A study of the sinking speed of longlines influenced by bait properties and anchor weights

Abstract

The paper presents investigations on to which degree the sinking speed of longlines is influenced by type ofbait, bait sinking orientations and anchor weights. The main aim of this study is to obtain further insight inthe ocean current displacement phenomena in demersal longlining. The sinking speed is one of the mainfactors deciding the current displacement. In an ongoing project, sinking speed experiments with longlineswith 6 kg and 10 kg anchor weights have been carried out in the Trondheim fjord. The longlines used in thefirst experiments were rigged without bait and hook. The results of these experiments with two differentanchor weights have revealed only a slight difference in the sinking speed, except for the part near to theanchors, even though the sinking speed of longlines in general is supposed to be much influenced by theanchor weights. The reason for the obtained result is supposed to be that the experiments have been carriedout at relative shallow waters. Further studies have included bait sinking experiments in the flume tank. Theexperiments showed that the drag coefficient of fillet type (flat)bait varied from 0.763 to 1.735, while itfor elliptic typebait varied from 0.62 to 1.483. Other activities have included calculation of the sinkingspeed of longlines as a function of the established resistance coefficients of bait of various shape and size forcommercial longlining. The calculated sinking speed of a longline with the fillet type bait was found to be12.4 to 16.5% lower than for a longline without bait. Keywords: Sinking speed of longlines, Baits, Anchor weights, Numerical methods, Ocean currentdisplacement of longlines