Introduction
Integrated Multitrophic Aquaculture (IMTA) stands as a viable avenue to pursue an increase and more sustainable production of marine resources while diversifying commercial output by little to no additional costs. In this regard, sea cucumbers, such as Holothuria arguinensis, are deposit feeders that can grow from organic matter deposited at sea floor, making them prime candidates as organic extractors in IMTA systems. However, the circular IMTA concept requires additional agents, like inorganic extractors from the water column like amonia (NH3), nitrites (NO-2) and nitrates (NO-3). Macroalgae are strong candidates for this role. The present study sought to determine the viability of producing Ulva lactuca in the same recirculating system (RAS) as H. arguinensis growing, in turn, in fish waste enriched sediment, in an Integrated Multitrophic Recirculating Aquaculture System (IMTRAS).
Materials and Methods
To determine the efficacy of U. lactuca as an inorganic extractor for aquaculture production, two experimental RAS were put in place, one with the macroalgae (U treatment) and one without (control). Each system was comprised of an individual sump and 3 flat bottom tanks with 50L capacity and substrate enriched with fish waste, provided by Piscicultura do Vale da Lama (Lagos, Portugal). Each tank housed 30 H. arguinensis and connected to the respective system’s sump. The two sumps were supplied with mechanical and biological filtration, but no protein skimmer. Each sump was then connected to 3 kreisels with 2.9 L capacity, where one system (U treatment) had U. lactuca at a density of 1g·L-1 and the other system (control) had empty kreisels. Water would then flow into the sea cucumber tanks, closing the circular system. This way, both RAS had the same water volume to better establish comparisons between them. Water quality parameters, mainly nitrogen compounds (NH3, NO-2 and NO-3) were measured daily and, once per week, all sea cucumbers had their biometrics registered, such as submerged length, fresh weight, and specific growth rate (SGR). The density (g·L-1) of Ulva in the U treatment system was also registered in the same weekly interval, to monitor macroalgae growth over time.
Results
After a period of 6 weeks, while there were no differences between treatments regarding H. arguinensis SGR (C = 0.23 ± 0.09 %·day-1; U = 0.24 ± 0.11 %·day-1), sea cucumbers grown with U. lactuca presented a higher mean survival rate (U = 97.78 ± 1.92 %) than the ones without (C = 84.44 ± 8.48 %). The inorganic compounds detected in the RAS system with U. lactuca were, inversely, at smaller concentrations (U: NH3 = 0.03 ± 0.03 mgL-1, NO-2 = 0.20 ± 0.31 mgL-1 and NO-3 = 1.26 ± 1.42 mgL-1) than the one without the macroalgae (C: NH3 = 0.05 ± 0.05 mgL-1, NO-2 = 0.58 ± 0.75 mgL-1 and NO-3 = 2.67 ± 2.18 mgL-1).
U. Lactuca reared in IMTRAS reached a peak of 2.44 ± 1.99 g·L-1 four weeks into the trial, with a maximum SGR of 2.77 ± 0.92 %·day-1.
Discussion
Sea cucumbers like Holothuria arguinensis have continuously demonstrated their aptitude for growth as organic extractors in IMTA settings. However, to reach a circular design and a more sustainable and resilient production system, macroalgae like Ulva lactuca are excellent inorganic extractors, mitigating the by-products of animal species in production, thus, improving water quality and showing positive effects on other organisms in the system. Furthermore, the lack of nutritional costs to produce additional trophic levels (commercial sea cucumbers and macroalgae) adds significant value to current production systems.