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



431 



Figure 1 



Map of the marine fishing areas (MFAs) of the South Australian rock lobster fishery. 

 Shading shows the MFAs that were considered in this study and where most fishing effort is 

 concentrated. 



sters in traps (Joll 1 ). Furthermore, there is a paucity of 

 quantitative data on the impact of fishing on octopus 

 populations, the proportion of lobster mortality that 

 is attributable to octopus predation, or the long-term 

 economic and ecological effects that octopus-induced 

 mortality may have on lobster fisheries. 



In this study, we examined the interaction between 

 O. maorum and J. edwardsii in the South Australian 

 rock lobster fishery (SARLF). The objectives were 1) to 

 determine the number of lobsters and octopus caught 

 and the number of lobsters killed in traps each year in 

 the fishery; 2) to describe the interannual and seasonal 

 patterns in lobster catch rate (CPUE L ), octopus catch 

 rate (CPUE ), and lobster mortality rate (M L ); 3) to 

 examine some factors that may affect lobster mortal- 

 ity rates; 4) to estimate what proportion of the lobster 

 mortality is attributable to octopus predation; and 5) to 

 determine whether the rate of lobster mortality through 

 octopus predation in traps is size dependent. 



Materials and methods 



South Australian rock lobster fishery 



The SARLF is divided into a northern zone (NZ) and 

 a southern zone (SZ), each of which is further divided 

 into marine fishing areas (MFAs) for statistical purposes 

 (Fig.l). There are 68 and 183 fishermen licensed to oper- 



ate in the NZ and SZ respectively. The fishing season 

 extends from November to May in the NZ and October 

 to April in the SZ. A quota management system was 

 introduced in the SZ in 1993, whereas the NZ is man- 

 aged by gear restrictions and temporal closures. 



Total annual catch and effort for the SARLF 



Catch and effort data are recorded on a daily basis by all 

 individual fishermen. Since 1983. a standardized logbook 

 for recording catch and effort has been used across the 

 fishery. Data provided by fishermen include MFA fished, 

 average depth fished, number of trap-lifts, number and 

 total weight of live lobsters, number of dead lobsters, and 

 number and total weight of octopus. This information 

 is stored in a South Australian rock lobster database 

 that is managed by the South Australian Research and 

 Development Institute, Aquatic Sciences. 



Interannual and seasonal patterns 

 in CPUE L , CPUE Q , and M L 



Although commercial fishing for lobsters occurs along 

 most of the South Australian coastline, the majority of 

 effort is concentrated in only a few MFAs. In the NZ 

 over the last 5 years about 72% of total trap-lifts were 

 made in MFAs 15, 28, 39, 40, and 49. In the SZ over the 

 same period 95% of trap-lifts were made in MFAs 51, 

 55, 56, and 58 (Fig. 1). 



