A caution for conservation stocking as an approach for recovering Atlantic eels

Abstract

Worldwide concerns about declining eel stocks have resulted in the increased use of conservation stocking (i.e. the translocation of early eel life‐stages from areas of high recruitment to areas experiencing recruitment declines) as a recovery tool.North Atlantic eels have complicated and incompletely understood life‐history strategies with major differences in life‐history parameters across the species’ broad distributions, and these differences may be important for migrating and spawning successfully.Sex ratio, growth rate and size and age at maturation of American eels (Anguilla rostrata) stocked into the St. Lawrence River basin from donor areas over 2000 km away with differing life‐history characteristics were compared with those of naturally recruited eels to assess the effectiveness of a potential stocking programme in maintaining a sub‐population with unique life‐history characteristics (largest from across the species range, and exclusively female).Stocked eels exhibited faster annual growth, had a different sex ratio, and matured and outmigrated at smaller sizes and ages than naturally recruited eels.Conservation stocking should be applied with caution, as stocked eels appear to be following life‐history patterns comparable with conspecifics in the geographic range of the donor streams where they were collected. These findings cast doubt on the generally accepted hypothesis that the mechanisms driving eel life‐history variation are environmentally induced, and suggest that more care should be taken in assessing and matching the life‐history characteristics of donor and recipient sub‐populations if conservation stocking is expected to be an important recovery option for eel restoration. © 2014 Her Majesty the Queen in Right of Canada. Aquatic Conservation: Marine and Freshwater Ecosystems © 2014 John Wiley & Sons, Ltd.

What happens to glass eels after restocking in upland rivers? A long‐term study on their dispersal and behavioural traits

“…This finding suggested that the G 2 individuals grew faster and could mature and migrate earlier than those in the other eel groups, leading us to question whether the G 2 group will have the energy to migrate successfully to the sea once arriving at the silver eel stage, compared with eels dwelling for a long time in rivers to reach larger sizes before maturing and undergoing seaward migration. According to Stacey, Pratt, Verreault, and Fox (), a substantially earlier maturation age associated with faster annual growth in eels may eliminate the benefit of conservation restocking because of the long migration that eels must undertake from the study area to the Sargasso Sea. In this study, the eels were in the fifth year of their lives in fresh water, after being restocked as glass eels in May 2013, and G 2 was the least abundant group.…”

RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel

“…Mathematical modeling efforts also indicate that within-generation selection can result in differences in quantitative traits, even in panmixia (see the Supplemental Discussion for more details) [31][32][33]. In the large-scale transplant (see above), the eels grew fast, matured early, and out-migrated at a young age [34]. Given that eels from that area have the longest migration back to the Sargasso Sea, it is unknown whether the transplanted young eels would have the energy reserves for the spawning migration.…”

Synergistic patterns of threat and the challenges facing global anguillid eel conservation

“…eels that have been either translocated to new areas or intensively grown out in captivity before being released) contribute significantly to the overall spawning stock biomass for a species (Westin, 2003;Limburg et al, 2003;Westerberg et al, 2014), also making population data analysis more challenging. Indeed recent studies suggest that translocated A. rostrata might in fact retain life history parameters (including sex ratios and size at, and timing of, out-migration), comparable with their previous growth or recruitment habitat and thus greater care should be taken to match donor and recipient characteristics (Couillard et al, 2014;Stacey et al, 2014). This debate will be critical to the future management and conservation of all anguillids as at present it is being implemented as a management strategyas part of EU Eel Management Plans for example -without a full understanding of its effectiveness.…”