Brock and Ward: Octopus bycatch and lobster mortality in the South Australian rock lobster fishery 



435 



A lobster Southern zone 



600 



500 



- 400 

 o 



o 



s; 300 



w 



CD 



« 200 



Oct Nov Dec Jan Feb Mar Apr 



octopus 



in 



% 6 



CL 



ra 5 - 



i *■ 



~S 3 



CD 



en o 



=3 ^ 



a. 



2 1 



o 



o 







Oct Nov Dec Jan Feb Mar Apr 



k dead lobster 

 20 



15 



1 1 1 1 1 1 1 



Oct Nov Dec Jan Feb Mar Apr 



B lobster Northern zone 



600 -, 

 500 

 400 

 300 

 200 - 

 100 

 



D 



— i 1 1 1 1 1 1 



Nov Dec Jan Feb Mar Apr May 

 octopus 



6 

 5 

 4 



3 - 

 2 



1 







1 1 1 1 1 1 1 



Nov Dec Jan Feb Mar Apr May 



Nov Dec Jan Feb Mar Apr May 



Month 



Figure 4 



Mean monthly catch rates in each fishing zone for lobsters lCPUE L ) (A and Bl, octopus 

 (CPUE ) (C and Di, and dead lobsters (M L )(E and F). Error bars are ±SD of mean. 



higher and more variable in MFAs 39 and 49 (Fig. 5B). 

 There were large interannual fluctuations in CPUE (1 

 in each MFA, and these trends were similar among 

 MFAs (Fig. 5D). M L was highest in MFA 40, where a 

 maximum of 12.5 dead lobsters/l 100 traps lifts) was re- 

 corded 1998 and lowest in MFA 15 where the maximum 

 was 5.2 dead lobsters/100 trap-lifts in 1997 (Fig. 5F). 

 No clear long-term trends in M L were apparent in any 

 MFA. 



Factors that affect within-trap lobster mortality 



Based on the mean square values, the number of octo- 

 pus had the greatest effect on lobster mortality in both 

 zones (Table 1, A and B). The number of dead lobsters 



increased with both octopus and lobster catches and 

 with soak-time and decreased as depth increased (Figs. 

 6 and 7). Based on the relative size of the mean square 

 values, the factor with the greatest effect on the number 

 of dead lobsters in the SZ was the number of octopus 

 caught, followed by soak-time, number of lobsters caught, 

 and depth. In the NZ, the number of octopus caught was 

 also the most important factor, followed by the number 

 of lobsters caught, depth, and soak-time. 



Source of lobster mortality and size-dependent mortality 



A total of 3627 lobsters from 635 trap-lifts were mea- 

 sured. In the sample there were 212 lobsters killed in 

 traps of which 207 (98%) were killed by octopus and 5 



