Cracking the code : defining roe quality of the long-spined sea urchin (Centrostephanus rodgersii) in Tasmania

In Tasmania, arrival of the long-spined sea urchin (Centrostephanus rodgersii) has presented economic opportunity along with ecological change where they occur. Over the last half century, C. rodgersii has undergone climate driven range-extension and is now distributed along the entire east coast of Tasmania. The highest densities of C. rodgersii occur in northern Tasmania around the St Helens region and become less abundant along latitudinal gradients. This pattern of distribution has resulted in a developing fishery for C. rodgersii being mostly focused between 'The Gardens' and 'St Helens Island' in the north east of Tasmania where catch rates are greatest. However, a lack of definitive information on the specific drivers of roe quality has been a significant hurdle to progress of the fishery in Tasmania and mainland Australia as a whole. This species is frequently implicated with high proportions of poor quality roe in commercial catches which has deterred interest from prospective entrants to the fishery. Identifying key parameters around high quality roe will help commercial fishers and processors target individual sea urchins with a greater probability of harvesting high quality roe hereby increasing economic return with the ultimate aim of maximising the potential of this new resource. This thesis explores the biological and environmental drivers of roe quality to determine factors that indicate high quality roe. Samples of Centrostephanus rodgersii were collected monthly from St Helens kelp and barrens habitats over an 18 month period spanning May 2014 to October 2015 and assessed for roe quality. Seasonal changes in reproductive biology were determined and assessed for effect on roe quality which comprised; colour, texture, granularity and quality index (possible score of 1-5 where 1 is the lowest score attainable and 5 is the highest, quality index was the sum of all individual criteria and ranged from 3-15). Examination of gonad histology showed a distinct annual pattern in reproduction and the highest levels of roe quality were recorded in the months prior to peak gametogenesis and spawning (i.e. during March, April, May and June of 2014 and 2015). Logistic regression showed high roe quality to be significantly affected by the proportion of nutritive phagocytes (NP's) within the gonad lumen (G21 = 47.864, P < 0.001) being 4.3 times more likely (95% CI = 1.647, 8.412) to encounter 'A' grade roe when NP's were in high proportions. Furthermore, gender was also found to significantly affect likelihood of harvesting 'A' grade roe, but it was the interaction between proportions of NP's and gender that increased the odds ratio to a highly probable 9.67 to 1 (95% CI = 1.976, 58.316) when individuals were male and contained a high proportion of NP's. Histological samples collected over an 18 month period in St Helens from 2003-05 (before commercial harvesting began in Tasmania) was made available for comparative analysis and showed remarkable gametogenic synchrony with samples collected during 2014-15. Both study periods demonstrated clear winter/spring spawning (August/September) with a small number of individuals persisting through to October/November. Spawning assays (response to KCl injection) were also conducted on separate sea urchins and indicated that the magnitude of spawning response was proportional to the density of NP's. This methodological procedure may be useful as a proxy measure of roe quality for industry (i.e. signal the end of the annual high quality period) considering high quality roe is significantly affected by the presence of NP's. To this end, the decadal consistency in the seasonal gametogenic cycle and the relationship between 'A' grade roe and NP's clearly demonstrates a temporally consistent harvest window which is critical knowledge for further development of this fishery. In addition, a suite of independent variables (exogenous and endogenous) were assessed for their effect on roe quality criteria using an ordinal logistic regression approach. Models were developed based on variables that could be; used in planning harvest operations, directly observed during harvest operations, manipulated post harvest or optimised through repeated harvesting. It was found that the independent variables; habitat, seasonality, age and test diameter (shell width) had the most influence on roe quality. Odds ratios (OR) for high quality roe were highest between April and June (OR 7.85 95% CI 4.97, 10.36) for seasonality and for sea urchins aged between 7-20yrs (OR 14.34 95%CI 9.67, 28.46). Odds ratios for sea urchins harvested from kelp habitats were also significantly increased (OR 4.78 95% CI 1.99, 4.87). The magnitude of odds ratios (particularly age) indicate that large improvements in roe quality are possible by tailoring harvest operations to accommodate these specific parameters.