Northern Ireland Estuarine Fish Community Surveys

The ichthyofauna of each transitional water is sampled using a variety of methods. These include seine netting, fyke netting and beam trawling. This multimethod approach is adopted to ensure that all representative habitats and species are sampled within each estuary. All netting is conducted from a 3.4 m inflatable boat equipped with a 15 hp outboard engine. The seine net is 30 m long by 2 m deep with a 14 mm mesh body and a 5 m long x 3 m deep central panel of 6.5 mm mesh. Seine netting is limited to unobstructed areas with gently sloping banks and is typically deployed in waters less than 1.5 m. Where possible, seine netting is conducted during the two hour period either side of low tide. A fleet of double fyke nets are set in each system. Each trap is 0.5 m high, 2.5 m long with a 10 mm mesh cod end and is joined by a 6 m long x 15 mm mesh leader. The fyke nets are deployed in deeper waters by boat and are laid diagonally across the flow of the river. They are anchored out of the main channel to avoid passing boat traffic and are retrieved after 24 hours. A lightweight shoeless beam trawl is used to sample benthic and demersal fishes. The mouth of the net is 1.5 m wide with a height of 0.5 m, the body of the net is constructed of 14 mm mesh and is 3 m long, tapering to a width of 0.5 m. A 1.0 m long x 0.5 m wide cod end of 6.5 mm mesh is attached to the end of the net. The beam trawl is towed 20 m behind the boat and is deployed and retrieved by hand. Towing speed is between 1 and 2 knots with each tow lasting approximately 5 minutes and covering a distance of between 150 and 300 m. Where possible, all fishes are processed alive in the field and returned to the water. Specimens are identified, measured (mm Total Length (TL)), and weighed (g) before being released. Specimens that cannot be identified in the field are placed in labelled plastic bags and kept cool for transport to the laboratory where they are frozen.