Culturing Northern Chilean scallop Argopecten purpuratus larvae in closed and recirculating aquaculture systems

GASPAR SORIA; GERMÁN MERINO; ELISABETH VON BRAND

Halifax, Canadá

Workshop; 16th International Pectinid Workshop; 2007

In Chile aquaculture is the main source of production of Argopecten purpuratus and the activity relies on an efficient, year-round, larvae supply. Scallop larvae are generally produced in closed aquaculture systems (CAS) and shows high variability in survival rates (from 0 to 80%), growths (vary by up to 800%), and periods of larval development (from 8 up to 40 days). These characteristics make larval production an uncertain venture. The use of recirculating aquacultural systems (RAS) could provide a method to increase production and significantly reduce seawater requirements and the mechanical stress of the larvae because there is no need of seawater exchange and sieving. In addition, seawater quality changes appear to be less dramatic. The main goal of this study was to compare, in terms of growth rates, the rearing of scallop larvae, subjected to a chemical treatment to induce a ploidy, in CAS and RAS obtained from several trials through 2004-2005. Mature scallops were obtained from Tongoy Bay (30º 15’S) and induced to spawn. Oocytes were fertilized (sperm-to-oocyte ratio 10:1) and zygotes were chemically treated to induce the required ploidy level. In the CAS zygotes and larvae were incubated in flat-bottomed fiberglass tanks (4000 L). Light aeration was provided at all times, and 24-h after fertilization larvae were fed Isochrysis galbana (5x103 cells/mL). Seawater was fully renewed daily. Larvae were sieved onto 35 µm mesh throughout the trials. Algae were added on a single day delivery and ration was adjusted (up to 30x103 cells/mL) by observing the gut content of the larvae. The RAS consists of four rearing units (three 500 L conical plastic tanks, two 1000 L conical plastic tanks) and one 1500 L tank as biofilter (0.5 m3 of Kaldness® media). Seawater was circulated through to each rearing tank (2-5 L/min for an 8h period) and partial flow was UV treated. Zygotes were transferred to 500 L or 1000 L conical plastic tanks. Larvae were daily fed I. galbana and food allotment was divided and provided in the morning and evening. Concentrated algae were harvested from a photobioreactor (25x106–45x106 cell/mL) in order to minimize culture seawater make up. After algae addition inlet flows were stopped (1 h) and then reestablished. Larvae were never sieved and the seawater was not fully exchanged during the experiment. Salinity, temperature, DO, TA-N, alkalinity were measured in both systems. NO2-N and NO3-N were determined only in the RAS. Larvae shell lengths (n= 30) were determined under microscope. Growth rates were estimated assuming a linear model (shell length (µm) = a + b x day). Equality of growth rates among regression lines was tested and then compared by T’ method. Shell larvae were photographed using Scanning Electron Microscope for further analyses of damage or abnormalities. Larvae survival (from zygote until the beginning of settlement) reared in CAS ranged between 1.1 and 10.6% and in RAS between 0.32 and 2.4%. Larval survival (from D-stage until the beginning of the settlement) in CAS had values between 8.0 and 85% and in RAS values between 15.7 and 65.8% (Table 1). Growth rates were significantly different (F11,2840 = 274.66;  p< 0.001) among groups (Fig. 1). All scallop larvae cultured in CAS showed lower growth rates (4.49 to 7.30 µm·day-1) and protracted period of culture (20 to 32 days) than larvae reared in RAS (9.56 to 13.15 µm·day-1). Larvae reared in RAS reached the settling stage within 12 and 15 days). Significantly higher growth rates (12.90 and 13.15 µm·day-1) were recorded in batches of scallop larvae reared in RAS (Table 1 and Fig 1. Analysis of larval shells did not suggest differences in shells damaged between systems. Higher growth rates observed in RAS could be attributed to a reduction larval manipulation and to the higher algae amount and availability. The relationship between TA-N levels (< 0.01 mg/L) and growth or survival rates does not show a clear pattern. Neither nitrite nor nitrate was recorded in CAS. In RAS, nitrite concentration ranged between 0.005 and 0.02 mg/L and nitrate between 1.2 and 2 mg/L. Survival is strongly influenced by the amount of viable zygotes after the chemical ploidy induction method. Therefore, survival percentages obtained in this research cannot be clearly attributed to the culture method used. However, survival rates from D-larvae until the beginning of settlement appears to be highly variable. Although the reduction in larval rearing time in RAS was high, the comparison between systems is more significant in view of the reduction obtained for seawater requirements. In this study a gross estimate of the make up seawater requirement per day needed for a batch of larvae reared in a RAS having a total culture volume of 5 m3 ranges from 15 to 30 L in contrast to the 80 to 128 m3 (tank volume x days of culture: 4000 x 20) needed for an equivalent amount of larvae reared in a CAS with a 4 m3 tank volume. All batches of larvae have the particularities that were air-provided from egg incubation and feeding began within the first 24h of incubation. Those procedures can lead a surplus in D-larvae survival during this early stage. Furthermore, it could be possible to reduce the cost of seawater heating from ambient sea temperature to culture temperatures, since the RAS was able to keep temperatures between 7ºC and 10ºC higher than CAS (13.7ºC to 15.5ºC). The information reported in this paper will be useful for the improvement of culture techniques for larval scallops under controlled conditions.