Is size grading important for farming the abalone Haliotis iris?

In New Zealand the abalone, Haliotis iris, is commonly known by its Maori name “paua”. Paua farmers normally purchase juveniles from hatcheries as un-graded stock and on-grow all the animals to harvest. To date there has been no clear guidance to farmers on whether it is advantageous to size grade and/or cull stock on receipt from the hatchery or at some other point during the production cycle. In this study a cohort of paua juveniles (6 months post hatch) were graded into three size classes and on-grown for 315 days under controlled conditions. The average growth rates achieved by each size class and the growth rates of individuals within each of the size classes were compared to those achieved in an un-graded population. The Large and Medium size grades and the un-graded population exhibited significantly higher growth rates (58, 57 and 55 μm/day respectively) than the Small size grade (37 μm/day). Comparison of population growth showed that the combined growth rate for the graded populations (50 μm/day) was lower than for the un-graded population, which was attributed to the small paua within the un-graded population growing faster than paua within the Small size grade. When individual growth rates were compared it was apparent that shell length at the start of the trial was not a good predictor of individual growth rate. Grading and culling of small paua is therefore not recommended during the early stages of on-growing, but is beneficial after a period of on-growing that allows slow growing animals to be identified.