Bottlenose dolphin (Tursiops truncatus) and harbour porpoise (Phocoena phocoena)

Candidate SACs for Bottlenose dolphin: Cardigan Bay, Moray Firth

Proposed SACs for Harbour porpoise: None

Bottlenose dolphin and harbour porpoise are two of the thirty five species of whales and dolphins which have been recorded in European seas. The bottlenose dolphin is commonly seen in coastal waters and resident or semi-resident groups are known from a number of locations around the UK. Large schools, which do not appear to be linked to any particular area, may also be seen in coastal waters. Harbour porpoise are also seen regularly in certain coastal areas with peak numbers between March and April and July to November. They are not confined to coastal areas, moving offshore at other times of year.

Cetaceans are accidentally caught by trawlers and seiners but set net fisheries, which include gill nets, drift nets & trammel nets, account for the majority of marine mammal by-catch in British waters23,34. The harbour porpoise is considered to be one of the more vulnerable cetaceans to entanglement in nets8,9,31,43,34,35. Analysis of stranding data collected between 1990-95 records this as one of the most frequent causes of death of harbour porpoises (38% of those examined) 23. The annual by-catch from the Danish set net fishery in the eastern North Sea has been estimated to be more than 5,000 animals.

There are reports of harbour porpoise being caught by long-line fisheries, entangled in creel or pot lines and salmon stake nets but the numbers are not thought to be significant19. There are also reports of dolphins (unspecified) being caught in anti-predator nets around fish farms19,59. These and other reports suggest that certain nets and locations may precipitate catches of cetaceans. It is reported, for example, that harbour porpoises are more likely to be entangled during storms or at night and it has been suggested that modification in fishing methods or use of reflective knots in netting and acoustic warning devices may reduce the occurrence of entanglement19. There are presently experiments to examine the effectiveness of these under the EU-funded BYCARE programme.

The impact of incidental capture on porpoises populations around the UK is not known. However it has been suggested that incidental by-catch could be a significant contributory factor in the overall decline in abundance of harbour porpoise in European waters9 and a serious cause of concern in relation to Celtic Sea populations in particular81. In other parts of the world there are examples where decline in populations are considered to be at least partly a result of entanglement in gill nets. A study of incidental catch of harbour porpoise in SW Bay of Fundy (Canada), for example, suggested that significant changes in length frequencies of the porpoises could be attributed to the fishery, and that sustained adult mortality in the gill-net fishery may have compressed the size, and possibly the age structure of the population31. Given the slow reproductive rate of the harbour porpoise, these catches were considered to be a serious threat to the relatively discrete harbour porpoise population in the area.

"Ghost fishing" by discarded and lost netting may also have an impact on marine mammal populations8,9,45 but no quantitative information on likely effects was found during this literature review.

Summary of the potential effects of fishing on bottlenose dolphin and harbour porpoise

Fishery

Methods

Potential effects

Mid-water Pelagic

Trawling

Accidental capture in trawls but insufficient data regarding species and numbers.

Demersal fin fish

Gill netting, drift nets, trammel nets set nets

Accidental entanglement and capture. It is considered that this is the most frequent cause of death of stranded harbour porpoise in the UK and, with their slow reproductive rate, means that there could be a serious threat to sustainability of discrete populations.

Salmon farming

Fish cage

Entanglement in anti-predator nets

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