Mixed-Age Releases of the Endangered Pinto Abalone (Haliotis kamtschatkana) to Maximize Hatchery Production and Survival in the Wild

The Washington State recovery programme for endangered pinto abalone (Haliotis kamtschatkana) relies on captive breeding. From 2009 to 2017, juveniles from wild parent crosses were released after an average of 20 months in the hatchery (average size 24 mm), and the results of a mark-recapture experiment suggested that size-at-release was not important to survival. The results of a pilot study suggested that abalone released at 9-month age survived at similar rates to previous releases at 20 months but that there was a significant cost to growth and survival for the 14-month releases. From 2019 to 2022, an average of 7000 mixed-age juveniles were released on a different subset of 24 restoration sites each year. Larger individuals (> 5 mm) from each family were released at 9 months (first years), with the remainder held in the hatchery to be released at 20 months (second years). The resulting survival at successful sites from 9- to 32-month age was 0.6%–6.1% for first years and 0.6%–4.5% for second years. In two out of three trials, there was little or no evidence of reduced survival or growth as a result of releasing almost a year early. When combined with the cost savings of rearing animals for a shorter duration, possible benefits to growth, reduction in hatchery acclimatization or selection, better rotation of hatchery resources and culture space and mixing 2 years of genetic crosses into one release, we suggest that mixed-age releases are the most efficient way forward for the conservation of pinto abalone.