Mackerel and salmon had different diets

In recent decades, the number of salmon in the Atlantic Ocean has fallen dramatically. In addition, they are not growing as big.

One theory to explain this is that herring and mackerel are eating the salmon’s food in the Atlantic, as these stocks have had good years. 

In a new study, researchers found no evidence to support this theory after combining stomach analyses with statistical data.

No correlation found

“We found little overlap in the diet of salmon, on the one hand, and herring and mackerel, on the other. We also found little geographical overlap between the herring and salmon, and no correlation between the fluctuations in the size of the various stocks”, says scientist Kjell Rong Utne, when summing up their conclusions.

Over the course of nine scientific surveys in 2008 and 2009, the researchers analysed the stomach contents of 750 young salmon, 678 mackerel and 204 herring caught between Ireland and Svalbard. Then they compared the diets of individuals caught in the same area during the same period. 

Different favourite dishes

All of the fish ate some amphipods and krill. But the herring and mackerel preferred Calanus finmarchicus and small zooplankton, whereas the salmon mainly ate fish larvae. 

“We were surprised at how different the diets of the species were, particularly in the case of fish caught in the same area at the same time. Sometimes even in the same trawl net”, says Utne. 

The mackerel population is estimated to be up to 1,000 times the size of the Atlantic salmon population in the ocean. Therefore, even a small overlap could have a big impact. And they did find some fish larvae in the mackerel stomachs. 

“In areas with few medium-sized zooplankton such as Calanus finmarchicus, the mackerel ate other prey. For example, west of Ireland and Scotland they fed on sand eel larvae, which are also important prey for young salmon in that area, so competition for food may have been higher in that location than in the Norwegian Sea. 

“But both mackerel and young salmon migrate quickly through this area in the early summer”, he adds.

The researchers know this because their research also looked at the extent to which the fish spent time in the same place at the same time.

Overlap between salmon and mackerel, but not salmon and herring

The researchers used catch data from the contents of 170 trawl nets in the Norwegian Sea to estimate the likelihood of salmon being found in the same areas as mackerel and herring. 

“There was little geographical overlap between salmon and Norwegian spring-spawning herring. Mackerel and salmon were at times present in the same area, but in the summer some of the salmon were a bit further north than the mackerel”, explains Utne.

More mackerel didn’t mean less salmon

Finally, the scientists used statistical data from the period 1982-2017 to look for a link between the number of salmon returning to their home rivers, and the size of the herring, mackerel and blue whiting stocks. 

Were increases in the other pelagic fish species associated with declines in the salmon population?

“We didn’t find any significant correlation with declines in the number of salmon entering the five European rivers we checked”, explains Utne. 

Can’t rule out competition

However, they did find a negative correlation between the spawning population of Norwegian spring-spawning herring and the total number of salmon migrating back to Northern Europe. 

“During periods when the herring stock grew, fewer salmon returned to their home rivers. But it is hard to know whether there is a causal relationship. It could be due to competition, but it could also be down to other factors that we haven’t investigated”, he says.

Parallel fluctuations in the populations are not expected

The researchers conclude by saying that they haven’t found any proof to support the theory that the reduction in salmon numbers is due to competition from herring and mackerel. 

“We don’t rule out that this may occur, but we wouldn’t expect large changes in salmon populations in response to the other pelagic fish stocks growing or shrinking”, says Utne. 

Will assess whether the situation has changed

“Competition may also occur locally in particular locations, and clearly the situation may have changed over time”, he continues. 

While the stomach analyses used in this study were from 2008 and 2009, the biggest increase in the mackerel population occurred after 2010. Since then, the Institute of Marine Research has taken routine stomach samples from herring, mackerel and salmon during its annual scientific surveys. 

“The salmon stomach contents have already been analysed, and in the case of the herring and mackerel, lots of material is ready to be analysed. Further research will put us in a better position to say to what extent diets and competition have varied over time”, says Utne. 

Through the SeaSalar project, the scientists will continue to study competition between the species.

References

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, (ja). Link: https://doi.org/10.1139/cjfas-2020-0037