Genetic diversity is maintained during lobster stocking

I have completed an important study that found critical genetic diversity of wild stocks is not impacted by hatchery lobster releases.  Working together with local pot fishermen, the National Lobster Hatchery (NLH) and colleagues at the University of Exeter, I recently completed the 20-month research project, funded by DEFRA under the Fishing Industry Science Partnership scheme.  Across the ‘GEMALOR’ project (Genetic Management of Lobster Releases), we set up and managed a series of strictly controlled rearing trials to assess the survival of individual sibling clutches within the mixed larval batches typical of hatchery culture systems.  This allowed me to trace release-age juveniles that survived communal culture back to mother lobsters using genetic parentage assignment, revealing how many juveniles each mother was providing.  This provided vital data with which to assess variation in the growth and survival of juveniles both within and between larval clutches, via which we were able show that, despite some sibling clutches dominating communal culture, sufficient genetic diversity is maintained to ensure that hatchery releases should not undermine sustainability objectives for wild stocks.

Adult lobsters on the quayside in Padstow, Cornwall, awaiting tissue samples for the project's wild fishery survey

Heightened survivability among cultured individuals is inherent to the viability of hatchery-based fisheries enhancement – naturally, more offspring have to survive in a hatchery than they would in the wild environment for hatchery stocking to work, and this is the case for lobsters.  An associated side-effect of this is that a batch of hatchery juveniles will tend to contain many more siblings and come from far fewer parents than an equivalent number of wild juveniles.  As a result, hatchery batches tend to exhibit lower genetic variation than wild equivalents.  Where this effect – ‘genetic bottlenecking’ – is severe and unmitigated it can have profoundly negative impacts on the health of the targeted population in the wild; in 2012, American scientists discovered that a hatchery release program had doubled the number of trout in a Pacific river system, but had drastically cut genetic diversity by almost two thirds.  Genetic variation is crucial to the health of animal populations, as it is the latent potential from which genetic adaptations may arise (i.e.  to help organisms respond to changes in their environment, such as those driven by climate change), as well as helping to prevent hereditary diseases and so-called ‘inbreeding depression’.  Our study assessed the extent of genetic diversity loss in hatchery lobsters for the first time, to ensure stocking releases aren’t harming the long-term viability of the wild stocks we’re attempting to conserve.

Juvenile lobsters of the same age and rearing environment showing pronounced size variation

The scientific logistics of GEMALOR proved a major challenge for our collaboration, but it was one our brilliant team were able to rise to!  For a start, each of our 56 larval rearing trials had to begin with precisely balanced representation from each sibling cohort, meaning that mother lobsters had to be separated and their larvae counted manually every day by NLH staff.  Across the course of the project, this meant some 116,000 newly-hatchery lobsters were exhaustively counted by dripping them slowly from jugs into our culture tanks, with sibling clutches from 2-5 mothers reared together in each tank!!  Over 3,700 release-age juveniles were tissue-sampled, and then the SNP genotyping tool developed in Exeter’s Molecular Ecology and Evolution research group was used compared their genetic profiles to those of 60 mother lobsters to resolve parent-offspring relationships.  This revealed that severe biases in survivability were common, with the offspring of some mothers doing far better than others in communal culture environments.  Of the 56 trial replicates, 43 exhibited survival variability which deviated significantly from normality, and the average pairwise skew in offspring success between was approximately 4 juveniles from the dominant mother to every 1 juvenile from the less successful mother.  Mother size was a factor, with larger mothers tending to produce larger juveniles which survived better, although there was extensive individual variation, with some mothers consistently over- or under-performing their expected breeding success, due to some apparent fitness qualities that may well be genetic in origin.  Importantly, our data showed no trade-off between offspring quality and quantity, so there is no evidence that juveniles from the most over-represented clutches were typically smaller (which might have indicated they were introducing genes associated with slow growth into wild stocks).  

Exeter researcher Karolina Gallagher in the midst of a mammoth task to measure over 3,500 juvenile lobsters!

Despite detecting big variability in offspring rearing success between mothers, this research suggests that genetic diversity losses across hatchery culture processes are negligible.  Levels of genetic diversity among released hatchery juveniles are similar to those found in the wild, and there is no evidence that over two decades of releases by NLH have negatively impacted the genetic diversity of wild lobsters around Cornwall.  Indeed, a survey of genetic profiles from over 600 wild lobsters showed that genetic diversity was actually slightly higher in two hatchery release sites than it was in two unstocked control areas, and full sibling pairings were found to be only one tenth as frequent in Cornish stocks as they are in a stock of low genetic diversity in the Netherlands.  We concluded that lobster hatchery ventures like NLH – which utilise a high number of local wild broodstock to rear and release juveniles into productive lobster fisheries – pose negligible threat to the genetic diversity of wild lobster stocks.  This is great news for the UK's growing number of lobster hatcheries; it means they don’t need to develop extensive modifications to current rearing techniques in order to conserve the genetic fitness of the juveniles they produce, and means that they can continue to increase their release efforts without threatening the long-term health of the fisheries they're working to safeguard.  My thanks go to my amazing collaborators at NLH and the University of Exeter who helped deliver this research, all the fishermen and shellfish merchants who generously gave up time to support it, and of course DEFRA for their support in funding it.  A scientific report detailing the full findings of the GEMALOR study is currently under independent review for publication by an academic journal, which I will share as soon as it becomes publicly available.    

Dr Charlie Ellis – Postdoctoral Research Affiliate, the University of Exeter.