WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2014 59 beneath the airlift intake (Fig. 3). The silo fits inside the bin with sufficient volume beneath the silo bottom to allow accumulation of gametes without entrainment by the flow generated by the airlift pump (Fig. 3). The system is operated for several hours or overnight. Then, the airlift pump and silo with broodstock are removed and the contents of the bin are treated with the assumption that broodstock had spawned and there are developing larvae in culture. A heater is returned to the bin to maintain the desired temperature for larval culture of that species. The airlift pump is replaced with an air diffuser placed in the bin to oxygenate water. Larvae in the bin are fed microalgae at a density appropriate for the species. On the second day, water in the bin is filtered through an appropriate (25-μm) mesh-size screen to retain the two-day old larvae (Fig. 4), similar to traditional methods. Test Spawns The bin-silo system was tested at the AIC during spring and summer 2012. To demonstrate the range of the system, three species of bivalves were successfully spawned using the bin-silo method: the Eastern oyster, the hard clam, and the ribbed mussel. The Eastern oyster is strip-spawned routinely, the hard clam cannot be strip-spawned and the ribbed mussel was a relative unknown in all aspects of its culture, including spawning. Adult broodstock were collected from local waters and held, separated by species, in silos. Seawater at ambient temperature (15-22 C) flowed through each system for the duration of spawning trials, which were conducted from late April through July. Eastern oysters had been conditioned but not used for strip-spawning for a different project, and were considered to have released all gametes. Hard clams and ribbed mussels had not been conditioned. A bin was filled with treated (sand-filtered and UV-sterilized) seawater and temperature was maintained at 26 C with an aquarium heater. A silo with broodstock was removed from the flow-through system at ambient seawater temperature, hosed off and placed in the bin. The sex of adults was not known. For all trials, the increase in temperature was between 2.0 and 12.3 C, depending on the difference between ambient seawater temperatures during the period and the target temperature in the bin (26 C). After placing the silo in the bin, the heater was turned off and the airlift pump was activated. A small volume (about 50 mL) of highdensity algal culture (either Chaetoceros, Isochrysis or Pavlova) was added to the silo to stimulate the animals to start siphoning water. This arrangement was left undisturbed overnight. The following morning, the silo with broodstock was removed and returned to the flowthrough system. The bin was treated as if it contained larvae. The airlift was removed and an air diffuser placed in the bin for aeration. The heater was activated to warm the water to the desired temperature, usually between 26 and 28 C. An appropriate volume of algae was added to achieve the desired cell density for a larval culture. This arrangement was left undisturbed overnight. The following morning, the bin was drained onto a 25-micron Nitex screen3 using a siphon. Larvae collected on the screen were washed into a beaker to estimate population size. Larvae are too delicate to screen before the second day of incubation. Larvae harvested from bins after two days were always D-larvae (Helm et al. 1986). Non-fertilized eggs were not observed, suggesting that fertilization was thorough and relatively concurrent. A small, instantaneous increase in temperature was a successful trigger for spawning. The results of successful spawns are listed in Table 1. Not every spawn attempt was successful. For hard clams, 4 out of 13 trials (31 percent) produced larvae. Overall production was approximately 5.5 million larvae or 2015 larvae/L. For the ribbed mussels, all (n=5) attempts were successful. Approximately 3.7 million larvae or 3700 larvae/L were produced. For the Eastern oyster, 6 out of 8 attempts (75 percent) were successful. Approximately 7 million larvae or 4357 larvae/L were produced. Advantages and Limitations of the Bin-Silo Method Practical complications associated with attempts to spawn hard clams prompted the development of the bin-silo method. Spawning hard clams can require several hours after a temperature shock and may be impractical without labor available to watch broodstock all day. The bin-silo method allows clams to spawn without interrupting the flow of a regular workday, minimizing time lost on unsuccessful spawns. This technique is especially (CONTINUED ON PAGE 60) LEFT, FIGURE 3. A key physical element of the bin-silo system is the position of the intake of the airlift pump. The intake is below the bottom of the silo but above the bottom of the bin so that gametes and embryos are separated from actively filter-feeding adults in the silo. RIGHT, FIGURE 4. The bin is drained on the second day by siphoning contents through a 25-μm mesh screen.
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