Ecological conditioning in hatchery-reared lobsters

‘Ecological conditioning’ is a term encompassing a range of processes by which captive-reared animals are encouraged to develop behavioural and physical traits which prepare them for life in the wild. Teaching an abandoned seal pup to catch fish for itself, or the ‘soft release’ of a rehabilitated bird (allowing it to leave its enclosure in its own time over the course of weeks or months) are examples of ecological conditioning. Common among most ecological conditioning programs is ‘environmental enrichment’, the provision of conditions in a rearing habitat which mimic those that the species may be used to interacting with in the wild, either to ready animals for release or simply to provide them with behavioural and neural stimuli in captivity.

For the past few decades, lobsters have been reared in hatcheries to facilitate their reintroduction to areas they had been wiped out, or their release to try and sustain commercial fisheries. To date, hatcheries have typically released ecologically naïve lobsters, whose rearing environment bears little resemblance to that of wild habitats. Fortunately, filmed observations of ecologically naïve hatchery-reared lobsters being released have shown that they retain some innate behavioural instincts which promote survival in the wild, such as aggressive use of the claws when confronted by other creatures, or a tendency to hide into cracks and crevices on the seafloor. Nevertheless, a growing number of studies have implied that the benefits of environmental enrichment are likely to extend beyond the captive welfare of hatchery fish and crustaceans, and into their fitness in the wild.

The most comprehensive study to date was published last month, by a collaboration between Institute of Marine Research and the local University in Bergen, Western Norway. The study, published by the journal PLOS ONE with unrestricted access online (http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0159807), shows that compared to naïve lobster juveniles reared in isolated plastic compartments, ecologically conditioned lobsters reared communally in tanks with natural substrates were more wary and more likely to occupy a shelter burrow. Clearly these are attributes likely to increase the survival of juvenile lobsters in the wild. When naïve juveniles were introduced into the enhanced communal habitat, they also grew and survived less well than the pre-exposed juveniles. This suggests that hatchery-reared lobsters have a better chance of supplementing existing wild stock (the main goal of hatchery stock enhancement initiatives) having been ecologically conditioned before release.

Providing lobsters with enriched environments in aquaria is tricky, especially when balanced against the need to constrain the spatial and financial requirements of juvenile rearing. The Norwegian researchers’ enriched environment consisted of a ~1.5m square tank filled with coarse sand, upon which scallop shells provided individual shelters for about 80 lobsters. By contrast, an un-enriched rearing upweller (http://www.aquahive.co.uk) can house over 4000 juveniles in the same space, so while it is desirable to introduce enrichment and conditioning processes to the rearing of hatchery-born lobsters, new techniques are required to achieve this in practice. One area of promise is the development of mariculture systems for the species – the on-growing of hatchery lobsters in containers submerged at sea. Having been reared through larval phases in the hatchery, maricultured lobsters are deployed into compartmentalised containers suspended in the sea, where they are able to feed on planktonic and encrusting organisms, experience natural sensory stimuli, and interact with other settling animals in a relatively complex and natural micro-environment.

Research by the National Lobster Hatchery in Cornwall has previously shown that rearing at sea has been found to accelerate growth and improve survival compared to traditional aquaria-based on-growing in mono-layer tray systems (https://www.researchgate.net/publication/277961947_Development_of_sea_based_container_culture_for_rearing_European_Lobster_Homarus_gammarus_around_South_West_England), with maricultured lobsters appearing better prepared for wild release, both physically and behaviourally. Lobster Grower 2 now gives us the opportunity to assess the practicalities of rearing juvenile lobsters at sea at a semi-intensive scale, and will provide us an unprecedented opportunity to explore how and why maricultured lobsters differ to aquaria-reared equivalents, and potentially establish marine on-growing as a transitional phase by which hatchery lobsters can be ecologically conditioned for wild release, and by which the benefits of hatchery stocking to lobsters fisheries can be improved.

Pic: Cultured juvenile European lobsters on-grown in sea containers in an open bay (c) and estuary (b) show increased growth and pigmentation compared with equivalents reared only in the hatchery (a).