ASSESSING THE SUITABILITY OF SUSPENSION- AND DEPOSIT-FEEDING SEA CUCUMBERS AS EXTRACTORS OF PARTICULATES IN INTEGRATED MULTI-TROPHIC AQUACULTURE (IMTA)  

Integrated multi-trophic aquaculture (IMTA) is a technique whereby both inorganic extractive species (e.g. seaweeds) and organic extractive species (e.g. bivalves, sea urchins, sea cucumbers, worms) are grown in close proximity to traditional fed finfish or shellfish culture to utilize the waste products and thereby reduce environmental impacts. One group of organisms that may be particularly appropriate for uptake of larger, heavier particulates that accumulate underneath aquaculture farms is the Holothuroids. In Canada, there are two species that are commercially fished and have established markets, which may be incorporated into IMTA systems - the orange-footed sea cucumber (Cucumaria frondosa), a suspension-feeding species on the east coast, and the California sea cucumber (Parastichopus californicus), a deposit-feeding species on the west coast. Research conducted within the Canadian IMTA Network (CIMTAN) over the past five years has evaluated these two species as potential organic extractive species in their respective IMTA settings in Canada.

When exposed to the assemblage of natural particles in the environment, ranging in organic composition from 30 to 50%, C. frondosa is capable of absorbing the material with an average efficiency of 70 + 3%. When C. frondosa is exposed to particles of higher organic content (> 60%), such as salmon food and faeces, their absorption efficiency (AE) can be elevated above 80%. Using the biodeposition method, it was revealed that C. frondosa cleared an average of 7.2 L/day in the field and 3.6 L/day in the laboratory. An experiment conducted in an annular flume tank, where changes in particle size class concentrations were monitored, showed that C. frondosa is capable of ingesting large particulates up to 1.2 mm in diameter and likely has a wide diet range in terms of particle size.

When exposed to natural benthic sediment with a relatively low organic content (1.6%), P. californicus exhibited a relatively low AE of approximately 24%. This value nearly doubled to ~45% when it was fed an organically-rich (86.8% organic content) diet of sablefish faeces. As with other Holothuroid species, both C. frondosa and P. californicus show elevated AEs with increasing organic content of the feed. Field trials with P. californicus have shown that the species can reduce total organic carbon and total nitrogen levels underneath a commercial sablefish farm by an average of 60.3% and 62.3%, respectively, showing its usefulness in reducing organic levels underneath fed finfish cages in an IMTA setting.

Both C. frondosa and P. californicus demonstrate a great deal of biological potential as organic extractive species in their local IMTA settings. The economic potential is still being investigated.