CTSA Project Update: Integrated Multi-Trophic Aquaculture of Shrimp and Sea Cucumbers
Feed typically represents the single largest operating expense for aquafarmers and one of the greatest inefficiencies on many farms is waste of valuable nutrients from feed. Furthermore, many aquaculture farms incur costs to treat and/or dispose of nutrient-rich sludge generated from production systems.
A promising area of aquaculture research that directly addresses these inefficiencies is Integrated Multi-Trophic Aquaculture (IMTA) where “waste” nutrients from a “fed” species is taken up and incorporated into the biomass of another commercially valuable “extractive” species. Such a management strategy improves nutrient use efficiency, reduces waste volume and disposal costs, and creates an additional revenue stream. An ongoing CTSA-funded project at the Oceanic Institute of HPU is investigating an IMTA approach using sea cucumbers to digest waste produced from shrimp production systems. The two-year project “Integrated Multi-Trophic Aquaculture of Shrimp and Sea Cucumbers for Nutrient Recycling, Sludge Reduction, and Creation of Additional Revenue Streams” has completed Year 1 activities and will begin Year 2 activities shortly.
The first objective of the project was to collect candidate species of sea cucumber in near-shore waters of Oahu, Hawaii and screen them for relevant shrimp pathogens. After some initial challenges, researchers were able to collect 73 Holothuria atra and 54 Actinopyga mauritiana specimens, which were transferred to quarantine. The sea cucumbers were cohabitated with “sentinel” Pacific white shrimp, P. vannamei, which had also been maintained during quarantine of several other batches of sea cucumbers from the same collection area. These specimens were utilized for Year 1 trials. No mortalities were observed in sentinel shrimp during the quarantine and near the end of the quarantine period sentinel shrimp were PCR-screened for relevant shrimp pathogens including White spot syndrome virus (WSSV), Taura syndrome virus (TSV), Infectious hypodermal and hematopoietic necrosis virus (IHHNV), and the bacterium causing Early mortality syndrome (Vibrio parahemolyticus) (AHPND).
Researchers then set up sea cucumber experimental systems where growth, survival, and sludge processing capability for candidate species of sea cucumber could be quantified. A high-density (350 shrimp/m2) RAS shrimp trial was initiated to supply sludge for Year 1 feeding trials. During the RAS trial, sludge was collected from a bead filter 3× per week and stored in a cone-bottomed tank for ~1 day. To estimate a sludge feeding rate (based on volume), sludge was sampled for proximate/nutritional analyses. The protein content of sludge filtrate dry matter was determined to be 21.8% (typical aquaculture growout feeds may contain 30% protein or more on a dry weight basis). Gross energy content of sludge filtrate dry matter was determined to be 2,807 kcal/kg (typical crustacean aquaculture feeds contain 3,000-4,000 kcal/kg gross energy (Cuzon and Guillaume, 1997). Gross lipid content of sludge filtrate dry matter was determined to be 2.4% (typical aquaculture growout feeds may contain > 5% fats/lipids). Based on these proximate analyses, sludge dry matter was determined to be slightly lower in crude protein and energy content compared to typical aquaculture growout feed. The sludge protein quality and amino acid profile is assumed to be inferior to that of a typical growout feed, but would require further analysis to determine.
During an eight-week feeding trial to compare three feeding treatments (Algamac, Sludge, and Unfed), the concentrated sludge matter was fed to sea cucumbers 3× per week, following batch water exchanges. Prior to the trial, commercially available sand was added to each 1.5-m3 mesocosm tank to create a sand bed approximately 5 cm deep and tanks were filled with seawater. Tanks were stocked at an initial targeted biomass of 200-250 g/m2 of tank bottom surface area (i.e. 400-500 g/tank). At stocking, “fresh” weights of individual sea cucumbers were recorded to determine total stocking biomass. In addition, length and width measurements were collected to provide secondary means of discerning individuals at harvest and estimating growth.
Feeding regime 1 (“Sludge”) tanks were fed RAS sludge at a targeted rate equivalent to ~2% body weight (bw)/day of a commercial pelleted aquaculture feed, based on proximate analyses, and adjusted based on visual observation of tank conditions. The diet was fed 3× per week, following water exchanges. Feeding regime 2 (“Algamac”; control diet) tanks were fed with Algamac Plus at a rate of 2% bw/day. The diet was fed 3× per week, following water exchanges. Feeding regime 3 was unfed (“Unfed”; only natural productivity in tanks).
During the course of the trial, mortalities were noted and tanks suffering complete mortality (i.e. no survivors) were eliminated from the trial. After eight weeks, remaining tanks were harvested and individual weight, length, and width measurements were taken for all remaining animals. Average survival rates for and H. atra and A. mauritiana were 43.8% and 4.2%, respectively, and were significantly different (p = 0.00). The low survival of A. mauritiana was likely due to skin infections arising from handling injury/stress. Thus, wild-caught individuals of this species were determined to be unsuitable for future trials.
No significant differences were detected in specific growth rates between H. atra treatments. However, this species was observed actively feeding during the course of the trial, and guts of specimens dissected for proximate composition and stable isotope analysis from Sludge and Algamac treatments were found to contain sand and algal matter, respectively. This shows that animals were feeding and potentially growing. Guts of specimens from the Unfed treatment appeared mostly empty.
For H. atra, there were significant differences in body composition between time of stocking and time of harvest, as well as between treatments at time of harvest. The most notable differences were between the Sludge treatment and the other two treatments. Sludge treatment composition was significantly lower in protein and higher in lipid and ash, compared to both the Algamac and Unfed treatments. It is unclear whether these differences indicate that specimens from the Sludge treatment were growing more or less than the other two treatments. Longer trials and/or trials using juvenile animals that would be expected to exhibit higher specific growth rates, would likely result in more discernable differences in growth and might provide insights into the relationship between tissue composition changes and growth.
Because A. mauritiana suffered nearly complete mortality during the course of the trial, it was not possible to adequately compare sludge processing or assimilation capacity between species. The 5-Day Biological Oxygen Demand (BOD) was measured and compared between species on one occasion during the course of the trial and determined to be 3.15 and 3.60 mg/L for H. atra and A. mauritiana, respectively. The 5-day BOD was measured again later in the trial for H. atra tanks only, in order to evaluate and optimize BOD sample volumes for future trials.
For H. atra, there were no significant changes during the course of the trial (T0 to Tfinal), or differences among treatments at harvest (Tfinal) for carbon (p = 0.47) or nitrogen isotopes (p = 0.93), respectively. It appears that the two-month period of the trial was too short to allow for sufficient growth, tissue turnover, or change in stable isotopes, using adult specimens. As with growth data, the use of juvenile specimens and/or a longer trial duration would increase the likelihood of obtaining useful stable isotope data. However, the sample processing and methodology for stable isotope analysis appeared to work well and might be applied in future trials.
Year 2 experiments will investigate whether sea cucumbers effectively remediate shrimp production effluent. Year 2 trials will also evaluate the suitability of alternative polyculture approaches (i.e. sea cucumbers in separate tanks/ponds from shrimp, comingled with shrimp, or in net pens within shrimp production units). Near the end of Year 2, an informational brochure will be prepared summarizing the knowledge gained during the project and distributed to USAPI stakeholders.