Worthmgton et al.: Alternative size limits for Haliotis rubra In New South Wales. Australia 



559 



not require any variation in the rate 

 of incorporation of calcium-carbon- 

 ate, which may continue regardless 

 of the rate of synthesis of the organic 

 matrix (see similar arguments for 

 fish otoliths in Gauldie and Radtke, 

 1990). Because calcium-carbonate 

 dominates a shell by weight (Palmer, 

 1992), if the rate of incorporation of 

 calcium-carbonate was similar among 

 different populations, the total 

 weight of the shell might be a reli- 

 able indicator of the age of the aba- 

 lone (see the literature on use of the 

 weight of otoliths to age fish, e.g. 

 Worthington et al., in press). The 

 ability to age abalone is obviously 

 desirable, but much doubt exists 

 about present techniques of ageing, 

 particularly for Haliotis rubra (Mc- 

 Shane and Smith, 1992). This uncer- 

 tainty emphasizes the need to inves- 

 tigate alternative methods to age 

 abalone, one of which may be shell 

 weight (Worthington et al., unpubl. 

 data). 



Potential for alternative size 

 limits 



The present 115-mm minimum legal 

 length limit is enforced along the 

 entire coast of NSW for both the com- 

 mercial and recreational fisheries. 

 This size limit was chosen by consid- 

 ering average rates of growth, mor- 

 tality, and reproduction in an attempt 

 to maximize yield from the entire stock. The intense 

 spatial variation in growth that we have described 

 creates several problems which combine to restrict 

 the effectiveness of the length limit. For example, 

 because the 115-mm length limit was chosen by con- 

 sidering average rates of growth, it is less appropri- 

 ate for sites where abalone grow faster or slower than 

 average. At sites where they grow quickly, abalone 

 rapidly reach the minimum legal length and may be 

 harvested before contributing significantly to levels 

 of egg production. Because of the limited dispersal 

 of larval abalone (Prince et al., 1987), such sites have 

 a limited capability to recover after fishing and hence 

 may easily become overexploited. In contrast, at sites 

 where they grow slowly, abalone take a longer time 

 to reach the minimum legal length limit, and few 

 are removed by divers. Consequently, rates of egg 

 production may be high but they do not contribute 



significantly to other populations because of the re- 

 stricted dispersal of larvae (see also Tegner, 1993). 

 In addition, because of the link between growth rate 

 and the maximum length of abalone at a site, as the 

 maximum length of abalone at the site is below the 

 minimum length limit, many populations can never 

 be fished. Different length limits related to the 

 growth of abalone at a site would be desirable, but 

 considering the small spatial scales over which sig- 

 nificant differences in growth can occur, their enforce- 

 ment would be impractical. 



An alternative approach to applying different 

 length limits would be to enforce a size limit based 

 on shell width. Because of the differences in mor- 

 phology of abalone among sites with different growth 

 rates, a width limit would allow individuals from sites 

 where they grow slowly to be removed at shorter 

 lengths than those where they grow quickly. This 



