BEHAVIORAL-RESPONSES OF THE PLANKTONIC STAGES OF THE AMERICAN LOBSTER HOMARUS-AMERICANUS TO THERMAL-GRADIENTS, AND ECOLOGICAL IMPLICATIONS

The behavioural responses of each planktonic stage (I to IV) of American lobster Homarus americanus to a thermal gradient were studied during complete day/night cycles using an experimental approach. The thermocline markedly influenced the vertical position of larvae of all stages, limiting their distribution to the upper part of the water column. In both absence or presence of a thermocline, older individuals (stages III & IV) were observed deeper in the water column than younger individuals (stages I & II). The vertical distribution of larvae was unaffected by the day/night regime. The ecological significance of this temperature-driven behaviour of the larvae relative to the recruitment dynamics of the adult stocks is discussed. We hypothesize that factors causing a breakdown in the thermocline should decrease recruitment to the adult population. The validity of this hypothesis was explored for the lobster stock of Iles-de-la-Madeleine (Gulf of St. Lawrence, Quebec). Significant negative correlations obtained between the frequency of strong winds (> 30 km h-1) during dispersal and settlement periods, and subsequent lobster landings (with an 8 yr lag) in this region over a 15 yr interval support the hypothesis.