What is a RSW System Aboard a Fishing Boat?

Today's fishing industry grows more competitive with every catch. Distances become greater. Quality demands increase. To not only survive, but to maximize profits, it is essential to utilize the best technology and finest equipment available. Whether it is a small boat or a large fishing vessel, refrigeration is essential for delivering top-quality products and receiving premium prices from today's demanding seafood buyers. RSW, or refrigerated sea water is a relatively new technology to the commercial fishing fleet. There are 3 major factors that make this system an asset aboard the common day sportfisher or seiner.

THE VALUE OF INDEPENDENCE: With either spray or flooded RSW systems, vessel owners can achieve maximum flexibility in marketing seafood. Further RSW systems give freedom from the availability and supply of ice and also give the freedom of traveling much further to reach the optimum price buyer.

QUALITY DIFFERENCE: Seafood quality is directly related to the speed with which the product is chilled after harvest. RSW systems not only chill the catch quickly and uniformly, but maintain that ideal temperature environment over the length of a voyage. The result is more value, and more freedom in landing that catch in the best ports.

LESS WORK LESS WEIGHT: A further advantage of the RSW system is on board. Because the RSW eliminates the slow and fatiguing work of shoveling ice and layering fish, the boat crew is more efficient. On smaller vessels with spray systems the bulk and weight of ice is eliminated conserving fuel and increasing speeds.

The purpose of the refrigeration system is to remove heat from the seafood products as fast as possible into the evaporator and discharge the heat overboard through the condenser. Sea water can be cooled on board the catching vessel in two ways, by the addition of ice or by means of mechanical refrigeration, and shipboard tanks can be either built into the hull or portable for easy transfer to and from road transport at the quayside. The factors that influence successful RSW implementation are listed below.

TUNA PHYSIOLOGY: Tunas are well known for their higher proportion of lateral dark muscle, which is physiologically designed for continuous movement. Tunas are also unique in that their vascular system is designed as a counter-current heat exchanger (rete mirabile), to maintain the temperature within the muscle at about 3°C above ambient temperature in the case of yellowfin tuna, or up to 7°C in the case of skipjack; this maintains muscle efficiency, increasing speed and endurance. Red muscle in tunas is located internally, to further conserve heat. Muscle glycogen provides an energy store. Post-mortem glycolysis uses this store after the fish is caught to provide contraction of the muscle, exhibited as rigor mortis. This is also associated with heat production. Given the depth of the red muscle, its elevated temperature to start with and the loss of blood circulation as a cooling medium, the internal temperature can rise to the extent of causing a partial denaturing of the muscle protein. In tuna, more than most fish, there is therefore a need for rapid chilling after the catch is brought onboard. The use of refrigerated sea water (RSW) and brine freezing provides the rapid heat transfer needed to cool the fish quickly.

BRINE FREEZING PRACTICES: Brine freezing is used for large fish to be frozen whole such as salmon and tuna. The technique is used extensively on board tuna fishing vessels. The fishing vessel is fitted with one or more insulated tanks containing refrigeration coils. Brine freezing practices vary depending on the type of vessel, size of tank and catch rate. The following describes a typical practice.Before fishing starts, brine tanks are filled with seawater, which is then cooled to around -1°C. As fish are caught, they are dropped or flumed into the tanks, where they are chilled and held at this reduced temperature. When a tank is full, the RSW is pumped out and refrigerated brine is pumped in, and the temperature is lowered so that the fish freeze. Alternatively, if the catch rate is high enough, the fish may be put directly into refrigerated brine.The final temperature that can be achieved depends on the concentration of the brine - the minimum, when the brine is saturated, is about -21°C. Figure 1 shows the effect of brine strength on freezing temperature of the brine. In practice, brine temperatures can be anywhere in the range - 9.5 to -17°C, depending on the fishing vessel practice. Bear in mind that the engineer will not operate the brine at its freezing point (otherwise an ice slush forms which cannot easily be pumped) but will need a margin of a few degrees. Therefore to freeze at say -12°C, will need a 18 or 19% brine (with a freezing point at -15°C). As a result brine freezing of fish results in the final temperature at the end of freezing being much higher than the ideal.

During this past decade, the refrigeration technology industry has worked closely with fishermen and boatbuilders to develop the most reliable freezing and refrigeration systems for the world's small and medium-sized vessels providing RSW systems that are custom designed and built according to customer’s requirements.

"Scholars have long known that fishing eventually turns men into philosophers. Unfortunately, it is almost impossible to buy decent tackle on a philosopher's salary."
Patrick F. McManus