FISHERIES RESEARCH BOARD OF CANADA. Translation Series No Regulation of the capelin fishery

Transcription

1 '4P'cFttùeS FISHERIES RESEARCH BOARD OF CANADA Translation Series No Regulation of the capelin fishery by Olav Dragesund, Terje Monstad, and Oyvind Ulltang Original ti.tle.: Regulering av Loddefisket From: Fiskets Gang (Fisheries Migration), 59 : 63-67, 1973 Translated by the Translation Bureau(PMJ) Multilingual Services Division Department of the Secretary of Staté of Canada.Department of the Environment Fisheries Research Board of Canada Office of the Editor Ottawa, Ont. 4 pages typescript

2 VI J / 1-/ Regulation of the Cape lin 'Fishery by Olav Dragesund, Terje Monstad and Oyvind Ulltang. p. 63 Fiskets Gang No. 3, January 18, p Introduction Since the middle of the 1960's the fishery in the Barents Sea has developed considerably and is today the most important industrial fishery in Norway. The capelin, which is caught by purse seine and midwater trawl, was earlier only exploited in the winter and spring when migrating to the coast of Northern Norway to spawn. In 1968 a summer fishery on the growth and feeding areas in the Barents Sea was initiated. This fishery is mainly being carried out between Bear Island and Hopen, but also at Spitzbergen and the central and southern parts of the Barents Sea. As a result of this development of the fishery, the immature and maturing part of the capelin stock is exploited, and questions regarding the regulation of the capelin fishery have become more important after the summer capelin fishery started. P. 64 A regulation of the capelin fisheryin the summer has as its primary purpose to obtain the best possible utilization of the growth potential of the capelin. Regulations which aim at keeping the spawning stock at a level where recruitment is normal, have so far been less emphasized and topical since no reduction in recruitment has been registered at moderate levels of exploitation. The increasing exploitation of both juvenile and mature capelin in the past few years may make it necessary, however, to initiate regulatory measures in order to secure a certain spawning ponntial. A group of scientists at the Norwegian Institute for Marine Research in collaboration with researchers from Christian Michelsen Institute have constructed a computer model for exploited fish.stocks. The model.gives an opportunity to simulate developments in a stock on the long term when this is subjected to various forms of exploitation. The model has so far been used on mackerel (Hamre and Ulltang 1972), but a simplified version which calculates the yield per recruit and spawning stock per recruit under various fishing strategies has also been adopted for North Sea herring and capelin. The purpose p. 65 of this article is to discuss some of the ïesults obtained with capelin.

3 2 Studies of Types of Exploitation of Capelin In the following case the recruitment has been kept constant, whereas empirical data for weight (Table 1) has been inserted in the model. It is assumed that all individuals in a year class become sexually mature at an age of 4 years and that it is exploited at the 2, 3. and 4 year stage. The exploitation of one year class has been chosen and the fishery is divided as follows: 1) Of the fishing mortality in the summer fishery, 20% is assumed to occur in July, 50% in August and 30% in September. 2) In the winter fishery, 20% of the mortality is in January, 50% in February, 20% in March and 10% in April. The spawning stock is calculated by weight per March 1, and it is assumed that the spawning has been completed the same date. This means that 70% of the winter fishery is carried out on capelin which has not spawned and 30% on spawned capelin. Yield/recruit (in weight) and spawning stock/recruit (in weight) is calculated for various instantaneous mortalities (F) the year class is exposed to. The following alternatives have been studied: (a) F 4 = 1.0F (b) F4 = 0.8F; F 3 = 0.2F (c) F4 = 0.6F; F 3 = 0.4F (d) F = 0.8F; F 4 3 = 0.1F; F2 = 0.1F (e) F4 = 0.6F; F 3 = 0.2F; F 2 = 0.2F F 4 is the instantaneous fish mortality in the winter capelin fishery. F 1 p. 66 and F' are the fish mortalities in the summer of respectively 2 and 3 year olds. The program has been run with the following values for the instantaneous natural mortality (M) = 0.2; 0.3; 0.4; 0.5.

4 3 /In Figures 1 and 2, F is plotted against Y/R (yield per recruit) and Y/R vs S/R (spawning stock per recruit) for the various values of M. For low values of M, Y/R is largest at alternative (a), (b) and (c). For M=0.2, there is in effect no difference between (a), (b) and (c). If M=0.5 it would be uneconomical to exploit 2 year olds. This shows up in both Y/R and in S/R. How uneconomical it is to catch two year olds is clearly seen when considering catching three year olds in relation to two year olds. Two year olds that are spared will yield 44% more as three year olds at M=0.3. If the fishery for three year olds first starts during the summer season, the percentage yield will be larger compared with the total yield (including catches of three year olds) that would be obtained if the exploitation started on the two year old level. Under the assumption of equal total mortality (F) for a year class in the two alternatives and M=0.3, an extra yield of ca. 30% will be obtained by sparing the two year olds. At M=0.4 and 0.5 the corresponding percentages will be respectively 14 and 8. One must have M=0.65 in order for the fishery for two year olds to be justified. not available. Calculated values of M for capelin to.insert into the model are so far For other fish species in the North Atlantic (herring, mackerel, cod, haddock, etc.), values of M between 0.10 and 0.30 have been found.. It is not too likely that the natural mortality for maturing capelin (2-4 year old individuals) is lower than 0.2. Even if natural mortality depends on the total age of the fish and is'greatest for species with a short lifespan, it rh has been found unlikely to set M>0.5. Assuming that 0.2,CM <0.5, it would be uneconomical to catch two year olds and these should therefore be protected. Earlier it has been recommended to set a minimum size of 12 cm during the summer fishery in order to protect the one year olds. It is shown in Fig. 3 that the minimum size must be raised to 14 cm in order to be sure that the two year olds.are protected. p. 67 In Table 2 the length distribution of the 1969 year class is shown as it was in the summer of and in Table 3 the length distribution of the 1970 and 1969 year class is shown later in the year. At aminimum size of 14 cm, practically

5 4 total protection of the two year olds will be achieved. Most of the three year olds will have passed 14 cm in July and August and the fishery will therefore mainly be based on three year old and older individuals with a minimum size of 14 am. If 0.3<«0.5, the most logical of the fishing strategies studied would be alternative (b) or (c). These will restilt in the best utilization of the growth potential of the capelin and at the same time give a relatively high spawning potential. 11.