Structure of the seabird assemblage associated with pelagic longline vessels in the southwestern Atlantic: implications for bycatch

Abstract

The region of the southwest Atlantic influenced by the Brazil-Malvinas Confluence (BMC) is important for globally threatened species of albatross and petrel. This applies particularly to the area of the continental slope, due to the high rates of incidental catch from pelagic longliners. We analyzed the temporal variation in the seabird assemblage associated with this fishery, identified the species that make use of discards, and evaluated their interactions while foraging. During 20 commercial fishing trips between 2005 and 2008, we completed 415 bird counts and recorded behavior in 172 of these. The observed species richness (≥38 species) is greater than that reported for any other fishery in the region and was highest from October to April, although many species in the assemblage were significantly more abundant between May and September. Only 14 species made significant use of discards, and all of these were albatrosses and petrels captured incidentally. We observed within-and between-species competition for access to discards. In general, the frequency of intraspecific competition was greater in the most abundant species, during the period of their greatest abundance. Albatrosses were more successful in interspecific competitive interactions, and we observed a dominance hierarchy related to body size. We conclude that the composition and seasonality of the seabird bycatch is determined by the spatiotemporal dynamics of the assemblage and by the observed pattern of interspecific interactions. Discards from pelagic longline fleets operating in the BMC may be an important food source for at least 8 species of globally threatened seabirds. Understanding the effect of discards on these populations could generate useful information for conservation, although reducing bycatch levels should be considered the main goal.

Requisite improvements to the estimation of seabird by‐catch in pelagic longline fisheries

“…This cannot be emphasized enough and can be readily illustrated with the Taiwanese data; Table 3 lists just two species (none of them albatrosses) sighted behind vessels or caught during daytime in the southwest Atlantic which were identified by Taiwanese observers. However, more than 38 species, of which 14 make substantial use of discards, were recorded behind Uruguayan longliners in an overlapping area (Jimenez et al, 2011). Yeh et al's recommendation that increased training be provided to observers on seabird identification therefore could not be more apt.…”

Risk assessment and relative impact of Uruguayan pelagic longliners on seabirds

“…In optimal demographic conditions, the population cannot increase by a proportion greater than λ max − 1 (that is, the sustainable incidental mortality cannot exceed λ max − 1) (Niel and Lebreton 2005). In the present study we estimated R max for all the species with a high level of association with the fishery (according to Jiménez et al 2011). Because s and α should represent optimal values rather than current value Fletcher 2008, 2011), we used the categories proposed by Dillingham and Fletcher (2011) as an approximation of these values.…”

“…The methodology is described in Jiménez et al (2009bJiménez et al ( , 2011. Basically, it included 3 counts d −1 (morning, around noon, and afternoon−evening) from aft of the boat, where an area encompassing about 200 × 400 m (∼200 m from the stern and ∼200 m to port and to starboard) was observed during approximately 30 min, during which the number of individuals of each species was counted (Jiménez et al 2009b(Jiménez et al , 2011.…”

“…Afternoon counts (initiated between 16:30 and 19:30 h) were during daylight hours, before or the early stages of gear setting. In some cases it was impossible to complete the 3 daily counts due to bad weather (Jiménez et al 2009b(Jiménez et al , 2011. For the purpose of the present study, counts realized during the setting and hauling of the longline (n = 349 counts) were used to calculate FO, discounting counts in which no fishing activity was carried out (i.e.…”

“…Furthermore, empirical evidence shows that species that are more likely to gain access to baited hooks during setting of pelagic longline gear, are those with greater ability to dive or with larger beaks that allow them to feed on large items such as baits. However, the existing dominance hierarchy related to body size (Jiménez et al 2011) establishes that larger species have greater access to baits . Given their low productivity and potentially high susceptibility to bycatch, great albatrosses would be in a high-risk situation.…”