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Monitoring Impacts

Since the 1980s, the tagging of released juvenile lobsters has established proof that hatchery-reared individuals can survive in the wild, recruit to the mature breeding stock, and contribute to fisheries landings.

However, key questions about the ultimate ecological and economic benefits of hatchery releases still remain, and we still have much to learn about how to maximise the effectiveness of lobster stocking as a fishery and wildlife conservation tool .

To that end, we are exploring novel ways in which we can distinguish hatchery-reared lobsters from natural counterparts, even after years of life in the wild. We have assessed the suitability and effects of implant tags in small juveniles, which are visible through the tail muscle, and continue to explore the potential of molecular markers to identify hatchery lobsters just from a sample of DNA. We hope these techniques will facilitate a new wave of monitored releases in the next few years, so that we can assess the impact of our release scheme more comprehensively, and compare its effectiveness to that of alternative strategies of sustainably managing lobster fisheries.

Lobster stocking impact assessments

2012-2015: Charlie Ellis – PhD thesis, University of Exeter. ‘Reproductive and molecular ecology of the European lobster: Implications for conservation management.’

Collaborative research between the University of Exeter & the National Lobster Hatchery, funded by the European Social Fund (ESF) with additional funding from the Worshipful Company of Fishmongers.

The European lobster (Homarus gammarus) is an ecologically important benthic decapod which supports fisheries that are critical to the economic prosperity of coastal communities. However, populations across its range are pressured by rising exploitation, from which management has failed to prevent stock collapses in the recent past. Fisheries management of the species is significantly hindered by deficiencies in our knowledge of fundamental characteristics of population biology, including the connectivity and genetic diversity of stocks. As a result, the effectiveness of strategies designed to conserve recruitment and ensure harvests are sustainable is poorly understood. This thesis focuses on elucidating aspects of reproductive and molecular ecology in H. gammarus which can be used to inform and appraise conservation management initiatives, currently applied via both the regulation of capture and the wild release of hatchery-reared juveniles. The size-specific fecundity of reproducing females was defined around southwestern UK, and spatial variation in clutch size between populations was linked to a longitudinal gradient in oceanic temperature range across Northern Europe. The reconstruction of paternal genotypes show that single males fertilise individual clutches, which hints at demographic stability within a productive Atlantic fishery. Population genetic structure, investigated at a fine spatial scale in the same region, evidenced high connectivity and suggests that the localised interventions of an active hatchery do not lead to juveniles being released beyond areas they might naturally recruit via planktonic dispersal. However, genetic differentiation and isolation-by-distance at a broad geographic scale indicate that direct gene flow between remote populations is limited, so that (i) a failure to maintain spawning stock biomass may negatively affect local recruitment, (ii) the utilisation of non-resident broodstock for hatchery stocking may cause a loss of adaptive potential, and (iii) the recovery of depleted stocks is likely to be problematic. Finally, simulations indicated that genetic parentage assignment will prove accurate in distinguishing cultured individuals from natural stock among admixed populations in the wild, an important development that should facilitate the optimisation of hatchery stocking and lead to rigorous assessments of the conservation value of releasing lobsters reared in captivity.

Ellis CD, Hodgson DJ, Daniels CL, Boothroyd DP, Bannister RCA, Griffiths AGF. (2015) European lobster stocking requires comprehensive impact assessment to determine fishery benefits. ICES Journal of Marine Science. 72 (S1): i35-i48, doi: 10.1093/icesjms/fsu196.

Historically, hatcheries in Europe and North America attempted to contribute to the conservation and enhancement of clawed lobster stocks, but lacked monitoring programmes capable of assessing success. In the 1990s, this perspective was changed by the results of restocking and stock enhancement experiments that inserted microwire tags into hatchery-reared juvenile European lobsters (Homarus gammarus) before release. This allowed recapture in sufficient numbers to prove that lobsters had survived and recruited to the mature fishable stock. However, evidence of recruitment still failed to answer key questions about the ultimate ecological and economic benefits. As a result, a growing number of lobster stocking ventures remain hindered by a lack of clear evidence of the effects of their stocking schemes. This review evaluates these experiments and related studies on other fished species, summarizes key findings, and identifies data and knowledge gaps. Although studies of fitness in cultured lobsters provide some of the most encouraging results from the wider field of hatchery-based stocking, the limitations of physical tagging technology have significantly hindered appraisals of stocking impacts. We lack basic knowledge of lobster ecology and population dynamics, especially among pre-recruits, and of the impact of stocking on wild lobster population genetics. We advocate the use of genetic methods to further our understanding of population structure, rearing processes, and stocking success. We also recommend that more focused and comprehensive impact assessments are required to provide a robust endorsement or rejection of stocking as a viable tool for the sustainable management of lobster fisheries.

Physical and genetic tagging

2012-2016: Charlie Ellis – University of Exeter, PhD thesis. ‘Reproductive and molecular ecology of the European lobster: Implications for conservation management.’
Collaborative research between the University of Exeter & the National Lobster Hatchery, funded by the European Social Fund (ESF) with additional funding from the Worshipful Company of Fishmongers.

Ellis CD, Hodgson DJ, Griffiths AGF. Evaluating parentage-based tagging for the identification of released hatchery lobsters. (2016 – Unpublished thesis chapter).

Recaptures of hatchery-reared European lobsters (Homarus gammarus) in the wild have provided a proof-of-concept that the release of cultured individuals can enhance the species’ valuable capture fisheries. However, several recent hatchery stocking initiatives are yet to monitor the wild stock for recaptures, in part due to unfavourable methods with which to tag released animals in order to distinguish them from natural conspecifics. To evaluate the suitability of parentage-based tagging to identify hatchery lobsters among admixed populations in the wild, we quantified the power and error of assignment to hatchery parents for stock samples simulated from known microsatellite genotypes. Assignment accuracy was improved where stock samples contained a greater proportion of hatchery individuals. Assignment solely via maternal candidate led to frequent false positives (>9.8% of allocations; >2.1% of natural stock) which increased in proportion to the number of candidates and always resulted in an overestimation of hatchery recaptures. In contrast, parent-pair assignment never overestimated the released component of the sample, greatly reducing false positives (to ≤2.0% of allocations; <0.3% of natural stock) and more accurately estimating hatchery stock size at all ratios of admixture. Parent-pair assignment yielded minor underestimates of the number of hatchery recaptures, but provided ≥86.0% power to distinguish hatchery and natural stock accuracy, and ≥96.8% power whenever hatchery recaptures comprised at least a fifth of sampled stock. Our results show that, where false positives can be controlled, genetic parentage assignment presents a powerful method for monitoring the contribution of released lobsters to admixed wild stocks, and should be used to inform the optimisation and appraisal of hatchery stocking programs.

2012: Sarah Neenan – University of Exeter, MSc thesis. ‘An investigation into the suitability and effectiveness of VIFE tags in juvenile European lobsters to aid fisheries stock enhancement.’ 

Neenan ST, Hodgson DJ, Tregenza T, Boothroyd D, Ellis CD. (2014) The suitability of VIE tags to assess stock enhancement success in juvenile European lobsters (Homarus gammarus). Aquaculture Research. 46 (12), 2913–2923, boi:10.1111/are.12445.

Assessments of stock enhancement programmes for European lobsters (Homarus gammarus) require mark-recapture analysis of stocked individuals. However, established tag technology is deemed unsuitable for extensive use by many current lobster hatcheries, particularly upon the early juvenile stages. We tested the suitability of fluorescent Visible Implant Elastomer (VIE) tags for use in 5-month-old juvenile lobsters. Three treatment groups comprising 348 cultured lobsters in total were used to examine survival, growth and tag retention, and to assess mobility, shelter use and moulting behaviours. Tagging had no significant effect on lobster survival, growth, mobility, shelter use or moult frequency. Survival over 7 weeks was 75% among lobsters tagged with two elastomers, 76% in those with one elastomer and 74% among untagged controls. Mortality during moulting did not increase in tagged (6%) compared to untagged lobsters (9%). We found no evidence that VIE tags cause any negative effects that would be expected to inhibit survival upon wild release, but tag loss had reached 12% in both tagged treatments after 7 weeks and showed no sign of abating. Our study suggests that VIEs effectiveness in discerning cultured lobsters long after wild release may be limited when used in smaller juveniles.