Grey seals may be the bane of some fishing skippers' lives, but scavengers like crabs appear to be causing more damage to catches in nets. This is the main conclusion of an EU-sponsored study on two fisheries in Mayo and Kerry.
But Halichoerus grypus may not rest too easily on those flippers yet. The study, which was carried out for Bord Iascaigh Mhara (BIM) and is published this week, notes that seals are most definitely "opportunistic" feeders, and that far more work needs to be done to gauge their impact on commercial fisheries. It also found evidence in these waters of migratory predators from Scotland where, significantly, seal culling has recently been reintroduced.
The study, entitled The Physical Interaction between Grey Seals and Fishing Gear, was commissioned on foot of claims by fishermen that an uncontrolled grey seal population was threatening the very future of some fisheries.
Certainly, grey seals are known to be hungry beasts; consumption has been estimated by scientists at between 4.5 kg and 10 kg of fish per seal per day. International experience points to seals as intelligent predators: a Royal Commission study in Canada in 1986 showed that grey seals were very effective at removing fish from the nets of salmon fishermen, with losses of 30 to 45 per cent in some areas.
Given that up to 65 per cent of this State's fleet is inshore, and accounts for some 50 per cent of employment in the sector, pressure was exerted by fishing organisations for some action.
The EU Commission's support for some research was enlisted by former BIM scientist, Ciaran Crummey, who did much pioneering work on the problem and gained the trust of many small-boat skippers.
This study focused on two "passive" or fixed-net fisheries - the north Mayo spring cod gillnet fishery, which is inshore, and the offshore hake fishery worked by boats from Dingle, Co Kerry. It also gathered information on seal diet.
The spring cod gillnet fishery involves eight vessels from Ballyglass and Porturlin, Co Mayo, working up to 25 nautical miles offshore in depths of 30 to 100 metres. The hake fishery is some 100 nautical miles offshore, and is worked by Dingle vessels of between 20 to 25 metres in length, and in depths of 90 to 270 metres.
The Mayo location was particularly significant, given the proximity to the Inishkea islands to the southwest.
The islands are known to host considerable numbers of grey seals throughout the year. The 1982 seal clubbing incident on the islands reflected the sort of level of frustration felt then with what was perceived to be an uncontrolled predator.
The scientists adopted two methods to assess damage rates to fish, and types. The first, a direct method, involved placing observers on board vessels. The second, an indirect method, involved studying landings. The study showed that in successive years, damage rates to catches in the Dingle fishery were 4.1 per cent and 7.7 per cent, using direct and indirect methods respectively.
A slightly higher rate of 10 per cent was recorded in the Mayo fishery, using indirect methods between 1994 and 1996. Significantly, a far higher incidence of interference by scavengers, such as crabs, was noted by observers on board vessels, at some 20 per cent of damage.
The dietary study showed that whitefish and non-commercial fish species were favoured by the seals, but that the average size of the commercial species consumed was much lower than those caught in the actual fishery.
It also recorded by-catches of seals in the Mayo gill-net fishery, and found that in most cases juveniles were involved. This may reflect inexperience; larger seals would tend to break free of the net on hauling, if entangled, and have been known to do so.
Acknowledging the limitations of what is the first published information on grey seal diet in Irish waters, the authors make some specific recommendations for future research. Stomach samples are a far more accurate barometer of seal diet than faecal samples, they note. Also, restricting samples to those taken from by-catches of seals is not adequate.
The authors say that simultaneous monitoring of seal diet, seal movements and fish stocks in a single area would permit seal foraging behaviour to be studied in context. And on-board observers are a much more effective way of examining the problem.