McShane: Exploitation models and catch statistics for Haliotis rubra off Victoria, Australia 



141 



Table 2 



Parameters used in computations of yield and egg-per-recruit. 

 Values for slow- and fast-growing populations of Haliotis 

 rubra are derived from mark-recapture studies in Victoria, 

 Australia (McShane 1990). Estimates provided are the von 

 Bertalanffy growth parameters (see text for details). 



(McShane et al. 1988a). Egg production under various 

 combinations of F, M, and minimum length-at-capture 

 was compared with egg production of an unfished 

 population (F = 0). 



Fast- and slow-growing populations ofH. rubra were 

 modeled. The generalised growth parameters (Table 

 2) were based on empirical estimates (McShane et al. 

 1988a, McShane 1990, McShane and Smith 1992). Both 

 yield and egg-per-recruit were expressed graphically 

 as a function of minimum length-at-capture (i.e., length- 

 at-recruitment) and F, using two rates of natural mor- 

 tality estimated for H. rubra (M = 0.1 and 0.2, Table 

 1). Length-at-recruitment was varied (in 10 mm incre- 

 ments) from 100 mm to 140 mm for fast-growing popu- 

 lations and from 70 mm to 130 mm for slow-growing 

 populations. The value of F was varied from 0.1 to 1.5. 

 A smooth surface was interpolated through points in 

 3-dimensional plots of yield and egg-per-recruit, follow- 

 ing the method of McLain (1974) in which negative 

 exponential weights are computed from distances be- 

 tween points in a regular grid and the irregularly 

 spaced data points in the X-Y plane (Wilkinson 1990). 



Results 



Fishery statistics 



Annual variations in catch, effort, and value of the Vic- 

 torian abalone fishery are described in Figure 1. Catch 

 is highly correlated with effort (r 0.98, n 25, P< 0.001). 

 Although the catch rose in 1965-66 (accompanying 

 development of export markets), the trend in both 

 catch and effort is one of a slight but unalarming 

 decrease followed by a slight increase during the 1980s. 

 The introduction of catch quotas in 1988 is reflected 

 in the decrease in catch in that year (Fig. 1). It is 

 noteworthy that prior to 1988, price of abalone is a 

 significant factor influencing the catch of the Victorian 

 abalone fishery. Allowing for inflationary increases, the 

 price of abalone doubled between 1967 and 1976-77 

 accompanying development of Japanese markets. Four 



i 



965 1970 1975 1980 1985 1990 



3. 



1965 1970 1975 1980 1985 1990 

 YEAR 



Figure 1 



Comparison of (upper) annual catch and effort (dashed line), 

 and (lower) actual price and CPI adjusted price (dashed line) 

 of abalone Haliotis rubra in the fishery off Victoria, Australia, 

 1965-88. 



exceptions to the steady rise in price have occurred. 

 In 1967-68, a slight fall in price resulted from ship- 

 ments of poor-quality abalone. Processing techniques 

 were, at the time, in a developmental phase. Second, 

 in 1976 an increase in price occurred concomitant with 

 high demand by export markets and increased competi- 

 tion between processors for supply. The introduction 

 of a competitive product, the Chilean "loco" Concho- 

 lepas concholepas, on Asian markets coupled with buyer 

 resistance to elevated prices of Australian abalone 

 resulted in a decrease in price during 1977 (Stanistreet 

 1978). Note that there is a lag between price variation 

 and catch and effort; the relative decrease in catch and 

 effort in 1978 reflects the price drop in 1977 (Fig. 1). 

 Buyer resistance also affected the price of abalone in 

 1981-82 and led to a decrease in effort and catch dur- 

 ing this period. 



More recently, the collapse of the large Mexican 

 abalone fishery and the imposition of catch quotas on 

 the Tasmanian and South Australian abalone fisheries 

 (Prince and Shepherd 1992) decreased the world supply 

 of abalone and increased the competitiveness of Vic- 

 torian suppliers (McShane 1990). This and a decrease 

 in the relative value of the Australian currency against 



