430 



Abstract— Octopuses are commonly 

 taken as bycatch in many trap fisher- 

 ies for spiny lobsters I Decapoda: Pal- 

 muridae) and can cause significant 

 levels of within-trap lobster mortality. 

 This article describes spatiotempo- 

 ral patterns for Maori octopus i Octo- 

 pus maorum > catch rates and rock 

 lobster (Jasus edwardsii) mortality 

 rates and examines factors that are 

 associated with within-trap lobster 

 mortality in the South Australian 

 rock lobster fishery (SARLF). Since 

 1983, between 38,000 and 119,000 

 octopuses per annum have been 

 taken in SARLF traps. Catch rates 

 have fluctuated between 2.2 and 6.2 

 octopus/100 trap-lifts each day. There 

 is no evidence to suggest that catch 

 rates have declined or that this level 

 of bycatch is unsustainable. Over the 

 last five years, approximately 240,000 

 lobsters per annum have been killed 

 in traps, representing ~4% of the total 

 catch. Field studies show that over 

 98% of within-trap lobster mortal- 

 ity is attributable to octopus pre- 

 dation. Lobster mortality rates are 

 positively correlated with the catch 

 rates of octopus. The highest octo- 

 pus catch rates and lobster mortality 

 rates are recorded during summer 

 and in the more productive southern 

 zone of the fishery. In the southern 

 zone, within-trap lobster mortality 

 rates have increased in recent years, 

 apparently in response to the increase 

 in the number of lobsters in traps 

 and the resultant increase in the 

 probability of octopus encountering 

 traps containing one or more lobsters. 

 Lobster mortality rates are also posi- 

 tively correlated with soak-times in 

 the southern zone fishery and with 

 lobster size. Minimizing trap soak- 

 times is one method currently avail- 

 able for reducing lobster mortality 

 rates. More significant reductions in 

 the rates of within-trap lobster mor- 

 tality may require a change in the 

 design of lobster traps. 



Maori octopus (Octopus maorum) bycatch and 

 southern rock lobster (Jasus edwardsii) mortality 

 in the South Australian rock lobster fishery 



Daniel J. Brock 



South Australian Research and Development Institute (Aquatic Sciences) 



2 Hamra Ave. 



West Beach, South Australia 5024, Australia 



Present address: Department of Soil and Water 



Adelaide University 



Adelaide, South Australia 5005, Australia 

 E-mail address: Brock. Daniel a 1 saugov.sa.gov au 



Timothy M. Ward 



South Australian Research and Development Institute (Aquatic Sciences) 



2 Hamra Ave. 



West Beach, South Australia 5024, Australia 



Manuscript submitted 28 April 2003 

 to Scientific Editor's Office. 



Manuscript approved for publication 

 2 March 2004 by the Scientific Editor. 



Fish. Bull. 102:430-440 (2004). 



Fishing traps are used throughout the 

 world to target a wide range of crusta- 

 ceans, fishes, and cephalopods. Com- 

 mercial trap fisheries, especially those 

 for decapod crustaceans, are often the 

 most valuable fisheries within a region 

 (Phillips et al., 1994). Traps are gen- 

 erally considered to be an efficient 

 and benign form of fishing because 

 they appear to cause relatively minor 

 damage to benthic habitats, can be 

 designed to target particular species 

 and size ranges, and produce live 

 catches in good condition while mini- 

 mizing bycatch (Miller, 1990). 



There are 49 species of spiny lob- 

 sters (Decapoda: Palinuridae) world- 

 wide, 33 of which support commercial 

 trap fisheries. The largest of these 

 are in Cuba, South Africa, Mexico, 

 Australia, and New Zealand (Wil- 

 liams, 1988). The main trap fisher- 

 ies in Australia are for western rock 

 lobster {Panulirus cygnus) in Western 

 Australia and southern rock lobster 

 (Jasus edwardsii) along the southern 

 coastline. Octopuses constitute a sig- 

 nificant component of the bycatch in 

 both fisheries (Joll 1 ; Knight et al. 2 ) . 



In South Australia, J. edwardsii 

 supports the State's most valuable 

 commercial fishery. Octopus maorum 

 is a significant bycatch species and 

 is thought to be the major cause of 

 lobster mortality in traps (Prescott 

 et al. 3 ). 



Although the octopus bycatch of the 

 South Australian rock lobster fishery 

 (SARLF) is saleable, the commercial 

 value of this product does not offset 

 the value of the large number of lob- 

 sters that are killed in traps by octo- 

 pus. Many fishermen are convinced 

 that incidental mortality of octopus 

 resulting from lobster fishing acts 

 to control octopus numbers and that 

 if these rates were reduced, octopus 

 abundance and levels of within trap 

 predation would increase. 



Despite the prevalence of octopus 

 bycatch in lobster fisheries, there 

 have been only a few studies on the 

 interaction between octopus and lob- 



1 Joll, L. 1977. The predation of trap- 

 caught western rock lobster {Panulirus 

 Longipes cygnus) by octopus. Depart- 

 ment of Fisheries and Wildlife, Western 

 Australia, Report 29, 58 p. (Available 

 from Department of Fisheries, 168-170 

 St George's Terrace. Perth, Western Aus- 

 tralia, 6000.] 



- Knight, M. A., A. Tsolos, and A. M. 

 Doonan. 2000. South Australian 

 fisheries and aquaculture information 

 and statistics report. Research Report 

 Series 49, 69 p. [Available from SARD] 

 Aquatic Science, 2 Hamra Avenue, West 

 Beach, South Australia 5022.] 



:! Prescott, J.. R. McGarvey, Y. Xiao, and D. 

 Casement. 1999. Rock lobster. South 

 Australian Fisheries Assessment Series 

 99/04, 35 p. [Available from SARDI 

 Aquatic Science, 2 Hamra Avenue, West 

 Beach, South Australia 5022.1 



