In a longline fishery, it is important to decide the water depth at which level the hooks should be set according to the type of fish to be caught. This subject is equal to determining the underwater shape of the main line. The shape of the main line can be considered as a catenary curve. However, as the equation of the catenary curve involves a hyperbolic function previously it has been difficult to calculate the shape. Yoshihara transformed the catenary into some other equations and using them and a certain table he obtained the shape of the main line that is the water depth of the main line. After that, studies about, the depth of the main line of longlines adopted Yoshihara' s equations and tables. However, the calculation using Yoshihara' s equation is complicated and requires the use of a special diagram in the calculation. This method lacks flexibility in its application and it is difficult to obtain the water depth of the longline under any given conditions. Present scientific electronic calculators are commonly equipped with a hyperbolic function. Consequently, I devised a catenary calculation using a scientific electronic calculator and showed that it is possible to readily calculate the water depth of the main line under any given conditions. For the application to the main line of the longline. I show a flow diagram (algorithm) and provide a practical example of the computation. I also show the application of catenary to the warp of a trawl. Catenary is very usable in the fishing gear dynamics. I hope this paper is able to contribute to the popularization of the use of catenary in the training of the fishing gear dynamics.
Catenary
Shape of the main line of a longline
Shape of the warp of a trawl
Hyperbolic function
Fishing gear dynamics