Retention of green-lipped mussel spat: The roles of body size and nutritional condition

Globally, mussel aquaculture relies mostly on the use of natural seed, or spat, to stock farms while the use of seed from hatcheries are also available in smaller quantities. The use of wild spat has a lower cost, but it is often highly variable in quantity, size, nutritional condition and performance after seeding onto longlines for growout. The inefficiency of seeding using wild spat of green-lipped mussels, Perna canaliculus, is a major issue in the New Zealand aquaculture industry, where over 80% of wild spat is lost shortly after seeding. Here, we examined the importance of spat size and nutritional condition on their subsequent retention in both the laboratory and on a mussel farm. After two weeks feeding in a laboratory, spat of 2.0-5.0 mm had higher retention (78.1 +/- 2.5%) than spat of 1.0-2.0 mm (65.7 +/- 3.3%) but did not differ to smaller spat of 0.5-1.0 mm (70.5 +/- 3.0%) in size. Furthermore, two weeks after seeding the spat out onto a mussel farm, both larger classes of spat (1.0-2.0 mm and 2.0-5.0 mm) had higher retention (80.5 +/- 3.5% and 83.6 +/- 2.5% respectively) than the smallest size class of spat 0.5-1.0 mm (65.8 +/- 2.9%). The calorific content of spat was used as a measure of nutritional condition at each stage of the experiment; i.e., spat collected from the wild (prefeeding), the wild spat after two weeks feeding in the laboratory (postfeeding), and post-seeded spat after two weeks after being placed on a mussel farm (post-seeding). Calorific content was found to be similar among the three size classes of spat but it was different among three stages of the experiment, with post-feeding and post-seeding spat having higher calorific content (3160 +/- 457.6 and 3055 +/- 404.6 cal. g(-1) of organic tissue) than the pre-feeding spat (1912 +/- 68.0 cal. g(-1)). This study results suggests that the propensity of spat to detach from grow ropes decreased with increasing size. Therefore, by size sorting the spat, and seeding out only the larger spat in good nutritional condition, whilst nursery culturing the smaller spat to a larger size and a good nutritional condition for their subsequent seeding onto farms could greatly improve the efficient use of spat.