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



883 



P. spinoxissima 



mersion process. Out of the 

 35 lively control animals a 

 similar proportion died dur- 

 ing the experiment as in the 

 re-immersed group (8%), but a 

 lower proportion of the control 

 group were lively following the 

 experiment (63%). The lower 

 proportion of lively animals 

 in the control group may have 

 been a result of interruptions 

 in the supply of oxygenated 

 water and the continual dis- 

 turbance on deck due to the 

 ship's motion. Consequently, 

 controls were not performed in 

 experiments 2 and 3. 



Experiments 2 and 3 took 

 a random sample of lively 



and limp crabs and subjected them to re-immersion. The 

 proportions of lively crabs at the beginning of the re-im- 

 mersion experiments were lower than the proportions 

 estimated for the population as a whole in Table 6, with 

 the exception off! spinosissima in experiment 2. However, 

 sample sizes were much smaller in the experiments and the 

 crabs were subject to greater handling times than those as- 

 sessed in Table 6; therefore the results presented in Table 6 

 are more likely to be representative of the condition of 

 discarded crabs than results presented in Table 8. 



In experiment 1 on cruise Gl, no limp animals were 

 subjected to re-immersion. In comparing the results of 

 immersing lively animals, there was no significant differ- 

 ence between the results for experiment 1 and 2 (cruises 

 Gl and G2 on the Argos Georgia) for either P. formosa or 

 P. spinosissima. Combining the results of the experiments 

 for Gl and G2, there was a significantly lower vitality 

 after re-immersion for both species on the Argos Helena 

 compared with the Argos Georgia (Table 7). On the Argos 

 Georgia there was no significant difference in vitality af- 

 ter re-immersion between P. formosa and P. spinosissima, 

 whereas on the Argos Helena vitality for P. spinosissima 

 was significantly lower than vitality for P. formosa. These 



2 It 

 x; JO 



TO 



ii 



c 

 _i 



25 



2 



Ln {carapace length) 



Figure 8 



Chela allometry in male (A) P. spinosissima and (B) P. formosa sampled at Shag Rocks in 

 2001, The intersections of fitted curves were used to determine size at maturity. 



results are similar to the initial assessment of vitality 

 prior to re-immersion (Table 6), where significantly fewer 

 P. spinosissima were lively in comparison with P. formosa 

 on the Argos Helena , but there was no difTerence between 

 the two species on the Argos Georgia. 



A single re-immersion experiment performed on 15 P. 

 anamerae crabs (11 were "lively" and 4 were "limp"), on 

 the Argos Helena in April 2001, resulted in a mortality 

 rate of 73%. Only 27% of the crabs that were "lively" before 

 re-immersion were still "lively" after the rehaul. Although 

 more data need to be collected on the survival rate of this 

 species, this high mortality rate, together with the higher 

 incidence of individuals off! anamerae found to be "limp" 

 during vitality assessments (Table 8; 10% compared to 3% 

 of P. spinosissima and 1% of P. formosa), seems to indicate 

 that this bycatch species might be particularly vulnerable 

 to onboard handling and discarding. 



Crabs that were physically damaged (i.e. had missing 

 legs or cracked carapaces) before being subjected to re-im- 

 mersion were less hkely to survive than undamaged crabs. 

 Of the 19 damaged P. spinosissima (13 of these were "lively" 

 and 6 were "limp"), 58% did not survive re-immersion and 

 only 32% of these were still "lively." The effect of damage 



