Berried Females & Temperature

Introduction:
This project was developed to obtain long-term records of the temperatures experienced by berried female lobsters (lobsters carrying eggs) and their eggs. To do this approximately 100 lobsters have been fitted with HOBO Tidbit temperature logging tags at 3 locations: 1) New Hampshire coast, 2) Mid-coast Maine and 3) offshore by commercial lobstermen.

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Objectives:
1. To measure the thermal history of berried females in the wild, in four distinct geographical locations, using temperature data-loggers attached to their carapace.

2. To determine if the movements of berried female lobsters results in an overall increase in the number of degree-days they experience.

3. To determine if the migration of berried females serves to increase the rate of egg development and subsequently decreases the incubation time.

Background

Management of the American lobster fishery is inextricably linked to a thorough understanding of their reproductive cycle. In particular, it is vital to know when females reach sexual maturity, where they release their larvae, and how larvae are transported in the water column and where they settle to the bottom, as this information influences the distribution and abundance of lobsters. A major goal of this project is to identify the factors that influence where female lobster release their larvae. If this question can be elucidated, it will greatly facilitate and improve models of larvae transport that are currently being produced by a number of marine biologists and oceanographers (Incze et al., 2003). These data will be invaluable for aiding in the determination of potentially distinct regional lobster stocks, a critical component of an effective management strategy for the American lobster fishery.

Female lobsters incubate eggs on the ventral side of their abdomen for approximately 9 months. However, gestation time can vary between 6 and 13 months, because development of lobster eggs is directly dependent on temperature (Perkins, 1972). In theory, females could speed up, or slow down, egg growth and development by moving into warmer or colder water. In fact, it is generally accepted that female lobsters move inshore during the warmer months and offshore in the colder months in order to expose their eggs to temperatures that cause their eggs to hatch at an optimal time for larval growth and survival (Campbell, 1986). However, recent work by Cowan et al., (in prep.) discovered that, while there was considerable variability in the distance moved by different berried females, the temperatures they experienced were very similar; as was the amount of time it took for the eggs to hatch. Furthermore, no matter how they moved, lobsters experienced at least some portion of the incubation period at temperatures < 5C. This period of time at a very low temperature may serve two purposes; first, it may serve to synchronize their reproductive cycle (Aiken and Waddy, 1976) and, second, it may actually delay development so that the eggs will hatch at the best time for their survival. For example, if a female extruded her eggs on August 1st, and was exposed to a constant temperature of 11C, larvae would start hatching on February 10th, when there are potentially lethally cold surface temperatures, and very little, if any, suitable food. Therefore, the view that most lobster movements can be explained in terms of a drive to seek higher water temperatures may need to be reexamined. The overall goal of this project is to test the long-standing hypothesis that the movements of female lobsters serve to increase the number of degree-days they experience and thus shorten the time period between egg extrusion and hatching.

Progress to Date

Tagging and Recapture:
Approximately 100 lobsters with freshly extruded eggs have been tagged and released in 3 different years. If and when these lobsters are recaptured their temperature loggers can be read while still attached to the lobster and she can be re-released. While the temperature loggers can log data for 4 years, the loggers will be lost when the lobster molts, likely in the early summer. This is okay, however, because we are most interested in information about movement and temperature exposure in the winter. This is the first year of two year project, so more lobsters will be tagged next fall.

Berried female lobster. Lobsters extrude eggs from a pair of oviducts at the base of their ventral abdomen. Eggs are fertilized with sperm that the female has stored in a specialized pouch located between her walking legs. Once the eggs are fertilized they flow down the tail and stick onto the swimerettes. Eggs grow here for 6-13 months before they are released as pelagic larvae.

Logging Data:
When a berried female lobster is caught participants log data into a datasheet that is later sent to us.

Downloading data to the computer. Once data is retrieved from the temperature loggers onto the shuttle it can be downloaded to the computer. To download to the computer the data is transferred from the shuttle to an optical basestation which is connected to a computer. Data can then be visualized using BoxCar software.

Sample output of temperature logger data displayed in BoxCar software. Participants save this data with the number assigned to the lobster and date and send these files to us via email.

Examination of the Eggs:
Staging guide for lobster egg development. Using a mathematical equation developed by Perkins (1972) the incubation time (time of extrusion to hatch) can be predicted for certain water temperatures as long as the size of the eye spot of the eggs is known.

Example of eggs collected for this project. Currently we are photographing and measuring the eyespots of eggs collected during initial tagging of berried females.

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