Franciscana bycatch is not reduced by acoustically reflective or physically stiffened gillnets

Abstract

The incidental capture of franciscana Pontoporia blainvillei in gillnet fisheries of Argentina, Uruguay and Brazil represents a major conservation threat to this species. We report on an experimental trial that compared franciscana bycatch rates in standard gillnets to gillnets with 1 of 2 modifications: increased acoustic reflectivity by infusion of barium sulphate (BaSO 4 ) or increased flexural stiffness of the nylon twine. Field trials were conducted in association with artisanal fishermen at San Clemente del Tuyu in Bahia Samborombon, Argentina, between October 2009 and March 2010. Depth sensors were used to record the underwater fishing behaviour of the 3 net types. Seventy-seven dolphins were observed incidentally captured in 807 monitored gillnet hauls, with similar fishing effort observed for each net type. There was no significant difference in franciscana bycatch rates (p > 0.05) or target fish catches (p > 0.05) among the 3 net types. The stiff net twine had a slightly but significantly higher flexural stiffness (FS) than the reflective or control net twine. The FS of the dry stiff twine was similar to that previously reported for BaSO 4 twine with a similar diameter; in contrast, the FS of the reflective nylon was, unexpectedly, much lower. However, the difference in the FS between wet, submerged stiff twine and control twine used in the field was estimated at 19.4%. The BaSO 4 net fished with a significantly lower mean float line height than either the control or stiffened net. These results show that the use of reflective or stiff nets does not lead to a reduction in franciscana bycatch rates; therefore, other management strategies need to be developed to reduce the impact of incidental captures of this species. KEY WORDS: Bycatch · Gillnet · Franciscana · Artisanal fishery Resale or republication not permitted without written consent of the publisher FREE REE ACCESS CCESS Contribution to the Theme Section 'Techniques for reducing bycatch of marine mammals in gillnets'Endang Species Res 21: [1][2][3][4][5][6][7][8][9][10][11][12] 2013 species is listed as Vulnerable by the IUCN (Reeves et al. 2008) and is considered to be the most threatened cetacean in the Southwest Atlantic (Secchi 2010). In coastal waters of Buenos Aires Province, Argentina, an estimated 650 dolphins are bycaught annually (Bordino & Albareda 2004). The most recent population estimate for this species in Argentinean waters is 14 000 individuals (Crespo et al. 2010), so it is unlikely that current bycatch levels are sustainable, and management measures are, therefore, urgently required to reduce the incidental bycatch of the species.The use of acoustic alarms has been shown to significantly reduce the bycatch rates of franciscana (Bordino et al. 2002) and other cetaceans in gillnets (Kraus et al. 1997, Carretta et al. 2008), but the financial cost of implementation and enforcement of pinger use limits their applicability in many smallscale fisheries (Read 2008). Concerns that the widespread use of acoustic alarms cou...

Reducing bycatch in gillnets: A sensory ecology perspective

“…However, it does seem clear that acoustic information is often not sufficient to detect gillnets. Furthermore, there is some evidence that improving the echo from gillnets by treating them with substances that increase their density, such as Barium Sulphate, might improve net detectability by cetaceans, but significant differences in effectiveness of such treatment have been reported (Au and Jones, 1991;Bordino et al, 2013;Cox and Read, 2004;Koschinski et al, 2006;Mooney et al, 2007;Trippel et al, 2003). Such treatments are unlikely to have any influence on the net detectability by non-echolocating species.…”

Population Genetics of Franciscana Dolphins (Pontoporia blainvillei): Introducing a New Population from the Southern Edge of Their Distribution

“…Like other coastal cetaceans, its distribution makes franciscana particularly vulnerable to anthropogenic activities, mainly to incidental by-catch [ 17 – 19 ]. Owing to its high incidental mortality in fishing gillnets [ 14 , 20 – 23 ], it is the most threatened small cetacean in the Southwestern Atlantic Ocean [ 24 , 25 ] and was classified as “Vulnerable” by the International Union for Conservation of Nature (IUCN) [ 26 ]. According to the International Whaling Commission Scientific Committee [ 27 ] a 1% incidental population mortality per year is a matter of concern in small cetacean populations, and a 2% mortality may not be sustainable.…”

Patterns of population structure at microsatellite and mitochondrial DNA markers in the franciscana dolphin (Pontoporia blainvillei)

“…Its distribution, restricted within the 30 m isobaths from the coast, is comprised between Itanúas (18°25′ S), Brazil, and Golfo San Matías (41°10′ S), Argentina (Crespo, 2009;Crespo, Pedraza, Grandi, Dans, & Garaffo, 2010;Pinedo, Praderi, & Brownell, 1989;Secchi, Danilewicz, & Ott, 2003). Due to its reduced movement patterns and to anthropogenic impacts, it is the most threatened small cetacean on the southwestern Atlantic Ocean; it is particularly susceptible to high incidental bycatch mortality throughout its distribution (Bordino, Mackay, Werner, Northridge, & Read, 2013;Bordino, Wells, & Stamper, 2008;Bordino et al, 2002;Cappozzo et al, 2007;Crespo, Corcuera, & Lopez Cazorla, 1994;Crespo et al, 2010;Di Beneditto, 2003;Negri, Denuncio, Panebianco, & Cappozzo, 2012;Secchi, 2010;Secchi et al, 1997Secchi et al, , 2003. Based on a projected species abundance decline of more than 30% over three generations (36 years;Taylor, Chivers, Larese, & Perrin, 2007), the franciscana dolphin was classified as "Vulnerable" by the International Union for Conservation of Nature (IUCN) in 2008 (Reeves et al, 2012).…”