Fishing effectiveness of trawl nets as resulting from experiments with wire cable bridles and with manilla covered bridles
Abstract
In the course of the described experiments the fishing effectiveness of trawl-nets was examined by alternatively using ordinary wire cable bridles and bridles made of combined materials (manilla covered wire cable). Randomized blocks served this purpose.
The experiments were carried out during the cruises of the Institute owned M/V BIOS south of the Cape Ploče, in geographical coordinates 43° 22,0’ N, 15° 53,2’ E and 43° 15,6’ N, 15° 55,6“ E, (see Fig. 1) falling between June 14th and 19th 1956 (26 hauls); October 21st and 23rd 1956 (12 hauls); November 25th and 28th 1956 (12 hauls); February 1st an 6th 1957 (24 hauls); and April 13th and 23rd 1957 (24 haute). The results obtained from October 21st to 23rd 1956 and from November 25th to 28th 1956 have been added together. A total of 96 hauls were performed, 48 of them with ordinary wire cable bridles and 48 of them with bridles made of combined materials. Identical techniques, i.e. the same vessel, net, locality, haul duration, etc. were applied with all the experiments.
Four standard hauls, each of an hour’s duration were performed, two of them by means of ordinary wire cable bridles (without manilla) and two of them by using a manilla covered bridle. The hauls were not systematicaly made during the trawling. The net used in the experiments was a locally produced one.
The aim of the experiments was to examine whether an ordinary wire cable can effectively be used for bridles instead of the customary but unpractical ones made of combined materials which are still applied by Yugoslav and Italian trawlers. The ordinary wire cable bridle, which was applied for the scope of our experiments, is shown in Fig. 2. This type of bridle constitutes a simple extension of the tow line and it is coiled on the winch in the same way. Its use is illustrated in Fig. 3.
Besides being faster and more easily handled, the advantage of ordinary wire cable bridles is also manifest in their fishing effectiveness. This is best confirmed by the obtained results of catches shown in the present paper, and by their statistical analysis as well.
The variance analysis method was applied to examine the statistical analysis of the obtained results. The outcome of the examination revealed the fact that there was no significant difference in fishing effectiveness between the two techniques (ordinary and combined bridles) in the course of the first three experiments, but that a significant difference appeared during the fourth experiment.
The catches of some express bottom species were negatively affected by manilia covered bridles, although, according to opinions prevailing among experienced fishermen, just the opposite result should be expected.
By examining the effect of tickler-chains with regard to Gadus merlangus, P a r i s h (1951) found that the disturbing action of additional gear such as tickler-chains can cause an active swimmer like Gadus merlangus to avoid the net and to get away before being trapped by its mouth. By far larger quantities of Gadus merlangus were caught when using the same kind of net without tickler-chains.
Our experiments have shown that the bridles made of combined materials reduce the fishing effectiveness not only in the case of better swimmers (Gadus capelanus, Maena smaris, Maena chrysellis, and Paracentropristis hepatus) but also in the case of some outstanding bottom species (Raja clavata, Mullus barbatus, Triglidae sp., Lophius budegassa, Uranoscopus scaber and Acantholabrus palloni), offering them the possibility of escape from the net-mouth as soon as they sense the danger of which, in this, case, they are warned by the bridles made of combined materials let alone if they be long and thick.
The obtained quotiens of cath (Table V) shows that the number of fish and shell fish varies according to the experiment and according to the species. It is evident from the above Table that the catches of Merluccius merluccius were more substantial during the first three experiments when bridles made of combined materials were applied (manilia covered wire cable) than when ordinary bridles made of wire cable (without manilla cover) were used. During the fourth experiment, however, the results were just contrary. The presence of Mullus barbatus in the catches of experiments II, III, and IV was more frequent when using ordinary bridles made of wire cable while the two techniques yielded the same results in the course of Experiment I. During all the four experiments Lepidorhombus whiff-jagonis was caught in larger numbers when the ordinary wire cable bridle was applied. The same was the case with Raja clavata. The most important edible invertebrate animal, Nephrops norvegicus, appeared in larger numbers only in the case of Experiment II when bridles made of combined materials were used, as was the case with Miscellaneous during the Experiment III. The total catch quotient for all the species shows that the catches taken with the application of ordinary wire cable bridles not covered with manilla were better numerically through all the four experiments.
Different reactions and behaviour of single species, due to their way of life (Table IV) were observed in the process of catching. So, for example, Merluccius merluccius and Scyliorhinus canicula were always caught in larger numbers when using bridles made of combined materials (with the exception of Merluccius during the fourth Experiment only). Other species, such as Raja clavata, Gadus capelanus, and Paracentropristis hepatus, on the contrary, were caught in much larger numbers with the application of ordinary wire cable bridles while Argentina sphyraena and Zeus faber manifested certain deviation in this respect.
In addition to different reactions and behaviour of fish within the framework of our experiments, there was a similar trend with regard to the average length of fish. The average length of the economically important fish species was generally greater when ordinary wire cable bridles were applied. (Table III). By analyzing the data given in Table II we find that the total weight and numerical difference, expressed in per cent rate, between the catches resulting from the alternative application of the two techniques (i.e. ordinary wire cable bridles made of combined materials) amounted to 7,04% (weight difference) and 2,94% (numerical difference). The difference in weight was greater than the difference in numbers. It is evident from the obtained ratio that application of ordinary wire cable bridles resulted in greater average length or weight of the caught species than it was the case with made of combined materials. The authors try to explain this difference in average length, particularly as regards the species of economic importance, with the assumption that the bridles made of combined materials have a cumbersome effect upon the net in the course of dragging since they cause meshes to scrink more closely.
