Hazell et al.: Somatic growth rate of Jasus lalandii 



517 



and intermolt period. Amongst adults, this variability 

 has been attributed to patterns of food availability and 

 gradients in environmental characteristics, particularly 

 oxygen content (Newman and Pollocii, 1974; Pollock and 

 Beyers, 1981; Pollock, 1982; Pollock and Shannon, 1987; 

 Pollock et al., 1997; Mayfield, 1998). Although there is 

 some spatial separation of juvenile and adult J. lalandii, 

 it is likely that the biotic structure of their environment 

 influences juvenile growth rates in a manner similar to 

 that of adults. 



In a study of the diets of juvenile lobsters at the Cape 

 Town harbor wall and Mouille Point, Mayfield (1998) 

 found that diets were similar at the two sites, but that 

 the benthic communities differed. He concluded that 

 lobsters on the harbor wall would have to spend more 

 time and energy feeding to maintain a diet similar to 

 those at Mouille Point, where more favored food species 

 (e.g. mussels, barnacles) were better represented. Similar 

 patterns were evident when comparing diets of adults 

 at Olifantsbos and Dassen Island (Mayfield, 1998), a 

 slow- and fast-growth site, respectively (Cockcroft and 

 Goosen, 1995). Subsequently, a more detailed study of 

 the diets of two size classes (<35 mm CL and 40-60 mm 

 CD of juvenile J. lalandii at the Cape Town harbor wall 

 and at Mouille Point was able to detect a significant 

 difference between the diets of inhabitants of the two 

 sites (Mayfield et al., 2000). Juveniles on the harbor wall 

 consumed significantly less black mussel, their preferred 

 prey species. This result, however, does not contradict the 

 suggested link between food distribution and juvenile 

 growth rates; specimens forced to subsist off suboptimal 

 food resources could be expected to grow more slowly than 

 those at liberty to pursue their preferred prey. 



The differences in growth rates between juvenile J. 

 lalandii at the artificial harbor wall and those at the 

 natural nursery reef at Mouille Point do not necessarily 

 preclude the use of the harbor wall as a legitimate site 

 to continue monitoring juvenile growth. The existence 

 of historical data (albeit for only two seasons) provides 

 a good reference point, and the easy and consistent 

 sampling conditions permit reliable data collection. 

 These sampling conditions are of particular importance, 

 given the established tendencies of early juveniles of 

 other species of rock lobster on natural reefs to be less 

 vulnerable to sampling than later stages (Annala and 

 Bycroft, 1985; Marx and Herrnkind, 1985; Breen and 

 Booth, 1989; Jernakoff, 1990; Childress and Herrnkind, 

 1994; Forcucci et al., 1994; Jernakoff et al., 1994). 

 Furthermore, knowledge of how growth rates change 

 over time is in itself important to the management of the 

 fishery (Cockcroft, 1997). 



Should spatial and temporal variability in juvenile 

 growth continue to be a contentious issue, a juvenile tag- 

 ging program would provide simpler and more rigorous 

 comparisons. It might also provide reliable estimates of 

 the intermolt period. Mitigating against this, however, are 

 the low rate of recapture of tagged specimens in the wild 

 and potential tag-related growth retardation and mortal- 

 ity. Nevertheless, continued monitoring of juvenile growth 

 may elucidate how close this link is between adult and 



juvenile growth, thereby improving our ability to manage 

 this valuable resource. 



Acknowledgments 



Numerous people assisted on sampling trips and in mea- 

 suring molt increments in the aquarium, but especially 

 Steven McCue, Marico Vercuiel, Eric Simpson, and Danie 

 van Zyl. UCT Zoology Department provided us with the 

 aquarium space that allowed us to collect molt increment 

 data. Many thanks also go to Dave Pollock, Andy Cock- 

 croft, and Retha Hofmeyer, who provided useful comments 

 in finalizing this manuscript. 



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