Determining the optimal scallop spat length and transport conditions for Pecten novaezelandiae during the seeding enhancement programme in Nelson, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
In the 1998/99 season the scallop dredge fishery was worth an estimated $30.2 million. The success of this industry relies heavily on a scallop spat enhancement programme run by the Challenger Scallop Enhancement Company. Spat collectors consisting of mesh bags, suitable for larvae settlement, are set during November or December and are harvested in March (primary spat). The harvested spat are dispersed into new areas where they are left to grow to a commercially harvestable size. Fall off from the primary harvesting process results in high densities of juvenile scallops on the primary harvest site, which need to be cleared. The clearing of these juvenile scallops is referred to as the secondary harvest. It is not known which length spat is the most robust and therefore most likely to survival through the primary and secondary harvesting processes or under what transport conditions during the secondary harvest survival is highest. Behavioural experiments were conducted on 9 length groups of spat collected during the 2002 primary harvest. When presented with a starfish predator, 10 mm spat had a significantly slower reaction time (8.30 ± 0.16 seconds) than other length groups and 17 mm length group had the fastest reaction time (2.55 ± 0.32 seconds). Small spat had significantly fewer adductions in their first swimming bout than large spat, (12 mm = 8.44 ± 0.61,27 mm = 10.90 ± 0.49). Also, for the total number of adductions this pattern was repeated with small spat exhibiting fewer adductions than large spat, (10 mm = 22.24 ± 1.58, 27 mm = 40.52 ± 1.20). These results indicate that larger spat are more robust after the primary harvest process. Glycogen analysis of primary and secondary scallops showed very low quantities of glycogen «0.3%) in whole scallops (wet weight). In the primary harvest the smaller spat «17 mm) had significantly higher glycogen levels than larger spat. The secondary juvenile scallops showed no variation in the glycogen content between length groups. Two methods were used to assess stress in primary and secondary scallops (stress on stress (SOS) test and recovery test). The SOS test had no variation in survival between length classes from the primary harvest. The recovery test suggested that larger spat (>20 mm) had a higher survival rate than small spat. The two experimental methods (SOS test and recovery test) showed opposite trends in survival with regard to length classes when used to test the same sample group of scallops during the secondary harvest, however, when all length classes were grouped together the assessment methods reported similar findings. The SOS test suggested that larger scallops are more stressed after the secondary harvest process, whereas the recovery test suggested that the smaller scallop were more stressed. Aerial exposure experiments suggested that transport of up to 8 hours was possible at temperatures <20°C with only 15% mortality when scallops were acclimated to 18°C. Exposure over 2 hours at a temperature of 30°C resulted in high mortality (60-100%). Findings in this study suggest that the primary harvest should target scallop spat that are >20 mm in order to seed the most robust scallop. The secondary harvest should also target larger juvenile scallops (>30 mm) when transporting long distance (>8 hours). Transport up to 8 hours could result in minimal mortalities when the air temperature is near current acclimation levels.