?
The third part of the work was to test if the number of salmon returning to their home rivers decrease when the abundance of herring or mackerel increase. Estimated return rates of salmon were correlated to biomass of herring and mackerel for the years 1982 - 2017. A total of eight datasets for salmon was tested against six datasets for other fish.
Of the 48 tested correlations there was one significant negative correlation: the biomass of Norwegian Spring Spawning herring and the return rate of salmon to northern Europe (including Scandinavia, Northern Iceland and Russia). The number of returning salmon was not correlated to the mackerel stock.
No evidence to support the hypothesis that survival of salmon is controlled by herring or mackerel
The conclusion of the work is that we did not find evidence to support the hypothesis that lower marine survival for salmon can be explained by competition with herring or mackerel. Competition between the species can still have a negative impact on salmon, but we do not expect large changes to salmon survival if the abundance of pelagic fish decrease.
However, the ocean is large and complex. Even though competition between the species does not seem to be important for salmon survival, there may be situations or areas where competition is more important than indicated by this work. Competition can also change with time and be more important now than only some years ago. Ongoing and future work in the SeaSalar project will therefore continue to explore if competition with other species, including herring and mackerel, can impact salmon growth and survival.
impact salmon growth and survival.
Contact: Kjell R. Utne
The new publication: Utne, K.R., Thomas, K., Jacobsen, J.A., Fall, J., Ó Maoiléidigh, N.P., Broms, C. & Melle, W. 2020. Feeding interactions between Atlantic salmon (Salmo salar Linnaeus) post-smolts and other planktivorous fish in the Northeast Atlantic. Canadian Journal of Fisheries and Aquatic Sciences https://doi.org/10.1139/cjfas-2020-0037
In Norwegian: Makrell og laks åt ulikt | Havforskningsinstituttet (hi.no)