60 MARCH 2014 • WORLD AQUACULTURE • WWW.WAS.ORG useful for a species that cannot be strip-spawned, such as hard clams, or if a spawning protocol has not been established, as in the case of the ribbed mussel. The minimal amount of labor needed after the initial start-up is an attractive feature of the bin-silo spawning method. Multiple bin-silo units can be put into rotation as sets, increasing the number of spawns possible with less time spent per unit if the process is approached systematically. Not all spawning runs are successful, regardless of method. However, the bin-silo method minimizes the labor time involved, including for spawns that are not successful. Based on the management protocol, cultures are harvested after two days of incubation, although the culture can be sampled at any time to assess the presence of eggs, embryos or larvae. If no larvae were produced, the only thing lost was the time to set up the bin-silo system, the microalgae fed to the bin and the time to check for larvae. The total time investment is at most two hours and the volume of microalgae used is incidental. Regardless of the outcome, the bin-silo system can be cleaned and set up to use again immediately, rotating in a fresh stock of adults potentially ready to spawn. If key physical elements are met, the entire set-up can be scaled accordingly. For example, small bench-top systems can be used for smaller animals and/or fewer adults. Because broodstock are not killed to harvest gametes, they can be reused as spawners. This is advantageous when a considerable investment has been made to obtain, grow to maturity and condition broodstock. Furthermore, if offspring from a particular spawn express a desirable trait, parental stock would be available for future spawns. In general, the bin-silo method is superior to methods that sacrifice broodstock. With traditional strip spawning, sex of the broodstock can be easily identified by type of gamete produced. With the binsilo method, sex cannot be determined prior to a spawning event because broodstock are not killed to obtain gametes. If larvae were successfully produced, at least one male and one female spawned but these cannot be differentiated from non-spawning broodstock. If explicit knowledge of parentage is required prior to spawning, the bin-silo method should not be used. By using sufficiently large population of randomly selected adults to spawn, it is reasonable to expect a mix of males and females. To the best of our knowledge, traditional methods used for spawning oysters or hard clams have been unsuccessful for producing ribbed mussel larvae. The bin-silo method minimizes handling stress on mussels. Mussels must be attached before spawning; disturbance of byssal threads disrupts spawning. Methods that require mussel broodstock to be separated by breaking the byssal thread connections stress the animals. Implications for Small-scale Hatcheries Small-scale shellfish hatcheries that need to produce larvae may find the bin-silo method well-suited to their needs. The required supplies and equipment are commonly available and relatively inexpensive; some can be homemade with some creativity. For example, a rough silo can be made from a bucket and some plastic mesh. The one item for which it is difficult to find substitutes is the 25 micron Nitex mesh screen used to collect D-larvae. Only a modest volume of seawater is needed, so this is not a prohibitive factor if it must be transported to the site. Some small-scale hatcheries use wild adults for broodstock, TABLE 1. Results from successful bin-silo spawns in 2012. Spawn Date No. Ambient Bin temp. Change Number of larvae adults temp. in temp. at first count HARD CLAMS 1 30 April 44 13.7 26.0 12.3 4,050 2 9 May 39 16.2 26.0 9.8 896,000 3 30 May 38 21.6 26.0 4.4 1,320,000 4 12 Jun 60 22.5 26.0 3.5 3,240,000 RIBBED MUSSELS 1 25 May 60 21.0 26.0 5.0 680,000 2 25 May 60 21.0 26.0 5.0 8,000 3 29 May 60 21.5 26.0 4.5 2,852,000 4 4 Jun 38 19.4 26.0 6.6 72,000 5 11 Jun 60 21.8 26.0 4.2 84,000 OYSTERS 1 28 Jun 20 23.0 26.0 3.0 2,280,000 2 2 Jul 20 24.0 26.0 2.0 14,000 3 4 Jul 20 23.7 26.0 2.3 132,000 4 4 Jul 20 23.7 26.0 2.3 152,000 5 6 Jul 20 23.8 26.0 2.2 1,600,000 6 10 Jul 20 23.8 26.0 2.2 2,800,000
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