Tasks Accomplished: 7) Culture Shellfish in Nurseries
Within six weeks, oyster seed in the nurseries grew to 11/2 inches.
Upweller Data Graphs
The culture bins and seed shellfish were pressure washed to control biofouling.
The Edgartown Shellfish Department successfully cultures one half million seed quahogs in an upweller nursery in 2000 for public stock enhancement.
7) Culture Shellfish in Nurseries --
1999-- Over the course of the 1999 growing season, over 0.5 million oyster seed and 1.4 million quahog seed were cultured by the growers in four upweller nurseries. Jack Blake grew 113,310 oysters and 900,000 quahogs; Tom Berry, 161,750 oysters; Scott Castro, 75,213 oysters and 500,000 quahogs; and Ray Gailey, 150,000 oysters.
The oyster seed (3-7 mm) was set out in batches throughout the growing season, between 21. June and 8. September. On average, the oyster seed was rinsed clean about twice a week, sieved to size every week or two, and thinned in half about every ten days. At the beginning of the culture, the seed was about 4-5 mm in size, and held at an initial density of about 20,000 per bin. Within about six weeks, the oysters had grown to about 1 1/2" (37 mm) and were at a density of about 1,400 per bin. When they had reached about 1 1/2", the oysters were transferred from the upweller nurseries into growout cages.
The quahog seed, at about 0.75 mm, was introduced into two of the upwellers on 25. June and 3. July at an initial density of about 180,000 per bin. It was rinsed clean about every two to five days (more frequently when it was smaller). It was first sieved after about two weeks in the nursery and then about once a week thereafter. After about three weeks it was thinned to a density of about 60,000 per bin. After about a month, it grew to an average size of about 5.7 mm and then some was thinned to floating sand box nurseries. After about two months in nursery culture, some of the quahog seed was sold by both growers to Edgartown for public stock enhancement.
Shellfish growth data for 1999 is presented on bar graphs titled "Upweller Data Graphs."
Throughout the culture season, the bins were regularly pressure washed to remove biofouling. Larger seed was also regularly pressure washed with little observed damaged and few reported mortalities. Both oyster and quahog seed were also brine dipped at least once during the growing season to help control biofouling. Brine dipping could not be done immediately after power washing because the power washing often chipped the edges of the shells preventing the oysters from sealing tight enough to keep the caustic salt from contacting the soft tissues of the animals. The most successful treatment consisted of immersing the seed in a saturated sodium chloride solution for one to two minutes followed by air drying for one and a half to two hours. This treatment is especially important in controlling the boring sponge on the oysters, which can riddle the shells and decrease market quality.
Quahog seed mortality was reported to be about 5-10%. Oyster seed losses were nearly nonexistent during the 1999 growing season. Jack Blake, however, reported a survival of only 44% for small oyster seed (3-20mm) he overwintered in the upweller. The highest mortalities were observed in the smallest seed.
The growers had to contend with only one significant storm event. Tropical Storm Floyd impacted the Island on 16.-17. September with sustained winds of 40 mph and gusts up to 60 mph. Prior to the storm, the growers made battens and literally battened down the hatches. The upwellers were turned around so that less of the structure faced into the current and the water flow was reversed, flowing down through the bins and helping to hold them in place. The nurseries and their seed sustained no damage.
In August, Jack Blake conducted an experiment to determine if bins built from pressure treated lumber would have any negative impacts on the growth of small seed. On 10. August, equal numbers of 2-3 mm seed were placed into upweller bins, one with a pressure treated frame, and the other with a frame of untreated, painted wood. After two weeks, a sample of seed from each treatment was measured and the difference in growth was found to be "highly non-significant". The results suggest that pressure treated wood could be used to construct the bins. Although bins constructed from pressure treated wood would be lighter and easier to build, most of the growers were quite satisfied with the plastic wood bins. Despite the initial added labor to construct them, they were thought to be more durable, less likely to split, and better able to hold up to power washing than trays framed with pressure treated wood.
2000-- Over the course of the 2000 growing season, over 1 million oyster seed and 0.5 million quahog seed were cultured by the growers in six upweller nurseries. Jack Blake grew 204,750 oysters; Tom Berry, 209,350 oysters; Scott Castro, 208,900 oysters; Ray Gailey, 216,430 oysters; Roy Scheffer 230,670 oysters; and Paul Bagnall and Warren Gaines, 500,000 quahogs.
On 1. July 2000, Warren Gaines, Edgartown Assistant Shellfish Constable, placed 500,000 1mm quahog seed produced in the MVSG Hatchery into four bins(125,000/bin) in a new nursery. The remaining empty bins were covered with plastic sheeting so as to direct the water flow through the bins with seed. The seed was rinsed to remove silt and feces about every three days. About every six days, the seed was sieved and thinned as necessary into the remaining bins -- eventually into eighteen bins at a density of about 28,000/bin. The seed grew at a rate of about 1 mm a week so that by 1. August, 150,000 5mm quahog seed were harvested from the unit and transplanted to floating sandbox nursery systems. By 15. August the remaining 350,000 quahogs reached 5mm and were transplanted. Mr. Gaines was very satisfied with the results and reported mortalities next to zero. Although the rate of growth observed was similar to that seen in the floating sandbox nurseries, he felt the tidal upweller nursery was a safer way to grow the small seed with much less risk of loss when compared to the sandbox nurseries.
The five growers cultured over one million oyster seed in the nurseries in 2000. This was about double the amount they grew in 1999. The seed oysters were set out in batches between 5. July and 16. August and ranged in size from 1 to 8 mm. Except for the higher densities attempted, culture protocol was similar to that followed in 1999.
The growers reported a greater prevalence of sponge biofouling in 2000 compared to 1999. This was controlled by brinedipping, discarding the oysters with sponge, and powerwashing the smaller seed which resulted in a 3-4% mortality. As the growers were culturing more seed than they could handle, these losses were not a problem. The powerwashing removed some of the new growth on the seed oysters This "preening" was in the long run beneficial as it caused the oysters to grow with a deeper cup which increased their market quality. It was reported that the seed oysters could not be brinedipped immediately after preening, as chips in the edges of the shells prevented the oysters from closing tightly enough to prevent damage to vital tissues.
Despite improved performance of the nurseries resulting from modifications to the structures and the experience of the operators, most of the growers reported that they attempted to handle too many seed in 2000. Through the season they were forced to cull the crop to prevent stunting. In some cases, they culled as much as half. The oysters they did keep, however, grew at a rate similar to that recorded in 1999 which averaged about 35 mm in six weeks. This exceptional growth was attained in spite of the fact that the growing season was colder and with less sunshine than 1999, both factors that would be expected to slow oyster growth. Some of the growers speculated that they may have seen even better growth had they tried to grow about 100,000 as in 1999, rather than the 200,000 that each attempted in 2000. Survival of the oyster seed they kept was reported to be excellent.
Some of the reasons given for the success in 2000 included cleaning the cultures and systems more often; growing the larger seed on larger mesh sieves (in 1999 the larger mesh sieves had not yet been constructed); and the added vents on two of the units which improved flow rates through the nurseries. Jack Blake ran one experiment to test the effect of increased flow on the growth of the oyster seed. He placed equal amounts of seed oysters in two bins. One bin was set in his original prototype, the other in the new "turbo-charged" nursery which was equipped with extra vents and had a faster rate of seawater flow through it. Surprisingly, after over three months of monitoring, Mr. Blake observed no significant difference in growth between the two groups. Although this issue needs further study, perhaps the phytoplankton levels in the growout area were such that the flow rate through the original design already provided for maximum oyster growth rates.