432 



Fishery Bulletin 102(3) 



Data from the database were used to calculate catch 

 rates of lobsters (CPUE L ), octopus (CPUE ), and M L on 

 an annual and monthly basis for the nine major MFAs 

 listed above. Catch rates from these MFAs for each 

 fisherman were calculated according to the formula: 

 catch rate = catch number/itrap-liftslday). Annual and 

 seasonal trends in CPUEj , CPUE , and M L were cal- 

 culated for each zone and MFA. 



Factors that affect within-trap lobster mortality 



Potential factors that affect within-trap lobster mortality 

 were analysed by using a general linear model (type-3 

 sums of squares) under the assumption that the number 

 of dead lobsters follows a log-normal distribution. 



The number of dead lobsters/trap-lift/day/license 

 (with a ln+1 transformation) was used as the measure 

 of lobster mortality. A model of the following structure 

 was used to examine factors that affect the numbers 

 of dead lobster: 



Dead lobster = License + MFA + Month + Year 



+ Effort + Depth + Octopus + Lobster catch 



+ Soak-time + I License xYear) + I License xMonth) 



+ (YearxMonth) + (YearxMFA) + iSoak-timexYear) 



+ < Soak-time xMonth). 



In the model. License represents an individual fisher- 

 man, MFA is the marine fishing area. Month accounts 

 for seasonal variation and Year accounts for interannual 

 variation. Effort is the number of trap-lifts/license each 

 day, Depth is the average depth fished by each License 

 on a particular day. Octopus and Lobster are the respec- 

 tive daily catches/license, and Soak-time is the number 

 of days that the traps remained in the water since the 

 previous trap-lift. 



The interaction terms License xYear and License x 

 Month account for variations in the catch characteris- 

 tics of the individual licenses over time that result from 

 changes in fishing practises and efficiency associated 

 with different boats, license holders, and skippers. The 

 interaction terms YearxMonth and YearxMFA account 

 for variation in the population dynamics of octopus and 

 lobster over time in different locations that could result 

 in differential trends in lobster mortality. The inter- 

 action terms Soak-time xYear and Soak-time xMonth 

 reflects the change in general fishing strategies over 

 time. In quota-managed fisheries the average soak-time 

 will be affected by a number of factors, for example, 

 that may include price, weather, and the fishermen's 

 perceived ability to catch their quota. 



The analysis was run separately for the SZ (/? = 493,629 

 traps) and NZ (ra=155,628 traps) because the respective 

 zones have different fishing seasons and management 

 structures. The relationship between the number of 

 dead lobsters and the factors depth, soak-time, and num- 

 ber of octopuses and lobsters were presented graphically 

 by the equation: 



Lobsters killed in traps <*. factor a , 



where a = the parameter estimated by use of the model. 



Source of lobster mortality and size-dependent mortality 



A sampling program was conducted on three commer- 

 cial vessels from the SZ during the 2001-02 fishing 

 season. Five days were spent on each vessel. All lobsters 

 caught were measured (carapace length, mm), and the 

 sex (male or female), maturity (mature or immature), 

 status (dead or alive), and cause of death (octopus or 

 other) were recorded. 



The method used to distinguish between lobsters 

 killed by octopus or other means followed that of Joll. 1 

 This suitability of this approach was confirmed through 

 examination of the carcasses of over one hundred lob- 

 sters killed by octopus in aquarium trials (Brock et 

 al. 4 ). Lobsters with shells that were partly or completely 

 separated at the juncture between abdomen and cepha- 

 lothorax but were otherwise undamaged were deemed 

 to have been killed by octopuses, whereas lobsters with 

 shells without this separation and with evidence of bite 

 marks were deemed to have been eaten by other preda- 

 tors (fish or cuttlefish). 



Anecdotal evidence from fishermen suggests that 

 larger lobsters are more susceptible to predation than 

 smaller ones. The effect of CL on the probability of 

 mortality was examined separately for males and fe- 

 males by generalized linear modeling. The probability 

 of mortality at a given size was modeled with a logistic 

 equation of the form: 



P{sex, CL) = l/(l+e- ,a+6CL, ), 



where P(sex, CL) = the probability of a lobster of a 

 given sex at carapace length CL 

 being dead; and 

 a and b are parameters to be estimated. 



Results 



Estimation of total lobster catch, octopus bycatch, 

 and lobster mortality 



In 1999, there were 1.6 million trap-lifts in the SARLF, 

 and 70% of this total effort was in the SZ (Fig. 2). The 

 number of traps-lifts in the SZ declined from 2.2 mil- 

 lion in 1983 to 1.2 million in 1999 (Fig. 2Ai. In contrast, 

 fishing effort in the NZ remained relatively consistent 

 with 406,000 trap-lifts in 1983 and 480, 000 trap-lifts 

 in 1999 (Fig 2B). 



The total annual lobster catch has generally increased 

 in each fishing zone since 1983 (Fig. 2, A and B). 



' Brock, D. J., T. M. Saunders, and T. M. Ward. In review. A 

 two-chambered trap with potential for reducing within-trap 

 predation by octopus on rock lobster. Can. J. Fish. Aquat. 

 Sci., 19 p. lAvailable from SARDI Aquatic Science, 2 Hamra 

 Avenue. West Beach, South Australia 5022.1 



