Refrigerated Sea Water System


Most fishermen use ice (icing) as a fish preservative for fresh fish products and use the freezer as frozen fish product. Handling of fish catches in the sea on fishing boats is the first stage that determines the quality offish that directly impact on the selling price. RSW to overcome the weakness of the use of ice in the fish preservation on the ship. It’s technique of cooling and preserving fish by inserting fish into a hold or chamber containing cold sea air temperature of about -5 C which is cooled by a refrigeration system. RSW system is the technology which is successfully used many countries especially Canada and Nordic Countries. It is the only system by which we can get the perfect water temperature control to get the fish quality maintained. 


All types of sea foods are perishable. Once bacteria action starts, as shown in Fig. 1, it cannot reverse; at best stopped from further deterioration, it is therefore imperative that preservation starts at the very beginning. It is said that “Fish of Pakistan is one of the Best in the World as long as it is in water”, the moment it is on deck deterioration starts from the way it is handled and then preserved. During the 1970’s FAO had passed a verdict that Pakistani wooden fishing trawlers can never be made hygienic. In the 70 years of history, fishermen being conservative in nature, local efforts have achieved half-baked lining and insulation of holds, albeit still using the same methods of stacking and manual handling. The result is encouraging as now to some degree B grade is achieved in some percentage of catch. Grade A quantities are small and still out of reach in most cases.

Modern Technology of RSW System

Modern technology of Refrigerated Sea Water (RSW) systems are used onboard fishing vessels to preserve large catches. Storing the catch in an RSW system is an effective and cost-saving method of maintaining A grade quality of the catch until offloading ashore or further processing onboard. In a minimum amount of time the catch is cooled rapidly to the freezing point of seawater, assuring its freshness and quality during transport. From these existing cooling techniques of fish using the cold water has many advantages compared to the other due to the fish faster cold caused can enter into the body of the fish and the fish are not subjected to the pressure caused by the power fish floating in the water.

Working of Refrigerated Sea Water System

The process of getting cold water into the fish hold starts from suction from sea, passing this water through the heat exchanger of a refrigerated system where in expanded cool gas is passed on one side and sea water on the other. This water is pumped into the fish holds from the top and sprayed evenly over the entire area.

Once the water had reached the desired level, it is recirculated through bottom suction to obtain the desired temperature. Depending on the quantity of catch, the water level and temperature are maintained for optimum results. Generally, this translates to thirty percent water and seventy percent catch. It is noteworthy that the normal capacity of fish holds is around 45 tons of catch

Refrigerated Sea Water System Storage Tanks

RSW tanks are normally constructed adjacent to one another spanning the width (beam) of the vessel. There are usually 2, 3, 6, etc. storage tanks on board a single vessel. The storage tanks are normally loaded to a capacity of approximately 80% fish and 20% water, depending on the type of fish. The tanks must be properly insulated from the hull, deck heads, machinery- and accommodation spaces to limit the possibility of heat ingress as much as possible. Furthermore the tanks must be equipped with over-flow and equalizing pipes. It is essential that the interior surfaces of the tanks have a smooth finish, with no obstructions or sharp edges.

Advantages of Refrigerated Sea Water System

RSW has the following advantages:

  • Greater speed of cooling
  • Reduced stress on the fish
  • Lower holding temperature possible
  • Quicker handling of large quantities of fish with little delay or labor involvement
  • By having 0 to -2 degree Celsius RSW Systems maintains the quality of fishes and prevents degeneration through bacterial action.
  • Improves the quality of fishmeal through better grade raw materials
  • Reduced quantities of catch with better quality means higher prices which give higher profit in international market.
  • RSW system is the most cost effective method that are being used for preserving the fishes until the arrival at harbour.
  • RSW system provides the benefit of not carrying the heavy load ice blocks that also occupy space leading to the increase in consumption of fuel.
  • As RSW system stands for Refrigerated Sea Water System, so according to its name it provides the quality fish products (canned seafood or fish meal) through refrigeration on board as well as off board.
  • Low Value Output can be easily eliminated through RSW system because its only due to the melting of ice blocks that are being carried by the fishermen, when ice block melts they have to throw the good quantity of fishes to save at least small amount of fishes otherwise they have to return with spoiled fishes.
  • Poor handing of fishes can also be avoided through the installation of RSW system in which the sea water is cooled to zero centigrade and the temperature is maintained until the fishes arrived at the harbour which also maintained freshness and hygiene of the fishes. RSW System can change the economic condition of the fishermen of Pakistan, and which will directly impact on the economy of Pakistan through exporting high quality fishes around the globe.

Disadvantages of Using Ice for Fishing

  • High cost of ice blocks.
  • Unhygienic water used in making of ice pollutes the fishes.
  • Sharp corners of crushed ice pierces the skin of fishes causing bacterial degeneration and loss of proteins.
  • Low quality of fishes due to delayed handling.
  • Low cooling efficiency for catches as compared to RSW System.
  • Stacked fishes stresses the lowest layer.
  • Carrying full load of ice increases fuel consumption.
  • Ice may or may not be utilized which leads to waste of ice and money.


  • A summary cost of each round trip of 15 days(maximum 10 days at sea) with 20 trips per year.
  • It is known that in the 90’s foreign trawlers did roaring business, in words of one local managing these craft they were earning over $300,000/- per month. We have conservatively extrapolated the catch figures as tuna in zone 3 is relatively unexploited, it will however take some time to get familiar with fishing tuna grounds and seasons, for this reason training has been emphasized. Foreign trawlers operating possibly have the data but it may not be easy to obtain. 
  •  The study indicates presently only 8% of the tuna is caught in zone 1, almost none in zone 3, Rodrigues shoal with isobars should be good fishing grounds, but these are suitable for well-equipped seaworthy craft. Of the 13 days voyage three days are marked for transit to and back from fishing grounds , depending on harvest the craft has bigger storage and endurance to be able to extend  the stay as necessary.
  •  It can be seen from Fig that breakeven is 4 tons per trip, a fairly low catch, this is deliberate to keep avoid high expectations right from beginning as many factors need to be improved. It would be nice to be able to get educated personnel but this is not possible at present as  no educated, trained crew is available, some can’t even read or write.
  • A grade sea food can quadruple bringing in foreign exchange and employment with pay back within one year.

PLS Sheet Trawler Operations 20 Voyages Per Year

Comparison of Trawler with ICE Vs Trawler with Refrigerated SEA Water System


  • As we are getting subsidy by Government of Sindh for this project of RSW System, it is the right time to avail and get this opportunity and take initiative.
  • For better economics and seaworthiness GRP craft be promoted.
  • The generous offer of M/s Izhar Steel Pvt. Ltd be availed
  • Installation of RSW be made a part of design  layout
  • As far as possible raw educated crew be given good incentives and trained for RSW and longline.
  • Liaison be made with good fish processing plants to export value added products
  • The craft be designed to suit our environment and practices with a blend of modern techniques and equipment
  • local materials be maximized and personnel trained in GRP boat building 

To know about the Refrigerated Sea Water System Cost, get in touch with us.