Vertical distribution and daily movements of larval lobsters Homarus americanus over Browns Bank, Nova Scotia

The vertical migration of lobster larvae was suspected from previous neuston collections taken off southwest Nova Scotia, Canada. An electronically-controlled Tucker trawl was developed for discrete depth sampling and used to study the diel vertical distribution of lobster larvae, together with environmental variables, over Browns Bank in late August. Significant vertical migration is demonstrated for stage I lobster, which were most frequently caught between 15 and 30 m water depth during daylight but were rarely caught below 10 m at night. Stage I lobster generally stayed below light intensities of 100 to 200 μ E m⁻² s⁻¹. Stage II and III lobster were collected throughout the upper 20 to 30 m of the ocean but were too rare to distinguish statistically between day and night depth-abundance patterns. Stages I, II and III lobster were all confined to the upper mixed layer, above the thermocline, which varied in depth with the tidal cycle. Stage IV lobster were caught almost entirely at the surface, with no significant difference between day and night abundances. These findings are novel and have important implications for the ecology and dispersal of larval Homarus americanus in offshore waters.