18th November

Sea trout collapse: a complicated issue

Martin Jaffa

In May 1989, many of the sea trout smolts returning early to their native rivers along parts of Ireland’s west coast after only a short time at sea were emaciated, and the flesh of others was white rather than pink. As to why these fish were starving and unable to find food are questions that have never been answered.

Aquaculture sea lice Bellacraher GF

Sea cages in Bealacragher Bay near Mulranny, Co Mayo

That same year, a population collapse occurred in most mid-western sea trout fisheries although most populations had been in slow decline for many years.

A subsequent investigation was directed towards the heavy infestations of sea lice; however in some fisheries, thin fish were observed that were lice free. The then director of Scotland’s Atlantic Salmon Trust later wrote that 'most of the sea trout sampled by the Irish Sea Trout Action Group (STAG) have been found literally to be starving and it is known that sea lice will more readily infect sick, weakened hosts.

'The question to be answered is whether sea lice are causing the starvation problem or whether the lice are affecting the fish that are already suffering from a primary disease or physiological disorder.'

Subsequent reports seem to imply that sea lice caused the starvation problem. Further investigations seem to be all directed at the sea lice infestations, leaving any other option as to why the sea trout starved unresolved.

By 2000, all reference to starvation and emaciation had been dropped from symptom descriptions of the collapse. The problem affecting sea trout was blamed on sea lice emanating from salmon farms along the west coast of Ireland and remains so up to this day. 

Low survival rate

A feature of the sea lice infestations recorded during the sea trout collapse from 1989 to 1991 was that most of the lice were juveniles. An observation in 1993 by Dr Alasdair McVicar - a Government fish disease specialist - which appears to have been lost over time may have some relevance to the sea trout collapse in Ireland.

He noted that sea trout caught from the River Ewe had very diverse levels of sea lice infestation; the majority of the lice were carried by only a few fish. He also noted that at least 97% of the juvenile sea lice that settle on salmon do not survive to become adults. The remaining 3% includes males and females and it is only the females that produce eggs. Thus, breeding females make up a small part of the total lice population.

Dr McVicar indicated that the presence of high levels of lice may indicate the presence of other problems with sea trout. He said that individual fish that are slow to recover selectively, accumulate high lice burdens in comparison to healthy fish in the same cage. He added that the possibility should be kept open that the lice burden on early returning finnock is a secondary attack on weakened fish that have suffered another debilitating problem.

Could it be that the high lice infestations observed in 1989 and 1990 were simply a secondary issue and that he fish were actually suffering from some other, yet unidentified problem?

At the time, STAG certainly considered a range of other possibilities as to why the sea trout fisheries collapsed. The theories put forward included predation, food chain problems and afforestation. All were eventually rejected as implausible.

The main theories considered in 1991 were sea lice; stress due to warm temperatures and unusual levels of rainfall, or the possibility of some yet unidentified disease.

Dr Bill Turrell, another Government scientist, explained that the Slope Current flowing along the edge of the Continental Shelf had brought warmer and saltier water from the south and was particularly strong from 1989 to 1991.

Evidence from Scotland showed that waters within enclosed sea areas and lochs was especially affected. For example, the problems in Ireland were more severe in the long sea loch at Killary Harbour that leads up to the fisheries at Delphi and Erriff.

Sea lice have been found associated with sea trout during the intervening years but there have been no reports of large numbers of thin and starving fish as were observed during 1989 and 1990.

Undoubtedly, young fish in marine waters near to salmon farms do pick up lice infestations; however these fish tend to be the weak or sick and the infestation is secondary. The fish are likely to die regardless of the presence of sea lice.

Mortality of young fish is high, which is why fish produce so many eggs. It is easy to forget that only a handful are expected to survive especially when salmon farms are a convenient scapegoat.

This article will appear in full in the Winter edition of Inshore Ireland publishing December 9

Spread the News

Sea trout collapse: a complicated issue
Irish shellfish safety reporting goes live
Increase in Irish aquaculture production
Aquaculture licensing in Ireland requires 'root and branch' reform