INFLUENCE OF A THERMOCLINE ON VERTICAL-DISTRIBUTION AND SETTLEMENT OF POST-LARVAE OF THE AMERICAN LOBSTER HOMARUS-AMERICANUS MILNE-EDWARDS

The hypothesis that the presence of a strong thermal gradient in the water column acts as a negative physical cue for settling lobster post-larvae (Homarus americanus, Milne-Edwards) (Thermally-Mediated Settlement Success hypothesis, TMSS) was tested in the laboratory. Our approach was to compare the relative distribution of competent post-larvae (mid-stage IV) in a thermally stratified water column and in a control, unstratified, water column. The proportion of post-larvae settled after 24 h was significantly lower in the presence of a thermocline than in the corresponding control situations (less-than-or-equal-to 25% vs. greater-than-or-equal-to 50%, respectively). Gradients from 18.5 to 13.5-degrees-C and from 16.5 to 12-degrees-C were sufficient to significantly reduce the proportion of settled post-larvae, even though these temperatures were well within the range encountered in nature. The proportion of post-larvae crossing the thermocline increased with a decrease in stratification, going from 13% for a thermocline of 6-degrees-C, to 15 and 28% for gradients of 4.5 and 2.5-degrees-C, respectively. Conversely, settled post-larvae left the bottom when the ambient temperature was reduced. The vertical distribution and settlement of post-larvae were unaffected by the day/night regime. The role of the thermocline as a mediator of settlement success is discussed.