Later coastal fishing favours wild salmon

Smolt (young fish) of both wild and stock enhancement salmon migrates from the rivers in northern Sweden to the Baltic Sea. The salmon live in the sea, until they return to their natal river to spawn some years later. 

‘The wild salmon arrive to the rivers earlier than the stock enhancement one, meaning that the stock of wild salmon can be protected by allowing fishing in well-defined “time windows”, that is, by starting the fishing only after some of the wild salmon have swum back up the rivers to spawn,’ says Professor Hans Lundqvist.

The researchers at SLU recommend coastal-close fishing from late June. The coastal fishing will then be focused on stock enhancement salmon, which will also reduce the risk of stock enhancement fish spreading to the spawning areas of the wild salmon and thereby genetically contaminate the few remaining wild stocks.

‘Modern fishing traps, with which you can catch the fish alive, may also help the wild salmon. All Swedish stock enhancement salmon have their adipose fin cut, making them distinguishable from the wild salmon. In the fishing traps, you can sort the salmon and release the wild salmon back to the Baltic Sea,’ explains Professor Hans Lundqvist.

Feed shortage and disease kill young fish
A large portion of the smolt dies during their first summer in the sea, after leaving the river. There has been a marked increase in mortality over the last twenty years. The researchers do not yet know what is causing it, but feed shortage may be one possible explanation.

‘Climate change causing environmental changes and human interactions may lie behind the increased mortality. Once in the sea, the smolt has a high demand of feed, why not enough organisms to feed on, such as insects, zooplankton and fry, creates a problem. The smolt cannot grow to the size required to feed only on fish, preventing it to become a fully grown salmon,’ says Professor Hans Lundqvist.

There is also a hypothesis about the seal population contributing to the increased mortality of the smolt – directly by eating smolt and indirectly by eating Baltic herring. Fewer herrings give fewer herring fry which constitutes a major feed source of the smolt. Another problem is the M74 syndrome, leading to the death of many fry. Last year 21 per cent of the stock enhancement offspring suffered from the syndrome, according to a survey by the Swedish Board of Fisheries. This is an increase by five percentage points since 2009. Previous research has demonstrated a connection between the disease and vitamin B1 (thiamine) deficiency.

Dioxin exception a hot topic
The environmental pollutant dioxin, formed during combustion processes such as waste incineration, constitutes another problem.  The level of dioxin found in oily fish from the Baltic Sea area, for example salmon caught in the wild, often exceeds the EU limits on the dioxin content in food. Dioxin may, for instance, affect reproductive capability, brain development and cause cancer. Sweden’s current dispensation from the EU limits on the content of dioxin in salmon and herring expires 31 December 2011. The Swedish government has decided to ask the Commission to prolong the dispensation beyond 2011.