440 



Fishery Bulletin 102(3) 



economic driving force for the fishery, the lobster catch 

 rates, and because the catch rates of octopus are dif- 

 ficult to predict. In addition, O. maorum is a solitary 

 animal that tends to be dispersed randomly throughout 

 areas of suitable habitat (Mather et al., 1985). 



The higher total catches and catch rates of both lob- 

 ster and octopus in the SZ, compared to the NZ, prob- 

 ably reflect the more extensive reef habitat and more 

 intense nutrient-enrichment upwelling in this portion 

 of the SARLF (Lewis, 1981). There have been large 

 interannual fluctuations in CPUE in both zones since 

 1983. Such fluctuations in population size are common 

 among other cephalopods, especially squid, and may 

 result from life history strategies that are characterized 

 by rapid growth, short lifespan (<two years) and almost 

 universal mortality after a single spawning event (Boyle 

 and Boletsky, 1996). Despite these fluctuations, CPUE 

 has not declined noticeably in any MFA since 1983, 

 which suggests that octopus mortality from fishing is 

 consistent with little impact on octopus populations 

 since the advent of fishing. This observation and the 

 poor relationship between octopus catches and effort re- 

 fute the belief of some SARLF fishermen that incidental 

 fishing mortality acts to control octopus abundance. 



This study, however, did confirm the view of fisher- 

 men that larger lobsters are killed more commonly by 

 octopus than are smaller ones. This effect was most 

 evident for male lobsters, which grow to larger sizes 

 than females. There could be several reasons for the 

 size-dependent mortality rates for rock lobsters. For 

 example, octopus could actively select larger prey, or 

 large lobsters could be captured more easily than small 

 lobsters in traps by octopus. Because large lobsters can 

 be worth more and produce more eggs than smaller 

 lobsters, the increased mortality rates of large lobsters 

 suggest that the total economic and ecological impacts 

 of octopus predation in the SARLF are greater than 

 indicated by the absolute number of lobsters killed. 



Octopus predation of lobsters in traps is a significant 

 problem in the SARLF. However, the economic effects 

 vary between the zones. In the quota-managed SZ, ad- 

 ditional lobsters are harvested to replace those killed 

 in traps, which increases the time and costs of catch 

 quotas, and imposes an impact on lobster abundance. 

 In the input-controlled NZ, where there is no direct 

 restriction on the quantity of lobsters taken, lobsters 

 killed in traps represent both a direct economic loss 

 and an impact on lobster abundance. 



Like most fisheries for spiny lobsters, the SARLF is 

 close to being fully exploited. Reducing rates of octopus 

 predation provides one option available for increas- 

 ing the value of the fishery. Some minor reductions in 

 lobster mortality may be achieved by minimizing soak- 

 times, especially in the SZ. More significant reductions 

 in the rates of within-trap lobster mortality may be 

 achieved by redesigning lobster traps (Brock et al. 4 ). 



Acknowledgments 



This research was funded jointly by the Fisheries 

 Research and Development Corporation, South Austra- 

 lian Rock Lobster Advisory Council, South Australian 

 Research and Development Institute (Aquatic Sciences), 

 and the University of Adelaide. The authors thank Thor 

 Saunders for his support and assistance during the 

 research. Jim Prescott provided advice for extracting 

 data from the South Australian Rock Lobster Database. 

 Yongshun Xiao provided statistical advice and assisted 

 with the numerical modeling. 



Literature cited 



Boyle, R. R, and S. V. Boletsky. 



1996. Cephalopod populations: definition and dynam- 

 ics. Phil. Trans. R. Soc. Lond. 351:985-1002. 

 Defeo, O., and J. C. Castilla. 



1998. Harvesting and economic patterns in the artisanal 

 Octopus mimus (Cephalopodal fishery in a northern 

 Chile cove. Fish. Res. 38:121-130. 

 Hernandez-Garcia, V., J. L. Hernandez-Lopez, and J. J. Castro. 

 1998. The octopus lOctopus vulgaris) in the small-scale 

 trap fishery off the Canary Islands (Central-East 

 Atlantic). Fish. Res. 35:183-189. 

 Lewis, R. K. 



1981. Seasonal upwelling along the south-eastern coast- 

 line of South Australia. Aust. J. Mar. Freshw. Res. 

 32:843-854. 

 Mather. J. A., S. Resler, and J. Cosgrove. 



1985. Activity and movement patterns of Octopus 

 dofleini. Mar. Behav. Physiol. 11:301-314. 



McGarvey, R., G. Ferguson, and J. H. Prescott. 



1999a. Spatial variation in mean growth rates of rock lob- 

 ster, Jasus edwardsii, in South Australian waters. Mar. 

 Freshw. Res. 50:333-342. 

 Phillips. B. F., J. S. Cobb, and J. Kittaka. 



1994. Spiny lobster management, 550 p. Blackwell 

 Scientific, London. 

 Quetglas, A., F. Alemany, A. Carbonell, P. Merella, and 

 P. Sanchez. 



1998. Biology and fishery of Octopus vulgaris Cuvier, 

 1797, caught by trawlers in Mallorca (Balearic Sea, 

 Western Mediterranean). Fish. Res. 36:237-249. 



Richards. L. J., and L. J. Schnute. 



1986. An experimental and statistical approach to the 

 question: Is CPUE an index of abundance? Can. J. 

 Fish. Aquat. Sci. 43:1214-1227. 



Rose, G. A., and D. W. Kulka. 



1999. Hyperaggregation offish and fisheries: how catch- 

 per-unit-effort increased as the northern cod iGadus 

 morhua) declined. Can. J. Fish. Aquat. Sci. 56(suppl. 

 11:118-127. 



Williams. A. B. 



1988. Lobsters of the world — an illustrated guide, 186 

 p. Osprey Books. New York, NY. 



