SWIMMING MIGRATION IN THE SEA

Many swimming migratory species in rivers, lakes and in the oceans are subject to some of the very same challenges: dam development in rivers, shipping routes affecting mi- grations due to noise, invasive species having an impact on their food chain, and illegal harvest, overharvest and bycatch (WCD, 2000; UNEP, 2001; UNEP, 2008).

Bycatch generally covers the accidental capture of non-target species in fisheries and threatens numerous migratory marine mammals, turtles, sharks and seabirds. It is the top threat to the majority of marine mammals being responsible for an an- nual loss of more than 600,000 individuals. Trawls, gillnets and driftnets, long lines and purse-seines are particularly prob- lematic with animals becoming entangled in fishing gear or attracted by bait.

A small population of Irrawaddy dolphins (Orcaella brevirostris) in the inner Malampaya Sound, Philippines, classified as “Crit- ically Endangered” in the IUCN Red List, is currently threat- ened by bycatch in the local crab net/trap fishery (Smith et al., 2004). Irrawaddy dolphins and finless porpoises (Neophocae- na phocaenoides) are bycaught regularly in gillnets and kelong (fish traps) and to a lesser extent in trawls in Malaysian waters (Perrin et al., 2005). Freshwater populations of Irrawaddy dol- phins in two rivers – the Mahakam of Indonesia and Mekong of Vietnam, Cambodia, and southern Laos – and one popu- lation in the Songkhla Lake in Thailand – are also classified in the IUCN Red List as ‘Critically Endangered’, with gillnet entanglement identified as the dominant threat (Beasley et al., 2002; Kreb, 2002; Smith, 2003; Smith et al., 2009). Although the data have not yet been collected, it is probable that there is a high level of Indo-Pacific bottlenose dolphin (Tursiops aduncus) bycatch throughout this region as well (Wang & Yang, 2009). Spinner dolphins (Stenella longirostris) and Fraser’s dolphins (Lagenodelphis hosei) experience substantial bycatch in the tuna

driftnet fishery in Negros Oriental, Philippines (Dolar et al., 1994), and similar fisheries for large pelagic species operate in other parts of the country (Perrin et al., 2005). Cetaceans may also be taken in round-haul nets; one estimate for the eastern Sulu Sea was 2,000–3,000 per year. In a recent ‘rapid- assessment’ of 105 fishing villages, 67 per cent were found to have some level of cetacean bycatch, with the bycaught dol- phins usually used for shark bait in longline fisheries (Perrin et al., 2005). Preliminary research indicates that the bycatch and entanglement of some small cetaceans in fisheries, espe- cially finless porpoises (Neophocaena phocaenoides phocaenoides and N.p asiaeorientalis), is also high in Chinese waters (Zhou & Wang, 1994).

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