Effect of thermoclines and turbulence on depth of larval settlement and spat recruitment of the giant scallop Placopecten magellanicus in 9.5 m deep laboratory mesocosms

Abstract

An experiment was conducted from December 1992 to February 1993 in a 10.5 m deep, 3.7 m diameter tank to examine the effect of thermoclines and water column turbulence on the depth of larval settlement and spat recruitment of the giant scallop Placopecten magellanicus (Gmelin). Polyethylene tube mesocosms set up within the tank were used to enclose 9.5 m deep columns of seawater which were then used as experimental replicates. Five different treatments were established as follows: (1) no turbulence and a 1.5°C thermocline, (2) no turbulence and no thermal gradient, (3) low level of turbulence with a 1.5°C thermocline, (4) medium level of turbulence with a 0.5°C thermocline, and (5) high level of turbulence with no thermal gradient. Various turbulence levels simulated turbulent energies ranging from open oceanic environments to near-shore and coastal conditions with vertical dissipation rates ranging from 10-7 to 10-3 cm2 s-3. Ropes with collectors positioned at every 1 m depth interval were suspended the length of the water column in each replicate tube to collect settled spat. Spat counts varied significantly with both depth and turbulence treatment and were dependent on the interaction between the 2 factors. Numbers of spat generally increased with increasing depth in the static and low turbulence treatments, but this relationship became less evident with increasing turbulence; spat recruitment in the high turbulence tubes appeared random with respect to depth. It is suggested that the trend of increasing recruitment with depth in the static and low turbulence tubes was driven primarily by larval behaviour at settlement. There was no indication of increased recruitment at or above the thermocline, in contrast to a previous mesocosm experiment with a stronger thermal gradient and a different population of larvae, suggesting that stratification intensity may affect depth of larval settlement and spat recruitment. Settlement rate did not appear to be a strict function of larval encounter rate with the spat collectors. Higher spat counts in treatments with a 1.5°C thermocline than in other turbulence treatments, even when results were corrected for differences in competency among the various treatments, suggest that thermal gradients have a potential commercial importance in both the collection and hatchery production of scallop spat.