Should slow growing pearl oyster (Pinctada margaritifera) spat (Runts) be discarded?

Abstract

In this laboratory, hatchery-produced Pinctada margaritifera juveniles are routinely graded at 3.5 mo of age, when spat of <5 mm ("runts") are generally discarded. This article reports on an experiment to assess the relative growth rates of three size classes (<5,5-10, and> 10 mm) of hatchery-produced blacklip pearl oyster (P. mararitifera) spat from the same cohort. The three size classes were classified as runts, normal growers, and fast growers, and had mean (±SE; n = 30) dorso-ventral shell heights (DVHs) of 4.5 ± 0.1, 8.6 ± 0.3, and 12.8 ± 0.2 mm, respectively, at the start of the 4-mo experiment. The mean DVH at completion of the study for each initial size class (<5, 5-10, and >10 mm) was 24.6 ± 0.4, 32.3 ± 0.4, and 35.6 ± 0.4 mm, respectively. All differed significantly from each other (P<0.001). The mean incremental increases in DVH for each size class (<5, 5-10, and> 10mm) over the 4-mo period was greatest in oysters from the 5-lO-mm size class (mean DVH 23.3 ± 0.4 mm) and lowest in oysters from the <5-mm size class (mean DVH 20.0 ± 0.5mm). lncremental increases in DVH were significantly different between oysters from the <5-mm size class and those from the larger size classes. The mean (±SE) percentage increase in DVH was greatest in oysters from the <5-mm size class (448 ± 17%) and lowest in oysters from the >10-mm size class (178 ± 7%). A number of oysters in the <5-mm size class grew very rapidly during the experiment and reached the same DVH as oysters in the larger size classes. This study shows that, given appropriate conditions, runts are capable of similar growth rates as larger spat. It may therefore be inappropriate to discard pearl oysters, which are classed as runts (<5 mm) at grading (3.5 mo). Furthermore, it is suggested that grading be delayed until 5 to 6 mo when a greater proportion of oysters are likely to be in the larger size classes.