Sea lice from fish farms were blamed when stocks of sea trout collapsed.The truth may not be so simple, reports Lorna Siggins.
When sea-trout stocks collapsed in 1989-90, the blame was put on fish farms, which were accused of infesting the population with sea lice. But two leading marine scientists say that early research on the collapse identified the wrong culprit - and that the official view that there are still no sea trout in Connemara is a myth.
In the current issue of Aquaculture Ireland, Dr Martin O'Farrell and Dr Neil Bass analyse the collapse and the considerable government-funded research commissioned as a result. They conclude that there is little or no evidence that fish farms were to blame.
Bass, who concentrates on the politicisation of the debate and interpretation of certain data by vested interest groups associated with angling, notes that an erratic decline in stocks of both salmon and sea trout was charted in Scotland as well as in Ireland. Much of it predated salmon farming by up to 30 years. The trend affected not only the regions where fish farms were located but also every region without salmon farms but where wild salmonids were heavily exploited.
As an example, Bass cites figures for the sea-trout catch in the north-east of Scotland, including the River Dee, where commercial fishery accounted for more than 90 per cent of the catch - and where there are no salmon farms. He notes that sea-trout populations had collapsed here before, as they did in the Waterville fishery in Co Kerry from 1900 to 1920. He also points out that a 1935 report of the Commission of Inland Fisheries highlighted overfishing and other "human interventions", such as construction of weirs and dams, as significant factors.
Much has been made of the impact of drift netting on wild salmon stocks in the mid 1960s and 1970s, but large sea trout would also have ended up on nets as a by-catch, subsequently being sold as salmon, according to Bass. This must have had "quite an impact" on sea-trout breeding stocks, he writes.
Some private-fishery owners, who would have been very critical of drift-netting, also contributed inadvertently to the decline, according to O'Farrell, of Aztec Management Consultants, who formerly worked in the state sector and later for both state and private fishery owners to examine the problem.
Many installed upstream and downstream traps, which intercepted salmon smolt but also immature sea trout with a consequent impact ultimately on adult fish numbers.
Taking a 20-year view of the issue, he notes that the 1989-90 collapse was mirrored in salmon populations in many parts of the world. He argues that sea-trout data from affected fisheries in Connemara were not interpreted in the historical context of a gradual decline in populations.
Global change, rainfall patterns and acidification caused by afforestation have also contributed to the decline, according to O'Farrell, but he challenges the view that sea trout have almost disappeared from Connemara.
In recent years the River Erriff has recorded sea-trout runs of more than 4,000 fish, although the Western Regional Fisheries Board notes that Erriff is among a group of rivers with a "small number" of fish. He does not believe there is sufficient evidence to back the continued prohibition on retaining rod-caught sea trout in certain areas.
Bass highlights the resilient nature of Lepeophtheirus salmonis, the sophisticated sea louse linked to the sea-trout collapse via fish-farm cages. An early hypothesis suggested lice in the cages were "propelled" into river mouths, where they were picked up by wild fish. This might apply in certain situations in Norway and Scotland, where fish-farm cages are close to trout and salmon rivers, but it has no bearing in Ireland, where farms are miles away, he argues.
Far more plausible is the scenario proposed by Aqua-Fact, an environment consultancy that started to research the issue in 1993: that sea lice used mature spring salmon as transport to river mouths, where they located their hosts, sea-trout smolt.