Ultrasonic Tracking

Tracking Lobsters in Their Natural Habitat

The American lobster, Homarus americanus, is one of the most well studied marine invertebrates in the world, as well as the basis of one of the most valuable fisheries in New England. Yet, despite the intense interest in this species, we still know little about its normal behavior in its natural habitat. This lack of knowledge is primarily due to our inability to conduct long-term observations in an aquatic environment.

In the summer of 2002 we created a large in situ mesocosm near the UNH Coastal Marine Laboratory (CML), within which we can continuously track the movements of up to 5 lobsters at a time, while simultaneously collecting time-lapse video recordings of lobsters in the same area. We are using these new technologies to address several unresolved questions concerning the behavior of lobsters in theirnatural habitat. In particular, we are investigating lobster territorial, mating and homing behaviors, as well as some of the environmental factors that influence their daily foraging activities.

Many thanks to the lobstermen who helped us in the summers of 2002 to 2004, by not fishing traps within the study site! Many thanks to the NMFS for allowing us to use their facilities for our base station.

Study Site

Based on our observations of approximately 24 videos obtained during the summers of 1998-2000, we have drawn a number of conclusions, a few of which are listed below. For more details about these studies see our first manuscript on the subject (Jury and Watson, 2001) in the Publication section of this website.

1. A large number of lobsters approach and enter traps, yet typically we only catch 1-3 per trap haul because the vast majority escape. We estimate that 10% of the lobsters that approach a trap enter, and of the ones that enter, only 6% are caught. Over 75% of the lobsters that escape the trap do so through the entrance. Video 1, on the right, shows a lobster escaping through the entrance to the kitchen.

2. Lobsters are very active around traps during the day, as well as the night. This confirms other field observations indicating that lobsters in their natural habitat are not as strickly nocturnal as previously thought.

3. Agonistic encounters around traps appear to limit entry and stimulate exits. Video 2 shows a large lobster chasing away smaller lobsters and then entering the trap. Small lobsters are very hesitant to enter, while larger lobsters tend to move right in like the one shown in this video.

4. Once in the trap, lobsters tend to "defend" the resource. Video 3 demonstrates this behavior. This also limits entry and it is probably one of the main behaviors that lead to trap "saturation".

Figure 1
Figure 1
Figure 2
Figure 2
Figure 3
Figure 3
Figure 4
Figure 4
Video 1 (click to play)
Video 2 (click to play)
Video 2 (click to play)

Preliminary Results from 2002

In the summer of 2002 we had four main goals:

  1. Measure the home range of lobsters by tracking them for at least 3 days.
  2. Determine if they exhibited homing behavior, naturally and if displaced.
  3. Measure the area of bait attraction.
  4. Determine if lobsters are nocturnal, diurnal, or neither.

During the past summer we tracked 36 lobsters for approximately 5-8 days each. Of these, 11 escaped from the mesocosm at some point during a typical 7 day trail, but some of these stayed in the area long enough to examine their behavior outside the mesocosm. In general, lobsters outside the mesocosm, that were in range of the telemetry system, moved about the same amount, and with the same general patterns, as they did when they were inside the mesocosm.

Many of the lobsters we tracked had some type of shelter affinity, even though their "shelters" were sometimes just pits in the sand (Figure 1). Occasion they would leave their shelters, move around within the mesocosm, and then return to the same location. Two example of this type of behavior are shown in Figure 3.

The data collected during the summer of 2003 were sufficient to make it possible to calculate the home range for a number of different lobsters. These calculations were performed using the Animal Movement Analysis Extension for ArcViewGIS. This extension was developed by Dr. Hooge and his colleagues. Based on our home range calculations so far, the typical home range of a lobster is 653.5 ± 149.4 m2. These home ranges are roughly 26% the area of the entire enclosure (2500 m2). An example showing how a home range was calculated is shown in Figure 4.

Although it is generally accepted that lobsters are nocturnal, the data we have collected so far does not support that view. We often observed lobsters moving about outside shelters during the day and many of the animals we tracked moved the same distance in the day as in the night (Figure 5).

One way we started to determine the area of bait attraction was to fish a single lobster trap, equipped with a transmitter, within the mesocosm. Because the local lobstermen were considerate enough to avoid dropping traps within the mesocosm, our trap was the only source of bait odor. On several occasions we were able to track lobsters approaching the trap (Figure 6) and, because the trap was also equipped with a video camera, we were also able to determine if they entered the trap or simply approached it. Based on these preliminary data lobsters appear capable of sensing a trap from distances of < ~ 10 meters. However, considerably more data are required before we can state with confidence that lobsters are attracted to bait from a certain distance away, taking into account major factors such as current speed and the activity or behavioral state of the lobster.

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