Marine mammal bycatch in commercial fishing is a secretive subject. But if you do a bit of digging, what little information that is available is truly horrific.
The IUCN lists the harbour porpoise as ‘Least Concern’, but this classification is based on woefully inadequate information.
Under ‘Threats’ the IUCN says this about the harbour porpoise –
“Today, the most significant threat in most areas is incidental catches in fishing gear, primarily gill nets. Incidental mortality in fishing gear is likely to occur throughout the range of the species, but substantial incidental takes have been documented (summarized in Donovan and Bjørge 1995) for the Gulf of Maine (1,200-2,900/year), Bay of Fundy (80-400/year), West Greenland (1,400/year), North Sea (4,600/year) Celtic Shelf (1,500/year), and also off central California during the 1980s and 1990s (tens to hundreds per year; Barlow and Hanan 1995). More recent monitoring programs of Danish set-net fisheries in the North Sea revealed an average of 5,591 porpoises taken annually in the period 1987-2001 (Vinther and Larsen 2002). However, most North Sea gillnet fisheries were not monitored for marine mammal bycatch (ICES 2002)”
The last sentence is crucial here “most gillnet fisheries were not monitored for marine mammal bycatch”. This makes the IUCN listing almost worthless. As the IUCN itself touches on, gillnet fishing is one of the most destructive forms of commercial fishing and the most likely to incur marine mammal bycatch.
An International Council for the Exploration of the Sea (ICES) report that studied two Norwegian gillnet fisheries found that the estimated total number of porpoises caught as bycatch in Norwegian gillnet fisheries in the period 2006-08 to be nearly 21,000 animals (Or approximately 3,500 porpoises per fishery).
The report also says that “according to the criteria advised by ASCOBANS (bycatches should not exceed 1.7% of the best population estimate), a population in excess of 400,000 is required to sustain an annual bycatch of 6,900 porpoises”.
And here’s where the problem lies. According to the IUCN “in the waters of the European Atlantic, abundance in 2005 was estimated at 385,600 [CV=0.20] (P.S. Hammond pers. comm.), of which about 335,000 [CV=0.21] were estimated in the North Sea and adjacent waters, where abundance was estimated at 341,000 [CV=0.14] in 1994 (Hammond et al. 2002)”.
On this basis, the porpoise bycatch in Norwegian gillnet fisheries alone is unsustainable. Add to this all the other European fisheries that have significant porpoise bycatch, and the long-term outlook for the harbour porpoise is dire to say the least.
The IUCN is the benchmark by which environmentalists gauge their response to what is needed to help to protect animals which are threatened and accuracy is essential to implement effective conservation measures. More accurate information on marine mammal bycatch is available from fishermen, but at what cost to the reputation of the commercial fishing industry? It seems that when it comes to marine mammals, and the harbour porpoise especially, the conservation world is turning a blind-eye to the most horrendous slaughter.