Efficacy of chemical markers and physical tags for large-scale release of an exploited holothurian

Hatchery-produced juveniles need to be distinguished from wild conspecifics in order to evaluate the success of restocking experiments and stock enhancement. The commercially valuable sea cucumber Holothuria scabra, or ‘sandfish’, has potential for stock enhancement but cheap, long-lasting tags have not yet been developed. We evaluated five non-genetic tags against several criteria: cost, ease of application, retention rate, and ease of detection. In a 1-month trial, T-bar tags and visible elastomer implants proved unsuitable on hatchery-produced sandfish juveniles. Coded-wire tags and two fluorochromes, tetracycline and calcein, showed more promise, so retention rates of these tags were tested on juveniles held in an earthen seawater pond for 1 year. Sixty percent of juveniles retained at least one of two coded-wire tags for 1 year. Under UV illumination, spicules from the body wall of sandfish immersed in solutions of tetracycline are yellow, whereas calcein-stained spicules are green. From months 1 to 12 after immersion, the average percentage of stained spicules declined from 40% to 8% with tetracycline and 54% to 5% with calcein. After 1 year stained spicules were detected in all immersed animals, i.e. mark retention was 100%. Stained spicules from the ventral body wall of juveniles were generally in greater proportions, and fluoresced brighter, than those from the dorsal body wall. Spicule density exceeded 7000 spicules per mm² of body wall in juveniles of 2–13 g, and decreased with sandfish weight. Fluorescence provides a simple diagnostic tool for identifying marked individuals and the sampling is non-destructive. Several mm² of outer body wall is ample for tag identification. Immersion-staining with fluorochromes is preferred to coded-wire tags and provided a cheap and reliable batch-mark for distinguishing released and wild sandfish.