A marine worm is helping to reduce the environmental impact of west coastfish farms. Lorna Siggins, Marine Correspondent, reports.
Environmental conditions at fish farms on the Irish coastline are significantly better than those in Scotland and Norway thanks to the hard work of an inconspicuous little worm.
A study published by the Irish industry in Galway yesterday says the importance of populations of Malacoceros fuligenosus, which live on the seabed, should not be underestimated.
The spionid polychaete worm has been recorded at high densities in a number of locations off the Irish west coast, with more than 10,000 per square metre at times.
The worms are avaricious and feed on detritus located at, or near, the sediment surface.
Their contribution towards maintaining good conditions was significant, according to the report's authors, Aqua-Fact International Services Ltd of Galway. They say more research is required to examine their population, propagation and persistence after harvesting has taken place.
The study, carried out for the Irish Salmon Growers' Association, acknowledges that salmon farming does affect the environment.
It indicates, however, that sea-floor conditions on this coastline are in a "relatively healthy state" - albeit different to the neighbouring ambient status.
Fin-fish farming has no great impact on the water column, as was demonstrated in a survey by Aqua-Fact for the Marine Institute in 1999. However, the excess organic waste on the sea floor directly beneath holding pens can make a difference.
The more exposed nature of the Irish Atlantic seaboard, where most farms are located, is the key factor in comparison to Norway and Scotland. About half of the fish farm sites here are in exposed or semi-exposed locations, and almost three-quarters are in areas with moderate or strong current flow.
The tidal range along the west coast is around five metres during highest spring tides, and this volume of water not only sweeps away uneaten food, but also dilutes any BOD (biological oxygen demand) or water with low oxygen levels near the farms.
The study applies the "redox" measurement of sediment, redox being a compound of the words "reduced" and "oxygen" as an important factor in determining the health status of the marine benthic environment.
IT notes that 61 per cent of farm sites have between one and 10 centimetre depths of reduced oxygen under cages, and only two Irish farms have undercage redox depths of less than half a centimetre. Reduced oxygen conditions leave these areas dead zones for animal life, but no farms show any detectable impact on the seafloor more than 20 metres from cage position, according to the report.
Topography and geological factors have an influence here. West coast bays are not silled systems that restrict water movement and exchange, such as those found in parts of Scotland and Norway.
"With the exception of Killary harbour (on the Galway-Mayo border) and Mulroy Bay (Co Donegal), entire bays can be entirely flushed over a short period of time," the study says.
Irish farms differ in several other respects to those in Norway and Scotland, featuring the presence of a substantially smaller production biomass, greater tidal influence and a higher number of sites offshore.
The study is a compilation of more than 100 separate environmental reports on farms, and has been peer-reviewed.
It points to sites where increased organic enrichment has been recorded, such as Lough Swilly in Co Donegal, but notes that impact in most cases is restricted to the area directly beneath the cages.
Mulroy Bay, which is described as "the most convoluted" of the marine inlets on the northwest coast, also performs relatively poorly.
However, the overall picture is positive, and the report states that those areas where organic enrichment has been recorded are able to assimilate that input.
The authors note that the current monitoring programme for fish farms is very progressive, and the development of new types of cages - such as the resilient Oceanspar offshore system tried in Killary harbour and at Clare Island in Co Mayo - and mitigating practices such as fallowing, site rotation and waste dispersal have made a difference to the environment.
Harrowing should be confined to ebb tides during the winter months when water temperatures are at their lowest and light conditions are least suitable for phytoplanktonic activity, it notes.
The Review of Benthic Conditions at Irish Fish Farms, Sept 2001 is published by Aqua-Fact International Services