Biological and economic characteristics associated with the body size of commercially important Aspidochirotide sea cucumbers

Abstract

As a delicacy, processed body-wall of sea cucumbers, bêche-de-mer, is among the most high-valued of seafood commodities harvested globally to supply, almost exclusively, demand from the Chinese market. Prompted by unsustainable fishing practices targeting more than seventy species, predominantly from the order Aspidochirotida, fisheries management is increasingly being implemented, with mixed success and ongoing difficulties of implementation. Knowledge gaps regarding holothuroids life-history traits are a common constraint, especially due to the dearth of methodologies suitable when studies need to consider their high morphological plasticity. This thesis investigated interdisciplinary aspects of size in holothuroids, from pricing of bêche-de-mer to methodological approaches for determining reproduction and growth of Holothuria leucospilota, used as a model species, in Hong Kong waters. In Hong Kong, the global hub of bêche-de-mer trade, a dozen high-value species were identified in the local market. A hedonic analysis revealed that prices varied mainly according to species, size and physical damage attributable to inappropriate post-harvest processing. Higher prices were significantly associated with larger bêche-de-mer, except for the temperate Apostichopus japonicus. As many bêche-de-mer were in their sexually immature size-ranges, effective application of minimum size-limits regulations would benefit from economic incentives derived from species-specific relationships between size and price. Reproduction in Holothuria leucospilota was determined in Hong Kong, which could be studied in a non-exploited state. The relationship between body size and gonad index was examined in detail. Risks of misinterpretation of gonad index are rarely considered in holothurian research, so this work explored the problems that arise when different body sizes are sampled over time and an allometric relationship holds between gonad and body weights. An innovative gonad index was successfully applied, removing the confounding effect of body size, and describing the same reproductive cycle as histological gonad analysis. To validate the use of fluorochromes to batch-mark Holothuria leucospilota, experiments were conducted in outdoor tanks. Preliminary calcein marking could be detected for a year in microscopic ossicles extracted non-lethally from dermal samples. Four fluorochromes, calcein, calcein blue, xylenol orange and tetracycline, successfully marked ossicles of juveniles and adults H. leucospilota for one year. Marked ossicles were detected after a year with 4% error in tag detection. Growth of holothurians was not affected by fluorochrome marking, although consistent weight loss occurred under experimental conditions. Several methods, from enclosure experiments and modal progression analysis to batch-tagging mark-recapture experiments, were investigated to determine growth of H. leucospilota. Although an effect of captivity was suspected, the variation in weight of H. leucospilota in the enclosure experiment appeared to be seasonal. Consistent negative growth rates during winter were confirmed with the other methodologies. Finally, as size is an important characteristic of the economic value of bêche-de-mer, further investigation on growth, longevity and reproductive capacity is essential for assessing stock renewal and potential for aquaculture. Based on life-history strategies exhibited by H. leucospilota in Hong Kong, further recommendations were proposed for fisheries management of Indo-Pacific species in the Asian subtropical area.