Applied the Improved Surplus Production Method to Assess the South Pacific Albacore Stocks (Thunnus alalunga),

Transcription

1 3vtv9 Coms% SGTB13 Working Paper ALB-2 SCTB 13 Noumea, New Caledonia 5-12 My 2000 Applied the Improved Surplus Production Method to Assess the South Pacific Albacore Stocks (Thunnus alalunga), Chien-Hsiung Wang Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC July 2000

2 Applied the Improved Surplus Production Method to Assess the South Pacific Albacore Stocks (Thunnus alalunga), Chien-Hsiung, Wang Institute of Oceanography, National Taiwan University No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan, ROC ABSTRACT Based on the catch and effort data of tuna longline fishery, the improved surplus production method was applied to assess the South Pacific albacore stocks. The carrying capacity (K), intrinsic growth rate (r) and catchability (q) were estimated by K=149733mt, r= and q= e-09, respectively. During , the change rates of biomass varied in the ranges of-0.27 to 0.36 with the mean The harvest rates varied in the ranges of 0.14 to 0.23 with the mean The productivity varied in the ranges of 0.73 to 0.94 with the mean The unique equivalent catch point is evaluated by fishing mortality= with biomass=4991 Imt and catch 3663 Imt. The basic information of the three reference points and four different fishery development stages were summarized in the "Basic Information Table of the Fishery Management Decision Making". The results revealed that the fluctuations of the South Pacific albacore stocks are rather stable and the current status is just closing to the equivalent catch point. Key words: albacore, surplus production, reference points 1

3 INTRODUCTION The South Pacific albacore stocks were mainly exploited by tuna longline fishery, especially by Taiwan (Wang 1984,1988; Wang et al. 1988). Total catch was about 36,466mt in Numerous studies tried to assess the stock (Skillman 1975; Wetherall et al. 1979; Wetherall and Yong 1984, 1987; Wang 1988; Yeh and Wang 1996). They mostly based on the equivalent catch method of the surplus production model. They tried to estimate the maximum sustainable yield. Fournier et al. (1998) suggested a new method MULTIFAN-CL. It is one of the length-based and age-structured models. Regardless the efficiency of this method, at least for those states of the distant-waters fishery, it is not so easy to get sufficient and reliable data for supporting such analysis. Generally, it is not so easy to get reliable length composition and age-structure data. It also need longer time to analyze the available data. It can not provide the in time information for decision making of the fisheries management. Comparatively, catch and effort data are easily obtained for distant-waters fishery states. Therefore, a simple, convenient, easier method, like as surplus production model, is still considerable and helpful method for providing an in time information of fishery management decision making. The problem is how to make the surplus production method to be more useful and meaningful when applied it to assessing fish stocks. Wang (1996) suggested an improved surplus production method and applied it to assessing the South Pacific albacore stocks (Wang 1999a). The results revealed that the South Pacific albacore stocks were still in rather safety condition. The fluctuation of this stock was deeply relative to the changes of the sea surface temperature (Wang 1999b). In order to make this idea more clear, Wang (2000, manuscripts) tried to prove it theoretically. 2

4 Based on the improved surplus production method, the author tries to assess the most recent status of the South Pacific albacore stocks. The final revised catch and effort data and the recent available 1998's data are involved in this study. MATERIALS AND METHODS Long terms catch and effort data of Taiwanese tuna longline fishery operating in the South Pacific Ocean are used to estimate the effective catch per unit of fishing effort. However, 's albacore catches were revised and provided by Overseas Fisheries Development Council of the Republic of China (OFDC). Overalls of the tuna longline catch of the South Pacific albacore stocks were adopted directly from the Table 1, p73 of the "Report of the Twelfth Meeting of the Standing Committee on Tuna and Billfish" (SCTB12, 1999). Overalls effective fishing effort was evaluated directly by the ratio of X t =Y t /U t. Where X t = overalls effective fish effort evaluated by hooks, 7, = overalls albacore catch of the South Pacific tuna longline fishery, U, = effective catch per unit offishingeffort evaluated by hooks based on Taiwanese tuna loneline fishery. The improved surplus production method suggested by Wang (1996,1999a, 2000 unpublished) was applied to assess the South Pacific albacore stocks. The recent status were descirbed and the basic information for thefisherymanagement decision making were suggested. RESULTS Based on 's catch and effort data of Taiwanese tuna longline fishery, the effective fishing efforts of the hooks operating in the South Pacific Ocean were estimated. The overalls of 3

5 the albacore catch and effective effort of tuna longline fishery were listed in Table 1. As shown in Figure 1. Although the fluctuations of the effort varied violently, the annual catches were rather stable. The nominal CPUE and effective CPUE are shown in Figure 2. Before 1974, both clearly revealed the strict decreasing trends. First, all of the 's catch and effort data are involved for estimating the parameters K, q and r. The results revealed that 's biomass are larger than the half of the carrying capacity. These data should be deleted and 's catch and effort data are used to re-estimate the parameters. The results revealed that 's biomass are still larger than the half of the carrying capacity. Again, 's data should be deleted and only 's data are used to reestimate the parameters. The results revealed that 1974's biomass had been lower than the half of the carrying capacity. As proved by Wang (2000), only those data after 1973 are appreciable for estimating the parameters. The results revealed that K-\A912>2>mt, q = '-09, and r = , respectively. Before 1974, Taiwan's distant-waters tuna longline fishery was still in development stages. The extension of the fishing grounds, unfamiliar with the migration of the fish stocks, and many other unknown factors might affect the exploitation of the albacore and hence the representation of CPUE. Moreover, in the early stage of the fishery development, the biomass began to reduce from the carrying capacity. The biomass in early stage was generally still larger than the half of the carrying capacity. Wang (1999,2000) showed that it is better to delete these data before estimating the parameters. Moreover, it is not necessary to assessing fish stocks in this stage. Next, F t = qx t will be used to evaluate the fishing mortality. As shown in Figure 3, the fluctuations of the fishing mortality are rather stable. They vary around the mean with the 4

6 ranges of to Before 1974, biomass was still larger than the half of the carrying capacity. Therefore, thefishingmortality was comparatively lower. In 1969, it was only. Corresponding to F n the biomass will be evaluated by B t = {K12)(1 - FI r) (Wang 1996, 1999, and 2000). The results are shown in Figure 4. They decreased depending on the increasing of the fishing mortality. They vary around the average of 57656mt with the ranges of 49723mt to 64176mt. The differences between two successive years are useful for knowing the actual fluctuations. The results are shown in Figure 5. The maximum increase is about 12681mt in 1994/95, two years later of the end of the gill net fishery. The maximum decrease is about 7782mt in 1980/81, two years before the heavier El Nino event in 1982/83. Wang (1999) pointed out that El Nino events and the development of the gill net fishery might be two main factors affecting the South Pacific albacore stocks. In average, it is about 55mt only. Under fishing, the maximum surplus production can be obtained by /, = (rk14)(1 -Fir) 2 (Wang 1999, 2000). The results are shown in Figure 6. They also respond to the fishing intensity F t. They varied in the ranges of 36356mt to 60563mt with the average of 49015mt. Annual biomass can be evaluated by B a - Y, I F a, where F a -1 - exp(-f,). If the fishing is excluded, then the surplus production can be evaluated by f a = rb a {\- B a IK). Hence, the ratio g a = f a lb a =r(l-b a IK) can be used to represent the annual increasing rate. The results are shown in Figure 7. The annual increasing rate varied in the ranges of to with the average of They varied around one. Theoretically, g a varies in the ranges of 0 < g a < r. Generally, they have higher increasing rate with the lower biomass. According to the logistic curve of the population growth, the increasing rate is g=>/2 if B = KI2. For the South Pacific 5

7 albacore stocks, r is estimated by r= It implies that g = if B = K/2. Actual change rate of the biomass under the fishing can be defined by c a = (B a+l -B a )l B a. The results are shown in Figure 8. They vary in the ranges of to with the average of This implies that the fluctuations of the South Pacific albacore are rather small. Since 1974, no one year of the change rate is over 0.4. Harvest rate is defined by h a = Y a l(b a + f a ). The results are shown in Figure 9. They vary in the ranges of 0.14 to 0.23 with the average of This implies only lower than one-fifth of the stocks were exploited by the fishing. Relating to the harvest rate, the productivity of the fish stocks is also an interesting information. Productivity is defined by the ratio of the surplus production and the biomass as follows. _f,(rk/4)(l-f l /rf_r F ' B, (K/2)(l-F t /r) 2 K r Without fishing, it is constant and just equal to the half of the intrinsic growth rate. Under fishing, the productivity decreases depending on the fishing intensity. Hence, the productivity varies in the ranges of r/2 to 0. During , the productivity of the South Pacific albacore varied around 0,8478 with the ranges of to This result implies that the productivity of the South Pacific albacore stocks are comparatively rather stable under long terms exploitation. As suggested by the improved surplus production method, the equivalent point is obtained by F = r 13. For the South Pacific albacore stocks, the equivalent point is evaluated by F , B, = 4991 \mt, Y t = 3663 \mt, with the fishing effort: X, = 173 million hooks. As shown in Figure 10, only in 1981, the fishing effort had a little over the equivalent point. Others are mostly lower than the equivalent point, especially for those years of the development 6

8 stages. They are far lower than the equivalent point. For the annual catch, some years are larger than the equivalent. In 1998, it has a little over the equivalent point. However, the fishing effort is still lower than the equivalent point. Except the current status of the fish stocks, the risk of collapse is also an important and useful information for fishery management decision making. Under constant environmental conditions, the equivalent point means sustainable yield of the constant biomass under fishing. Hence, the risk of collapse of this point should be zero. Over this point, the larger fishing intensity implies the higher risk. The collapse of the fish stocks means that the risk is 100%. In this case, the risk of collapse can be defined by R t =(F-r/3)/(r-r/3) = (F-r/3)/(2r/3) The risk of the precautionary point is 25%. At dangerous point, it is 50%. Over 50%, it has been in severe over fishing. As suggested by Wang (1999, 2000), three reference points can be concluded directly from the improved surplus production method. They are equivalent point, precautionary point and dangerous point. These points separated the fishery development into four stages; under fishing, light over fishing, heavy overfishingand severe over fishing. For the South Pacific albacore stocks, the basic information, includingfishingmortality, surplus production, biomass, theoretical catch, productivity and risk of collapse, of the different reference points andfisherydevelopment stages of the South Pacific albacore stocks are summarized in Table 2. Comparing to the current exploitations, the present status is clearly closing to the equivalent point. This implies that the present status may be the most preferable status. However, the fishing effort is far lower than the equivalent point. Some reasons might be considered. 7

9 1). It is relative to the concentration of the fishing grounds. Experimentally,fishermenlike to operate in the higher density area. Generally, they are notfishedrandomly. (2). It is relative to the low coverage rate of the reported catch and effort data. For Taiwanese tuna longline fishery, although the total catch is completely accumulated, the coverage rate of the reported logbook data is comparatively rather low. Evenly, it was lower than 10% for some years (Yeh and Wang 2000, unpublished). Low coverage rate implies low appearance of the fishing grounds. This implies a wrong concept that our fishermen operated only such small ranges of fishing grounds, and so large amounts of the albacore are fished from such small fishing grounds. For estimating the effective fishing effort, up to now, no useful method can be used to mend the catch and effort data of such no information area. Of course, if the coverage rate of the reported logbook data can be risen to 100%, then no any mended work is necessary. Unfortunately, the actual coverage rate is still rather low. Therefore, 1) how to mend the catch and effort data of no information area? 2) how to rise the coverage rate of the reported data to 100%? and 3) how to make the migration and distribution of the fish stocks more clear to reflecting the variations of the fishing grounds? Maybe this is the focus works of the future research. Yeh and Wang (2000) and Hsu et al (2000) tried to discuss these problems, primitively. DISCUSSIONS Traditionally, equivalent catch method or non-equivalent catch method was applied to assess the fish stocks. When the observed catch data are fitted to the equivalent catch method, it was assumed that the annual catch is just equal to the surplus production. In this case, larger catch implies larger surplus production. This implies that the maximum sustainable yield can be obtained by increasing 8

10 the catch as large as possible. This can not be accepted for fishery management goal. Fishery management goal can not be target on maintaining the constant biomass through the nonequivalent catch method, too. Constant biomass implies the equivalent catch. This is similar to the equivalent catch method. On the other hand, if constant biomass can not be expected then the sustainable yield is doubtful. Therefore, regardless the equivalent or non-equivalent methods they are, both implied that the concept of the maximum sustainable yield is trivial and meaningless. Wang's suggestion (1996) provided a new consideration of the surplus production model. There is only one equivalent point. This unique equivalent catch point provides us a very clear and definite fishery management goal. This point depends on the fishing intensity, carrying capacity and intrinsic growth rate. Following linear equation can be used to estimate the population parameters. r U t +U M r Where, t/=cpue, X=fishing effort. It was derived directly from the definition of the theoretical catch inspite of the equivalent catch or the non-equivalent catches (Wang 1999,2000). This method is applied to assess the South Pacific albacore stocks. The estimated equivalent catch has a little higher than which estimated by other methods (Skillman 1975; Wetherall et al 1979; Wetherall and Yong 1984,1987; Wang et al 1988; Yeh and Wang 1996). However, they have the same results that the South Pacific albacore stocks are still in safety condition. The equivalent fishing mortality is equal to one-third of the intrinsic growth rate. It is far lower than which concluded from the traditional method. It is more appreciable for the conservation of the marine resources. 9

11 CONCLUSSIONS Based on 's catch and effort data^ the improved surplus production method is used to assess the South Pacific albacore stocks. The results were summarized as follows. 1. Before 1974, Taiwanese tuna longline fishery was still in developing. These data are not suitable for estimating the parameters. 2. The carrying capacity, intrinsic growth rate and catchability of the South Pacific albacore stocks were estimated by K=149733mt, r=2.2018, q= e-09, respectively based on 's catch and effort data. 3. Under long term's exploitation, no strict variation of the biomass can be found. The change rates varied in the ranges of to 0.36 only. Maybe this is relative to the rather low harvest rates (ranges of 0.14 to 0.23) and the stable productivity (ranges of 0.73 to 0.94) 4. The unique equivalent catch point was evaluated by the fishing mortality=0.7339, biomass=4991 lmt, catch=3663 lmt. This is the first reference point offisherymanagement decision making. The second reference point corresponds to F=r/2= where the maximum catch can be expected. The third reference point corresponds to F=2r/3= where the difference between the theoretical catch and the surplus production is maximum. Three reference points separate the fishery development into four different stages: under fishing, light over fishing, heavy over fishing, and severe over fishing. 5. Fishing mortality, surplus production, biomass, theoretical catch, productivity, and the risk of collapse corresponding to the three reference points and four different development stages form the Basic Information Table of Fishery Management Decision Making. This Table provides the important and referable information of fishery management and conservation of marine fish stocks. 10

12 6. The present status of the South Pacific albacore stocks is just closing to the equivalent point. This implies that the present status might be the most preferable status under the fishing. 11

13 CITED PAPERS Foumier, D.A., J. Hampton & J.R. Sibert (1998) MULTIFAN CL: a length-based, age-structured model for fisheries stock assessment, with application to South Pacific albacore {Thunnus alalunga). Oceanic Fisheries Programme, Secretariat of the Pacific Community, Noumea, New Caledonia, 43pp. Hsu, T. L. (2000, unpublished) Preliminary study on the distribution and migration of South Pacific albacore {Thunnus alalunga). (Master degree thesis) Biology and Fishery Division of the Institute of Oceanography, National Taiwan University, Taipei, Taiwan. SCTB12, (1999) Report of the twelfth meeting of the standing committee on tuna and billfish June 1999, Papeete, Tahiti, French Plynesia Skillman,R.A.(1975) An assessment of the south Pacific albacore, Thynnus alalunga.,fishery, Mar. Fish. Rev. 37(3):9-17 Walters, C. J. and R. Hilborn (1976) Adaptive control of fishing systems. J. Fish. Res. Bd. Can., 33: Wang, C. H. (1984) Review of the development of Taiwanese far seas tuna longline fisheries (in Chinese). China Fisheries Monthly, 375: Wang, C.H., M.S. Chang and M.C. Lin (1988) Estimating the maximum sustainable yield of the south Pacific albacore, ACTA Oceanographica Taiwanica, 21: Wang, C. H. (1988) Seasonal changes of the distribution of south Pacific albacore based on Taiwan's tuna longline fisheries, Acta Oceanographica Taiwanica, 20: Wang, C. H. (1996) Reconsideration of assessing fish stocks with the surplus production model (in Chinese). ACTA Oceanographica Taiwanica, 35(4): Wang, C. H. (1999a) Reconsideration of assessing south Pacific albacore stocks {Thunnus alalunga). ACTA Oceanographica Taiwanica, 37(3):

14 Wang, C. H. (1999b) Fluctuation of the south Pacific albacore stocks (Thunnus alalunga) relative to the sea surface temperature. TAO, 10(2): Wang, C. H. (2000, manuscript) Some comments on logistic curve applied to assessing fish stocks. ppl6, (submitted to this Journal) Wetherall, J.A., F.V. Riggs and M.Y.Y. Yong (1979) Assessment of the south Pacific albacore stocks. U.S. Nat. Mar. Fish. Serv. Southwest Fish. Center Admin. Rep. H-79-6,17pp. Wetherall J.A. and M.Y.Y. Yong (1984) Assessment of the south Pacific albacore stocks based on changes in catch rates of Taiwanese longliners and estimates of total annual yield from 1964 through U.S. Nat. Mar. Fish. Serv. Southwest Fish. Center Admin. NPALB/87,14pp. Wetherall J.A. and M.Y.Y. Yong (1987) South Pacific albacore stock assessment and related issues. U.S. Nat. Mar. Fish. Serv. Southwest Fish. Center Admin. Rep. H-84-11,7pp. Yeh, Y. M. and C. H. Wang (1996) Stock assessment of the south Pacific albacore by using the generalized production model, ACTA Oceanographica Taiwanica, 35(2): Yeh, C. C. (2000, unpublished) Research on how to mend the catch and effort data of the no information area before estimating the effective fishing effort. (Master degree thesis) Biology and fishery division of the Institute of Oceanography, National Taiwan University, Taipei, Taiwan. 13

15 YEAR Fig-1. Catch (1 OOOmt) and fishing effort (million hooks) of south Pacific albacore tuna longline fisheries. 100 nominal CPUE O -O -O effective CPUE 0 I i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i YEAR Fig-2. Trends of nominal CPUE and effective CPUE I H , 0.4 i YEAR I -1 1"! Fig-3. Variations of fishing mortality (Ft). max=0.7395, min=0.3144, mean= /<

16 70 mt) 65 o W5 W5 T BI i i i i i 1 r i I l r YEAR Fig-4. Fluctuation of biomass (B»). -i i i i i i i i i max=64176mt, min=49723mt, mean=57656mt ' YEAR Fig-5. Differences of biomass between two successive years. max=12681mt, min= mt, mean=55mt YEAR Fig-6. Fluctuation of net growth (ft). max=60563mt, min=36356mt, mean=49015mt /{

17 YEAR Fig-7. Fluctuations of increasing rate (g a =fa/ba). max=1.421, min=0.846, mean=1.124 G i i i 1 i i i i l r i r I YEAR Fig-8. Fluctuations of change rate :c a =(B a+i -B a)/b a. max=0.3594, min= , mean= i H 0.20 i <j % YEAR Fig-9. Fluctuations of harvest rate (h a =Ya/ta). max=0.2289, min=0.1440, mean= A

18 surplus production equivalent point theoretical catch I I "? T U FISHING MORTALITY FIig-10 Current status of south Pacific albacore stocks. / ;

19 Table 1. Catch and effort of South Pacific albacore tunalongline fishery, year total longline catch (mt) effective effort xloooh max min mean max min mean Note: 1. ( ) Taiwan's LL catch, adopted from Table 4, p27 of WP-5/SCTB-1 L May 30 to 6 June 1998, Noumea, New Caledonia ( ) adopted form Table 1 of NFR-18, SCTB-12, June 1999, Papeete, Tahiti, French Polynesia 2. Total LL catch, adoptedfromtable 1, p73 of Report of SCTB-12, June 1999, Papeete, Tahiti, French Polynesia catch catch effort effort /«*

20 Table 2. Basic Information Table of the Fishery Management Decision Making. q= E-09 K= mt r= item Fr = /' = Bt = Yt = Pt = Ri = under fishing 0- r/ rk/4 - rk/ lmt K/2-K/ lmt O-rK/ lmt r/2 - r/ equivalent point Peq r/ rk/ mt K/3 4991lmt rk/ mt r/ % light over fishing r/3 - r/ rk/9 - rk/ mt K/3-K/ mt rk/9 - rk/ mt r/3 - r/ % precautionary point Pea r/ rk/ mt K/ mt rk/8 4l210mt r/ % heavy over fishing r/2-2r/ rk/16-rk/ mt K/4-K/ mt rk/8 - rk/ lmt r/4-4/ % dangerous point Pdn 2r/ rk/ mt K/ mt rk/ mt r/ % severe over fishing 2r/3 - r rk' Omt K/ Omt rk/ Omt r/ % extinction r % F-r/3 F-r/3 r-r/3 2r/3