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What are the limits of salmon abundance?

Record number of salmon returning to spawn in the northern Baltic Sea rivers in 2016 raises question of the optimal number of salmon, as management moves from recovery to sustainable yield.
Published: 6 June 2017

​​In the summer of last year, record numbers of Atlantic salmon heading upstream from the Baltic Sea to spawn in the Finnish river Simojoki were documented, while numbers returning from the sea to the nearby river Tornionjoki on the Swedish-Finnish border approached the all-time high (101 000 salmon). Also recorded were juvenile salmon (smolts) heading downstream to the sea, with a rise in the number of those migrating from the Tornionjoki. In neighbouring Sweden, record high quantities of spawners were also counted in the northern Piteälven and Byskeälven rivers.

The data from Simojoki and Tornionjoki, recorded by the Natural Resources Institute of Finland (LUKE) using echosounders, and from Swedish rivers equipped with counters in fish ways, point to a strongly reduced exploitation, something that has characterized salmon populations of the northern and central Baltic Sea as they bounce back from decades of overfishing.

Recurring catch advice for Baltic salmon fishing

The story of the upturn in returning adult salmon was part of the Finnish and Swedish national reports on salmon populations and catch statistics presented to the recent ICES Assessment Working Group on Baltic Salmon and Trout (WGBAST). Attended by experts from all the Baltic countries, each of whom contribute similar data, the annual meeting sees the status of the salmon – and sea trout – stocks evaluated. This new data formed part of the overall puzzle the group tried to put together for the provision of catch advice. Total allowable catch (TAC) is the most important regulatory tool of salmon fishing in the Baltic Sea, thus, annual catch advices form the basis for sustainable use of this salmon resource.   

Fundamental questions                                                                      

One particular challenge for the group is assessing the limits of population growth.

"Because some salmon stocks are fast approaching their upper limits of abundance, there is an increasing need for this type of information, which allows for identification of biological reference points for management," explained Atso Romakkaniemi, member of WGBAST and February's WKBaltSalmonbenchmarking workshop.

"In other words, for the stocks with the best status, recovery-oriented management aiming only at an increase in abundance trend is not a satisfactory approach any longer. Now we should also find an answer to the question "at what level should these stocks be maintained in order to keep them at biologically sustainable level? At the same time how can we effectively use the ecosystem services they provide?"

MSY approach

The concept of fishing at or above MSY level is currently widely applied in fisheries advice. However, defining MSY for tens of individual salmon stocks is not an easy task, especially in the face of unforeseen high salmon abundance levels in some of the stocks, which question the earlier estimates of limits for salmon production.

In the case of salmon, the topic is in theory easier to study and a result better understood than in the case of most other fish species. This is due to the separate, well-defined reproduction areas which often allow for the effective monitoring of stock-recruitment dynamics. The time-series from many Baltic salmon stocks contain stock-recruitment information from a uniquely wide range of stock abundance levels, which further helps for accurate and detailed estimation of stock-recruit dynamics.

Even if biological reference points could actively be estimated for each stock, applying the MSY approach for Baltic salmon fisheries is extremely challenging. This is due to, as WGBAST chair Stefan Palm explained, "the presence of multiple stocks with varying states that are harvested in offshore and coastal mixed-fisheries."

The role of recreational fishing

Other dynamics the group has to contend with for the assessment are those of recreational fishing and disease. For the former, this is obtaining reliable data from growing recreational salmon fisheries in the Baltic and in surrounding rivers; for the latter the occurrence of the reproductive disease known as M74 among the salmon. 

The M74 disease has been more prevalent in the salmon's yolk-sac fry phase, something observed in Tornionjoki, Simojoki, and other rivers during the summer of 2016, and it is expected to increase further in 2017. This is a trend which continues to be taken into account in the assessment, although deaths of spawners returning back upstream are not yet included.

ICES advice for Baltic Sea fish stocks​, including salmon, was released re​cently.

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​Photo: ​​​Rune Muladal

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What are the limits of salmon abundance?

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