F. X. Bard 1. SUMMARY

Transcription

1 SCRS/00/119 REVISED EXTENSION OF GEOGRAPHICAL AND VERTICAL HABITAT OF ALBACORE (THUNNUS ALALUNGA) IN THE NORTH ATLANTIC. POSSIBLE CONSEQUENCES ON TRUE RATE OF EXPLOITATION OF THIS STOCK. F. X. Bard 1. SUMMARY The history of albacore (Thunnus alalunga) fisheries in the north Atlantic is a long one. A particular feature is that overall production peaked during the decade of the sixties and never reached the same level in the following decades. Nowadays, total production is levelling at half the production of the 1960s. This has generated much research and discussion, particularly during the ICCAT Albacore Research Program, but up to now there has been no clear interpretation of this situation of depressed catches, particularly for surface gears. The purpose of the present paper is to bring new information or recall some past studies on the geographical and vertical extension of habitat of albacore, and associated oceanographic conditions. In the light of the above mentioned changes, the information suggests an alternative interpretation of the actual status of exploitation of the north Atlantic stock. RÉSUMÉ La pêche au germon dans l Atlantique nord a une longue histoire. Un de ses aspects particuliers est que la production globale a atteint pendant les années 1960 un niveau maximum qu elle n a plus jamais atteint pendant les décennies suivantes. A l heure actuelle, le niveau de la production globale est la moitié de celui des années Ceci a donné lieu à beaucoup de recherches et de débats, notamment dans le cadre du Programme ICCAT de recherche sur le Germon, mais il n y a pas eu jusqu à maintenant d interprétation claire de la situation des prises en baisse, en particulier pour les engins de surface. Le présent document a pour but d apporter de nouvelles informations ou de rappeler des études antérieures sur l étendue géographique et verticale de l habitat du germon, ainsi que sur les conditions océanographiques associées. Au vu des changements susmentionnés, l information suggère une autre interprétation de l état actuel de l exploitation du stock nord-atlantique. RESUMEN Las pesquerías de atún blanco (Thunnus alalunga) en el Atlántico norte tienen una larga tradición. Una de sus caracterísitcas es que la producción global aumentó mucho en la década de los sesenta, pero en las siguientes décadas no llegó nunca a alcanzar el mismo nivel. Hoy día, la producción total se está estabilizando a un nivel que representa la mitad de la cifra alcanzada en los años 60. Este hecho ha provocado gran cantidad de investigación y discusiones, sobre todo en el curso del Programa ICCAT de Investigación sobre el Atún Blanco, pero hasta el momento no ha habido una clara interpretación de esta situación de bajas capturas, sobre todo respecto a los artes de superficie. El objetivo de este documento es aportar nueva información o bien recordar estudios efectuados en el pasado sobre la extensión geográfica y vertical del hábitat del atún blanco y las condiciones oceanográficas asociadas. A la vista de los cambios arriba mencionados, al información sugiere una interpretación alternativa de la actual situación de la explotación del stock del Atlántico norte. KEY WORDS Thunnus alalunga, stock assessment, habitat. 1 Fishery Biologist at IRD (ex ORSTOM) Xavier. Bard@ird.ci

2 1-INTRODUCTION During the last decades 80 s and 90 s, the production of albacore from the North Atlantic stock decreased steadily, (Figure 1). Analysis carried out by SCRS showed that an apparent drop in the recruitment level, along with changes in capturability could be the cause of this failure. The surface fisheries, troll and baitboat, which rely on young fish, aged 1 to 4 years, were the most affected. Santiago (1997) hypothetized that large environmental oceanographic changes, reflected by the North Atlantic Océanographic (NAO) index could be the at the origin of new conditions prevailing over the fisheries. Particularly the NAO index after a long series of negative index has become generally positive since Two mechanisms could be involved. Santiago favored the hypothesis that recruitment level could have been affected but Ortiz de Zarate and al.(1997) advocated that environmental condition in the fishing grounds could affect directly the catches. Bard and Santiago (1999) showed that some areas of the North Atlantic are particularly affected by anomalies of sea surface temperature (SST) in June August, correlated with NAO index, (Figure 2). The most important area is large body of water in the Eastern Atlantic, corresponding roughly to the fishing ground of French and Spanish surface fishery. This observation suggests that changes of catchability to the surface gears could have been induced by change of the regime of SST, giving support to the hypothesis of Ortiz de Zarate and al. Another feature is that the level of CPUE of Asiatic longliners which is the only source of abundance index for large adult fish, always remained at a low level since the years 70 s in the North Atlantic, in spite of important changes in the fishing effort exerted either directly by longliners on large fish, either indirectly with the decline of the surface effort on young fish prior to escapment to older ages. The purpose of the present paper is to bring new information or remind some past studies on the geographical and vertical extension of habitat of albacore and oceanographic conditions associated. Such information when put in perspective of the changes quoted above, suggest an alternative interpretation of the actual status of exploitation of the North Atlantic stock. 2- GEOGRAPHIC EXTENSION OF YOUNG ALBACORE HABITAT IN NORTH ATLANTIC. Every year, young albacores less than 5 year old, undertake seasonally a trophic migration, associated with sea surface waters ranging from 16 to 21 C, (Havard Duclos, 1973). Sizes groups are separated according to fine patterns of sea surface temperature (SST). Large fish, around 7 to 8 kg are associated to SST of C, medium sized, about 5 kg are associated to SST of C, and very large, about 10 kg or more, associated with small one, about 2-3 kg are associated with SST of C. The French and Spanish surface fisheries, operated traditionally on these young fish from June to October within an area limited by latitude 40 to 50 N and longitude limited for long time at W. It was believed that presence of young albacore was restricted to the Eastern part of the North Atlantic But, as the range of ships increased, the troll fishery extended its operation to 30 W, catching the same sizes of albacore. On an other hand, some experimental fishing were conducted in North West Atlantic, demonstrating the existence of young albacore there. Aloncle et al, (1978) explored by trolling in summer 1975 the West Atlantic from Azores to an area South of Newfoundland. The ship encountered

3 young albacores from 30 W to 60 W. Sizes of the fish were similar to the ones in Eastern Atlantic. The Western limit of fishing area was the strong thermic front at the Eastern boundary of Gulf Stream. Recently Vinnichenko (1996) reported another experimental fishing conducted by USSR during the years 80 s. Young albacore was captured in mid Atlantic in surface waters, West of the traditional fishing ground of French and Spanish boats. A casual observation, quoted by Bard (1981, fide Postel) is the fact that French cod fishermen used to catch albacore by trolling during summer, south of Cape Farewell, during transit towards Newfoundland bank. Consequently it appears that young albacore inhabit surface waters of suitable SST, in summer, in the North West Atlantic. It has been commonly accepted that quantities of these young fish were minor, but, in fact, there have never been any objective evaluation, because of the absence of directed fishery. The food availability should not be a limiting factor, given the high productivity of the frontal zone between cold waters of Labrador current and warm waters of Gulf stream. Comparing to the Pacific a similar area exists at the confluence of Oya Shivo and Kouro Shivo, off Japan, and this area is a conventional albacore fishing ground by pole and line. In the North Atlantic, during summer young albacore begins to migrate in April-May, associated to the seasonal warming of surface waters of which SST range from 16 C to 21 C. Observation of SST maps in summer show that such body of water affects the shape of a «funnel». (The «funnel» is more apparent on a globe, than on a Mercator map which exaggerates horizontal distances at high latitude, as noted by Fonteneau) The main component of the migrating fish follows a path along an axis from Azores towards the Bay of Biscay and Celtic sea. But it does not exclude the possibility of a component of young albacore migrating farther in the West, and particularly concentrating along the front of Gulf Stream. By August the migration ends in two feeding areas, continental slope of Bay of Biscay and Celtic shelf in one hand, and front of Gulf stream, south of Grand Banks on the other hand. Very young (age 1 : 2.5 kg), fish of these two components could be homologated to the Azorian and Cantabrian sub population proposed by Aloncle (1979), but it is probable that they mix progressively during the next years when growing to age 2-4, (Bard, 1981). The interesting point is that in the study of correlation between NAO and SST anomalies in summer, four geographical areas appear, (Figure 2). Two areas are noteworthy. In the North East Atlantic, a large area corresponds roughly to the migration area of young albacore and in the North West Atlantic a zonal area stretches across the front of Gulf Stream at the location where young albacore have been observed by Aloncle. These two areas have correlation of same sign with NAO. This observation lead to the following hypothesis. Anomalies in SST, reflected by NAO could alter the migration pattern between the two paths leading to the Eastern or Western feeding grounds. Such effect of alteration of migration path has been demonstrated for North Pacific young albacore by Laurs and Lynn (1977). The SST during spring season in a «transition zone» far offshore (about 25 S, 130 W) trigger the migration of two components of albacore towards either South California, either Oregon and Washington state coasts. The conclusion of the hypothesis proposed here is that apparent decrease of abundance of young albacore in the surface fishery of NE Atlantic could be caused by a change in the migration paths and /or changes in catchability of fish in traditional fishing grounds. These changes of behavior could be connected to climatic and environmental, lasting several years. (Multi-decadal changes, Bakun, 1992) 3- VERTICAL EXTENSION OF HABITAT OF LARGE ADULT ALBACORE; VULNERABILITY TO LONGLINE Japanese longliners began to catch large albacore in the North Atlantic by 1956 (Uozumi, 1996). Fishing effort concentrated on two fishing grounds. In summer the spawning grounds in the Western Atlantic between latitudes 20 N 5 N; in winter the feeding grounds in the Central Atlantic at latitudes 35 N to 20 N. Separately Koto (1969) and Beardsley (1969), analysing of the catch statistics

4 of Japanese longliners proposed a model of migration for large albacores, which can be considered as adults. A particular feature of the fishery at its early beginning is the evolution of CPUE, (Shiohama, 1973, 1978). For the North stock, it was very high in 1957, decreased rapidly from 1958 to 1959 and and stabilized at a low level since 1960 (Figure 3). A similar evolution was observed for CPUE of South albacore stock, but with a decline less steep from 1959 to This feature has been observed for other tunas longline fishery such as Atlantic yellowfin, ( Honma 1975). These observations are the source of J. Gulland paradox of the «stupid tunas». When Korean, then Taiwanese longliners substituted to the Japanese longliners, using same gear, the albacore CPUE remained low in the North Atlantic. And here is the remarkable fact: If CPUE, even corrected by Honma or GLM methods are accepted as index of abundance of adults albacore, it appears that adult stock was severely depleted as early as 1960, and never recovered. However this view contrasts with some observations. First, Uozumi (1996, see figure7) showed that mean length of longline caught albacore remained very stable as well in the feeding grounds ICCAT area 31, than in spawning grounds (area 32) till recent years. Second, the development in 1973 of a Spanish and Portuguese baitboat fishery by October November near Azores (Bard and Gonzales Garces, 1978). which caught very soon considerable quantities (up to tons in 1975 to compare with longline at its early peak year 1964 : tons). Sizes of the albacore caught are similar to the ones caught by longline in winter feeding ground. Landings of this fishery decreased in the 80 s because of problem of access to Azores EEZ, but resumed its activity by the 90 s and now is operating at a level of t/year, modulated by meteorological conditions. Another fishery for large albacore, although less important, the French pair trawlers, catch large albacore in summer along the continental slope of Bay of Biscay and Celtic shelf.(ortiz and Cort, 1998). And eventually it can be noted that in , Taiwanese longliners caught up considerable quantities of albacore (up to MT in 1986) without clear changes in CPUE. All these observations raise the question of the real extension of habitat of large albacore in the North Atlantic. The vertical habitat of large albacore was explored by Japanese scientists in the Pacific, (Suzuki and al 1977). They showed that albacore can be fished in the depth well beyond the maximum range of conventional longline which is -150 m. Development of monofilament longline afforded to fish commercially down to 500m. But as frozen albacore prices are no more attractive, this technique did not lead to huge development of fisheries targeting albacore, on the contrary of bigeye. An exception is a developement of a local longline fleet in French Polynesia. A scientific program of studies associated, named ECOTAP (Abbes at Bard, 1999), showed that large albacore is currently present from -100 m to 450 m with a maximum abundance at m, at latitude 15 to 20 S. Such habitat is associated to waters layers ranging from 25 C to 15 C and oxygen content superior to 2 ml/l. Albacore average weight is 21 kg and they feed actively in the deep layers. An estimation of tuna density by echo-integration was carried out by Bertrand and al, (1999). At latitude 15 to 20 S they computed an average density of 1.33 tuna per Km², of which albacore is certainely a major part. Consequently, the biomass of tuna was estimated to 9940 tons by 5 x 5 square to be compared with past catches by conventional Asiatic longliners in this area : maximum 1200 tons in years , average : 300t. This low level of catches compared to the biomass suggest a poor catchability to conventional longline and could be interpreted according the hypothesis of a cryptic biomass unavailable to some gears (Fonteneau at al, 1998). A similar situation could occur in the the North and South Atlantic. Large bodies of tropical water well oxygenated with 15 C isotherm as deep as 500 m exist in the center of the great currents gyre. They are located in the west of the ocean (Merle, 1978) precisely where conventional longliners fished for albacore in the past. This similarity, along with the considerable catches of large albacore by

5 baiboats and pair trawl far at the East of the conventional longline fishing grounds suggest that hypothesis of cryptic biomass hold for North Atlantic large albacore. (Bard et al, 1998). 4- DISCUSSION AND CONCLUSION The present paper presents two hypothesis derived from considerations on real habitat of albacore in North Atlantic. The consequences of these hypothesis could strongly affect the appraisal of the real level of exploitation of North stock. a) Is the decrease of abundance of young fish in the North Atlantic apparent or real?. If it is only apparent, the explanation lies in a decrease of catchability to the traditional gears operating in NE Atlantic. Two mechanisms could be involved. Hydroclimatic changes, reflected by NAO index, in North East, such as losses of thermic contrast, could affect directly the catchability of fish in the traditional fishing grounds, and this is the hypothesis of Ortiz and al, But the occurrence of young albacore in the Atlantic North West attested by some observations recalled here, suggest that the change could be indirect because the summer migration pattern have changed. On an other hand if the recruitment level has really changed, which is the alternative hypothesis proposed by Santiago (1997), it means a direct effect of hydroclimatic changes on recruitment processes. And indeed it is apparent for some past cohorts; The 1969 strong cohort is associated to a negative index of NAO of 5.9, and the very poor 1973 cohort is associated to a positive NAO index of But until now the 1996 NAO index of 3.8 does not seem to be associated to a particular strong cohort. Analytical approach of direct effects of hydroclimate on recruitment request a particular study of the processes of recruitment itself (Bakun Triad). Exploring them in the case of North Atlantic albacore, where even the geographic location of larvae remains imprecise is a difficult task. Finally the two categories of hydroclimatic changes can coexist. It is worth to recall here that the similar recent apparent decline of abundance of North Pacific albacore in surface fisheries has not received full explanation yet. b) Does the abundance of adult fish is correctly indexed by CPUE of longliners? Vertical extension of habitat of the large albacore appears as extending far deeper (-450 m) than range of action of conventional longline which was used when albacore was targeted by Asiatic fleets: 150 m. Thus a major part of the deep swimming albacore remains far from attraction of the baits. These deep swimming albacore forage on micronecton. They follow the diel vertical migration of micronecton (Bertrand et al, 1999) and stay in the depth close to the preys, most of the daytime, except in very short time at dawn and sunset. Such behavior might keep the albacore at vertical distance of hooks set in upper layers, and could generate a case of cryptic biomass. However cryptic biomass involves a «viscosity» of the local fish population ( Fonteneau et al 1998) which means that albacore stay permanently in deep layers away from hooks range of conventional longline. Such a behavior has been shown for bigeye (Figure 4) and it is utilised by superfreezer longliners, when fishing with deep longline till 300 m, targeting bigeye. Sonic or archival tagging could help to solve the matter, but large albacore are particularly difficult to tag, because they seldom survive to hauling from the depth as their fragile swimming bladder explodes (Bard et Josse, 1996). Another behavior could increase this unavaibility to hooks during daytime. It seems that only albacore (and other tuna) with low level of stomacal repletion attack the baits. (Bard, paper SCRS/00/120, presented at the same meeting). This low level of repletion caused by an unsuccessful feeding at dawn (when micronecton begins to dive) increases the pressure for feeding during daytime in deep layers, below the average hooks level.

6 Geographical extension of habitat of large albacore is also a matter. From longline data, the simple model of Koto, Beardsely postulated that population of large albacore concentrated in spawning grounds in summer, and migrated to feeding ground in winter. The development of new fisheries for large albacore, pair trawl in summer and baitboat in fall suggest that migrations of large albacore encompass a much larger part of ocean than believed before. Eventually, we propose the following hypothesis. When conventional longliners began to fish large albacore, they initially exploited an accumulated biomass, of adults. Fishing grounds initially centered on spawning grounds (Koto) facilitated this exploitation. The CPUE of the South stock, virtually unexploited, stayed longer than the Northern one at a high level (Figure 4). On the reverse, the North where the young fish component of the stock was widely exploited since about 1950, resisted less to exploitation. But of a fair quantity of adults albacores remained unavailable to conventional longline shortly after the early development of this fishery. The development of deep longline did not changed much this situation as the ships target bigeye in different fishing grounds. The ultimate consequence would be that conventional longline CPUE series cannot index properly the evolution of abundance of deep swimming albacore. As an example, the evolution of CPUE of yellowfin caught by longliners (Figure 4), if accepted as representative of large yellowfin abundance would have concluded that the Atlantic stock of adults was severely depleted by the 70 s. The onset of a massive purse seine fishery on large yellowfin by the 80 s proved the contrary. Eventually, the hypothesis lead to the conclusion that if a decrease of fishing pressure on young fish had left an increased escapment to adult component of the stock, it could have remained undetected. (Bard and Joanny, 1996). The use of conventional longline abundance indices in the tuning VPA could therefore be misleading. 5- REFERENCES ABBES R. et BARD F.X., ECOTAP, Etude du Comportement des Thonidés par l'acoustique et la Pêche en Polynésie Française, Rapport final.convention Territoire /EVAAM,/IFREMER/ ORSTOM n , 523 p. ALONCLE, H. et DELAPORTE, F., Nouvelles remarques sur la structure du stock de germon Thunnus alalunga dans le Nord-Est Atlantique. ICCAT Rec. Doc. Sci. VIII SCRS/78/34 : ALONCLE, H., Campagne de prospection du germon de surface dans le Nord Ouest Atlantique (11 juillet - 11 août 1979). ICCAT, Rec. Doc. Sci. IX SCRS/79/47 : ALONCLE, H.,, F Delaporte A Forest, C Leroy, 1978 Campagne de prospection de germon de surface entre la longitude de Terre Neuve et les Açores SCRS/77/27, ICCAT Rec Doc Sci Vol 7 (2) : BAKUN, A Global greenhouse effects. Multi decadal wind trends and potentail impacts on coastal pelagic fish populations. ICCAS mer Sci Symp., 195 : BARD F X et A. GONZALES GARCES, Données de base sur les pêcheries de surface de germon (T alalunga) Nord Atlantique. ICCAT Rec Doc Sci. 7(2) : BARD F. X. and J. SANTIAGO, 1999 Review of albacore (T alalunga) historical surface fisheries data ( ), for possible relationships with North Atlantic Oscillation. SCRS/98/106, ICCAT Rec. Doc. Sci 49(4).

7 BARD F X., JOSSE E., Peculiarity of swimming bladder of large albacore (Thunnus alalunga) caught by longline. Doc. SPAR 6th meeting. 3p. BARD F.X., P BACH et E JOSSE, Habitat, écophysiologie des thons: Quoi de neuf de puis 15 ans? Actes Symposium ICCAT Sao Miguel, Juin 1996, J. Beckett Ed, : BARD F X et T JOANNY The North Atlantic Albacore Assessment problem. ICCAT Final meeting of the ICCAT Albacore Research Program ICCAT Vol 43: BARD F.X., Le thon germon (Thunnus alalunga) de l'océan Atlantique. De la dynamique de population à la stratégie démographique. Thèse Doctorat ès Sciences Naturelles, Université de Paris VI, 330 p. BEARDSLEY G.L., Proposed migrations of albacore Thunnus alalunga in the Atlantic Ocean. Trans. Am. Fish. Soc., 98 (4) : BERTRAND A, E. JOSSE et J. MASSE, 1999 Détection acoustique et estimation de l abondance des thons en Polynésie Française, Chapitre 4, pp in ABBES R. et BARD F.X., 1999, ( ibid.) BERTRAND A., F.X. BARD et E. JOSSE, Environnement biologique des thons exploités par le pêcherie palangrière en Polynésie. Française. Chapitre 5, pp in ABBES R. et BARD F.X., 1999, ( ibid.). FONTENEAU A., D. GASCUEL et P. PALLARES Vingt cinq ans d évaluation des ressources thonières de l Atlantique. Quelques réflexions méthodologiques. Actes Symposium ICCAT Sao Miguel, Juin 1996, J. Beckett Ed, : HAVARD DUCLOS, F., 1973 La pêche au germon dans le Golfe de Gascogne. Influence de la température sur le déplacement des mattes. ICCAT Rec Doc Sci 1 : HONMA, M Overall fishing intensity and catch by length class of yellowfin tuna in Japense Atlantic longline fishery, ICCAT Rec Doc Sci 50:82-85 KOTO, T., Studies on the albacore 14. Distribution and movement of the albacore in the Indian and the Atlantic oceans based on the catch statistics of japenese tuna longline fishery. Bull. Far Seas Fish. Res. Lab., 1: LAURS, R.M. and LYNN, R.J., Seasonal migration of North Pacific albacore (Thunnus alalunga) into North American coastal waters. Distribution relative abundance and association with transition zone waters. U.S. Fish. Bull., 75 (4) : MERLE, J., Atlas hydrologique saisonnier de l océan Atlantique intertropical. Trav. et Doc. ORSTOM, n 82: MORITA, S., 1977a - Estimated age composition of albacore harvests of Japanese and Taï wanese longline fisheries in the Atlantic Ocean. ICCAT Rec. Doc. Sci. VI (2) SCRS/76/32 : UOZUMI, Y An historical review of Japenese longline fishery and albacore catch in the Atlantic Ocean. ICCAT Rec Doc Sci 43 : ORTIZ DE ZARATE, V., A LAVIN and X. MORENO -VENTAS, 1997 Existe algun relacion entre las variables ambientales y las captura de superficie de atun blanco(t alalunga) en el Atlantico norte?. SCRS/97/54,ICCAT Rec. Doc. Sci. 48(3) :

8 ORTIZ DE ZARATE,V and J CORT, Albacore stock structure in the Atlantic Ocean as inferred from distribution and migration patterns.? Actes Symposium ICCAT Sao Miguel, Juin 1996, J. Beckett Ed: SANTIAGO, J., 1997 The North Atlantic oscillation and the recruitment of temperate tunas, ICCAT SCRS/97/40, 21p. SHIOHAMA, T., Overall fishing intensity and yield by the atlantic longline fishery for albacore, ICCAT Rec. Doc. Sci. 5 (2) SCRS/75/25 : SUZUKI, Z., WARASHINA, Y. and KISHIDA, M., The comparison of catches by regular and deep longline gears in the Western and Central Equatorial Pacific. Bull. Far Seas Fish. Res. Lab., 15 : VINNICHENKO V New data on distribution of some tunas (Scombridae) in the North Atlantic. (in Russian) Voprosy ikhtiologii 5 :

9 Catches of North Atlantic albacore, separated in youngs and adults fish, Metric Tons Adults Youngs Years Figure 1: Catches of North Atlantic albacore separated in two components: young and adults fish.( ) Figure 2: Spatial distribution of correlations between the winter index of NAO and June-August SST anomalies for Source Figure 8 in Bard and Santiago, 1999

10 Evolution of CPUE of Japenese longliners, Nominal CPUE (number of fish /100hks) Years Atlantic YFT N Atlantic ALB S Atlantic ALB Figure 3 : Initial evolution of nominal CPUE of Japanese longliners in the Atlantic, Sources : Shiohama, Honma. 0 Heure 16:00 20:00 0:00 4:00 8:00 12:00 16:00 20:00 0:00 4: Profondeur (m) Figure 4 Vertical movements of a large bigeye tuna (50 kg) bearing a sonic tag. 18 S, 150 W, June 1997, Source Abbes et Bard, 1999.