The Environment Agency is actively investigating the widespread occurrence of wild salmon with swollen and bleeding vents.
There has been an increase in the number of wild Atlantic salmon showing signs of nematode infestation. The condition is characterised by swollen bleeding vents (the waste and reproductive outlets) in female and male salmon. There is no evidence at present to suggest the condition causes mortalities in salmon, or adversely impacts on spawning, and any affected fish caught should be treated as normal and can be released following capture without additional concern.
In England and Wales the situation is being monitored closely by the Environment Agency and scientists at the Centre for the Environment, Fisheries and Aquaculture Science (Cefas), Weymouth. Similar monitoring is taking place in Scotland and Northern Ireland.
â€œWe are uncertain at present as to the exact cause of the inflamed vents,â€ explained Environment Agency Fisheries Manager Godfrey Williams. â€œThere is some evidence from fish caught to suggest the cause could be a reaction to the presence of the Anisakis simplex nematode parasite. However, as Anisakis are a common parasite of salmon and many other fish, it remains unclear as to why these symptoms are now being seen so widely.
â€œTo date there is no evidence the condition affects spawning or causes mortalities in salmon, but the situation is being monitored closely. There is no evidence of the condition in other marine species, including wild trout or farmed species of salmon or trout.
The Anisakis simplex is a common parasite of marine fish throughout the world. Whilst the parasite can be carried into the freshwater by migrating fish, it is unable to complete its lifecycle in freshwater as it requires the presence of marine invertebrates or mammals to complete its life cycle. The nematode has a complex life cycle, with the adult form infecting seals, whales and other marine mammals. Wild salmon ingest the nematode larvae during feeding at sea.
Low incidences of this condition have been noted in wild salmon in previous years but the number of cases increased during 2006 and has been noted at their highest levels so far during 2007. The condition appears to be widespread and has been reported in many salmon rivers in England and Wales, including the rivers Exe, Dee, Taff, Camel, Tamar Eden, Tyne, Itchen, Ribble, Esk, Exe, Wye and Lune, and in a number of Scottish and Irish rivers.
The symptoms are not unusual in individual fish. What is unusual now is the number of fish involved and its widespread distribution. In 2005 5% of trap-caught fish on the River Dee had the condition. In 2006 that had risen to 10% (119 individual fish). The provisional figure for 2007 is 15% to date. Reports suggest the frequency of occurrence may be higher in some other rivers.
Both male and female salmon have been affected. The majority of affected salmon to date have been freshly-run grilse. However, two-sea-winter fish displaying the symptoms have also been reported from a number of rivers. To date we have not seen these symptoms in sea trout. Apart from the specific vent symptoms, the fish generally appear in good condition.
â€œWe have been keeping a close eye on developments and keeping records so that we can determine numbers of fish affected and where these are located. We will continue to monitor the situation and will report our findings in due course,â€ continued Godfrey Williams.
â€œWe have obtained a small number of fish from the Rivers Eden (2006) and Tamar (2007) for analysis and post mortem. It is our intention to obtain additional samples from a number of rivers in 2007.
â€œAny additional threat to already depleted salmon stocks must be of concern. However, from our observations it is unlikely that there will be any additional impact at the individual stock level. There have been no reports, either last year or this year, of mass mortalities of salmon. Any affected fish should be treated as normal and can be released following capture without additional concern.
â€œA number of affected fish were taken to our hatcheries last winter and all of these were successfully stripped of their eggs (and the eggs developed normally). In addition, an affected salmon tagged on the River Dee in 2006 was recaptured this spring as a kelt. On inspection this fish appeared to have spawned successfully in the wild and had recovered condition. It appears the affected fish will survive to spawning time and will spawn successfully in the wild or in hatcheries.
â€œWe will continue to monitor the situation over the coming months. As well as recording all incidents we are made aware of we will obtain additional samples of fish for post-mortem and analysis. What we want to do now is to work with colleagues in Cefas to try and find out what is causing symptoms to appear in so many fish. In addition, we will share experiences and knowledge with our counter-parts in Scotland and Ireland and report our findings in due course.â€
The Environment Agency cannot provide advice on the consumption of fish. The Food Standards Agency supervises a raft of hygiene legislation applying to food businesses that protects consumers from the risk of parasitic infection from consuming fish. The FSA advices cooking fish for at least two minutes at a temperature of 70C will kill any parasites, including nematodes. Further advice may be obtained from the FSA website at www.food.gov.uk.