A pilot-scale wet-holding installation for increasing bay scallop sales

Final report

Étang Ruisseau Bar Ltd

AIMAP2011-G05

This project aimed at the set-up of a pilot-scale wet-holding commercial facility for the bay scallop Argopecten irradians, to test the effect of two stocking densities under a flow-through and in a heated recirculation mode, and to do a cost analysis of the various wet-holding scenarios. The physical set-up consisted of 24 insulated tanks which housed 44 Dark Sea trays with a central lifting pole. Each holding tank was modified such that an air injection ring is located at the base of each stack of trays. Air delivery is provided by means of two Sweetwater air blowers through a manifold and quick connects to each tank. Water delivery is arranged such that water is controlled by means of a valve and a solenoid valve, a switch being assigned to each tank. A stocking density of 300 kg per tank or 7.5 kg per tray in the heated recirculating option offered the best survival. However, the economic analysis indicated that the combination of declining inventory value due to mortality and the cumulative increase in maintenance costs meant that both holding systems became non-profitable in late December. While it may be advantageous to hold sufficient quantities of bay scallops to supply the fresh market through November-December, profitability disappears after the 9^th or 10^th week depending upon the scenario. Increasing the turnover rate of the inventory in the wet-holding system is the key to achieving a higher level of profitability. Another option would be to develop, based on the calculated costs, a winter pricing strategy for the bay scallops in order to justify the maintenance of this product in the wet-holding system. It appears that sales of large tonnages of bay scallops would require alternative strategies, such as processing using Individually Quick-Frozen (IQF) techniques. 

Background

Sustained commercial sales of market-size bay scallops require the installation of an efficient wet-holding facility in order to ensure the healthy transition of the product from the multiple production sites to the certified packing plant, as well as to augment the window of opportunity for product sales in the winter months. L'Etang Ruisseau Bar has a CFIA-approved wet-holding facility which is primarily used for the winter storage of Eastern oysters. No recirculation technology has been installed; at present ambient seawater is pumped through the tanks once and exits the facility. Previous R&D trials showed that maintaining seawater temperatures above 0oC greatly improved the survival and maintained the tissue weight of the bay scallops over time. We also showed that the optimal Dark Sea tray stocking density was 9 kg per tray, and that air injection greatly enhanced survival and improved the quality of the bay scallops for marketing. This project aimed at incorporating these features into the design of a pilot-scale wet-holding system, and evaluating the survival and economic performance of bay scallops stocked at commercial levels under flow-through and re-circulating conditions.

Objectives

The general objective of this project was to compare the performance of bay scallops stored in flow-through ambient conditions versus in a re-circulating system designed to maintain a minimum temperature level above zero. The specific objectives were as follows:

Task 1: Set-up and test a pilot-scale recirculation system for the wet-holding of bay scallops
Task 2: Optimize the holding conditions and operational protocols for the wet-holding system
Task 3: Prepare an economic analysis of the costs of wet-holding bay scallops

Task 1: Deployment of the bay scallops in the wet-holding system

In October 2012 the final stages of the wet-holding installation were completed including the set-up and labelling of the tanks. Each tank was equipped with four air injectors to ensure adequate water circulation and oxygenation. On October 30, 1400 kg of bay scallops from the field were distributed among four 1200-L holding tanks - 2 tanks with 300 kg (40 trays @ 7.5 kg/tray) and 2 tanks with 400 kg (40 trays @ 10 kg/tray). Two paired tanks (300 kg and 400 kg) were set-up in flow-through mode while the other pair was equipped for recirculation. The initial recirculation set-up was activated in early December when the incoming water temperature fell below 2oC, but was subsequently modified as the reservoir proved to be too small. The trays were removed from each tank every 2 wk using the central hoist and the tank was rinsed.

Task 2: Optimize the conditions for maintaining bay scallops in the wet-holding system

Temperature probes were deployed in each treatment and bay scallop samples were taken for initial dry tissue weight and shell weight. Mortality was assessed every 2 wk (6 trays per tank) and samples were collected for assessing scallop condition (wet/dry tissue weight) and for faecal coliform analysis (IRZC - L'Institut de Recherche de Zones Cotieres). The re-circulation system was activated in early December and the flow of 8oC fresh water through the "heating" coil was adjusted to maintain the tank temperature in the 2-4oC range. The initial set-up was found to be insufficient to maintain the desired temperature and was re-designed in mid-December. Unfortunately the 15-hp freshwater pump failed on December 31 and was repaired on January 9. Hence the scallops in the re-circulating treatment were exposed to ambient conditions for several days. A similar low-temperature exposure event occurred in early March.

As of November 30 the survival of scallops in both the flow-through and re-circulating tanks was >98%. Substantial mortality was observed in last two weeks of December in the two higher density treatments (10 kg/tray). By the end of December there were more scallops remaining in the low density treatments than in the high density treatments. During January the numbers in the two flow-through treatments declined more rapidly than in the two re-circulating treatments. By the end of January only the low density (7.5 kg/tray) re-circulating treatment contained >50% of the original number of scallops.

Assessment of scallop condition index (dry tissue weight/shell height) showed a steady decline in all four treatments over time. Interestingly, the scallops in the re-circulating system, which had higher survival values overall, generally exhibited a lower condition index. It is possible that individuals will survive even with low tissue weights as long as the water temperature does not fall below a certain threshold. Conversely, low tissue weight scallops in the flow-through treatments would die earlier and therefore not be represented in the sampling pool.

Task 3: Prepare an economic analysis of the costs of wet-holding bay scallops

The economic analysis of the four storage treatments has three components: the change in inventory value over time; the initial set-up and incremental maintenance/utility costs; and the determination of the utility and labour costs.

The utility costs per week for the flow-through system were higher than for the re-circulating system because they were averaged over a shorter time period (106 versus 134 days). It should be noted that the labour costs for the removal of large numbers of dead scallops (i.e., at the end of December) would actually be higher than the average values specified. Also note that while the value of the remaining inventory effectively increases over time, the sale price ($0.35) remains stable. 

Relatively high survival rates over the first two months ensured that harvesting the tanks remained profitable until mid-to-late December. However, it is apparent that in the case of the Flow–Through -7.5 kg treatment, there was no advantage to maintaining the tank after January 9 when survival fell to 69%. Likewise, the Re-Circulating-7.5 kg treatment was no longer profitable after January 9, even though overall survival was still 77%. The two 10-kg treatments had a higher inventory value initially ($3164 - 10 kg versus $2492 - 7.5. kg) but the relatively high losses meant that neither tank was profitable after December 30 when survival fell to 41% (Flow Through) and 54% (Re-Circulating). Even though the inventory in the two re-circulating tanks survived approximately 4 wk longer than in the flow-through tanks, maintaining this inventory was no longer profitable. Under commercial conditions it would be necessary to combine tanks to reduce the higher operating costs per scallop due to mortality.

Discussion

Advantages of the wet-holding system

The installation of the wet-holding system at our storage facility in October 2012 greatly facilitated the holding and marketing of the bay scallop harvest. Previously, bags of bay scallops were harvested from the field sites in late October, size-graded for market and then returned to the field either in floating boxes or in bags attached to submerged longlines. Both these strategies were expensive in terms of labour and the stock was vulnerable to storm-related equipment damage, mortality associated with early ice freeze-up and burial/smothering by shifting beds of dead eelgrass. Bags of bay scallops retrieved from the field, sometimes from under the ice in December, had to be re-cleaned and checked for mortality. All these problems were successfully avoided with the immediate grading and transfer of the bay scallops into the storage tanks.

Issues with the design of the wet-holding system

In general, the water and air delivery systems functioned effectively over the four-month study period. Occasionally, the tubing connected to the air distribution rings at the bottom of each tank became blocked by ice formation. It may be necessary to change the type of tubing installed at the base of the tank in order to insure no bends between the main manifold and the entry into the injection rings. Another issue with the storage tank design is the inability to fully drain the tanks when the trays are in place; at present each tank must be emptied and tipped on its side in order to rinse out any accumulated sediment. The installation of the hoist for lifting all four stacks of trays as well as tipping over the tank has served to facilitate the cleaning procedure.

The water quality conditions in the tanks appear to be suitable for holding bay scallops over several months. The intense air bubbling effectively restricted the development of anaerobic bacteria populations, even when there were high numbers of dead scallops in the tanks. Faecal coliform sampling (November-January) yielded consistently negative results which confirmed that there were no issues with harvesting product from these tanks. Water flow rates were set at approximately 20 L/min in the flow-through tanks, which seemed to be sufficient. It should be noted that the water circulation pattern in the tanks could have been designed differently; at present the water enters at the top of the tank in the center and flows out through the surface drains located in the upper corners of the tank. As long as the air bubbling continues to promote circulation, there seems to be sufficient mixing, but given an air blockage/stoppage, there would be no water renewal at the base of the tank. This could lead to substantial product losses; protocols need to be implemented to ensure that any interruptions in air bubbling are addressed immediately.

Limitations of the experimental re-circulation system

Previous bay scallop trials in the winter of 2011-2012 suggested that survival rates were higher when they were maintained at temperatures above 0oC. Unfortunately, the re-circulation system set up in December 2012 encountered certain technical issues. In particular, the main freshwater pump which ensured that 8oC water was running through the heating coil in the reservoir, stopped on two occasions. Although this freshwater system is a new installation, the plumbing contractor should have installed a "soft starter" on the large 15-HP submersible pump. The frequent 24-h stop and start cycle of this pump, coupled with the lack of a soft starter, resulted in the severing of the electrical wire in the well on three occasions. Also, variable flow rates in the incoming seawater line contributed to fluctuations in the storage temperature in the re-circulation system. If scallops are to be maintained through December-January next year, a dedicated re-circulation system with an associated temperature probe and control system needs to be installed to adjust the flow rates through the coil on a 24-h basis and maintain a more stable temperature profile.

Effect of storage conditions on scallop survival

Comparison of bay scallop survival profiles indicated that the higher stocking density (10 kg/tray) was acceptable in November but appeared to negatively impact performance in December, possibly due to the delayed effects of overcrowding. While it may be possible to stock at the higher density initially (October), it would be advisable to steadily reduce the load of scallops in the tank through harvesting. Further trials would be required to determine whether the increased mortality observed in December was related to the total biomass of scallops per tank or specifically the density of scallops per tray. An increase in the flow rate to the 10-kg tanks, particularly when the water was warmer in early November, might also have promoted better survival.

Despite the erratic temperature conditions in the re-circulating system there was clearly an advantage in terms of improved survival over time. It should be noted, however, that the data on scallop condition (dry tissue weight per unit shell length) did not suggest any significant advantage. This contradicted previous results from 2011-2012 trials which indicated that maintaining a higher temperature was consistent with higher tissue weights. This issue needs to be re-visited when a more stable re-circulating system becomes available.

Economic analysis

The economic analysis clearly indicated that the combination of declining inventory value due to mortality and the cumulative increase in maintenance costs meant that both holding systems became non-profitable in late December. The inability to profitably maintain bay scallops in storage for over two months was attributable to both the low sale price per scallop and the cumulative labour/utility costs. While it may be advantageous to hold sufficient quantities of bay scallops to supply the fresh market through November-December, profitability disappears after the 9th or 10th week depending upon the scenario. Amalgamating tanks in late December would slightly reduce the cost per scallop, but the mounting weekly maintenance costs do not allow for a storage period longer than 10 weeks given the current sale price structure. Increasing the turnover rate of the inventory in the wet-holding system is the key to achieving a higher level of profitability. Another option would be to develop, based on the calculated costs, a winter pricing strategy for the bay scallops in order to justify the maintenance of this product in the wet-holding system. It appears that sales of large tonnages of bay scallops would require alternative strategies, such as processing using IQF techniques.