Purse Seine Fishing Operations
Once the vessel has identified a fish aggregation, the skipper will evaluate the species composition, school size, and chances of capturing the aggregation. Often electronic devices like the echosounder and sonar are used to make these determinations. If the aggregation is at a FAD, these tools are critical because most FAD fishing is initiated before daylight so other visual cues, such as fish on the surface or birdlife, cannot be used to evaluate the aggregation. If the decision is made to make a set, the vessel is positioned by the skipper and then the skiff is released, towing the end of the net. The purse seine vessel tows the purse seine net around the aggregation to encircle them within the net. Most often, the corkline will be in a circular or oblong shape when the purse seine vessel and skiff come back together. At this point, cables and towlines are exchanged between the two vessels, and the skiff commences towing the purse seine vessel in order to maneuver it away from the net. The net is then closed underneath the school by hauling the purse line running through the rings at the bottom of the net. This process is called “pursing.” Once pursing is completed, the net hauling (commonly referred to as “net rolling”) process begins. The net is stacked back on the back deck of the vessel with the aid of the power block and the crew. As the volume of the net becomes smaller, the fish become more concentrated. At the end of net rolling, the “sacking up” point is reached, where the final slack in the net is removed and the catch is concentrated and scooped out using a brailer.
A sorting bin, commonly known as a hopper, is often used to sort the catch before it is directed into wells for storage. The hopper typically holds the capacity of the vessel’s brail of fish, which are loaded in the hopper one by one as the crew sorts through the catch. The crew can remove unwanted bycatch, mutilated or damaged catch, and other bycatch at this stage. A door or hatch is actuated with a hydraulic control and allows the catch to be dumped into the loading hatch of the vessel in a controlled manner. Since the hopper stalls the loading of the catch into the wells, this is a good point for observers to evaluate catch composition for their reporting.
If a hopper is not used, brails are emptied directly into the loading hatch of the wet deck. This makes it difficult to assess the type and amount of bycatch species in each brail.
Once on board, the fish pass below deck to be loaded into wells. While most vessels employ conveyor belt systems to direct catch into desired wells, some use chute systems that also quickly deliver catch to wells. Some vessels are equipped to bulk-freeze the catch, but most keep the fish in refrigerated brine tanks (brine at 0° C), which occupy much of the lower part of the hull and are equipped with batteries of seawater pumps for circulation. In the larger vessels, the tunas are preserved in wells of 20 to 120 metric tons each, (vessel total 800 to 2,000 metric tons) with brine freezing at -20° C. In the more artisanal purse seiners, tuna are generally kept in iced seawater.