Female lobsters lay their eggs between August and October. The eggs remain attached to appendages (pleopods) on the underside of the tail until they hatch between March and September the following year. When the eggs are first laid they are very small and dark green. As they get closer to hatching, they develop, changing colour to black and then to red just before they hatch.
In Cornwall there is a fisheries byelaw in place, preventing fishermen from landing egg carrying female lobsters (berried hens) within the six mile limit. However, special permission has been granted to a number of fishermen which allows them to land berried hens for the hatchery. These fishermen, along with shellfish merchants and restaurants, lend berried hens to the hatchery, where they are kept in our broodstock facility until the eggs hatch. The mother lobsters are then returned to their rightful owner.
Larval collection and rearing
Hatching normally takes place overnight in the broodstock tanks, with a single female spawning up to 12,000 eggs over a three day period. The larvae are collected using rigid plastic mesh strainers. Before being transferred to the larval rearing containers they are sterilised with a mild disinfectant to prevent introduction of harmful pathogens into the larval rearing system.
The vessels used to rear the lobster larvae are cones containing 70 litres of seawater which is constantly recirculated through a filtration system. Water is injected at the bottom of the cone and overflows into a central standpipe. This system, known as a Kreisel system, is important in keeping the larvae suspended throughout the water in the cone and imitates their natural environment where, in the wild, they would live as part of the plankton. The larvae are normally stocked in the cones to a density of 20 to 25 larvae per litre. At lower stocking densities, cannibalism is reduced and therefore the overall survival rate is increased. The larvae remain in the Kreisel system for the planktonic phases of their lifecycle (metamorphic stages I, II and III) which normally lasts between two and three weeks, dependent on water temperature.
Whilst the larvae are being reared they are fed a mixture of diets. The earlier stages, stages I and II, are fed live brine shrimp nauplii (Artemia). The Artemia are hatched from dry cysts in saltwater at 28 degrees C. The hatched Artemia nauplii are then enriched with essential nutrients required by the lobster larvae. Pre and pro-biotics are also added to the Artemia enrichment to aid survival of the larvae. The older lobster larvae, late stage II and stage III, are also fed frozen copepods, krill and mysid shrimp. Food densities are monitored closely throughout the day because surplus food in rearing vessels may cause problems with water quality.
In the wild, when the larvae metamorphose into stage IV of their lifecycle, they move from a planktonic to a benthic lifestyle (they settle out of suspension and live on the seabed). At this stage in the lobster hatchery, they are removed from the larval cones and placed in individual on-growing containers. The lobsters are kept separate to avoid cannibalism and harm caused by aggression with one another. The lobsters and on-growing containers are kept in long raceways containing recirculated seawater. As the lobster moults and increases in size they are transferred to larger on-growing containers because their growth can be stunted when living in a confined space.
The juvenile lobsters are fed a diet of dried pellets every other day. This is quite a time consuming process because each lobster is given one pellet each and when the raceways are full, there are up to 4000 juveniles to feed in the hatchery. Any uneaten food must also be removed from the on-growing containers at the end of the day, in order to maintain high water quality. As a result of our research at the NLH we are able to use our own formulated pellet that has been supplemented with pre and pro-biotics that has been proved to boost survival.
Juveniles are kept in the hatchery for up to three months before they are released into the sea surrounding Cornwall. For more information on release sites and techniques see the ‘Lobster releases’ page.