SEASONAL MOVEMENTS AND ACTIVITY LEVELS OF AMERICAN LOBSTERS (HOMARUS AMERICANUS) IN AN ESTUARY
American lobsters (Homarus americanus) are traditionally found in coastal and offshore waters, but they may also reside in estuaries, where environmental conditions can be more challenging than in the ocean. Previous studies have suggested that, in order to survive in such a seasonally harsh habitat, lobsters migrate into the estuary in the spring, and out of the estuary in the fall. Moreover, it has been suggested that the differential seasonal movements of males and females is what leads to a male skewed population in the areas furthest from the coast. In order to test these hypotheses further two different studies were carried out, one in the field and one in the laboratory. First, acoustic telemetry was used to investigate the seasonal movements of immature and mature lobsters of both sexes (Chapter 1). Second, the influence of photoperiod and water temperature on the year-round activity of lobsters was studied using accelerometry.
Estuarine lobsters did not move very far at any time during the year (Mean net distance traveled = -0.75 ± 0.23 km; Lowest position in Estuary -4.4; Highest position in Estuary +2.5). Importantly, no lobsters, including females with eggs, left the estuary, so there appears to be a resident population of lobsters in the Great Bay Estuary. It appears as if the animals in this study did not move as far as in previous studies for two reasons: First, we tagged and released them in a region of the estuary that is characterized by a benthic habitat that is very favorable for lobsters and resembles, in many respects, coastal habitats. Second, during this two-year study, there were very few major storms and therefore the salinity remained higher than during previous studies.
For the accelerometer study I fitted a total of 90 lobsters with small waterproof accelerometers and placed them into individual mesh containers that were, in turn, placed inside larger containers that were continuously perfused with ambient seawater. The accelerometers made it possible to continuously monitor their locomotor activity, throughout an entire year. Overall, there was a good correlation between water temperature and activity, with a steady increase in activity from the spring until October, before declining throughout the winter. In addition, the lobsters tended to be very nocturnal, so as the days became longer, they were active for a longer period of time. Combined, the results suggest that lobsters in estuaries tend to remain within the estuary, and potentially reproduce there, despite increased activity in the fall, characterized by a tendency to move toward the coast.