The California sea cucumber (Parastichopus californicus) has been identified as an ideal species for Integrated Multi-Trophic Aquaculture (IMTA) due to its nutrient recycling capability and high market value. The overall objective of the study was the design of a cage that effectively contains juvenile sea cucumbers in a way that maximizes extraction of large-particulate waste (i.e. faeces/pseudofaeces) within an IMTA system with Pacific oysters (Crassostrea gigas). Because of the sea cucumber's morphology and behaviour, small mesh sizes are required for effective containment, which could restrict the flow of farm particulates to the sea cucumbers, hence reducing overall IMTA system efficiency. That factor, as well as animal behaviour and habitat preference, were carefully considered when developing new suspended cage designs. High Flow™ oyster grow-out trays (L × W × H: 56.25 × 56.25 × 21.25 cm, mesh size: 1.2-3.0 cm) were chosen as the basic cage since they are commercially available, relatively cheap, and because, at present, there are no commercial cages designed specifically for sea cucumber culture. Three basic cage designs were tested: (1) unmodified High Flow™ oyster cage with lid ("Unmodified"), (2) High Flow™ cage with fine mesh (mesh size: <1 mm) on the sides and lid ("Fine Mesh"), and (3) High Flow™ cage with fine mesh (mesh size: <1 mm) on the sides, with no lid, and an encircling mesh fringe (mesh size: 250 µm) along the top rim to potentially restrict emigration ("Fringe"). Each of these cage designs incorporated either no oyster shell or oyster shell (weight: 2 kg) as a substratum for the juvenile sea cucumbers, for a total of six cage types.
These six cage treatments (n=5) were tested at a Pacific oyster (Crassostrea gigas) farm where they were suspended beneath rafts from May to Nov. A control Fine Mesh cage with oyster shell was established at a reference site 320 m away. Containment and growth of the sea cucumbers were monitored over time. Oyster faeces/pseudo-faeces deposition and total organic matter (TOM) in each cage were also examined. The effects of oyster shell presence/absence were non-significant for the parameters studied and the values below reflect combined treatment means (i.e. n=10). The Fine Mesh cages contained significantly more sea cucumbers (mean ± SE: 77.1 ± 7.9%) than the Unmodified (21.4 ± 5.4%) and Fringe (28.6 ± 4.8%) cages. The Fringe cages at the oyster farm contained significantly more sea cucumbers (38.7 ± 6.3%) than the ones suspended at the control site (20.0 ± 2.7%). All three cage types had significantly different sediment retention and TOM rates: Fringe cages (3.93 ± 0.96 g m-2 day-1, 0.37 ± 0.10 g m-2 day-1, respectively), Unmodified cages (1.60 ± 0.60 g m-2 day-1, 0.15 ± 0.06 g m-2 day-1), and Fine Mesh cages (0.12 ± 0.03 g m-2 day-1, 0.03 ± 0.01 g m-2 day-1). Absloute growth rates (g d-1) of sea cucumbers in the fall (Oct./ Nov.) were negative and not significantly different among cage types. Absolute growth rates in the summer (Aug./Sep.) were postive with those in the Fine Mesh cages (0.01 ± 0.02 g d-1) being significantly lower than those in the Unmodified (0.22 ± 0.05 g d-1) or Fringe (0.19 ± 0.05 g d-1) treatments. Our results indicate that there is a trade-off between waste capture/sea cucumber growth and sea cucumber containment efficiency, dictated by cage mesh size.