Bycatch of Cetaceans from Dutch Fishing Vessels, a long Described problem in Irish waters.
By-catch of cetaceans (whales, dolphins, porpoises), are a common problem all over the world. By-catch are the unwanted fish and other marine creatures trapped by commercial fishing nets during fishing operations for commercially valuable species. This occurs because these nets trap everything larger than the net's mesh. This often includes sharks, seabirds, marine turtles and cetaceans.
Harbour porpoise by-caught in fishing net. Photo source: WDC, U.K.
The numbers of cetaceans accidentally caught by Dutch fishing vessels between 1989 and 1994 in Irish waters was studied extensively. Although at first, it was difficult at first to persuade these vessels to cooperate and record the amount of by-catch of cetaceans, by 1994, 12/12 Dutch fishing vessels agreed to cooperate. Each vessel was equipped with a poster with clear illustrations of numerous cetacean species and information about the project.
Atlantic white sided dolphin on Faroese dock. Source: fodderforthinking Wordpress.
(A total of 61 dolphin stomachs were analysed to see which fish species were the most commonly consumed. The by-caught individuals examined included 47 Atlantic white-sided dolphins ( Lagenorhynchus acutus ), 11 short-beaked common dolphins ( Delphinus delphis ), 2 bottlenose dolphins ( Tursiops truncatus ) and 1 white-beaked dolphin ( Lagenorhynchus albirostris ). Sieves were used to separate hard prey remains, (e.g. otoliths), of fish from the dolphin stomachs. These remains were then carefully examined. The results showed that 44 white-sided dolphins had intact or partly digested fish in their stomachs most of which consisted of mackerel. Horse mackerel remains were mainly found in stomachs of common and bottlenose dolphins but not at all in white-sided dolphins. Silvery pout (which is a mesopelagic species) were most frequently found in stomachs of white-sided dolphins.
So, why was it that mainly white-sided dolphins that were captured most often? It is believed that Atlantic white-sided dolphins migrate from deeper oceanic water to more shallow water (about 200m) when mackerel arrive on their migration.
So, why is it that mainly white-sided dolphins were being captured most often? It is believed that Atlantic white-sided dolphins migrate from deeper oceanic water to more shallow water (about 200m) when mackerel arrive on their migration.
Strandings of Atlantic white-sided dolphins are less frequent in recent years along the Irish coasts. This can possibly be due to the quantity of by-catch caused by the fishing industry which greatly reduced their numbers in Irish waters or changes in distribution as a result of climate change.
Reference
:
SHARE THIS ARTICLE
