A Detailed Explanation of Tuna Fishing Methods and Operational Procedures

There are quite a few methods for catching tuna, and the most popular one today is longline fishing.

Longline fisheries are divided into three types: surface longline, bottom longline, and midwater longline. Tuna longline fishing gear is mainly composed of a main line, branch lines, floats, float lines, and hooks, that is, a tool for catching tuna by hanging several hooked branch lines from an extended main line. Several floats are tied to the main line, and the float lines between the main line and the floats as well as the length of the branch lines serve to adjust the fishing depth. The hooks need to be baited. The bait mainly includes frozen saury, squid, sardine, chub mackerel, and so on.

I. Exploratory Fishing

First set a small number of lines, generally about 500 hooks, and then after the next day, judge the central fishing ground based on the catch.

II. Setting the Line

1. The time for setting the line should be based on the feeding characteristics and vertical patterns of tuna; the best time to set the line is preferably 3-4 in the morning.

2. As the rise and set of the moon is delayed, the time for setting the line should also be delayed. If the line is set too early, the bait will be swallowed by other fish before tuna feed.

3. The direction of setting the line should be cross-current or diagonal-current. Generally speaking, tuna prefer to feed against the current. One method is to set the line perpendicular to the direction of the ocean current; this method intercepts a large cross-section and can provide a reference for the width of the next set. A better method is to form a "V" shape perpendicular to the direction of the current, which can increase the chances of hooking the same school of fish.

4. The boat speed when setting the line should preferably be 8-9 knots, and satellite navigation can be used for observation. If fishing for bigeye tuna, the boat speed can be reduced accordingly so that the line is set deeper.

5. When setting the line, it does not matter much if two boats cross, but do not set lines parallel to another boat within 5 nautical miles, otherwise the lines may become tangled together. When setting the line, keep the starboard side exposed to the wind or current, and the bait should be thrown upstream; otherwise, the branch lines will foul the main line, preventing the hooks from extending effectively and affecting the fishing effect.

III. Moonless Water Operations (referring to nights without moonlight)

Because tuna are phototactic, the catch increases on nights with good moonlight. Conversely, it decreases. Moonless nights do not mean there are no fish; rather, because the light intensity is weaker, the depth to which small fish rise cannot reach the original depth, and in addition, during dark-water weather, because the current is faster, the line forms a relatively large slope, so the hooks cannot reach the original depth, resulting in very poor fishing results.

The following methods can be adopted to overcome this problem:

1. Extend the float-to-line ratio

2. Lengthen the branch lines, change the float-to-sinker ratio, and add lead weights

3. Avoid strong currents and find fishing grounds with moderate currents

4. Set the line at slow speed so that the line is set deeper

IV. Hauling the Line

When hauling the line, first locate the hooks based on the electrical signal measured by the direction finder. Make the boat form a 30-degree angle with the line, and let the boat move along with the line, but do not go too fast; a boat speed of 6 knots is suitable. When fish are brought up, slow the boat and reverse. This keeps the fish from swimming under the boat and makes it easier to bring them in. Temporarily remove the floats, branch lines, radio buoys, and buoy lights from the main line and organize them, and inspect the main line and branch lines; if any damage is found, cut it off immediately and reconnect it. The main line is directly reeled into the main line drum at the stern. After the branch lines are organized and placed in baskets, they are transported to the hook-setting area on the aft deck, and the floats are also transported to the float compartment on the aft deck.