Impacts of fisheries on the Critically Endangered humpback dolphin Sousa chinensis population in the eastern Taiwan Strait

Abstract

Biological and fisheries data were analysed to assess the impact of fisheries mortality on a Critically Endangered subpopulation of <100 humpback dolphins Sousa chinensis in the eastern Taiwan Strait (ETS). Substantial interactions between ETS S. chinensis (hereafter Sousa) and fishing gear are known to cause dolphin mortality. In 2009, a total of 6318 motorised fishing vessels were operating from ports within Sousa habitats. An average of 32 fishing craft per kilometre was observed along a 200 km stretch of Sousa habitat. Based on a photo-identification catalogue, > 30% of the ETS Sousa subpopulation exhibited injuries caused by fishing gear. Three individuals were photographed with fishing gear attached to their bodies, and 1 dolphin was found dead with fresh injuries caused by fishing gear. To ensure recovery of ETS Sousa, mortality due to human causes should be reduced to <1 individual every 7 yr. Fisheries bycatch is the most serious threat to these dolphins and needs to be eliminated as soon as possible to avoid extinction. Preventing the use of trammel nets, other gillnets and trawling throughout their habitat would be the single most effective conservation measure for ETS Sousa in the short term. Other fishing methods are available, and using the most selective, sustainable fishing methods available will benefit not only dolphins but also fish stocks, seabirds and other species, as well as the fishing industry, which depends on these species for its long-term viability. However, in the short term, there are costs associated with switching to more selective fishing gear.

Viability of the Critically Endangered eastern Taiwan Strait population of Indo-Pacific humpback dolphins Sousa chinensis

“…Bottom trawls are restricted to waters farther than 3 nautical miles (~5.6 km) from shore, but continuing illegal inshore trawling with minimal enforcement has resulted in overlap with much of the ETS dolphin habitat (J. Y. Wang unpublished data). In contrast, trammel nets are plentiful and are used with no restrictions (Slooten et al 2013). There is no direct estimate of bycatch levels (or even fishing effort), but gillnets are used everywhere within the study area (Slooten et al 2013).…”

“…There is no direct estimate of bycatch levels (or even fishing effort), but gillnets are used everywhere within the study area (Slooten et al 2013). Even the bycatch capture of a single individual from this small population each year would be unsustainable based on a calculation of the potential biological removal (PBR) level, which was 1 individual every 7 years (Slooten et al 2013).…”

“…The levels of bycatch were chosen based on preliminary information from government reports and field data (Wang et al 2007b, Slooten et al 2013). The 7 yr period was based on the PBR calculations that were performed by Slooten et al (2013), who found that the removal of even 1 individual every 7 yr exceeded the PBR for this population.…”

“…For example, several proposals for more large-scale industrial development that require massive land reclamation (for some examples, see Wang et al 2007b) signal that habitat loss and degradation are likely to worsen in the near future, thus compromising habitat quantity and quality for humpback dolphins. Regarding the impacts of fisheries, field observations have shown that more than 30% of the ETS dolphins exhibit serious scars/wounds that are most likely fisheries-related (Slooten et al 2013). …”

Diagnosability and description of a new subspecies of Indo-Pacific humpback dolphin, Sousa chinensis (Osbeck, 1765), from the Taiwan Strait

“…Some bathymetric contours in the Taiwan Strait are also shown. Wang et al 2004b;Ross et al 2010;Dungan et al 2011;Slooten et al 2013), and an assessment of this population against the criteria of the IUCN Red List of Threatened Species resulted in a 'critically endangered' status (Reeves et al 2008). The population's current very low potential biological removal level (Slooten et al 2013) and precarious viability under various likely scenarios (Araújo et al 2014;Huang et al 2014) further confirm the direness of its Red List status.…”