11/ This paper not to be cited without prior reference to the author. I'

Transcription

1 / 11/ This paper not to be ited without prior referene to the author. I' International Counil for the Exploration of the Sea C. M /H : 3 0 Pelagi Fish (N) Committee THE EFFECTS OF REGULATIONS OF THE MACKEREL FISHERY IN THE NORTH SEA By Johannes Hamre and 0yvindUlltang Institute of Marine Researh Bergen, Norway INTRODUCTION Sine 1970 the Norwegian makerel fishery for meal and oil has been regulated by a minimum legal size of 30 m, losed season during winter and spring, and annual ath quotas for reduing the fishing mortality (HAMRE 1970 and 1971). The present paper is a theoretial study of the effets of the various regulation measures on stok and yield. For this analysis a simulation model has been developed. The paper also gives a brief aount of the present state of the makerel stok and the regulation measures.imposed on the fishery in THE MODEL In order to study the effet of seasonal distribution of fishing effort, all alulations in the model are done on a monthly basis. Input data is monthly mean weight of fish by age, total number in eah yearlass at the beginning of the first year run, reruitment to the stok during the time period inluded in the analysis, monthly fishing mortality (Ft) oeffiients and oeffiients aounting for redued fishing mortality of the age groups not fully

2 - 2 - reruited to the fishery. The model alulates the yield in number (C) and weight (Y) by yearlasses and month (t) by the equations: C = N t. F [ -(Ft + M)j t t I - e J Ft + M Y = W C t t t and the number in the yearlasses (N) at the beginning of the next month by N + t l = N t. e-(ft +M) The programmes are run with a onstant natural mortality rate (M) of 0.2. W t is the monthly mean weight of fish. The model summarizes size and omposition of the ath,and the orresponding stok size during the time interval under study. The weight of the stok in June, inluding the yearlasses 2 years and older, is used as index of the spawning stok. January I is regarded as the birthday of the fish. For onstant reruitment a simpler programme has been used. Input data of growth and mortality is also arranged here on a monthly basis, and the programme alulates yield per reruit and spawning stok per reruit for equilibrium state of stok and yield. EFFECTS OF CLOSED SEASON The effets of losed season regulationson stok and yield have been analysed by using the programme for onstant reruitment. The results are illustrated in Fig. 1 (A, B and C).

3 - 3 - Fig. la shows yield per reruit (Y/R) in perent of the orresponding yield when F is equally distributed throughout the year. Fig. lb shows similar figures of weight of spawning stok, and Fig. le the average weight of fish in the ath. The programme is run for two ages at first apture (t ). For illustration purposes two alternatives of losed season regulations are seleted, those whih yield maximum and minimum ath per reruit respetively. The first one (the solid line) illustrates the ase when the fishery is restrited to July August only, and the latter (the dotted line) to the period January-Marh. In pratie the seleted seasons orrespond to the Norwegian purse seine fishery during summer in the Shetland area and the winter fishery on the Viking Bank. Any other seasonal distribution of fishing effort lies between these two urves. The figure shows that restriting the harvesting of the stok to partiular seasons may effet the obtainable yield per reruit within a range of some + 20% ompared with equal distribution of effort throughout the year. It is further seen that the variation range inreases with inreasing F when t is low, but beomes more or less independent of F for t = 3 or above. In general, it an be onluded that restritions on unfavourable fishing seasons may inrease the yield per reruit onsiderably, partiularly when the fishery exploits younger age groups. The losed season regulations have a similar effet on the size of the spawning stok, but the effets are more influened by the rate of exploitation (Fig. lb). Thus for t = 2 and F = 0.2, SIR may vary within a range of + 10% ompared to equal seasonal distribution of effort, but the range may inrease to + 30% if F is inreased to o.6. For low values of F, the effet of losing unis naturally low and not signifiantly favourable fishing seasons influened by t. But when the stok is heavily exploited the losed season regulations are also important for maintaining a high

4 - 4 - level of stok size during the spawning season, and most important when the fishery exploits younger age groups. The effets of either winter or summer fishery on the average weight of the fish in the ath are illustrated in Fig. lc. The variation range of W is some + 20% and is little influened by F and t,exept for very high values of F when t is low. The regulations imposed on the Norwegian makerel fishery in 1970 and 1971 prohibited meal and oil prodution from January to July and during the last months of the year when the allowed ath quota was filled. This resulted in an approximate perentage distribution of the fishing mortality (F) as follows: January-Marh 0%, April-June 6%, July-August 14%, September Otober 80% and November-Deember 0%. These figures refer to the North Sea stok only, and are derived from the Norwegian ath statistis after subtrating the estimated ontribution to the ath from the Irish stok (HAMRE 1971). The broken urves in Fig. 1 show the effet of the above mentioned atual distribution of fishing mortality on the stok and yield. The figure shows that the present restritions on the fishery for meal and oil has resulted in a fishing strategy whih approahes the expeted optimum ath as far as seasonal distribution of effort is onerned. Sine the stok has been very heavily exploited in reent years the losed season regulation during winter and spring is regarded as a very important onservation measure beause it ontributes signifiantly, both to obtain optimum yield per reruit and to maintain a high level of spawning stok. Further studies are based on the observed seasonal distribution of F outlined above. EFFECTS OF MINIMUM LEGAL SIZE Aording to previous studies it is found that the gain in yield per reruit by, inreasing the age at first apture, is rather insignifiant (POSTUMA 1969, HAMRE 1970). This is also the ase

5 - 5 - when the distribution of F is regulated by losed seasons (Fig. 2). As the figure shows, the Y/R does not inrease signifiantly by inreasing age at reruitment (t ), at least not for F-values below 0.4. The parameter, t, has however a onsiderable effet on the size of the spawning stok when the population is heavily exploited. This is shown in Fig. 3. The figure shows the size of spawning stok per reruit for onstant reruitment at equilibrium state of stok for different values of age at first apture (t ), as a funtion of F. It is seen that an inrease in t from 1 to 3 may double the size of the spawning stok when F approahes the level of optimum yield per reruit (0.4). Therefore, if a positive orrelation exists between reruitment and the stok size at the orresponding level of exploitation, a minimum legal size regulation will be quite deisive for the obtainable sustained yield. It may further be noted that the urves are rather steep in the interval below F = 0.4, but fl~ten out above that value. This means that under the onditions that the reruitment is redued by redued. stok size, the loss in yield per reruit by reduing F may be more than ompensated by the inreased probability of reruitment. The stok-reruitment relationship is thus one of the most important fators in the onsideration of the relationship fishing mortality / sustained yield. SUSTAINED YIELD Hamre (1970) estimated the size of the North Sea makerel stok to a level of 4 million tons before the purse seine fishery was introdued in Before that time the stok was very poorly exploited (less than 3% annual fishing mortality). The yearlasses 1962 to 196~ whih were reruited from that stok size level of spawning stok,w~eestimated to an average of 1910 million reruits at 2 years of age. From these figures, together with an estimated yield per reruit under the most favourable seasonal distribution of effort, he onluded that the expeted sustained yield would approah a maximum of 400 thousand tons when F approahed 0.4, provided that the redued stok size did not influene the reruitment.

6 The reruitment to the makerel stok sine 1966 has been far below the said value. The 1967 and 1968 yearlasses were extremely poor and are estimated to some two and four hundred million reruits (HAMRE 1971). The parent stok was estimated to 2.7 and 1.8 million tons respetively (HAMRE 1970). This low reruitment has no doubt been due to unfavourable onditions of reprodution. Compared to these yearlasses the 1969-yearlass is however strong,and a preliminary estimate referring to August 1971 indiates a yearlass strength of 1200 million individuals (HAMRE 1972). The data on whih the estimate is based is poor, and may also be biased towards an underestimate of the real value. The parent stok of the 1969-yearlass has been estimated to 1.1 million tons. The 1970-yearlass has not so far shown up in the Norwegian athes and may thus be very poor. The spawning stok in 1970 is estimated to about 0.7 million tons. Although the present knowledge of reprodution is 'far from adequate to formulate a density dependent relationship of reruits and stok, some valuable information an be obtained by inorporating suh a realtionship in the stok model. For this study we have used the stok-reruitment'relationship of Beverton and Holt (1957): R = S as + b where R denotes reruits, S stok size and a and b are onstants. In aordane with the existing data on reruitment in the years when exploitation was low ( ), the asymptoti Vllue ~ was set to 2000 million individuals.. The onstant b determines the urvature of the stok-reruitment urve, and the aim of the study is to investigate how this parameter may effet the sustained yield. In this respet three values of b are seleted, and the orresponding urves are shown in Fig. 4. The upper urve (b = 0.1) illustrates a stok-reruitment relationship where the reprodution is very slightly effeted by exploitation rates below F = 0.4.

7 - 7 - The two lower urves postulate that a redution in reruitment does exist. Inorporating the said stok-reruitment relationship litothe model, the expeted yield at equilibrium state is alulated and the results are shown in Fig. 5, for the respetive values of b. The important onlusion whih may be drawn from this study is that the risk of overexploiting the stok is onsiderably redued by maintaining a high t, even when the exploitation is high and the reruitment is signifiantly effeted. It is thus seen that a ontrol on the fishing mortality, limiting F to 0.4,may be adequate if the age at first apture ould be kept as high as 3 years, even if the reruitment is onsiderably redued at that level of exploitation. This high exploitation rate may on the other hand be fatal if the immature fish are unproteted (Fig. 5), and even with the present t in fore (2 years), the danger of over exploiting the stok an not be ruled out when F approahes 0.4. Sine an inrease in t above 2 years of age may not be pratial, and the possible loss in yield may in any ase be rather small by reduing F to 0~3, it is felt that this level of exploitation is preferable for the future exploitation poliy,rather than the level on whih the fishery has been regulated in the past (F = 0.4). THE PRESENT STATE OF STOCK As mentioned previously, the 1969-yearlass seems to be relatively strong ompared to the yearlasses 1967 and Due to the depletion of older yearlasses, the 1969-yearlass is now dominating the mature stok. 'This yearlass ontributed with 80% in weight on the purse seine athes landed from the North Sea south of 59 0 N during autumn A preliminary estimate of the yearlass strength amounts to 1200 million individuals, the estimate referring to August This is however regarded as an underestimate, beause the yearlass may not yet be fully reruited to the athable stok. The total stok size was estimated to 460 thousand tons. The Norwegian ath after 1971 amounted to some 23 thousand tons and the bulk of this ath originated from the North Sea stok. The prospets of reruitment from the yearlasses 1970 and 1971 are rather poor. The 1970-yearlass whih was expeted to turn up in

8 - 8 - the autumn athes in 1971 seems to be extremely poor. The strength of the 1971-yearlass is more unertain but may also be far below average. This is indiated by the fat that no ourene of the O-group in the Norwegian fjords was reported during late autumn 197~ whih is usual when a good yearlass is reruited. It is therefore fair to assume that at least two years will pass without substantial reruitment to the mature stok, and under these irumstanes the fishery must be kept under strit ontrol if the stok shall not be further depleted. CONSERVATION MEASURES IN 1972 An analysis of expeted ath and size of spawning stok, by various rates of expioitatio~ has been arried out by simulating a fishery strategy whih orresponds to the distributi6n of fishing effort by age groups, season and area, resulting from the regulation measures imposed in Size and age struture of the mature stok was established on the basis of data obtained during The future reruitment was set to 500 million individuals at 2 years of age or about 1/4 of the estimated average reruitment from the yearlasses The programme was run through the years for various rates of exploitation on a onstant annual basis and the alulated ath and orresponding stok size are shown in Table 1. table shows that if F exeeds 0.2 the spawning stok may ontinue to, deline unless the reruitment should turn out to be substantially better than antiipated. The failing reruitment in reent years gives reason to fear that the stok size is already brought down to the level where the reruitment is effeted, and the exploitation poliy suggested for the oming years is therefore aimed at rebuilding the stok, disregarding expeted loss in ath per reruit. The The regulations of the Norwegian makerel fishery refers to the onservation of the stok spawning in the Skagerak and North Sea only.

9 - 9 - Sine 1970 a purse seine fishery for makerel has been developing in the area around Shetland during late summer. Aording to re~ults obtained from tagging, this fishery is based on mixed stoks, and the ontribution from the North Sea makerel stok has so far been of minor importane (HAMRE 1971). The restrited area inludes the Skagerak and North Sea east of \1, In order to exlude the Shetland fishery during summer this area has been divided in a northern and southern part using 59 0 N as boundary. In the area south of 59 0 N industrial fishing will not be permitted in 1972, whereas the fishery north of 59 0 N has been open for the industrial fishery from July 17. No ath quota for redution purposes has been set, but should the ontribution from the North Sea makerel stok beome dominant the fishery an be losed with short notie. Purse seining for makerel for human onsumption has been prohibited for vessels above 70 ft during most of the period January-June Due to this, and to the market onditions, the quantity landed for human onsumption is limited. It is expeted that the regulations in fore will keep the total Norwegian ath of makerel from the North Sea stok below 100 thousand tons in SUMMARY AND CONCLUSIONS 1. The paper deals with a study of the effets of regulating the makerel fishery by minimum legal size, losed seasons and annual ath quotas for reduing the fishing mortality. The future exploitation poliy is onsidered in view of the present state of the North Sea makerel stok. For this analysis a simulation model has been developed. 2. Regulations by losed seasons have a ~onsiderable effet on the yield per reruit, the spawning stok per reruit and the average weight of fish in the ath, espeially when the fishing mo~tality is high and the younger age groups are unproteted (Fig. 1). Fishing during summer is most favourable, the winter

10 fishery represents the most unfavourable fishing strategy. The seasonal distribution of effort resulting from the regulations in fore is lose to the distribution giving optimum values of stok and yield. 3. The gain in yield per reruit obtained by inreasing age at first apture is rather small (Fig. 2). This parameter, however, effets the size of the spawning stok, espeially when the stok is heavily exploited (Fig. 3). More than a 100% inrease in stok size may theoretially be obtained by inreasing age at first apture from 1 to 3 years when the fishing mortality is above Optimum sustained yield is onsidered by assuming density dependent reruitment (Fig. 4). It is found that the probability of overexploiting the stok is low if an age of 3 years at first apture ould be maintained (Fig. 5). This is however impratial, and under the present irumstanes the risk of overexploitation an not be ruled out if the fishing mortality exeeds 0.3. Sine the expeted gain in sustained yield by inreasing fishing mortality above 0.3 in any ase is rather small, this value is suggested as the limit hosen as a basis for a system of ath quota regulation of the fishery. 5. A prognosis of ath and stok size for the years is given in Table 1. With the present regulations in fore it is expeted that the Norwegian ath of the North Sea makerel stok in 1972 will be kept below 100 thousand tons. REFERENCES EEVERTON, R.J.H. and S.J. HOLT On the dynamis of exoloited fish populations. Fish.Invest., Lond., Ser.II, 19: HAMPE, J Size and exploitation of the makerel stok in the Sl:agerar: and the north-eastern North Sea. Coun.Meet. int. Coun.GxplC!!~.?e~, 1970 (II:26): 1-5, 1 tab, 3 fi0s. )1imeo~ " 1~71. Further results of the Norwegian makerel investier'") t i () n :;. C () un. ~~ E' (' t. i n t. Co un. F x n 1 0 r. S (~a, ( H : 2 3): 1-14, ~. -._-- --.~ ~ ~

11 tabs, 2 figs. [Mimeo] HAMRE, J Some notes on the state of the makerel stok. Int.Coun.Explor.Sea, 1972 (Li:8): 1-2.lMime~. POSTUMA, K.H On the abundane of makerel in the northeastern North Sea. Coun.Meet.int.Coun.Explor.Sea, 1969 (H:21): 1-8, 5 figs.[mime~. Table 1. Prognosis of ath and spawning stok of North Sea makerel for values of fishing mortality (F). For further explanation see text. Spawning stok Cath Mean weight (in 1000 tons) (in 1000 tons) (in grams) Year F = F = F = F = F =

12 - 12- VIR 120 t ::: 2 VIR ~ t = 3 ~ BO, A 1 A 2 SIR SIR ~ z 120 w u 100 a:: w ", " B 1..., ::;;;;~ ". '" B 2...,... '" w w 120""!.. -:== C 1 C F F Fig. 1. Effets of losed season regulations by fishing mortality (F) and age at first apture (t ) on yield per reruit (Y/R 2 R), spawning stok per reruit (SiR sa) and mean weight of fish in the athes (W ), in perent of the orresponding figures when fishing mortality is equally distributed throughout the year. The solid line illustrates a fishery limited to July-August only, the dotted line January-Marh. The broken line represents a fishing strategy orresponding to the losed season regulation in fore.

13 VIR 0.22 t : 2. Fig /- // /;;:>~,.? : = 3 1,= Yield per reruit (Y/R in kg) by fishing mortality (F) and age at first apture (t ). Seasonal distribution of F as at present (see text). 0.11, ' 0 I I I I.-.---y r- -h--l , F SIR 1.8 Fig , 1.2 Spawning stok per reruit (SIR in kg) by fishing mortality (F) and age at first apture (t ). Seasonal distribution of F as at present (see text) o..s F R =---== ~ , r r S Fig. 4. Stok-reruitment urves aording to Beverton and Holt (1957). S denotes stok in million tons, R denotes number of reruits For further explanation see text.

14 \ ' y o --- le = 3 b.oi 0.3 "~ i r I I I I I 1-' F /~~;;;rle.1 "- Y b.0.3 le 3 I ~ ~ le r r r-_... _-...,... i ""! F " y 02 b.os 01 le' le' T '," ,----r-\ I ~ 0: F Fig. 5. Estimated sustained yield (Y in million tons) by fishing mortality (F) and age at first apture (t ) for density dependent reruitment (see text).