Purves et al.: Distribution, demography, and discard mortality of crabs caught as bycatch in the South Atlantic 



875 



operational restrictions on the fishery, and low levels of 

 bird mortality still occur (CCAMLRM. 



Pot fishing for toothfish has recently been tried around 

 South Georgia (Agnew et al., 2001), and although pots do 

 not catch birds, they do take lithodid crabs as bycatch. 

 Crabs are largely a "nuisance" catch when fishing for 

 toothfish, but this bycatch is clearly of concern in rela- 

 tion to crab populations, and must be considered within 

 the CCAMLR's ecosystem approach. A small amount of 

 exploratory crab fishing has already taken place around 

 South Georgia Island and Shag Rocks. Only 798 metric 

 tons (t) of crabs have been taken in directed crab fisheries 

 since 1992; by the FV Pro Surveyor (July-August 1992; 

 299 t; CCAMLR2), the FV American Champion (September 

 1995-January 1996; 497 t; CCAMLRM), and the FW Argos 

 Helena (August 1999; 2 t) (CCAMLR''). A pot fishery for 

 toothfish is likely to take place in deep water where cur- 

 rent longline fishing concentrates (around the 1000 m con- 

 tour; Agnew et al., 1999) rather than in shallower waters 

 (<400 m) where crab fishing has taken place (Otto and 

 Macintosh, 1996; Watters, 1997 ). Toothfish pot fishing may 

 therefore impact different crab population components 

 from those impacted by the crab fishery. 



We investigated the likely effects of toothfish pot fishing 

 on crabs caught as bycatch on board two commercial fish- 

 ing vessels. These vessels conducted three separate trials 

 fishing for toothfish around South Georgia between March 

 2000 and April 2001. This paper reports on the species of 

 crab taken during these trials, as well as the distribution of 

 crabs and their biological characteristics. In common with 

 many other crab fisheries (Hoggarth, 1991; Schmidt and 

 Pengilly, 1993) retention size limits are set for the fishery 

 around South Georgia. Only males greater than 102 mm 

 carapace width for Paralomis spinosissima and 90 mm for 

 P. formosa may be retained. All undersize and female crabs 

 from both the toothfish pot fishery and the crab fishery 

 must be discarded. We also report results of experiments 

 on survival of such discards. 



Methods 



During the first cruise (March to May 2000), two observ- 

 ers were deployed on board the FV Argos Georgia (cruise 

 Gl). Detailed information was collected on the number 

 of toothfish and the numbers and species composition of 



' CCAMLR (Commission for the Conservation of Antarctic Marine 

 Living Resources). 1999. Report of the Working Group for 

 Fish Stock Assessment, 110 p. Annex 5 in the report of the 

 eighteenth meeting of the Scientific Committee. CCAMLR, RO. 

 Box 213, North Hobart, Tasmania 7002, Australia. 



- CCAMLR. 1992. Report of the working group for fish stock 

 assessment, 164 p. Annex 5 in the Report of the eleventh meet- 

 ing of the Scientific Committee. CCAMLR, RO. Box 213, North 

 Hobart, Tasmania 7002, Australia. 



^ CCAMLR. 1997. Report of the working group for fish stock 

 assessment, 169 p. Annex 5 in the Report of the sixteenth meet- 

 ing of the Scientific Committee. CCAMLR, P.O. Box 213, North 

 Hobart, Tasmania 7002, Australia. 



■• CCAMLR. 2000. CCAMLR statistical bulletin, 153 p. 

 CCAMLR, RO. Box 2 13, North Hobart, Tasmania 7002, Australia. 



I  P. spinosissima 

 n P formosa 

 aP anamerae 



Depth range (m) 



Figure 1 



Catch in numbers per pot of the three dominant crab 

 species by depth range as found during fishing opera- 

 tions of the Argos Georgia from May to April 2000. Pots 

 were set between 211m and 1651 m depth. 



crabs caught in the pots. Information was also collected on 

 fishing results, such as catch rates, fish bycatch, and the 

 commercial viability of this fishing method (Agnew et al., 

 2001), and the diet of toothfish (Pilling et al., 2001). In the 

 second and third toothfish pot cruises single observers were 

 deployed on the Argos Georgia (cruise G2) and another 

 vessel, the FV Argos Helena (cruise H), which fished simul- 

 taneously from January to April 2001 . Fishing gear and the 

 configuration of gear was similar for all three cruises. The 

 semiconical pots of approximately 80 cm height were con- 

 structed of steel frames and covered with 80-mm polysteel 

 (Movline) mesh. A collapsible funnel entrance was situated 

 on the side of the pot, orientated horizontally, and tapering 

 to the pot's interior. A drawstring held the bottom mesh 

 together in the middle. This configuration allowed the 

 pots to be emptied easily when hauled aboard and to be 

 stacked on top of each other when not in use. A panel was 

 sewn into the pots by using biodegradable sisal twine to 

 ensure that crabs could eventually escape from lost pots 

 and to prevent "ghost fishing." However, catch handling 

 methods were different on the two vessels; the pots were 

 emptied directly onto the factory conveyor belt on the Argos 

 Georgia and emptied down a chute to the factory level on 

 the Argos Helena. 



During the first cruise (March to May 2000) depth of fish- 

 ing, determined as water depth by onboard echo sounders, 

 was related to the species of crab caught. Paralomis spino- 

 sissima were generally caught in relatively shallow waters, 

 whereas P. formosa tended to be caught in much greater 

 numbers in deeper waters (Fig. 1). Paralomis anamerae 



