Cold Storage of Eastern Oysters, Crassostrea virginca, to Reduce Winter Mortality in an Increasingly Variable Environment

Final report for FNE19-931

Project Type: Farmer
Funds awarded in 2019: $14,845.00
Projected End Date: 04/15/2021
Grant Recipient: Betsy's Cape Shore Salts
Region: Northeast
State: New Jersey
Project Leader:
Betsy Haskin
Betsy's Cape Shore Salts
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Project Information


I was testing two methods for overwintering oysters to reduce winter mortality on intertidal flats and improve oyster health throughout the growing season. The two treatments were cold storage and subtidal storage. No significant improvements were found in mortality, growth, oyster condition index, Dermo disease, MSX, or Polydora infestation. A mild winter with no extreme temperature fluctuations probably contributed to these results.  

I did learn that oysters can be kept in cold storage for three months without suffering negative impacts so this is still a viable option to deal with extreme weather. More work could be done to improve the possibility of reducing Polydora.

I have spoken informally with other local oyster farmers about this project. A poster was presented at National Shellfisheries Association, and Partnership for the Delaware Estuary.

Project Objectives:

This project seeks to determine if cold dry storage or subtidal high salinity winter handling methods can be used as a means to reduce Dermo oyster disease, eliminate polydorid worms and, most importantly, enhance overwinter and post-winter survival of intertidal farm-raised eastern oysters, Crassostrea virginica. The comparison is among three handling methods: intertidal, subtidal and cold storage. The oysters will be monitored for mortality, growth, condition, disease, and shell blisters in each treatment. If the project is successful oyster farmers on the intertidal flats of Delaware Bay and other similar regions will learn that winter cold storage is an option that improves winter and post-season oyster mortality and reduces losses to disease. They will have the incentive to invest in winter cold storage to improve profitability of their farms. In addition they may suffer fewer losses from fouling by polydorid worms.


Increasingly variable winter weather coincides with increasing winter and post winter oyster mortality on intertidal oyster farms in lower Delaware Bay. Currently local oyster growers say that their winter losses are in many thousands of dollars. If, as proposed, anomalously variable winter conditions, both directly and indirectly, lead to increased oyster mortality throughout the year, then a possible solution to the problem is to maintain oysters at a consistent cold temperature through the winter. Stable cold storage could resolve the problem of increasing winter and post-winter oyster mortality and provide significant savings for the farmers.

Cold storage during winter months has long been used in the Northeastern US where winter ice can crush oysters. Reportedly, Native Americans pulled oysters out of the water in late fall, buried them in the sand during the winter, and returned them to the water in the spring (ECSGA listserve 2018).  This method, called “pitting” has been adopted by modern day oyster farmers who use root cellars or similar underground storage in New England where winters are consistently cold.  Walton and Murphy (2005) report that these “pits” where oyster are stored for three months or more are typically 36 to 40 F and humid (90-100%). They compared survival of small seed oysters held in pits with those remaining on the intertidal or moved to subtidal waters below any ice. As expected, the intertidal oysters suffered large mortality from ice, while survival in both deep water and in pits was 80% or more. Seed grew slightly while overwintered subtidally, but subsequent growth in pitted oysters exceeded overall growth of subtidal oysters, possibly due to increased winter metabolic costs in cold water with little food.

In Maine, Hidu et al. (1988) demonstrated that small seed oysters held under refrigeration could survive well for six months.

Pitting has not been used in South Jersey where our winter soil temperatures are not consistently cold enough to safely use the “pitting” method of oyster storage, even though it is less costly than using mechanical storage. Therefore, I propose storing oysters in a climate-controlled chamber sensu Hidu et al. (1988). 

Dana Morse of Maine Cooperative Extension provides the following recommendations for using a commercial cooler to safely store oysters for several months. The temperature should be stable between 32-38 F and the oysters must be kept damp. Wet burlap which is periodically sprayed with water will keep the humidity high. His information does not include any follow up on post-winter oyster health.

Access to subtidal leases free of winter ice is not readily available or feasible for most intertidal shellfish farmers in South Jersey.  Overwintering oysters on the tidal flats has proven risky.  To overcome this liability, we have planned to use commercial cold storage for sub-market size oysters which are approximately 1.5 years old at the beginning of the winter because this is the size class where we experience the greatest loss with significant economic impact. We will monitor oyster mortality and growth every month starting at the end of winter to track their progress, or lack thereof, for the rest of the year.  In addition to the baseline of oyster mortality and growth we will sample for oyster meat condition, disease prevalence, and Polydora blisters at key points during the year. Dermo disease will be assessed in June and October following overwintering to capture the seasonal low point of remission and the peak infection intensity. MSX disease will also be assessed in May or June when infections might be expected to peak.  This sampling regimen should give us information on possible effects of our treatments on oyster health. If more oysters from the cold storage treatment survive and grow into market size by the end of the year we can link improved outcomes to stable cold storage in winter. Likewise outcomes for oysters overwintered subtidally will offer new insight on that treatment for those oyster farmers who do have access to deep water storage. The disease assays will provide more information about possible links to the progression and subsequent impact of Dermo disease and MSX. Additionally, Rawson et al. (2015) report that cold storage benefits oyster farmers by reducing the shell blisters caused by the fouling worm Polydora websteri . We may find that fouling from the worm Polydora cornuta significantly reduced in the spring. P.cornuta  biofouling has been a summer problem for oyster growers  for many years.

This study could provide valuable information to other oyster farmers about the effectiveness of several treatments for improving outcomes for intertidal oysters in South Jersey. Both treatments proposed are time-consuming but if survivability and improved health result, then these methods can be worth using. We are exploring the possible connection between winter stress and Dermo disease which has not yet been done, as well as adding an assay for MSX.

If there is an indication of reduced Polydora sp.fouling and also shell mud blisters, then that is another advantage of cold storage as the polydorid worms increase labor costs to remove bio-fouling and create unsightly mud blisters which reduce marketability.





Description of farm operation:

I have been working a small oyster farm on the intertidal flats in Delaware Bay about eight miles north of Cape May for 12 years. My farm uses the rack and bag method of growing oysters which is the one currently most used on these flats as it stands up to high winds and rough water. My main market is local restaurants and the busiest season is the summer when the Jersey shore is flooded with tourists wanting local seafood..
With some help from family members, I am able to keep the business running without employees. That means I am out in the water everyday watching what goes on and thinking about how to keep healthy oysters. My observations of changing weather conditions and oyster survivability led me to propose this SARE grant to answer a few questions.


Click linked name(s) to expand/collapse or show everyone's info
  • Dr. David Bushek - Technical Advisor (Researcher)
  • Lisa Calvo (Educator)
  • Mitchell Tarnowski - Technical Advisor (Researcher)
  • Aislinn Mohyla
  • William Schroer - Technical Advisor (Researcher)


Materials and methods:

My project compares three methods of overwintering oysters in South Jersey: leaving them on the inter-tidal flats of Delaware Bay, exposed to cold snaps and warm spells, putting them into coolers and kept at stable temperatures (32-38 degrees), and keeping them underwater in a deeper basin. While my project was funded last February, it could not start until the beginning of the winter this past December, 2019. In early December we collected baseline data on sixty oysters for condition and size and Dermo disease. The presence and intensity of mud blisters (Polydora websteri) in the shells was also noted. We then filled fifteen oyster grow-out bags each with 225 sub-market oysters (year class 2018) and then fifteen more with 230 young seed oysters (year class 2019) in each. Five bags of each year class were placed in a cooler. Five more bags of each age group  were deployed underwater in a nearby harbor and the last set was secured on racks on the inter-tidal flats on my oyster farm. A total of thirty bags make up the set of samples: fifteen are older sub-market oysters which are one and a half years old (2018 year class) and the other set of fifteen are approximately 6-8 months old(2019 year class).

Oyster condition, an indicator of the health of the oyster, is determined by a method suggested by Crosby and Gale (1990). Oysters are weighed, then shucked, dry empty shells are weighed, and meat is dried and weighed. The equation for Condition Index is:

CI=dry soft tissue weight (g)x 1000/ internal shell cavity capacity(g)  The denominator is whole oyster weight-empty shell weight.

Oyster size is measured as height in mm.

When the Delaware Bay warmed up with water temperatures close to 50 degrees all the bags of oysters were returned to my farm on the inter-tidal flats. Generally it is the last half of March when waters are warm enough that there is food for oysters as they open up and start filtering water but this year with milder temperatures and Covid lurking, we moved faster. On March 11, 2020 oysters were removed from the cooler and placed back on the intertidal flats next to the oysters which had spent the winter there on the flats. The same day we retrieved oysters from the subtidal location where they had been stored for the winter and returned them to the flats.


We proposed that one week after oysters were returned to their place on the flats we would sample again for oyster condition, size, P. websteri mud blisters, and mortality. Our plan was take five oysters from each bag to measure so that we will have a total sample size of 75 oysters for each year class (2019 and (2018). The the number of dead oysters in every bag would be counted and recorded.

Because of Covid restrictions on travel and complications on working indoors to complete sampling we were not able to sample until two and a half weeks after oyster returned to the bay. We completed all measurements as proposed.

Every month after from April 2020 to October 2020 we counted the number of dead oysters in every bag and recorded that mortality. On June 20 we completed our second sampling for Condition Index, growth and disease. Again we were delayed by Covid restrictions on travel for a month. We took a sample of five oysters from each bag to measure condition and growth. Another 20 oysters  was taken to test for two diseases: MSX and Dermo. We had planned to sample in May for MSX because that is generally the month when it would be most prevalent if oysters have the disease but Dr. Bushek thought that the June sample was adequate.  Both Dermo and MSX are cumulative in oysters and would not be expected to have built up to high levels in younger oysters as compared to the older oysters.

In October 2020, we did the final sampling. Condition, growth, P. websteri,  and Dermo were measured along with the final mortality counts.

At this point the data is being analyzed to look for any differences among treatments.  There are several questions which we are asking. Is the condition of oysters coming out of cold storage different from the underwater storage or the condition of oysters remaining in the inter-tidal flats? Does a difference in condition correlate to a difference in disease or survivability? Are mortalities higher in some treatments at different times of the year? Does removing oysters from water for several months reduce the prevalence or intensity of P. websteri mud blisters in shells?




Research results and discussion:

Research results and discussion:

As proposed, after measuring initial condition index in December 2019, we tracked the condition index and the growth of oysters from March, when treatments were returned to the intertidal flats, until October when the growing season was winding down.  Every month from March until October we recorded mortality in every bag of oysters and sampled twice for two diseases, Dermo and MSX. We reported the intensity of mud blisters on shells from the fouling worm Polydora websteri.

Condition index data show that, other than in March, when 2019 year oysters overwintered on the intertidal flats were in better condition, treatment did not have a discernible effect.   A three-way ANOVA test was run to elucidate the impact of treatment, year class, and month on condition index. There was no significant three-way interaction (F(4, 432) = 2.27, p = 0.06). The only statistically significant simple main effect of treatment on condition index occurred in the 2019 year class during March (F(2, 432) = 18.3, p = 2E-8). Within this year class and month, pairwise comparison showed there was a significant difference in means between cold storage (mean = 85) and intertidal (mean = 119) (Emmeans test p = 8E-7), also between intertidal (mean = 119) and subtidal (mean=85) (Emmeans test p = 7E-7). 

Beyond what the data analysis shows there are some interesting things we noticed about condition index information Figure1, right panel. In December, the condition index was much higher than other months. Oysters “fatten up” for the winter, storing glycogen to last until spring when there is food again in the bay. The December condition index was highest in the younger oysters because their shells are thin and the ratio of meat weight to shell weight is higher.

By March the condition index of all oysters had dropped as they had used up their winter food supply. In June they were feeding again and had gained meat weight but by October the condition index was lower. We attribute this loss to a combination of disease (see Figure 3) and to energy lost to summer and fall spawning.

Shell height data (Figure1, left panel)shows growth of the 2019 year class, catching up with the 2018 size class by October as oysters grow more slowly as they age. While not statistically significant the subtidal oysters were somewhat larger than those from the cooler and the intertidal flats, perhaps benefiting from being submerged through a mild winter.

 A three-way ANOVA test was also run on shell height data. There was no significant three-way interaction (F(4, 432) = 0.834, p = 0.51). There were two statistically significant simple main effects of treatment on shell height: The 2018 year class in June (F(2, 432) = 4.98, p = 0.007) and the 2019 year class in October (F(2, 432) = 5.73, p = 0.003). Within the 2018 year class month of June pairwise comparison showed there was a significant difference in means between cold storage (mean = 67mm) and subtidal (mean = 72mm) (Emmeans test p = 0.010) also between intertidal (mean = 68mm) and subtidal (mean = 72mm) (Emmeans test p = 0.047). Within the 2019 year class month of October pairwise comparison showed there was a significant difference in means between cold storage (mean = 79mm) and subtidal (mean = 83mm) (Emmeans test p = 0.003). The lack of widespread effect of treatment on shell height suggest that treatment did not impact oyster growth during this experiment.

Mortality/survivorship data (Figure 2) show strong survivorship through the spring and early summer, with the greatest mortality occurring in mid to late summer. We attribute this mortality to Dermo disease. A three-way ANOVA run on survivorship data shows no significant three-way interaction (F(2, 198) = 1.3, p = 0.26). The only statistically significant simple main effect of treatment on mortality  occurred in the 2018 year class during October (F(2, 198) = 3.78, p = 0.024). Within this year class and month, pairwise comparison showed there was a significant difference in means between cold storage and intertidal (Emmeans test p = 0.018) also between cold storage and subtidal (Emmeans test p = 0.0048). Overall, treatment did not have a meaningful effect on mortality, but it was higher in the 2019 year class.

The oyster disease Dermo did not appear to be affected by treatment. It was negligible in December (Weighted Prevalence mean= 0.166) and slightly more prevalent by June (WP<1). By October Dermo in all treatments and year classes was well beyond lethal levels (WP >1.5)  (Figure 3) and most likely is responsible for the increases in mortality in all oysters. Mortality in the younger oysters was twice that of mortality in 2018 year class oysters. There could be two explanations for that. Either the more susceptible oysters die of Dermo in the first year and therefore the 2018 year class oysters are the more resilient survivors. Alternatively, oysters bought from the hatchery are not genetically identical every year and the 2019 year class may have been more susceptible to Dermo than the 2018 year class.

The tests for the disease MSX showed that it was inconsequential. We had postulated that oysters kept in a stable environment  would be in better condition to withstand a springtime MSX infection.

There appeared to be no differences among treatments in mud blisters from Polydora websteri (Figure 4).  This experiment showed no benefit from cold storage on Polydora infestation.

The big picture is that while there were some differences among treatments and year classes, they were not consistent across time and among treatments. The winter of 2019-2020 was very mild with no long freezes nor warm spells between late December and mid-March and this probably explains much of these results.

Cold storage worked very well as oyster mortality was minimal, but likewise few oysters died on the intertidal flats or in the subtidal basin. Probably because the weather was mild the intertidal oysters suffered no mortality but that might not always be the case. The absence of extreme temperature fluctuations and warmer conditions during the overwinter phase of this study relative to previous years (Figure 5a) negated the advantages of a constant, controlled environment afforded by the cooler. As seen in Figure 5, the temperature variability is greatest in the intertidal zone and lowest in the cooler. In winters when temperatures fluctuate widely cold storage for overwintering oysters could be a good option.

Winter on the flats
Research conclusions:

The goal of this project was to evaluate different overwintering methods to reduce winter losses  on the intertidal oyster farm exposed to extreme temperature variability. We also hoped to determine if stable winter conditions resulted in better spring/summer health.  While we tested overwintering oysters subtidally for comparison,  I was interested in the practicality of using cold storage. It is easier and less expensive for me and, perhaps other oyster growers, to access a large cooler than a deep-water subtidal site and boats to get there.

Based on the results of the experiment, we can say that cold storage is a viable method for overwintering oysters. However, the mild conditions experienced during the winter of 2020 confounded the comparison among treatments. In this experiment cold storage did not show an improvement in survival, condition, or growth over oysters left on the intertidal flats, but there was no loss from storing oysters in a cooler for three months. Given the unpredictability of winter weather in a changing climate it may be beneficial to use cold storage in future winters and I have chosen to continue to do so.

The total rental cost for the cooler was approximately $780. It was a small cooler that held 60,000 seed and 50,000 near market-size oysters. Costs for a larger cooler can be much higher. The labor involved in moving this small farm into a cooler was approximately 4 person-days ($480). For the price of $1,260, 110,00 oysters can be overwintered safely. At a market price of $0.70 per oyster that is 1,800 oysters sold to pay the cost of cold storage. Winter losses have been as high as 20% or 22,000 oysters, so it can be far more cost effective to invest in a cooler in a hard winter.

One last comment on coolers: the cooler we used had cooled steel plates on the walls and ceiling and it retained humidity. Standard reefer coolers use cold forced air and require extra effort to maintain high humidity for the oysters. At other times we have used the forced-air cooler and needed to cover oysters with wet burlap and then with a plastic tarp to keep the air from blowing directly on the oysters.

Participation Summary
1 Farmers participating in research

Education & Outreach Activities and Participation Summary

5 Consultations
1 Curricula, factsheets or educational tools
1 On-farm demonstrations
1 Online trainings
2 Webinars / talks / presentations

Participation Summary:

2 Farmers participated
Education/outreach description:

One member of our team was a high school senior participating a science program through her high school to get college credit. She helped in field sampling when possible. With help from us she put together a Power Point Poster which she presented at two meetings. She participated in a virtual poster session at the Partnership for the Delaware Estuary 2020 Science Summit. She also presented at the National Shellfisheries Association 2021 Annual Meeting in two virtual events. The poster is attached here. Research Poster

Lisa Calvo, the outreach coordinator will work with me to record a webinar at the Haskin Shellfish Lab. We plan to do this in September because summer is  an extremely busy season for oyster farmers and the timing is right to plan for winter. We will post the video on You Tube so an oyster grower  searching for information on winter handling of oysters can find it. I will be working with Lisa Calvo to develop a brief informational FAQ sheet presenting information on winter cold storage with a link to the You Tube video. This will be distributed through the East Coast Shellfish Growers Association and the Haskin Shellfish Research Lab website.

Dr. David Bushek, my main technical advisor, will be working with me to publish our results in the Journal of Shellfish Research. We plan to do this in the fall when we both have the time. We will provide  SARE the information on the proposed publication at that time  as well as the links to the You Tube video.


Learning Outcomes

3 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

Local farmers wanted to know primarily about the survival of oysters coming out of the cooler. They were pleasantly surprised when they came down to look at my farm to see lots of healthy oysters. Several have expressed an interest in putting their crop into cold storage and are looking into the logistics.

Reducing infestations of both Polydora websteri and P.cornuta is also of great interest to farmers and it was disappointing to report no reduction in mud blisters.

I am planning to work with Rutgers and Lisa Calvo, the education outreach coordinator, on a webinar this spring. Perhaps by next fall Rutgers will allow in-person meetings and I will give a seminar for oyster growers to reach a broader audience.

Project Outcomes

1 Farmers changed or adopted a practice
Project outcomes:

I plan to continue to overwinter my oysters in a cooler for the foreseeable future as a hedge against the unpredictability of winter weather.
While the last two winters have been mild, the extreme cold weather this past winter in Texas and the central US could happen here next winter and could be devastating to intertidal oyster farms. Even the floating farms where oysters are submerged under rafts would be impacted by an extreme cold spell.

Climate change seems to be inevitable and a report in the New York Times May 13 speaks of a new normal in terms of our climate. ( It seems wise to be thinking about options for dealing with it. Cold storage is manageable economically and for oyster health, it is viable. Deep water storage works well too but is more capital intensive as it require boats with the gear to load and unload large quantities of shellfish.

One of the actual benefits for me is enjoying a winter with much less anxiety about the weather.  The spring placement of the oyster farm back onto the intertidal flats provides an opportunity to reorganize the farm after one busy season so that its easier to manage for the next busy season. An additional benefit is that bio-fouling is reduced when oysters spend a few months in a cooler. Bio-fouling by the mud worm Polydora cornuta adds significant labor costs because we have to wash our oysters almost every week as the water warms up.

Assessment of Project Approach and Areas of Further Study:

The main confounding factor in the project was the weather. A mild winter was not what we needed to test our hypothesis. While we did not find big gains from cold storage, we were able to show that oysters can be kept in a cooler for 3 months without any significant losses.

I will continue to use cold storage for overwintering oysters but will monitor temperature and humidity more closely. High humidity and temperatures close to 33-35 degrees are reported to be beneficial for oysters. I accidentally did not start the humidity data logger and did  not realize it until we were downloading the cooler. Keeping a closer eye on humidity would be wise.

Looking further into the Polydora websteri mud blisters could be useful. If oysters were allowed to dry for 24-48 hours after leaving to cooler and before returning to the bay, the Polydora might not survive. This method could also kill any of the bio-fouling Polydora cornuta which survived on the shell exterior or the oyster bags.

These results are most useful to shellfish farmers in the mid-Atlantic of the United States where winter temperatures fluctuate widely.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.