Skewed sex ratio in an estuarine lobster (Homarus americanus) population

A total of 19,485 lobsters were caught sites in the estuarine and coastal waters of New Hampshire from 1989 to 1992, and their size and sex were determined. The sex ratio of lobsters caught farthest from the coast, in Great Bay, was heavily skewed in favor in males. Sex ratios in other estuarine and river sites were also skewed toward males, and there was a tendency for the number of males per female to decline as one moved down the estuary toward the coast, where the sex ratio was nearly 1:1. The single offshore site was dominated by females, with about 0.6 males for each female. There were also seasonal trends in the sex ratios in the upper estuarine sties, where the number of males per female tended to decline from summer through autumn. In general, differences in the sex ratios between sites were those of primarily adult lobsters larger than 80 mm carapace length (CL). At all sites, the sex ratio of lobsters smaller than this size was close to 1:1, whereas in the upper estuary the mean sex ratio of lobsters greater than 80 mm CL was more than 14:1. These data, in conjunction with seasonal variations of sex ratios, suggest that differential movements of adult male and female lobsters is the primary cause of skewed sex ratios in the Great Bay Estuary

Lobster movements in response to a hurricane

There are several anecdotal reports of the American lobster Homarus americanus moving to deeper water, or suffering increased mortality, in response to storm events. It has also been reported that changes in salinity and temperature, similar to those resulting from storm events, can elicit avoidance behaviors. In 1991, during an ongoing lobster study, Hurricane Bob caused a substantial drop in salinity in the Great Bay Estuary, New Hamphire, USA. This freshet, and possibly related environmental perturbations, was associated with increased movement of lobsters down the estuary toward the coast and a rise in catch in areas closer to the coast. Thus, our data support the hypothesis that storms can induce movements of lobsters and subsequent transient shifts in the demographics of the lobster population

Lobster movements in response to a hurricane

There are several anecdotal reports of the American lobster Homarus americanus moving to deeper water, or suffering increased mortality, in response to storm events. It has also been reported that changes in salinity and temperature, similar to those resulting from storm events, can elicit avoidance behaviors. In 1991, during an ongoing lobster study, Hurricane Bob caused a substantial drop in salinity in the Great Bay Estuary, New Hamphire, USA. This freshet, and possibly related environmental perturbations, was associated with increased movement of lobsters down the estuary toward the coast and a rise in catch in areas closer to the coast. Thus, our data support the hypothesis that storms can induce movements of lobsters and subsequent transient shifts in the demographics of the lobster population

Comparative Growth and Survival of Juvenile Atlantic Cod (Gadus morhua)Cultured in Copper and Nylon Net Pens

Bio-fouling on net pens has been a major concern for the marine aquaculture industry. As cage systems increase in size, so does the surface area for the attachment of colonial organisms that create drag on the net, reduce water flow important to fish health, and increase operational expenses due to net cleaning. To solve this problem, the International Copper Association (ICA) has been developing copper alloy netting for sea cages. Copper netting has unique properties that minimize bio-fouling, reduce the risk of fish escapement, prevent predators from entering the net pen, and is recyclable. To test the alloy netting, an experiment was conducted to compare juvenile cod cultured in traditional nylon nets with cod grown in Seawire copper netting ([email protected]). Six, 0.78 m3 cages were each stocked with 200 Atlantic cod (Gadus morhua) averaging 29 ± 2.2 g and grown for 4 months in coastal waters of New Hampshire, USA. Results of the study indicated no significant differences in cod growth, survival, feed conversion ratio (FCR), specific growth rate (SGR), or Fulton’s condition factor (K) between the fish grown in the copper alloy and nylon nets. A chemical analysis was conducted on the cod and indicated no differences in copper levels in muscle, liver and gill tissues taken from the net treatments. Nylon nets with antifouling paint accumulated significantly more bio-fouling than the copper nets. Materials that were in direct contact with the copper netting (plastic cable ties) fouled heavily with hydroids indicating minimal leaching to the environment. This study describes some of the beneficial attributes of copper netting, however future studies need to be conducted over a longer period of time, on a larger scale, and in a more energetic environment to definitively test the utility of this new product

Lobster movements in an estuary

The extent to which the American lobster, Homarus americanus (H. Milne-Edwards), utilizes estuarine habitats is poorly understood. From 1989 to 1991 we examined lobster movements in and around the Great Bay estuary, New Hampshire using tag/recapture and ultrasonic telemetry. A total of 1212 lobsters were tagged and recaptured at sites ranging from the middle of Great Bay, 23.0 km from the coast, to Isles of Shoals, 11.2 km offshore. Twenty-six lobsters equipped with ultrasonic transmitters were tracked for periods ranging from 2 weeks to \u3e1 year. Most lobsters moved \u3c5 km toward the coast, with those furthest inland moving the greatest distance. Lobsters with transmitters moved in a sporadic fashion, with residency in one area for 2 to 4 weeks alternating with rapid movement to a new location (mean velocity = 0.3 km d−1, 1.8 km d−1 max.). Site of release influenced distance moved, but there was no significant relationship between lobster size and distance traveled, days at large, or rate of movement. Most movement into the estuary occurred in the spring, while during the remainder of the year there was a strong tendency to move downriver, toward the coast. These seasonal migrations of estuarine lobsters may enhance their growth and survival by enabling them to avoid low salinity events in the spring and fall, and to accelerate their growth in warmer estuarine waters during the summer

The Influence of Stocking Density on the Swimming Behavior of Adult Atlantic Cod, Gadus morhua, in a Near Shore Net Pen

Cage design and stocking density are important aspects of aquaculture farm design, therefore understanding how fish behave at different stocking densities is critical information for farm managers. In this study, high resolution acoustic telemetry was used to investigate the swimming behavior of adult Atlantic cod, Gadus morhua, that were stocked at four densities (5, 10, 25, and 45 kg/m3). Acoustic tags were placed into the abdominal cavity of five fish per density treatment so their swimming behavior could be continuously monitored throughout the study. An array of hydrophones made it possible to calculate the position of each fish in three dimensions, at ∼2–5 sec intervals, for 4–30 d. Three underwater cameras were used to obtain additional data about the distribution of fish in the cage during the daytime. At the lowest density, the cod spent the majority (64.3 ± 0.08%) of their time in the bottom third of the net pen. As density increased, the fish moved higher in the water column, and this behavior was most evident at night, at all densities. At no time throughout the entire study were there any obvious occurrences of schooling behavior, even at the highest density (45 kg/m3)

A self-contained system for observing and quantifying the behavior of Atlantic cod, Gadus morhua, in an offshore aquaculture cage

A self-contained data collection system is described that was used to investigate the behavior of Atlantic cod (Gadus morhua) in an offshore net pen (Sea Station 3000) located 13 km off the coast of New Hampshire, USA. The entire system was housed inside a modified U.S. Coast Guard (USCG) navigational buoy that was retrofitted for this purpose. Power was provided by a combination of eight 12 V batteries, two solar panels and a wind generator. The behavior of the population of cod as a whole, during daylight hours, was monitored using four waterproof cameras connected to a four channel digital video recorder. The behavior of 4–12 individual fish implanted with ultrasonic transmitters was continuously recorded, during each of four study periods, using a HTI model 291 ultrasonic telemetry system. Laboratory studies showed no influence of transmitter implantation on swimming or feeding behavior. Transmitters were programmed to “ping” at intervals between 1.7 and 3.3 s and they typically lasted for about one month. The system successfully detected and plotted 84.9 ± 6.0% of transmissions, resulting in an average of 1283.4 ± 252.5 positional fixes for each animal, during each hour of the study. This preliminary evaluation of cod behavior in a net pen demonstrated that they are diurnally active and have a tendency to mill about, rather than school. Cod predominately used the lower half of the cage, except when rising to the feeding area during periods when feed was delivered. The system that was developed proved to be ideal for investigating the behavior of fish within a net pen, and it can be used by both inshore and offshore farms to gather behavioral data that can lead to improvements in the efficiency of aquaculture operations

The behavior of cod (Gadus morhua) in an offshore aquaculture net pen

Despite the rapid growth in cod (Gadus morhua) aquaculture, relatively little is known about the fundamental aspects of cod behavior within net pens. To investigate some aspects of their behavior, we used a high resolution ultrasonic telemetry system that makes it possible to continuously and accurately monitor the position of multiple fish, in 3D, for days at a time. The data presented in this manuscript were obtained from a total of 32 cod (29.3 ± 4.9 cm, range: 22.5–43.0 cm), tracked under a variety of conditions, inside a net pen located 13 km off the coast of New Hampshire, USA. Typically, cod exhibited clear diurnal rhythms, with the highest swimming activity during daytime hours (mean swimming speed = 17.8 ± 5.5 cm/s; 0.6 ± 0.2 body lengths per sec (BL/s)) compared with nighttime hours (6.6 ± 0.5 cm/s; 0.2 ± 0.03 BL/s). Analysis of net pen utilization revealed that: a) the entire volume of the net pen was not utilized (volumes used by individuals overlapped); and b) there was a spatial preference for the lower half of the net pen. Adding lights and increasing stocking densities elicited dramatic changes in behavior. When lights were turned on at dawn and dusk, to extend the day photoperiod, cod increased their swimming activity by 66.4% and 202.6%, compared to day and night levels, respectively. When the density of cod reached high levels (~ 48.5 kg/m3) as they grew, their typical diurnal pattern of independent swimming changed to schooling behavior, with no significant difference in day and night activity. The results of this study could be used to optimize various aspects of cod aquaculture including: net pen geometry, feeding schedules, stocking densities, and the use of artificial lights. Implementation of these modifications could increase production efficiency and animal welfare

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

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

The behavior of lobsters in response to reduced salinity

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 (\u3c one body length) out of their shelter when the salinity reached a level of 18.4 ppt ± 1.42 (SEM), and definitively moved away from their shelter (\u3e 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) 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) when exposed to low salinity of sufficient magnitude, they attempt to avoid it, and; (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