Brock and Ward: Octopus bycatch and lobster mortality in the South Australian rock lobster fishery 433 



A 



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5 



4 



3 



2 



1 







Southern zone 

 I lobster —  — trap lifts 



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T 2 ' 5 



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1 



05 







B 



7 



6 



5 + 



4 



3 



2 



1 







Northern zone 

 I lobster —  — trap lifts 



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1983 



1987 



1991 



1995 



1999 



1983 



1987 



1991 



1995 



1999 



1983 



1987 



1991 



1995 



1999 



1983 



1987 



1991 



1995 



1999 



Season 



Figure 2 



Annua] total catch and effort for each zone of the South Australian rock lobster fishery for number of 

 live lobsters caught (A l, number of trap-lifts used (B). number of dead lobsters caught (C), and number 

 of octopuses caught (D) (Note: change of scale in D). 



In the SZ, the annual lobster catch rose from 3.8 mil- 

 lion lobsters to a peak of 6.4 million lobsters in 1991 

 and was 5.4 million lobsters in 1999 (Fig. 2A). In the 

 NZ, 560,000 lobsters were taken in 1983 compared to 

 850,000 in 1991 (Fig. 2B). 



The total annual octopus catch varied among years in 

 both zones, but between 70% and 907c of the total octo- 

 pus catch were landed in the SZ (Fig. 2). The total num- 

 ber of octopus ranged from 36,000 in 1986 to 109,000 

 in 1992 (Fig. 2C) in the SZ, and from 4700 octopuses in 

 1985 to 11,200 in 1998 in the NZ (Fig. 2D). 



In 1999, over 226,000 lobsters were killed in traps in 

 the SARLF (Fig. 2). Since 1983, the mean proportion of 

 dead lobsters out of the total catch has been approxi- 

 mately 4%. In the SZ, the number of lobsters killed in 

 traps has generally increased from 118,000 in 1983 to 

 196,000 in 1999; a peak of 274,000 dead lobsters oc- 

 curred in 1992 (Fig. 2C). In the NZ, there has also been 

 a general increase in the number of lobster killed in 

 traps each year; 24,000 dead lobsters were recorded in 

 1983, compared to 31,000 in 1999 and a peak of 39,000 

 dead lobsters recorded in 1998 (Fig. 2D). 



Interannual and seasonal patterns in 

 CPUE L CPUE ,and M L 



Southern zone Mean annual CPUE L in the SZ increased 

 from 175 to 466 lobsters/I 100 trap-lifts/day) between 

 1983 and 1999, and the largest increase occurred 

 between 1997 and 1999 (Fig. 3A). Mean annual CPUE 

 ranged from 1.8 to 6.2 octopus/dOO trap-lifts/day) in 

 1987 and 1992, respectively (Fig. 3C). Mean annual 

 M L rose from 5 to 17 dead lobster/(100 trap-lifts/day) 

 between 1983 and 1999 (Fig. 3E). Peaks in both CPUE 

 and M L occurred in 1985, 1992, and 1995. 



Mean monthly CPUE L declined during the fishing sea- 

 son from 310 to 164 lobster/(100 trap-lifts/day) between 

 October and April (Fig. 4A). In contrast, mean monthly 

 CPUE increased from 2.6 to 3.7 octopus/dOO trap-lifts/ 

 day) between October and December and declined to 1.8 

 octopus/dOO trap-lifts/day) in April (Fig. 4C). Similarly 

 mean monthly M L increased from 10.7 to 12.8 dead lob- 

 ster/(100 trap-lifts/day) between October and November 

 and declined to 6.7 dead lobster/) 100 trap-lifts/day) in 

 April (Fig. 4E). 



