Patterns of Diel activity and movements of the American lobster, Homarus americanus, as determined by ultrasonic telemetry

Christian, John R. (John Robert) (1995) Patterns of Diel activity and movements of the American lobster, Homarus americanus, as determined by ultrasonic telemetry. Masters thesis, Memorial University of Newfoundland.

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Abstract

The diel activity and nocturnal movements of the American lobster, Homarus americanus, were studied using a fixed hydrophone array tracking system at Broad Cove, Conception Bay, Newfoundland. Twenty lobsters tagged with ultrasonic transmitters were monitored at various times between September, 1987 and August, 1990, providing 291 nights of tracking data of which 246 were complete (i.e. an animal was monitored for the entire period of darkness). Positional information on active lobsters was recorded every two minutes, providing both temporal and spatial information on activity and movements. It was assumed that lagging did not have a pronounced or prolonged effect on the activity or movements of subjects. -- Lobsters were active on 28.5 % of all tracking nights. Time of onset of activity in the initial shelters averaged 1.5 h after sunset while time of onset of movement from them averaged 2.4 h after sunset. Lobsters were active at or within the final shelter an average of 52 min while the average time of cessation of activity at the final shelter was 5.7 h after sunset. The duration of the activity period, which corresponded to the interim between onset of activity at the initial shelter and cessation of activity at the final shelter, ranged from 6 min to 12.8 h with a mean of 4.0 h. Total out-of-shelter time ranged from 4 min to 12 h with a mean of 2.3 h. The out-of-shelter period was characterized by movement or stationary behaviour. Behaviour was considered stationary if the transmitter position remained the same for a minimum of five minutes. Otherwise, the lobster was considered to be moving. Average movement times and out-of-shelter stationary times were 1.3 h and 1.2 h, respectively, while respective ranges were 4 min to 6 h, and 0 to 10.2 h. -- There was also considerable variation within and between individual lobsters with respect to distance moved. Two measures of distance were calculated; the maximum distance (MD) of a lobster from its initial shelter and the approximate total distance (ATD) moved by a lobster while out-of-shelter. The averages of maximum distance from the initial shelter and approximate total distance moved were 24 m and 64 m, respectively. The respective ranges of these two variables were 2 to 74 m and 5 to 361 m. On average, large lobsters (> 81 mm carapace length) moved a maximum distance of 27 m from the initial shelter compared to 21 m for small individuals (< or = 81 mm carapace length). Average approximate total distance moved was essentially the same for both size groups. Movements were either simple (mostly straightline movement with little or no path crossover) or complex (numerous abrupt direction changes with much path crossover). Both types were characterized by extensive, wide-ranging patterns and restricted, light patterns. -- Thirty percent of the lobsters returned to the same shelter on the same night while 25% of the lobsters returned to a particular shelter after periods of absence exceeding one day. Certain shelters appeared to be preferred by either one individual or numerous individuals. Shelter fidelity was quite variable between individuals. Some lobsters used the same shelter numerous times while others changed shelters during each activity boul. The average entrance height: width ratio of shelters occupied (0.96) by monitored lobsters was only slightly lower than that of unoccupied shelters (0.97). -- Over seventy percent of the total stationary time occurred in areas with boulder and outcrop substrate. The greatest percentage of total stationary time (49 %) occurred in areas with scattered horse mussel and green sea urchin distributions. Large lobsters (> 81 mm carapace length) moved more frequently (32.5 % vs. 28.X %) than smaller ones (< or = 81 mm carapace length) but the smaller individuals averaged longer activity bouts (5.1 h vs. 2.8h). Large males (35.3 %) and small females (38.7 %) were active more frequently than small males (21.4 %) and the large female (21.3 %). Tagged ovigerous and molted individuals displayed activity consistent with the behaviour described in literature for lobsters in these physiological conditions. Tagged individuals were less active at times of oviposition and molting. In general, activity was greatest at times when water temperature exceeded 8° C (after July 1). Storm events sometimes resulted in the downslope movement of tagged individuals. Lobster activity was slightly higher at times of new moon and first quarter moon compared to times of full moon and last quarter moon.

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