Among all the professions currently engaged in by humans, fishermen are the oldest. Fishery is the last hunting behavior of humans.
The fishermen are the last hunters, but they are not using guns but nets. What they roar is not the forest, but the stars and the sea.
From the earliest spearfishing, to the invention of bone and stone hooks, and to later learning to make nets, the level of human fishing continues to improve with the advancement of technology. Today, mechanized ocean fishing boats It has become the main force that provides us with seafood. There are different kinds of seafood, and different fishing boats use different methods to catch them.
Let me introduce to you how the trawl is used for fishing.
Trawler
Fishing objects: shrimp, crabs, bottom and middle water fish
Trawl fishing means that a boat drags the net forward, "pockets" the fish in the sea, and then collects the net.
In order to catch more aquatic products, the fishing net must be opened as large as possible, but under the traction of the rope, the net mouth will inevitably shrink inward. How to overcome this difficulty? In the actual operation of trawlers, it is often solved in three different ways.
The first is to use a truss rod, that is, a rigid long rod to open the mouth of the fishing net to a certain width, as shown in the figure below:
Of course, we have also seen that the mouth of this kind of fishing net is often a long strip, which is still not big enough, and because of the stringer, the net is large in size and it is inconvenient to operate.
So after the 1950s and 1960s, a new type of trawl-net plate trawl appeared. This kind of trawl is equipped with two baffles on both sides of the net. When the fishing boat is dragging the fishing net, the current pushes the two plates to expand outward at a certain angle, driving the fishing net to open the net opening, as shown in the following figure:
In addition to these two, there is a simpler and more rude way, that is, two boats drag the fishing net together, but this is easier said than done, it requires two boats to cooperate closely, and coordinate the course and speed according to the position of the fish. , So the frequency of use is not as high as the above two single-vessel operation modes.
Trawl fishing is currently the most mainstream fishing method because of its convenient operation and wide application area. Some trawlers are suitable for trawling with floats to catch fish in the middle of the sea; some trawlers use beam trawls with counterweights to catch shrimp and crabs on the seabed. It should be noted that the nets are actually It is very fragile, so bottom trawlers need to operate on the bottom of the sea, which is dominated by silt, to avoid the rocks (or coral reefs) on the bottom of the sea from scraping the fishing nets.
A fishing boat that uses bottom trawl to catch crabs can be seen with a small beam trawl on each side.
Although trawl nets are versatile, they are not omnipotent. For example, fish on the top of the sea are difficult to catch with trawl nets. We can suppose that there is a school of fish on the water, and your fishing boat drives straight through the middle of the school of fish. The school of fish is scattered and fled, and the fishing nets dragged behind the boat are naturally gone. Therefore, to catch the fish at the top of the sea, another method is needed-purse seine fishing. In the next article, we will introduce the method of purse seine fishing.