WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2016 45 dimensions for Sydney rock oysters, but it was clear that preferences exist and that NSW farmers are looking for characteristics slightly different from those selected as the optimum for Pacific oysters. The average ratio of height:length:width for Sydney rock oyster entries was 3: 2.22: 0.80, while the average for winning entries was 3: 2.29: 0.93. Winning entries tended to be longer and wider than the average oyster submitted. Farmers were clear in their preferences for Sydney rock oysters, with the winning entries attracting approximately 55 percent of the total vote within that class. Indeed this is likely to be an underestimate of their preference because there were instances where it was clear that farmers were voting for their own entry. Assuming each farmer did this and one vote was deducted from the total of each entry’s score, the proportion of the total vote received by the winners exceeded 65 percent. Pacific oysters The culture of Pacific oysters in NSW is unique within an Australian context in several respects. Like Tasmania and South Australia, the majority of oysters cultivated are hatchery produced, but a major, although unquantified, proportion of NSW Pacific oyster production arises from wild catch from a single estuary, Port Stephens. This is the only estuary where wild Pacific oysters can be collected legally because this is the only location where government approval to culture diploid Pacific oysters has been granted. All other Pacific oysters cultured within the state are required to be triploid and eight estuaries in NSW have the appropriate government approvals for this. Accordingly the vast majority (>80 percent) of entries within this class were hatchery produced, triploid oysters. Perhaps reflective of the comparatively high growth rates of Pacific oysters in NSW and the inherent growth advantages of triploid stock, all but four of the 32 entries were large oysters (> 75 g each). However, unlike Sydney rock oysters, the weight and dimensions of the winning Pacific oyster entries were significantly less than the average for all submissions. This could reflect a NSW industry accustomed to production of the comparatively smaller Sydney rock oysters and one in which the major market is for oysters in the half shell. Three of the 32 entries consisted of oysters with individual weights exceeding 250 g, beyond the measurement capacity of the scales provided. These entries were frequently regarded as a novelty and were not among the winners. Their dimensions were included to calculate a ratio to be indicative of shape, but they were not included in the overall weight and measurement averages shown in Table 1. There was little difference between the shape of the winning Pacific oysters (3: 1.84: 1.03) and the average for all submissions (3: 1.87: 1.05). This average is close to the southern Australian Pacific oyster producers target ratio of 3:2:1 (Kube et al. 2011) and well within the range considered good (< 3) for Irish Pacific oysters (BIM 1996). Indeed every Pacific oyster entry within the competition had a “good” score of less than 3. The winning Pacific oyster entries attracted approximately 38 percent of the total vote within this class. This is less than observed with Sydney rock oysters and could reflect a number of factors. First, Pacific oysters are comparatively new to culture in NSW and are only permitted to be cultured in five of the state’s 32 oyster-producing estuaries when these data were collected. Many of farmers who voted in the Pacific oyster category were not experienced in Pacific oyster culture and may have been less certain about their preferences. More importantly however, the numerical dominance of triploid oyster entries from Australia’s single hatchery producer of triploid Pacific oysters is likely to have led to greater inherent uniformity in the stock, making choice more difficult. This may also explain why the general shape of all oysters submitted differed little from the overall winners. Flat oysters As the name suggests flat oysters have a markedly different shape to the two cupped oysters (Sydney rock oysters and Pacific oysters) and this was reflected clearly in their overall dimensions (3: 2.85: 0.83). Although there were too few submissions to warrant great consideration, those entered were generally representative of oysters currently marketed. In most instances, oysters were approximately round in shape, with a width similar to that measured for Sydney rock oysters. Often the oysters submitted had a “rumbled” appearance with recent shell growth knocked off due to handling or movement in culture. However, the winner of this class (Fig. 1) at the south coast field day (Batemans Bay, NSW, 2009) did not have this appearance and was the most popular exhibit at any show in any class, attracting over 82 percent of the total vote. It is also worth noting that this entry was also the smallest ever submitted in the class (84 mm mean shell height). Do as I Do: Implications for the Sydney Rock Oyster Breeding Program Is it a case of “do as I do, not do as I say”? Despite showing a clear preference for large cultchless plate-grade Sydney rock oysters, NSW farmers most commonly sell smaller, often stick-grown oysters (O’Connor and Dove 2009). There are commercial drivers that influence sale size, such as meat condition, variable growth rates and disease. However, a premium price is paid for larger grades of oysters. Regardless, many farmers complain that selectively bred Sydney rock oysters are too flat and in a recent survey 44 percent of respondents listed shape as their greatest concern with current breeding lines. This has undoubtedly been exacerbated by some growing practices and the age of selectively bred oysters at harvest, but it has led to changes in selection processes and has forced shape to be included as a characteristic in ongoing selective breeding programs. This trait has always been carefully considered and monitored in selectively bred oysters that may be cultured under different methods and conditions compared to commercial stocks. The inclusion of additional traits within the breeding program is likely to detract from the rate of progress with other factors and thus there should be an economic incentive for its inclusion. Unfortunately, there is limited evidence for this. Australian surveys have shown that oyster appearance (presentation, shape and size) is a factor that explains only 4.4 percent of the variance associated with consumer oyster attribute scores and these scores are not species specific (Kow et al. 2008). Given that appearance is arguably more variable in Sydney rock oysters than Pacific oysters, it may well be more important to consumers of this species. At this time, shape in Sydney rock oyster family lines is being monitored and decisions (CONTINUED ON PAGE 46)
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