NOTE Laptikhovsky: Survival rates for rays by the bottom trawl squid fishery 



759 



Discussion 



Acknowledgments 



The survival of discarded rays during trawling opera- 

 tions in the Falkland waters is quite important. Although 

 65.2% of the individuals were initially assigned as dead, 

 the actual mortality was 40. 99c, although it took some 

 rays up to six hours to recover. Survival of shallow-water 

 shelf species such as Psammobatis sp., in particular, but 

 also B. brachiurops and B. magellaniea, was somewhat 

 higher than relatively deep-water species such as B. 

 albomaculata, B. griseocauda, and Bathyraja sp., which 

 inhabit the shelf edge and upper part of the slope. This 

 survival rate was most likely related to the greater 

 resilience to environmental changes for shallow-water 

 species, whose habitat is more changeable both season- 

 ally and spatially. Male survival was lower, which is in 

 accordance with data for rays and skates obtained in 

 northern Australian waters (Stobutzky et al., 2002). 



Recent data from a tropical prawn fishery off northern 

 Australia showed that on average 449c of individuals of 

 a number of ray and shark species survived a trawl- 

 ing event (Stobutzky et al., 2002). The Falkland ray 

 survival rate was higher. This difference may be due 

 either to the higher metabolic rates of tropical ray spe- 

 cies (and therefore a higher vulnerability to asphyxia), 

 or to an overestimation of their mortality, which was 

 assessed immediately after individuals where landed on 

 deck (unlike the recovery time allowed in the present 

 study). The latter factor is more probable because in the 

 present study 41.9% of rays initially recorded as dead 

 (D and DD) eventually recovered. 



Despite the demonstrated ability of skates to survive 

 after being caught and stored in fish bins, their contin- 

 ued survival is not guaranteed once they are discarded. 

 They may fall prey to the hundreds of albatrosses and 

 other scavenging birds that are associated with trawl- 

 ers (author's pers. obs.). The consumption of differ- 

 ent discarded fish species and squids from trawlers in 

 Falkland waters by seabirds, primarily by black-browed 

 albatrosses, has been studied (Thompson, 1992), but it 

 is not known whether rays are also taken by sea birds 

 and to what extent. Despite the great abundance of 

 seabirds around vessels in the Southwest Atlantic, it 

 is likely that they consume a minor part of discards as 

 found in Australia (Hill and Wassenberg, 2000). Most 

 of the discarded fish probably fall to the sea floor and 

 attract and are consumed by bottom scavengers and 

 bottom dwellers (Laptikhovsky and Fetisov, 1999; Lap- 

 tikhovsky and Arkhipkin, 2003). Consequently, even 

 after recovering and successfully avoiding the seabirds, 

 the discarded skates may be consumed or mortally in- 

 jured by these bottom scavengers during the recovery 

 time, which appears to be about 0.5-1.5 hours. 



I would like to thank the crew of FV Sil for their valu- 

 able help during sampling procedures and their hospital- 

 ity onboard; the Director of Fisheries, John Barton, for 

 supporting this work; A. I. Arkhipkin and an anonimous 

 reviewer for valuable comments; and Helen Otley (FIFD) 

 for language editing. 



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