FACTORS AFFECTING THE GROWTH OF UNDERSIZE WESTERN 



ROCK LOBSTER, PANULIRUS CYGNUS GEORGE, 



RETURNED BY FISHERMEN TO THE SEA 



R. S. Brown and N. Caputi^ 



ABSTRACT 



The Western Australian fishery for the western rock lobster, Panulirus cygnus, yielded about 12,400 t, 

 valued at $A1G0 million, in 1982-83. It is the largest single species fishery in Australia and one of the 

 largest rock lobster fisheries in the world. 



During a season, between 16 and 20 million undersize rock lobsters are brought aboard the vessels 

 by normal fishing operations, despite the escape gaps in all professional and amateur pots. All undersize 

 animals must be returned by fishermen to the sea, but to accomplish this it took from a few minutes 

 to hours, depending on the sorting technique used. The negative effects of handling on the survival of 

 the undersize lobsters have been previously reported, but another important aspect is the effect of handling 

 (damage, exposure, and displacement) on the growth rate of returned undersize rock lobsters. 



Two laboratory experiments showed that growth increment at the first molt after air exposure was 

 significantly reduced, and in one of the experiments it was also significantly reduced for the second molt 

 after exposure 



Three field tagging trials were conducted with 6,700 undersize rock lobsters. One trial showed that 

 exposure had a significant detrimental effect; the other trial in which exposure was tested, there was 

 a negative, but not significant, trend. Damage (number of appendages lost) and displacement from the 

 home range significantly reduced the growth increment in each of the three tagging trials. The growth 

 increment of damaged animals was inversely proportional to the number of appendages lost by the animal 

 with sizes ranging from 0.33 to 0.48 mm smaller per appendage missing. The losses to the fishery and 

 other associated problems caused by the reduced growth of the undersize lobsters are discussed. 



The fully exploited stocks of western rock lobster, 

 Panulirus cygnus George, support the largest single 

 species fishery in Australia and one of the largest 

 rock lobster fisheries in the world, averaging 10,000 

 t for the last 9 yr to 1982-83. In 1982-83 the fishery 

 recorded its best season; the 12,400 t worth about 

 $A100 million, were landed by 780 boats licensed to 

 participate in the industry. The amateur catch from 

 the fishery, estimated at 1.6% of the professional 

 catch (Norton 1981), is considered to be a relatively 

 insignificant component of the total fishery, though 

 it may be locally important and occurs in shallow 

 areas where large numbers of undersize rock 

 lobsters (i.a, those with a carapace length <76 mm 

 and referred to below as undersize) are caught. 



Fishing pressure on the rock lobster stock has 

 been increasing steadily during the past 20 yr, even 

 though it has been a Hmited entry fishery since 1963 

 (Morgan 1980a, b; Hancock 1981). This has led to 

 the need for constant monitoring of the fishery and 

 the updating of management procedures and regula- 



Western Australian Marine Research Laboratories, Department 

 of Fisheries and Wildlife, P.O. Box 20, North Beach 6020, Western 

 Australia. 



tions to ensure the stability and viability of the rock 

 lobster stock (Bowen 1980; Morgan 1980a, b; Han- 

 cock 1981; Morgan et al. 1982). An important com- 

 ponent of management of a fully exploited stock is 

 to reduce waste, ag., by predation and poor handling 

 techniques. 



Two of the most important regulations that aid in 

 conservation of the western rock lobster stock per- 

 tain to undersize: 



1) Compulsory use of a 54 x 305 mm escape gap 

 in each of the 76,000 professional and all amateur 

 pots (traps) in the fishery allows many undersize 

 animals to escape before the pot is pulled (Bowen 

 1963). 



2) Undersize lobsters that do not escape and are 

 brought aboard must be returned to the sea. 



Although use of escape gaps reduces the retention 

 of undersize lobsters by over 50% (Bowen 1963; 

 Brown unpubl. data), between 16 and 20 million 

 undersize animals are still handled each season by 

 professional fishermen (Brown and Caputi 1983). 

 The latter authors found that the handling practices 

 of fishermen, which cause exposure, damage 



Manuscript accepted December 1984. 

 FISHERY BULLETIN: VOL. 83, NO. 4, 1985. 



567 - 



