The role of the sea cucumber, Stichopus mollis (Hutton) in a semi-recirculating polyculture system

Abstract

The bioenergetics of Stichopus mollis sea cucumbers collected from Mahanga Bay, Wellington were investigated to determine if S. mollis could increase the productivity of a polyculture system by utilizing the wastes of the main species, the paua Haliotis iris. The energy budget components - organic matter (OM) food consumption rate (FCR) and faecal production rate (FPR) were calculated for 3 size classes (small, medium and large) of sea cucumbers when fed 3 diets made of uneaten paua food (kelp flake or KF diet) and two types of paua faeces, one where paua were offered a 100% algal diet (algal waste or AW diet) and the other where a 50% algal and 50% artificial pellet diet (mixed diet or MW diet) was offered to the paua, at 3 temperatures (14, 16 and 18°C). Using the FCR and FPR values the change in organic waste volume in the polyculture system, if one sea cucumber was included, was estimated. Proximate analyses conducted on the diets determined their chemical composition. Oxygen consumption rates (OCR) and Total Ammonia Nitrogen production rates (TANPR) were also calculated. Energy budgets were used to calculate the production (P) of S. mollis when offered each diet at each temperature. However, to culture sea cucumbers it is also necessary to understand juvenile bioenergetics. A reliable source of juveniles was required to calculate the juvenile energy budget components however efforts to spawn wild adults and rear the larvae were unsuccessful. Information on the bioenergetics of juveniles remains limited. All sizes of sea cucumbers reduced the volume of organic wastes in the system, except for small and medium sea cucumbers when offered the MW diet at 14°C. OCR (14°C = 0.07 ± 0.02, 16°C = 0.11 ± 0.02 and 18°C = 0.15 ± 0.06 ml O2 g DW-1 h-1) displayed a positive correlation with dry weight and temperature. The TANPR of small and medium sea cucumbers demonstrated a positive correlation with temperature but the TANPR of large sea cucumbers did not. The low OCR and TANPR values indicate that the sea cucumbers will not deplete the oxygen concentration or increase the Total Ammonia Nitrogen (TAN) concentration of the water to toxic levels and are unlikely to have a negative effect on paua production. The interaction between diet energy content (ED total) and FCR of S. mollis have significant effects on the energy available for production (P). At the proposed system operating temperature of 18°C, the sea cucumbers have positive P values when offered the KF diet (ED total = 14.07 J mg-1, FCR = 1.01 ± 0.46 mg OM g DW-1 h-1, P = 8.87 J mg-1), and the AW diet (ED total = 10.93 J mg-1, FCR = 1.10 ± 0.32 mg OM g DW-1 h-1, P = 6.70 J mg-1), and a negative P value when offered the MW diet (ED total = 7.7 J mg-1, FCR = 0.26 ± 0.12 mg OM g DW-1 h-1, P = -1.96 J mg-1). Only the KF and AW diets were suitable energy sources for S. mollis. Including S. mollis in the polyculture system as a secondary product that reduces the volume of organic wastes could increase the overall productivity of the system if the paua are offered an algal diet or a kelp flake diet.