Steven H. Jury

Lobster trap video: in situ video surveillance of the behaviour of Homarus americanus in and around traps

A lobster-trap video (LTV) system was developed to determine how lobster traps fish for Homarus americanus and how behavioural interactions in and around traps influence catch. LTV consists of a low-light camera and time-lapse video cassette recorder (VCR) mounted to a standard trap with optional red LED arrays for night observations. This self-contained system is deployed like a standard lobster trap and can collect continuous video recordings for >24 h. Data are presented for 13 daytime deployments of LTV (114 h of observation) and 4 day and night deployments (89 h of observation) in a sandy habitat off the coast of New Hampshire, USA. Analyses of videotapes revealed that traps caught only 6% of the lobsters that entered while allowing 94% to escape. Of those that escaped, 72% left through the entrance and 28% through the escape vent. Lobsters entered the trap at similar rates during the day and night and in sandy and rocky habitats. Lobsters generally began to approach the trap very shortly after deployment, and many appeared to approach several times before entering. These data confirm the results of previous laboratory-based studies in demonstrating that behavioural interactions in and around traps strongly influence the ultimate catch.

The behavior of lobsters in response to reduced salinity

Abstract Two experiments were conducted to measure the behavioral responses of lobsters, Homarus americanus (Milne-Edwards), to reductions in salinity. In the first experiment animals were placed in a 3 ft diameter tank that was divided in half by plastic mesh. Spontaneously active lobsters were able to move between the two halves of the tank by passing through either of two conduits. The conduits were equipped with optical sensors to monitor the passage of animals, and a perfusion system to control the salinity of the area in, and around, the conduit. When the salinity in the vicinity of both conduits was the same (28–32 ppt), lobsters exhibited no preference for either conduit. However, when the salinity in one of the conduits was lowered, lobsters preferred to pass through the high salinity (20–25 ppt) conduit rather than the one with low salinity (10–15 ppt). In addition, females appeared to be more selective in their preference and exhibited higher overall activity than males when exposed to reduced salinity. In the second experiment, individual lobsters were placed in a shelter at one end of a long seawater table and exposed to seawater of gradually decreasing salinity. The salinity required to cause a movement out of a shelter, i.e. an avoidance response, was recorded. On average, lobsters first ventured small distances ( one body length) when levels approached 12.62 ppt ± 1.59. Although it was not statistically significant, females again seemed to be either more sensitive to salinity or found it more aversive, because they tended to initiate movements at salinities greater than those required to influence males. These behavioral data indicate that: 1. (1) adult lobsters are capable of detecting changes in salinity which are comparable to the levels found during natural fluctuations in coastal bays and estuaries; 2. (2) when exposed to low salinity of sufficient magnitude, they attempt to avoid it, and; 3. (3) females appear to be more sensitive to drops in salinity and/or they find it more aversive. Previous studies have demonstrated that estuarine lobster populations are dominated by males and that there are seasonal migrations of lobsters into, and out of, estuaries. We conclude that the behavioral responses of male and female lobsters to low salinity may determine, in part, the distribution and movements of lobsters in estuarine habitats.

Thermosensitivity of the lobster, Homarus americanus, as determined by cardiac assay.

It is generally accepted that crustaceans detect, and respond to, changes in water temperature, yet few studies have directly addressed their thermosensitivity. In this investigation a cardiac assay was used as an indicator that lobsters (Homarus americanus) sensed a change in temperature. The typical cardiac response of lobsters to a 1-min application of a thermal stimulus, either warmer (n = 19) or colder (n = 17) than the holding temperature of 15 degrees C, consisted of a short bradycardia (39.5 +/- 8.0 s) followed by a prolonged tachycardia (188.2 +/- 10.7 s). Lobsters exposed to a range of rates of temperature change (0.7, 1.4, 2.6, 5.0 degrees C/min) responded in a dose-dependent manner, with fewer lobsters responding at slower rates of temperature change. The location of temperature receptors could not be determined, but lesioning of the cardioregulatory nerves eliminated the cardiac response. Although the absolute detection threshold is not known, it is conservatively estimated that lobsters can detect temperature changes of greater than 1 degree C, and probably as small as 0.15 degrees C. A comparison of winter and summer lobsters, both held at 15 degrees C for more than 4 weeks, revealed that although their responses to temperature changes were similar, winter lobsters (n = 18) had a significantly lower baseline heart rate (34.8 +/- 4.4 bpm) and a shorter duration cardiac response (174 s) than summer lobsters (n = 18; 49.9 +/- 5.0 bpm, and 320 s respectively). This suggests that some temperature-independent seasonal modulation of cardiac activity may be occurring.

The effects of reduced salinity on lobster (Homarus americanus Milne-Edwards) metabolism: implications for estuarine populations

Abstract During periods of substantial freshwater runoff, lobsters that inhabit estuaries, such as the Great Bay Estuary in NH, are exposed for several days to weeks to seawater that is diluted as low as 10 ppt. To assess the physiological stress imposed by these conditions, we measured the oxygen consumption, heart rate, ventilation rate and hemolymph osmolarity of lobsters while sequentially exposing them, for 24-h periods, to seawater of 20, 15, and 10 ppt. Measurements of hemolymph osmolarity confirmed previous results which demonstrated that at salinties below 20 ppt lobsters are limited osmoregulators; allowing their hemolymph osmolarity to drop as the environmental salinity is reduced, but always maintaining it higher than the ambient osmolarity. All animals exposed to 10 ppt, at 15 °C, were capable of surviving for at least 72 h. There was a nearly linear increase in oxygen consumption, heart and scaphognathite rates in animals exposed to dilute seawater, with almost a twofold increase in metabolic rate when animals were moved from 20 to 15 to 10 ppt. At the lowest salinity tested (10 ppt) the average oxygen consumption was higher for females than for males. We conclude that at low salinities the energetic demands of osmoregulation are greater for females than males, and for both sexes the physiological stress imposed may determine, in part, their distribution and/or movements in estuarine habitats.

Detection of Salinity by the Lobster, Homarus americanus

Changes in the heart rates of lobsters (Homarus americanus) were used as an indicator that the animals were capable of sensing a reduction in the salinity of the ambient seawater. The typical response to a gradual (1 to 2 ppt/min) reduction in salinity consisted of a rapid increase in heart rate at a mean threshold of 26.6 ± 0.7 ppt, followed by a reduction in heart rate when the salinity reached 22.1 ± 0.5 ppt. Animals with lesioned cardioregulatory nerves did not exhibit a cardiac response to changes in salinity. A cardiac response was elicited from lobsters exposed to isotonic chloride-free salines but not to isotonic sodium-, magnesium- or calcium-free salines. There was little change in the blood osmolarity of lobsters when bradycardia occurred, suggesting that the receptors involved are external. Furthermore, lobsters without antennae, antennules, or legs showed typical cardiac responses to low salinity, indicating the receptors are not located in these areas. Lobsters exposed to reductions in the salinity of the ambient seawater while both branchial chambers were perfused with full-strength seawater did not display a cardiac response until the external salinity reached 21.6 ± 1.8 ppt. In contrast, when their branchial chambers were exposed to reductions in salinity while the external salinity was maintained at normal levels, changes in heart rate were rapidly elicited in response to very small reductions in salinity (down to 29.5 ± 0.9 ppt in the branchial chamber and 31.5 ± 0.3 ppt externally). We conclude that the primary receptors responsible for detecting reductions in salinity in H. americanus are located within or near the branchial chambers and are primarily sensitive to chloride ions.

Use of a treadmill to study the relationship between walking, ventilation and heart rate in the lobster Homarus americanus

A passive treadmill was designed and built that measures the speed of lobster walking, while simultaneously recording heart and scaphognathite pumping rates. The treadmill only moved when the lobster voluntarily walked either forward or backward. Lobsters (n = 5) on the treadmill typically walked in bouts lasting an average of 5 min (range 8 s to 53 min). During these bouts, the mean walking rate was 0.9 0.6 m/min (n = 100 bouts from 5 lobsters), and the maximum speed recorded was 2.5 m/min. At the onset of walking, and sometimes 5–10 s before, the heart and ventilation rates rapidly increased. Ventilation rate increased more than heart rate during walking (ventilation rate: 194% increase; heart rate: 69% increase) and returned to baseline faster (4.5 3.9 min) than heart rate (6.6 3.9 min) after a bout. Heart and ventilation rates during walking do not appear to be related to the duration of the bout. Heart and ventilation rates were positively correlated with the average walking speed attained in 3 of the 5 lobsters tested. Heart rate, in particular, appeared to have an upper limit of approximately 90–100 beats/min, regardless of walking duration or speed.

The relationship between American lobster catch, entry rate into traps and density

Abstract Management of the American lobster (Homarus americanus) fishery depends on accurate estimates of lobster abundance. Catch from traps remains a widespread indicator, as it is widely accepted that catch per unit effort (CPUE) is correlated with density. However, surprisingly few studies have rigorously tested this relationship. We fished traps, and conducted concurrent SCUBA surveys to determine lobster densities, in the same study area along the New Hampshire coast, USA. We found that catch in standard commercial lobster traps loosely correlated with density (r 2=0.471), particularly for pre-recruit lobsters at lower densities. Video observations revealed that small lobsters entered early in the soak and larger individuals entered later, which likely influenced trap saturation. When traps were pre-stocked with a single adult lobster there was a reduced rate of entry by pre-recruit lobsters, but overall CPUE was unchanged relative to normal traps. These data indicate that, while standard commercial traps are a reasonable indicator of pre-recruit lobster abundance under some conditions, due to the behavioural characteristics of lobsters, the relationship between catch and density is limited, particularly at higher densities and for legal-sized lobsters. A better understanding of the behavioural mechanisms that influence catch could improve the use of catch data to predict abundance.