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

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 (BaSO4) 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 BaSO4 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 BaSO4 net fished with a significantly lower mean float line height than either the control or stiff- ened 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.