The eastern oyster, Crassostrea virginica, is a commercially and ecologically important species in the Delaware Bay. However, the Bay’s oyster fishery has declined with current harvests representing less than 10% of peak harvest levels. This decline is largely attributed to the emergence of two oyster diseases that have caused significant losses of the resource. The increasing availability of disease resistant hatchery produced oyster seed creates an opportunity for sustainable and profitable aquaculture to occur in the face of disease.
The purpose of this project was to demonstrate the production potential of subtidal cage culture of disease resistant oyster stocks as a means to revitalize production on the largely abandoned leased planting grounds of Delaware Bay and to improve culture handling and husbandry methods. Two types of cages and two husbandry regimes were evaluated in respect to labor costs and production benefits.
Oyster growth varied little between the two cage types and two cleaning regimes evaluated; however, an on-bottom flip-top cage design offered higher oyster survival and reduced labor effort in comparison to a tiered rack and bag system.
The eastern oyster, Crassostrea virginica, is a commercially and ecologically important species in the Delaware Bay. However, the Bay’s oyster fishery has declined with current harvests representing less than 10% of peak harvest levels. This decline is largely attributed to the emergence of two oyster diseases that have caused significant losses of the resource. The increasing availability of disease resistant hatchery produced oyster seed creates an opportunity for sustainable and profitable aquaculture to occur in the face of disease. In New Jersey, oyster aquaculture has emerged as a viable industry with intertidal areas of the lower Delaware Bay serving as an ideal environment for rack and bag culture of disease resistant oysters.
In 2010, a small group of oystermen began pilot studies to evaluate the production potential of subtidal cage culture of disease resistant oyster stocks as a means to revitalize production on the largely abandoned leased planting grounds of Delaware Bay. These grounds historically served as grow out grounds for transplanted wild oysters. Initial evaluations of the subtidal cage culture systems produced promising results.
The purpose of this NE SARE Farmer Grant project was to improve subtidal oyster culture handling and husbandry methods and further demonstrate the potential of cage- culture as a means to increase oyster production on underutilized leased grounds of the Delaware Bay. Two types of cages and two maintenance regimes were evaluated in respect to labor costs and production benefits.
The goal of this project was to improve cage-oyster culture technologies and methodologies to minimize handling costs and enhance profitability of subtidal oyster aquaculture on leased bottoms of the Delaware Bay.
Specific objectives were to: (1) evaluate the production of oysters grown in two types of cage systems, tiered rack and bag and stacked flip top; (2) examine fouling and ease of handling of the two types of cage systems; (3) optimize husbandry practices to maximize production and minimize labor costs; and (4) modify vessel gear to improve handling of cage culture systems.
Two types of oyster cages were evaluated. The first type, which was used in previous trials, was a three tier-cage system that accommodated 3 standard 7/8th inch mesh ADPI bags. The second cage type was a 36” x 48” x 4” interlocking vinyl covered steel 1”x1” mesh tray, purchased as a kit from Ketchum Traps (www.lobstering.com). The finished cage consisted of a top unit with a flip top lid, affixed to a bottom legged cage stacking to an of bottom height of 12” (Fig. 1). Both cage types were deployed on the bottom as independent units. A line with a buoy served for marking and hoisting the cages.
Experiment 1. Four production regimes, or treatments were evaluated—2 cage types (flip top (FT) and 3-tiered (3T) cage and bag) and 2 husbandry routines (cleaning every 2 or 4 weeks (2W and 4W, respectively). An initial investigation was initiated in May 2012. Four replicates were established for each treatment. However, due to limited availability the preferred size class of 1” seed was not available and the study was initiated with 2” submarket oysters. On the first sample date at week four, a large number of market oysters were removed from the trays and as a result of a misunderstanding of the experimental design the remaining submarket oysters were pooled and redeployed. Due to the small number of remaining oysters and loss of replication, the investigation was terminated in favor of a new start.
Experiment 2. The second trial began in on July 9, 2012. Submarket hatchery reared oysters were obtained from a farm located in the lower Delaware Bay and placed in the cages at a volume of 30L (10 L per bag) for the 3-tier cage, and 60 L (30 L per tray) for the flip top system. Duplicate cages were established for each treatment. Oysters at the start of the experiment averaged 57 mm shell height. The cages were deployed on a lease located at Maurice River Cove. Oysters were removed from the cages monthly to maintain similar densities in the bags and cages. Initially, the removed oysters represented all size classes; howver, beginning with the August 27, 2012 sample date, market size oysters (>3”) were removed from the experimental cages to better mimic production practices. Duplicate cages were sampled monthly through October 2012 and the following parameters were assessed: oyster mortality, growth, and market yield; Dermo disease (Perkinsus marinus) and fouling. Dermo disease prevalence and intensity was diagnosed using standard Ray’s fluid thioglycollate assays at the Haskin Shellfish Research Laboratory. Fouling was ranked on a scale of 0 to 4 as follows: 0 – no fouling evident, 1 – very light fouling, less than 25% of the oysters and cages fouled, 2 – light fouling, 26-50% of oysters and cage fouled; 3 – moderate fouling 51-75% of the oysters and cage fouled, and 4 – heavy fouling, 76-100% of the oysters and cage fouled. Statistical analyses included two-way repeated measures ANOVA (cage type x cleaning regime). Analysis was conduced using Matlab software. When appropriate data was arcsine transformed prior to analysis. Labor invested in the husbandry of the cages was also documented. Oyster production (number marketed in respect to number planted) was recorded during the course of the grow-out season.
The oysters grew well during the experiment with many growing to harvest size (shell height = 76 mm) by late August. Market oyster yield was significantly higher in the flip top cages than in the 3-tier cages, but there was no significant difference in respect to husbandry regime (Figure 2). Overall, market yield in the flip top cages was on average 39% higher than in the 3-tier cages. The best performing treatment in respect to the market oyster production was the flip top cage with a two-week husbandry regime, in which 38% of the oysters reached market size during the course of the study.
Growth rates from the start of the experiment through the end of August ranged from 6 to 10 mm per month, with the exception of the 3-tier, 2-week treatment group, which exhibited a growth rate of only 1.4 mm per month during the first sample period (Figure 2). The growth rate in this group exceeded all other treatments during the second interval. The effects of cage, cleaning regime, and time on oyster growth as indicated by shell height, were statistically significant. A multiple comparison test indicated that differences among treatments were significant on the first sample date (July 30). Differences in mean shell height among treatments were not significant on subsequent sample dates; however, the analysis is confounded by the harvest of market oysters from the experimental trays beginning on the August sample date.
The effects of cage and husbandry on oyster mortality were statistically significant (p=0.0321 and p=0.0399, respectively). Oyster mortality was notably lower in the flip top two-week treatment group than in all other groups (Figure 2 and 3). Analyses were conducted using only one of the replicate trays for the FT 4W treatment because the replicate cage was turned upside down post-deployment and experienced significant mudding and high oyster mortality. Final cumulative percent mortality was respectively 29.5%, 39.5%, 40.8%, and 52.4% in the flip top two-week, flip top four-week treatment groups, 3-tier two-week, and 3-tier four-week treatments.
Oysters maintained in the flip top cages exhibited lower levels of Dermo disease than those maintained in the 3-tier cages. Dermo disease was diagnosed monthly from July through September. Average prevalence across the three dates was 53%, 57%, 77% and 82% in FT2W, FT4W, 3T2W, and 3T4W treatments respectively. Similarly, Dermo weighted prevalence was respectively 0.925, 1.225, 2.325, and 2.738 in FT2W, FT4W, 3T2W, and 3T4W treatment groups (Figure 2). The highest disease levels were observed in August for all groups, while the lowest disease was observed in July.
Biofouling during the study period varied temporally, both in terms of the fouling organisms and the intensity of fouling. Fouling was lower in the cages cleaned every 2 weeks than in cages cleaned monthly for both cage types (Figure 2). Fouling intensity did not differ between 3-tier and flip top cage designs; however, crewmembers preferred the flip top design for ease of cleaning. Polydora cornuta was the most significant and damaging pest. Other principal fouling organisms included a tunicate, Mogula sp. and an unidentified brown macroalgae. During heavy fouling periods a two-week cleaning regime was preferred due to ease of cleaning and better survival. However, the less intensive 4-week cleaning regime did not significantly negatively impact oyster growth (Figure 3).
Documentation of the man time invested to clean the cages of the various treatments proved challenging. Typical workdays involved a crew of six men and the farm was tended an average of 8 days per month for six months of the year. One hundred and thirty thousand of the 150,000 oysters planted were harvested for market. All oysters were nearly one year old and larger than 1” at the time of planting. Oysters at 2.75 to 3.0″ shell height were selected for market. Labor costs represented the largest production cost, about 50% of all costs. It was estimated that the flip top cages took half as much time to clean as the 3-tier cages, presenting significant labor cost savings.
Other significant costs included equipment, materials, and vessel operations, which were estimated to be 34% and 13% of production costs respectively. Despite these costs the farm was profitable.
Equipment investment included modification of a vessel boom to facilitate cage hoisting and handling. Shortly after installation the vessel had mechanical problems that resulted in dockage, so we were unable to evaluate the efficiency of the boom during the grant period.
The project has further demonstrated the potential of subtidal cage culture of hatchery reared oysters in the Delaware Bay, New Jersey. The evaluation of two cage types and two husbandry regimes will enable growers to employ science-based management practices to improve handling efficiencies and oyster production.
Overall the flip top cages were preferred over the 3-tier stacked bag systems. Oysters maintained in the flip top cages exhibited lower levels of Dermo disease, better survival, and slightly faster growth than those held in the 3-tier systems. However, with the exception of survival, differences were not demonstrated to be statistically significant. Crew preferred the flip top cages to the 3-tier cages as they were easier to clean and work over.
Biofouling during the study period varied temporally, both in terms of the fouling organism and the intensity of fouling. Fouling was significantly lower in the cages cleaned every 2 weeks than in cages cleaned at 4-week intervals. Polydora sp. was the most significant and damaging pest. During heavy fouling periods a two-week cleaning regime was preferred due to ease of cleaning and better survival. However, the less intensive 4-week cleaning regime did not significantly negatively impact oyster growth. Due to the variable nature of the timing and intensity of fouling in the future cages will be checked at least every two weeks.
Though this study design was limited due to limited replication of the treatment groups, the findings are still important. Given the demonstrated advantages in terms of growth and survival we estimate that use of the flip top cage design with biweekly cleaning could improve production yields by 20%. Although the flip top cage systems offered advantages to the three tier systems, observed differences in oyster growth and mortality between the two systems may not warrant a complete replacement of 3-tier systems that farmers already have in operation. In the future the 3-tier cages will be used primarily for small seed oysters early in the growth cycle.
Education & Outreach Activities and Participation Summary
This work was presented at the joint meeting of the Northeast Aquaculture Conference Exposition, Milford Aquaculture Seminar, and International Conference on Shellfish Restoration, which was held Dec 12-15, 2012. The work was also presented at a local forum of the Bayshore Discovery Project and a poster presentation has been displayed at various local festival and events, including an open house at the Aquaculture Innovation Center (May 8, 2013), Cape May Harbor Fest (June 15, 2013), Baydays (June 1, 2013). The poster is prominently displayed at the Haskin Shellfish Research Laboratory, which hosts monthly and special meetings of the Delaware Bay Shellfisheries Council and the shellfish industry community. An oral presentation will be made to fellow industry members at an upcoming Delaware Bay Oyster Growers Forum.
This study captured only a single summer of a two-to-three summer grow out cycle, however, the results suggest that subtidal cage culture of oysters is a viable option for growing oysters on the traditional planting grounds of the Delaware Bay. The project has demonstrated two suitable cage designs for the grow out of oyster seed in subtidal areas of the Bay, presenting an alternative culture system to the more widely employed rack and bag culture systems used in the intertidal areas of the lower Delaware Bay. Of the two cage designs the flip top cage was preferred for both ease of handling and production benefits. Additionally, the results suggest that a biweekly cleaning regime may be beneficial over a monthly cleaning regime during high fouling periods.
Profit margins at present are still narrow; however, continued attention to optimize cage oyster densities, husbandry routines, stock performance, and handling efficiencies will continue to help identify means to decrease production costs and improve profits. Future studies should be conducted to evaluate overwintering survival of oysters and post-winter cage retrieval.
- Overall the flip top cages were preferred and are recommended over the 3-tier stacked bag systems for the grower out of oysters 1.5 inch and larger.
The flip top cage system offered some advantages to the three tier systems; however, differences in oyster growth and mortality between the two systems were only marginally significant and may not warrant a complete replacement of 3-tier systems that farmers already have in operation.
The 3-tier cage system is recommended for smaller oysters, given the ease of varying mesh size as the oyster grow.
Cleaning of oysters and cages at biweekly intervals during summer is recommended to avoid heavy fouling, ease cleaning, and reduce oyster mortality.
Though this study captured only a single summer season of a two-to three summer grow out cycle, the results suggest that subtidal cage culture of oysters is a viable option for growing oysters on the traditional planting grounds of the Delaware Bay.
Future research should focus on optimization of grow out densities, creating efficiencies along the entire production process, overwintering survival and cage recovery, and improvement of stock performance.