Ulltang, Øyvind

NORA - Norwegian Open Research Archives

/ \\~~~ !-,his paJ?,er no_~ __ ~ be cited wi tho':!!.....E!'..ior reference to the author International Counoil for the Exploration of the Sea C.H, 1972!H: 8 Pelagic Fish (Northern) Committee Yield curves of North Sea Herring By 0yvind Ulltal1.g Institute of :lVlarine Research, Bergen, Norway INTRODUCTION The catches of North Sea Herring have shown a steady decrease since 1965, while the fishing mortalities have increased to a level of about 1.0 for adults and 0.5 for juveniles (AlTON, 1972). A regulation of the fishery seems required, and the aim of this paper is to investigate the effeot of different oonservative measures on yield per recruit (Y/H) an.d spawning stocl!:: per re .. cruit (siR). l!1ATEHIAL AIm HETHODS A computer model originally applied on the Horth Sea r..mckerel stock (HAl/lRE and ULLT.ANG 1972) was adjusted to the fisheries for North Sea herring (autumn spawners). As a first approach constant recruitment has been assumed, and the progran calculates yield per re crui t and spawning s t ocle per re crui t. All c al cula t ions are done on a monthly basis. Inpu'/i data ore) monthly l:1ean 'weights by age, fishing and natural mortality coeff'icients and coefficients expressing the distribution of the yearly fishing mortality on the different raonths. The fishing t:lOrtality coefficients and the coefficients giving their distribution on the different 1:1Onths are split on O"ringers (1 year old), l~ringers (2 years old) and adults (2-ringers and older). The seasonal distribution of the 2 estinated fishery on adult herring was frOL1 data on nonthly catches of North Sea herring by Norway, Iceland, Faroe Islands and Scotland, and the distribution of the fishery for juveniles was estir:mted fror.l data froD the Danish young herring fishery. The distri ... butions applied are shown in Table 1 together with the distri-butions used to study the effects of different alternatives of closed seasons, Monthly nean weights by age in the catches (Table 2) are t~cen from-Report of the North Sea Herring Assess-ment lrJorking Group (ANON, 1972), RESULTS In Fig. 1 the yield per recruit (Y/R) is plotted against fishing I:HJrtality for adults (2 ... ringers and older) for three different values of the fishing nortality for l~ringers, F1~IO.5, 0.25 and 0, The fishing t:lortality for O ... ringers I is put equal to 20 % of Fl • The ourves are given for two alternative values of natural mortality, M: 0.1 and 0.2. In Fig. 2 the oorresponding curves are given for spaw~ing stook per reoruit (SiR). The autulTI ... 1'1 spal'm.ing herring was assuI.:led to spawn for the first tir..le as 2-ringers (3 years old) .. From the figures it is seen that (i) under OOi.~stant fishing nortality for l ... ringers, r.laxir.1Utl y/R occurs for Fadult= 0.5 (H= 0,1) or Fadult= 1,0 (H= 0,2). By decreasing the fishing mortality for adults frOtl 1,0 to 0.5, y/R would increase by 2-3 % if H= 0.1 and decrease by 4-5 % if H= 0,2, How-ever, a deorease in adult fishing l:lortality fror:1 1.0 to 0.5 would nore than double the spawning stock per recruit, If an increase in spaw.a.ing stock of this order would result in increased recruitnent, the result could be a oonsiderable gain in long term yield. (ii) If H= 0.1 and F adult = 1.0, a decrease in l!"'l.... fr 0 1:1 0.5 to 0.25 would give about 20 % higher yield per recruit. A total stop i~hrishery for juveniles would result in about 46 % higher y/R. The increase in. spa'WUing stock per recruit would be about 35 % and 82 % respectively. If N= 0.2, the corresponding figures for increase in Y/R would be 18 % ru~d 41 %. The increase in SiR would be the sarae as for 11= 0,1. 3 In Fig. 3 the e£feot of fishing on O-ringer is illustrated. It is seen that when Fl == 0.5 and Fadult= 1.0, no fishing on O-ringers would give 7-8 % higher yield per reoruit than when FO == 0.1. All figures given above are oaloulated assut:.1ing no olosed season in the fisheries. To study the dependence of the yield on the seasonal distribution of the fishery, yield per recruit ruLd spawning stook per recruit was oaloulated for three different alternatives of closed season: (i) Closed seasons in IJ~ay and Septol:1ber, (ii) Closed season from 1 April to 15 June, (iii) Closed season from 1 February to 15 June. This corresponds to the different alternatives of closed seasons whioh have been, or are, imposed 011. the fisheries for :North Sea herring. The seaSonal distribution of the fishing nortalitiE)s applied £01' the different alternatives in the caloulations are shown in Table 1. The resulting values of yield per reoruit and spawning stock per recruit at the actual level of fishing Elor-talities are given in the table below. Yield per recruit and spawning stock per reoruit (g) for different alternativeb of closed season. F == adult 1.0, F -1- 0.5, FO= 0.1. ~![ = 0.2 Yield of Y/R % juveniles % siR % No closed per recruit season 63.3 100 14.2 100 45.4 100 Closed seasons in Nay and Sep. 63.5 100.3 14.4 101.4 44.5. 98 Closed season 1 Apr .... 15 Jun. 66.3 104.7 15.3 107.7 48.7 107.2 Closed season 1 Feb, -1.5 JUll., 68,8 108.7 17.2 121.1 48.7 107.2 4 M ::: 0.1 Yield of YjR % juveniles % SjR % No closed per reoruit season 83.2 100 16.4 100 62,2 100 Closed seasons in gay and Sep. 83.5 100.4 16.6 101.2 60.9 97.9 Closed season 1 Apr .... 15 Jun, 87.3 104.9 17.7 107.9 66.7 107.2 Closed season 1 Foo.-15 Jun. 90,6 108.9 20,2 123.2 66.7 107.2 It is seen that closed season 1 February - 15 June will give about 9 % greater yield per recruit and 7 % greuter spmming stocl: per reoruit than no olosed season. The greatest relative inorease in yield per recruit occurs in the fisheries for juveniles, For example, closed season 1 February ~ 15 JU1~e will give above 20 % inorease in yield per recruit, It should be pointed out that the oalculated increase in yjR and SiR are strongly dependent on the assur;1ed seasonal distribution of the fishing r;lortalities. As an exar.lple, it is seen in Table 1 that 62 % of the yearly fishing l:1ortality for adults is assigned to the second half of the year when there is no closed season, If a smaller percentage had been applied, tIle calculated gain in yield per recruit and spa~lUing stock per reoruit by olosed seasons in the first half of the year would have been greater. If closed seasons have the effect of reducing the fishing t:lOrt al i ty for juvenile s, the inoreas e in YjR will be greater than shown in the table, For exm:lple, assuning that a closed season from 1 February to 15 June gives the sane total yield in weight of l~ringers as no olosed season~ Fl would be reduced from 0.5 to below 0.4. With Fadult= 1,0 1 this would give an Y/R-value about 16 % greater (111= 0,1) than when F;t. = 0,5 and there is no closed season, DISCUSSION Oonsidering the whole stock of North Sea herring (autt:u:m spawners) as one unit, it has not been possible to show any correlation between stock size and recruitnent (AlTON. 1972). HO~lever, as pointed out by the North Sea AssessElent Horking group 5 it is possible tnat an existing stoc1~/recruitnent relationship might be nasked w'hen several stocks (the Ban.le, Downs and Buchan stocks) are considered together .. lJhen evaluating the different fishing strategies one should therefore tcl(e into account that a considerable decrease (increase) in spal'ming stock nay give a decrease (increase) in recruitoent, Oonsequent1y the curves of spal'n:ling stock per recruit (Fig, 2) are of' great interest. If' a positive corro1ation exists between stock and recruitnent, a certain increase in spawning stock per recruit will give rize to an increase in recruitnent wich again will increase the spawning stock and so on until equi1ibriuD state is reached. The spawning stock will thus increase nore than. the value of siR, For a given stool:/recruiti:lent relationship, R= f ~S), the state .. gy giving naxinuI:1 long tern yield is easily fround fro[1 the Y/R ... curves and S/n ... curves (HAI1RJ] and ULLTAI·JG 1972), 1Vithout knowing anything about the relationship between stock and recrui tlClent for North Sea herring, such considerations have not been atter.1pted here. SUMI1ARY (i) Fro[1 tho yield per recruit criteriul:l, there is 1itt10 to gain or 1:cfe'~ by reducing the fishing Dortality of adults by say one half. Hm'lever, by reducing F adul t by one half, spawl1.ing stock per recruit would be Dore than doubled. L considerable gain in yield could then be obtained if a positive correlation between stock and recruitnent exists, (ii) A total banning of the young herring fishery would give an increase in yield per recruit between 40 % and 50 %, depen~ ding on the value ofH. Spawning stocl: per recruit would in" crease by about 80 %. (iii) Of the alternatives of closed season studied here, a closed season 1 February ... 15 June is best fro!:.l the cri teriur.l of yield per recruit and spawning stock per recruit, giving an increase of' about 9 % in Y/R ruLd 7 % in SiR .. 6 R:;J::FERElTCEG AlIOH, 1972, Report of the North Sea herring assessnent working group. Coun.Meet.int.Coun.ExE10r.Sea, 1972(H:5). [1/1i1:1eO .] Ellil:lRE, J, and ULLT.A1JG, 0. 1972. The effeots of regulations of the Hackere1 fishery in the North Sea. <l.0un.~1eet. int,Cq"un.ExE10r.S,e.a, 1.,27.2(H:30): 1-14. Dl1ineo.J Table 1. ill Jan O .... ringers l.-ringers 0.10 Adults 0.04 Liil Jan O-ringers 1-ringers 0.12 Adults 0.06 iiii} Jan O-ringers l-ringers 0.10 Adults 0.04 (iv:) Ja,n O-ringers l-ringers 0.12 Adults 0.04 7 The proportion of the yearly fishing 1:10rta1i ty coefficient assigned to the different nonths~ No closed season Feb Mar Apr May Jun Ju1 Aug Sep Lc. ~ Nov 0.2,5 0.25 0.25 0.12 0.12 0.08 0.08 0.01 0.08 0.16 0.10 0.08 0.04 0 0 01 0.01 0.01 0.05 0.26 0.26 0.17 0.09 0.04 0.04 Closed seasons in Nay and Septel:1ber Feb I/iar Apr May Jun Ju1 Aug Sep Oc:t :Nov 0.33 0.33 O.lL~ 0,14 0,08 0.02 0.08 0.16 0.12 0.08 0.02 0.01 0.01 0,28 0.28 0.18 0.06 0.06 Closed season 1 April ... 15 June Feb Mar Apr May Jun Jul Aug Sep Oct Nov 0.2,5 0.2,5 0.25 0.12 0.12 0.01 0.12 0.20 0.14 0.10 0.05 O.Ol 0.01 0,20 0.31 0.20 0.11 0.05 0.05 Closed season 1 February ... 1,5 June Feb Mar Apr 'May Jun. Jul Aug Sap Gc:; Nov 0,25 0.25 0.25 ... .... .... ... 0.02 0.18 0.26 0.20 0,12 0,05 ... ... 0.21 0.32 0,20 0.11 0.05 0.05 Des 0.25 0.03 0.02 Dec 0.33 0,06 0.04 Dec 0.25 0.04 0.02 Dec 0.25 0.05 0.02 8 TabhY 2. :Wean. Vleights (€ ) by Nonth and Age (Total North Sea) AGJ] IN WINTER RINGS r'lonth 0 1 2 3 4 .5 6 7 8+ Jan. ... 29 84 131 159 19.5 207 222 232 Feb. ... 29 82 112 142 161 177 181 202 jlJ~ar. .. 30 94 121 144 174 195 210 222 Apr. 34 106 134 157 177 192 207 219 Fay 40 112 146 169 190 20.5 219 231 Jun. 47 147 17.5 197 218 233 247 2.58 Jul. .5 56 184 216 242 264 ~84 300 314 Aug. 7 64 170 20.5 230 252 273 291 304 Sep. 15 70 157 191 216 242 264 284 303 Oct. 22 75 157 185 212 234 25.5 272 289 Nov. 27 77 144 166 194 215 232 248 260 Dec. 28 78 133 160 187 207 224 239 253 S a:: ...... >-:::. 140 120 100 80 60 40 20 100 80 60 40 20 ";. 0.2 Fig. 1. 'I ,. . ~ 9 M = 0.1 ---~----------------- FI-RINGERS = 0 --------------------- FI-RINGERS: 0.25 ---~------------------ FI-RINGERS : 0.5 M = 0.2 FI-RINGERS = 0 FI-RINGERS = 0.25 - FI-RINGERS = 0.5 , i I I i 0.4 0.6 0.8 1.0 1.2 1.4 1.6 FADULT Yield per recruit (Y/R) plotted against fishing mortality for adults (Fadu1t) for three different values of fishing mortality for 1-ringers. F. :: 0.2 • O-rl-ngers F1 · • -rl-ngers , . • <. ' ..... .~ t" • 800 700 600 500 400 300 200 100 500 400 300 200 100 0.2 0.4 Fig. 2. M = 0.1 M = 0.2 10 F1-RINOERS = 0 FI-RINOERS = 0.25 F1-RINOE RS ; 0.5 ::::=========== FI-RINOERS ,,0 --- t- I  = .25 FI-RINOERS = 0.5 0.6 0.8 1.0 FAouLT 1.2 I I 1.4 1.6 Spa'\ming stock per recruit (sin) plotted against fishing mortality for adults (F d lt ) for three a us. different values of fishing mortality for l-ringers. FO' = O. 2 • F l . • -r1ngers -r1nGers 100 80 § 60 Cl: ..... >-40 0.2 Fig. 3. 11 Iv! : 0.1 ------~------------- FO-RINGERS : 0 -------------------- FO-RINGERS : 0.1 0.4 0.6 0.8 M: 0.2 1.0 FADULT 1.2 1.4 Fo -RINGERS = 0 FO-RINGERS : 0.1 1.6 Yield per recruit (Y/.n) plotted against fishing' [1ortality :for adults (Fadult) for t'To'lO dif:ferent values of fishing mortality for O-ringers. F - 0 5 l-ringers- •• 

Yield curves of North Sea herring

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