142 



Fishery Bulletin 90(1). 1992 



that of export markets resulted 

 in a rapid increase in price of 

 abalone during the 1980s. 



Exploitation models 



The yield and egg-production- 

 per-recruit for individuals of vari- 

 ous lengths under various levels 

 of exploitation are shown for 

 fast-growing (Fig. 2) and slow- 

 growing (Fig. 3) populations of 

 H. rubra. It can be seen that the 

 relative yield-per-recruit is great- 

 er for fast-growing compared 

 with slow-growing populations of 

 H. rubra. The minimum lengths 

 producing maximum yield-per- 

 recruit are 130 mm for fast-grow- 

 ing populations and 120 mm for 

 slow-growing populations. These 

 maxima occurred at high exploi- 

 tation rates (F~l) and were in- 

 dependent of the natural mortal- 

 ity rates applied to the model 

 (0.1, 0.2). Natural mortality had 

 an obvious effect on the decline 

 of yield-per-recruit with mini- 

 mum length. For M = 0.2, yield- 

 per-recruit was less sensitive to 

 variation in minimum length 

 compared with M = 0.1. Note that 

 at realistic levels of F (0.1, 

 McShane and Smith 1989a), 

 yield-per-recruit is comparative- 

 ly low (Figs. 2 and 3). For such 

 low rates of exploitation, the 

 model shows that for fast-grow- 

 ing (in contrast to slow-growing) 

 populations, yield-per-recruit is relatively insensitive 

 to variation in the minimum length-at-capture. Similar 

 results were obtained from yield-per-recruit analyses 

 of other species of abalone, provided that rates of 

 fishing mortality are relatively low (F<0.3) (Ishibashi 

 and Kojima 1979, Sainsbury 1982a, Sluczanowski 1984, 

 Breen 1986, Sloan and Breen 1988, Tegner et al. 1989, 

 Nash 1992). 



For fast-growing populations, exploitation rates pro- 

 ducing maximum yield-per-recruit are associated with 

 minimum egg production. Indeed, values of F>0. 3 are 

 associated with egg production of less than 50% of an 

 unfished population. Low egg production may cause 

 recruitment failure in abalone stocks (Sloan and Breen 

 1988, Tegner et al. 1989). Egg production increases 

 with minimum length; results of other studies show 



Figure 2 



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

 fast-growing population oiHaliotis rubra subject to natural mortality (M) of 0.1 and 0.2. 



that fecundity ofH. rubra is directly related to length 

 (McShane et al. 1988b, Prince et al. 1988). 



Egg production is less sensitive to variation in fishing 

 mortality in slow-growing than in fast-growing popula- 

 tions oiH. rubra. For minimum lengths over 120mm, 

 egg production rates are over 50% of an unfished 

 population. At F = 0.1, egg production is over 60% of 

 that of an unfished population for both M = 0. 1 and 0.2. 



Discussion 



Catch levels for the Victorian abalone fishery suggest 

 a robust fishery. But catch data are poor indicators of 

 the stock abundance of abalone because fishermen can 

 maintain catch rates by exploiting substocks (Breen 



