Sea ranching release techniques for cultured sea cucumber Holothuria scabra (Echinodermata: Holothuroidea) juveniles within the high-energy marine environments of northern Australia

Abstract

The valuable sea cucumber Holothuria scabra has potential as a ranching species to contribute to the economic growth of northern Australia. However, the high-energy environments of north Australia present certain atypical environmental challenges for sea cucumber ranching, proving commonly practiced ranching techniques ineffective. This study aimed to develop a suitable method for the effective deployment of cultured sea cucumber juveniles within the constraints of north Australian environmental conditions. Two boat-based release techniques were trialed: temporary, floorless cages; and a chute. Sea cucumber recovery rates, dispersal distance and direction were compared to assess the relative merit of each. The proportion of sea cucumbers recovered from the chute release treatment (18.9%) was significantly higher than those from the cage release treatment (10.6%). Recaptured individuals were assigned to their nearest release point, as the tagging method allowed differentiation between methods but not between sites. However, unanticipated sea conditions and equipment failure caused a flawed deployment at one site, which corresponded with atypical recapture results, relative to other sites, and led to the analyses of two possible scenarios explaining sea cucumber movement post-release. Scenario 1 assigned individuals by proximity to release points, and Scenario 2 assigned a group of far-lying individuals to the site where the flawed deployment occurred. There was no significant difference in mean angle (a−) of movement, between the chute and cage methods, for both scenarios (Scenario 1 - 80.3° and 71.16° respectively, Scenario 2 - 81.04° and 74.84° respectively) and the directionality statistic (r) and the angular dispersion (s*) for both methods and scenarios indicate that the strength of any preferred direction was very weak. There was a significant difference in mean distance travelled between the two methods for both scenarios (Scenario 1: cage and chute 10.8m and 20.2m with 95% CI's of 2.57m and 4.42m respectively; Scenario 2: cage and chute 11.1m and 20.4m with 95% CI's of 2.99m and 4.52m respectively). However, this difference is caused primarily by the group of individuals thought to have washed away from the flawed deployment–if these are treated as outliers and removed from overall distance and dispersion analysis, no significant difference is found between distance travelled (F1,184=1.25, p=0.264) or dispersion (F1,165=0.385, p=0.636) between the two methods. The cause of the flawed deployment at one site is easily rectified and, given the significantly higher recovery rates, lower cost, ease of construction and efficiency, the chute release method proved superior. The improved sea ranching release techniques identified in this study will provide practical and economically sound release methods for sea cucumber ranching, and stock enhancement activities across northern Australia.