A fishing experiment with multifilament, monofilament and monotwine gill nets in Lofoten during the spawning season of Arcto-Norwegian cod in 1974

摘要

A fishing experiment with multifilament, monofilament,and monotwine gill nets in Lofoten during the spawning season of Arcto-Norwegian cod in1974. FiskDir. Skr. Ser. HavUnders., 16: 531-550.From 6 February to 30 March 1974 during the spawning migration of Arcto-Norwegiancod, a fishing experiment with gill nets made of continuous multifilament nylon, nylon monofilamentand nylon monotwine was carried out in Lofoten.The different types of nets were combined to make upone gill net setting consisting of 40 to92 single nets, half of which were multifilament nylon nets and one quarter each monofilamentand monotwine nets. The sequence of the single nets was varied during the experiment.The result for the total experiment was that the monofilament nets caught 26% (innumbers) more cod than the multifilament nylon nets and 38% more than the monotwine nets.For saithe the monotwine nets were apparently the most and the multifilament nylon nets theleast efficient.The average length of the captured fish was slightly higher for the multifilament nylon thanfor the monofilament nets whereas the fish caught by the monotwine nets were somewhatsmaller.Taking the length frequency of cod caught by purse seine in the same area during theexperiment as representative for the cod available to the gill nets, a log-normal distributionselection curve was fitted for each of the three types of gill nets.The mesh size used in the experiment (186 mm) was clearly too small to obtain maximumcatches of the available cod. Assuming proportionality between mesh size and mean selectionlength gave optimum mesh sizes of 224 mm for nylon, 222 mm for monofilament and 234 mmfor monotwine. The ratios between the theoretical maximum catches thus obtained were:Monofilament: Nylon = 1.46; Monotwine: Nylon = 1.48; Monotwine: Monofilament = 1.02.Assuming that all length groups are equally numerous among the cod available to the nets,ratios between the catch efficiency of the three nets, which should represent a moregeneral situation, were calculated, giving: Monofilament: Nylon = 1.23; Monotwine: Nylon= 1.15; Monofilament: Monotwine = 1.07. However,the accuracy and the general validity ofthese ratios are dependent on several factors of which the environmental conditions may be themost decisive.