Marine mammal bycatch

Marine mammals can be divided into two groups in UK waters: whales, dolphins and porpoises (collectively known as cetaceans) and seals (pinnipeds). Although generally not considered to be marine mammals, otters also occur in estuarine and coastal waters. Otters have been caught and drowned in fyke nets set for eels and in creels/pots set for crustaceans. All fyke nets must now be fitted with otter guards which are provided by the Environment Agency. The introduction of mandatory otter guards in fyke nets has reduced the otter bycatch, but illegal nets still catch otters on an annual basis.

 

Very little data exists on the bycatch of seals in UK waters, but bycatch deaths are thought to be in the low hundreds per year. These are mainly grey seals associated with the herring and mackerel fisheries. In a recent study, 2% of tagged seals were killed in fishing gear, mainly gill and tangle nets. The impact of this bycatch level on the seal population is not thought to be significant. However, deaths from shooting and those associated with fishfarms have not been taken into account and may be significant in some areas.

 

In contrast, cetaceans are one of the more high profile victims of accidental capture in fishing nets. It was estimated that almost 60,000 cetaceans are killed globally each year through entanglement in fishing gear. In European waters, there have been a number of studies of such incidental bycatch. These, though not complete or comprehensive, indicate that the bycatch of cetaceans is widespread, and may threaten the integrity of dolphin and porpoise populations. In particular, the harbour porpoise bycatch from bottom-set gill nets is estimated to be over 7000 animals annually in the North Sea. This exceeds 2% of the population and is considered unsustainable. Dolphins tend to be caught more often in pelagic trawls. For example, large pelagic trawlers targeting horse mackerel south west of Ireland are known to catch white-sided and common dolphins and long fin pilot whales, with a catch rate of one dolphin per 93 towing hours. Whilst observers recorded 91 common dolphins caught in 313 hauls in the pelagic trawl fisheries for bass (south west England) between 2001 and 2003. Prior to the introduction of EU legislation to ban the use of driftnets for tuna, dolphins, particularly stripped and common, were caught in large numbers (over 750 individuals in 1420 hauls). In contrast, larger whales may break free of fishing gear. However, injuries and entanglement in broken warps etc may cause illness, starvation and death at a later date.

 

The UK is party to several agreements with legal obligations to protect small cetaceans, including the EU habitats directive, the Agreement of the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Sea (ASCOBANS) and the UN Convention on the Law of the Sea. Technical, educational and legal measures can be taken to avoid or reduce cetacean by-catch. Most fishermen would not want to catch cetaceans, so a number of projects have been undertaken with their co-operation to test measures such as the attachment of acoustic devices ('pingers') to nets to alert cetaceans of the presence of the nets. More recently, tests have been made on devices to allow dolphins to escape from pelagic trawl nets.

 

Pingers © Simon Northridge/SMRU