McShane: Exploitation models and catch statistics for Hahotis rubra off Victoria. Australia 



143 



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Figure 3 



Variation in egg and yield-per-recruit with shell length and fishing mortality (F) for a 

 slow-growing population ofHaliotis 'rubra subject to natural mortality (M) of 0.1 and 0.2. 



1980, Sloan and Breen 1988, McShane and Smith 

 1989a). Yet the available evidence is that the fishery 

 is underexploited (McShane and Smith 1989ab; 

 McShane 1990). Catches ofH. rubra can be adjusted 

 opportunistically by increasing effort when incentive 

 (price) is high. Although Victorian abalone fishermen 

 have the capacity to serially deplete substocks, rates 

 of exploitation of H. rubra are generally low (see 

 McShane and Smith 1989a). A surplus of harvestable 

 individuals is maintained in substocks by the conser- 

 vative fishing practices employed by Victorian abalone 

 fishermen (McShane and Smith 1989a). With low ex- 

 ploitation rates (F<0.3), the egg-per-recruit model 

 shows that there is adequate egg production by in- 

 dividuals above the present legal minimum length of 

 120 mm. However, as a consequence of reaching har- 

 vestable size in about 4 years, prerecruit individuals 

 from fast-growing populations have fewer years of egg 



production than those H. rubra 

 from slow-growing populations 

 which reach harvestable size in 

 about 10 years. Fast-growing 

 populations of H. rubra are 

 therefore vulnerable to recruit- 

 ment overfishing should exploita- 

 tion rates increase (F>0.3). This 

 is unlikely in the Victorian aba- 

 lone fishery because both the 

 number of operators and the an- 

 nual catch are controlled. 



Most abalone fisheries are gen- 

 erally subject to pulse fishing. 

 Substocks are fished, then left to 

 recover (Sluczanowski 1984, 

 McShane and Smith 1989a). 

 Fast-growing populations are 

 important in this regard because 

 they can be fished at a higher fre- 

 quency than slow-growing popu- 

 lations (Sluczanowski 1984). 

 Thus fast-growang populations 

 are subject to higher exploitation 

 rates than slow-growing popula- 

 tions. Slow-growing populations 

 are often characterised by large 

 accumulations of prerecruit aba- 

 lone which are food-limited 

 (Sloan and Breen 1988, McShane 

 1990). Egg-per-recruit analysis 

 shows that even at extraordinar- 

 ily high rates of fishing mortal- 

 ity (F> 1) egg production in slow- 

 growing populations of H. rubra 

 is above the assumed "safe" level 

 of 50% of an imfished population 

 (Sloan and Breen 1988, Tegner et al. 1989). The model 

 shows that slow-growing populations could be "fished 

 down" at a reduced size limit and high exploitation rate 

 without endangering egg production. Such a harvesting 

 strategy could reduce the abundance of the accum- 

 ulated stock to a level where food is no longer a limiting 

 factor (McShane and Smith 1989b). To date in Victoria, 

 such slow-growing stocks are rarely fished because of 

 a paucity of abalone of harvestable size (McShane and 

 Smith 1989b). 



Why is the Victorian fishery for H. rubra apparent- 

 ly robust in contrast to other abalone fisheries? The 

 viability of the Victorian fishery can be attributed to 

 a relatively low number of operators (limited entry has 

 operated in Victoria since 1968) and an associated low 

 exploitation rate (see McShane and Smith 1989a). A 

 minimum length that maintains a safe level of egg pro- 

 duction provides further safeguards against recruit- 



