Tuna resources have been classified by scientists as well as by lawyers (law of the sea) as being highly migratory species. Among the 49 species of scombrids fished world wide, one should note that only six tuna species constitute the group of major tunas that are heavily fished by various industrialised countries and targeting the two main markets for tunas, namely canneries and sashimi. On the opposite, various species of small tunas are often fished by coastal small scale fisheries for local consumption, but most of these stocks and fisheries are poorly followed by scientists and they will not be examined in this paper.
The major tuna are the two species of bluefin (Thunnus thynnus and Thunnus thynnus maccoyii), yellowfin (Thunnus albacares), skipjack (Katsuwonus pelamis), bigeye tuna (Thunnus obesus) and albacore (Thunnus alalunga). World catches of these tunas have been showing during the last half century a quite steady increase (shown by figure 1). Such trend is nowadays seldom observed when many fish stocks are increasingly overfished; this positive trend seldom observed for coastal resources, could suggest that tuna stocks world wide are not yet overfished. This paper will compare the status of the various tuna species and stocks world wide, discussing their present rates of exploitation, in relation with their potential overfishing. The serious difficulties faced by scientists world wide to estimate the real status of tuna stocks and their overfishing risks will also be discussed. This document will also present and discuss the past and potential problems of by-catches by the various tuna fisheries and the collateral potential effects of tuna fisheries on the pelagic ecosystems, knowing that these effects could have serious negative impacts on various tuna fisheries.
1. DEFINITION OF OVERFISHING
It is important first to give the basic definition of the biological overfishing. "Overfishing" can be used to mean different things.
Growth overfishing corresponds to a situation where the fishing mortality (or fishing pattern) is such that fish are caught before they have a chance to reach their growth potential. Fishing less or with a better exploitation pattern would result in higher total catches. The low catch rates corresponding to this situation may not be profitable in economical terms (unless when these fishes are sold at a very high price).
The term Local overfishing describes a peculiar case of growth overfishing. This is a situation where the observed catches are not increasing as a response to higher local fishing mortality. This type of local overfishing can be sustainable in biological terms when the stock itself is in good shape, but the low catch rates and stable catches corresponding to this situation may not be profitable in economical terms.
The term recruitment overfishing describes a situation where an excessive fishing mortality has produced an excessive decline of the spawning stock, and consequently low recruitment. This recruitment overfishing may lead to a temporary or long term collapse of the stock.
2. WORLDWIDE SIMILARITY OF TUNA STOCKS, TUNA FISHERIES AND PELAGIC ECOSYSTEMS
Tuna are fished worldwide in all the oceans between 50°N and 45°S, but mainly in the intertropical areas (figure 2). All tuna species are classified as being highly migratory species. This typical status is often fully justified: this is for instance the case for southern bluefin tuna, a species showing large migrations between its feeding and spawning areas around the Antarctica in the Atlantic, Indian and Pacific oceans (see figure 3). Northern bluefin in the Atlantic and Pacific oceans are also showing such permanent wide scale migration. However, it should be noted that there is an increasing recognition among tuna scientists that most tropical tuna species often do show limited movements, at least during long periods of time. This problem was first expressed in the provocative paper published in 1986 by Hilborn and Sibert: Is international management of tunas necessary? Since that time, many results obtained from tagging have confirmed that in many cases the geographical scale of these movement patterns is quite limited. As a consequence, it appears that if tuna should still be considered as being highly migratory species inhabiting wide oceanic areas, they also often do show some stock viscosity, as proposed by Mac Call 1990. Following this concept, significant fractions of the tuna populations could be highly viscous and showing limited movement during large periods of time.
The first important point to keep in mind in the comparison of overfishing risks faced by the various tuna stocks world wide is the between oceans similarities in the Pacific, Indian and Atlantic oceans:
All these tuna species are the same in each ocean and show very similar biological and behavioural characteristics.
They are fished by multigear fisheries (mainly purse seiners, longliners and pole and line vessels) that are nearly identical in all the oceans world wide; furthermore many of the industrial fleets are mobile from one ocean to the other.
They live in offshore and coastal pelagic ecosystems that are typical of each ocean, but showing similar global characteristics in their structure and fauna (similar associated species: sharks, billfishes, turtles, etc) and similar rates of specific by-catches (see section 6 below).
There is then a major potential field of investigation in the between oceans comparisons of tuna stocks and their increasing exploitation by multigear tuna fisheries. Furthermore this potential for comparative analysis of tuna overfishing mechanism is widely facilitated by the histories of the fisheries in each ocean: while the Eastern Pacific has been showing 30 years of apparent growth overfishing, the Western Pacific stock is still experiencing a moderate exploitation rate. The same comparison will be also valid between the Atlantic (where tuna purse seine fisheries have been very active for 40 years) and the Indian Ocean (where active tuna fisheries have been developed only since the early eighties with the arrival of a large fleet of purse seiners).
However, one of the difficulties faced in these interesting and comprehensive between oceans comparisons is due to the heterogeneity between the various tuna Commissions: each tuna Commission has been created at a given time and when the pressure on tuna stocks became a source of worry (IATTC in 1949, ICCAT in 1968, IOTC in 1993 and the beginning of 21st century for the Western Pacific). Their scientific work is thus not covering comparable periods. Furthermore each of these tuna bodies has a particular framework and uses particular models to do their stock assessment. Subsequent detailed comparisons of their results are then quite difficult. Comparative analyses between oceans were seldom done, and it is often quite difficult to do in depth comprehensive comparisons of the tuna stock status between oceans.
3. HETEROGENEITY BETWEEN TROPICAL AND TEMPERATE TUNAS
It is increasingly admitted that tuna species are showing a great resilience to overfishing. This characteristic is probably due to their very large habitat, as tunas are living in wide areas (a total area of about 140 million nautical miles2), scattered in a wide range of depth (over 500m). This situation leads to cases where, in relation with stock viscosity, various fractions of tuna stocks remain cryptic because they are not highly mobile and/or too deep or too remote to be caught significantly. It is expected, that over time, the refuges will become progressively fewer and fewer and the risk of overfishing will therefore, increase.
However, the biological and behavioural differences between tropical tunas (such as yellowfin, skipjack and bigeye) and temperate species (such as bluefin) are important factors to keep in mind in relation with the risks of overfishing faced by the various tuna stocks. The increasing fishing pressure world wide on these two groups of tunas tends to be quite different.
Tropical tunas such as skipjack (and yellowfin) tend to show a high biological productivity, a short life span, a fast growth and an early massive spawning done successfully in a wide range of time and area strata. Furthermore these species tend to be widely scattered in vast equatorial and subtropical areas in most of their life. The market price of these species tend to be quite low (most often less than 1.5 US$/kg).
On the opposite, temperate tuna stocks such as bluefin tend to have quite large standing stock but with low biological productivity, a long life span over 20 years and showing a late spawning (an age of over ten years for southern bluefin); furthermore, their spawning is done during short seasons and in given small areas. An additional risk faced by bluefin is due to the seasonal concentration of adults in their feeding and spawning zones, where they are easily caught by fishing vessels. The market price of these species tends to be very high increasing the risk of overfishing.
As a consequence, it appears that de facto tropical tunas like skipjack (or yellowfin and bigeye) tend to be much more resistant to both growth and recruitment overfishing than temperate tunas such as bluefin. This resistance is also well confirmed by simulation studies that are built combining the biological characteristics of these stocks (Fromentin and Fonteneau 2001).
4. COMMON DIFFICULTIES FACED BY SCIENTISTS TO ASSESS TUNA STOCKS
It can be concluded, after 50 years of active research done world wide upon tuna stock assessment, that it remains extremely difficult to estimate tuna stock status with a full confidence. As tuna stocks cannot be sampled directly by scientific vessels, most of the analyses done by scientists are entirely dependent on fishery data. On the other hand, it is now well proven that most tuna fisheries are permanently changing their behaviour, their fishing zones and fishing depth, improving their fishing efficiency, as well as changing their size and species selectivity.
All these changes have been introduced by fishermen in order to improve their catches and their catch rates, leading to drastic changes and spectacular increases of fishing efficiencies. For instance, one should note among the major changes observed in tuna fisheries the effects, on tuna fisheries as well as on tuna populations, of multiple Fish Aggregating Devices (or FAD) that are widely increasing the catches of small tunas world wide. These FADs have been introduced massively in all equatorial areas by tuna fishermen; they have increased the efficiency of most purse seiners world wide (about 50 percent of purse seine catches being taken world wide under FADs), but the negative impacts of these FADs are still widely unknown (mainly because natural mortality of juvenile tunas is also poorly known).
These increases of fishing efficiency are probably observed for all gears catching tunas world wide (purse seiners, longliners, baitboat and others), but they remain very difficult to measure in the absence of scientific data. Tuna fisheries represent a typical case were the growth overfishing of stocks can be generated by technological progress, even when the nominal carrying capacity of the fleets remain stable (case of the Atlantic and Indian Ocean). Unfortunately there is very little use of acoustics to allow estimating stock sizes, primarily because tuna schools and isolated individuals are very rare, and extremely difficult to locate in their vast oceanic areas.
In general, data sets that are somewhat independent of fisheries have been provided by large scale tuna tagging programmes (allowing the evaluation of stock sizes, as well as natural and fishing mortalities). However, such programs are expensive and seldom conducted by the tuna commissions. One of the consequences of this lack of tagging data is that the movement patterns of most tuna stocks remain insufficiently known by the tuna commissions, when this information would be essential to establish the best stock structure hypothesis used by scientists to evaluate stock status and by commissioners to manage these stocks (Hunter et al 1986). Among these typical scientific uncertainties, one should note the major uncertainties still faced for the stock structure of bluefin tuna in the North Atlantic: a two stock hypothesis has been used by the ICCAT since the early eighties, but is increasingly in contradiction with tagging results.
These difficulties that are typical of tuna stock assessment have been worsened during recent years by the increasing difficulties to obtain reliable statistics on major fleets: increasing numbers of ghost fleets are poorly followed world-wide and increasing catches by Illegal Unreported and Unregulated (IUU) vessels were observed world wide during the last 10 years by most tuna commissions. These statistical difficulties are obviously worsening the intrinsic difficulties already faced by models that are working exclusively on already biased fishery data.
As a consequence, and despite a wide variety of models used by the various Tuna Commissions, too many retrospective analysis done upon previous stock assessments are showing that the previous diagnosis on stock status were quite often dubious or inaccurate. It appears a posteriori that these diagnoses were valid ones, but only in the context of the fisheries active at that time (in a given area and at a given depth), but were not correct at the level of the real global stocks. Most often, tuna stocks were more resilient and more productive that estimated by the various models used by scientists.
5. BRIEF OVERVIEW OF PRESENT STATUS OF TUNA STOCKS WORLDWIDE
5.1 General points on tuna overfishing
Many tuna stocks are not yet showing serious symptoms of overfishing, the permanent increase of total tuna catches observed during the last 50 years being dominated by tropical tuna catches (mainly skipjack and yellowfin). However, there is in the world of tunas fisheries a wide range of situations, varying between healthy stock with still large potential for increased catches (mainly skipjack), when several other stocks are being growth overfished, and a few are already facing a dangerous recruitment overfishing (such as southern bluefin). One difficulty to evaluate the real status of tuna stocks is to interpret the changes in the Maximum Sustainable Yield (or MSY) estimated by the various tuna commissions: it is clear for instance that the Eastern Pacific and Eastern Atlantic yellowfin stocks have been classified during many years as being growth overfished. However, wide increases of MSY were estimated by scientists (figure 4) when the fisheries were expending their geographical range of operation towards offshore areas (Laloë 1989) (see figure 5, the Atlantic yellowfin example) and also fishing deeper and more efficiently. This tendency to underestimate tuna stock MSY and to overestimate their rates of overfishing should always be kept in mind in the discussion of past tuna overfishing.
The following paragraphs will briefly review the present situation and prospects for overfishing of some major tuna stocks, tropical and temperate: skipjack, yellowfin, bigeye and bluefin stocks being taken as examples.
5.2 Skipjack tuna
Skipjack tuna catches have been showing steadily increasing trends in the Indian and Western Pacific oceans (figure 6). These two stocks are producing record catches (reaching 400 000 tonnes in the Indian Ocean and over 1.2 million tonnes in the Western Pacific), but they are not yet fully exploited. The same diagnosis of low exploitation rate has been obtained by the IATTC in the Eastern Pacific, where skipjack catches are widely fluctuating, at less than 200 000 tonnes the highest catches being observed during recent years in relation with FAD fisheries. In the Atlantic, skipjack production has been flattening during the last six years at about 150 000 tonnes despite of a massive use of FADs that were used primarily to increase skipjack catches; there are serious indications, for instance its seriously declining average weight, that this stock is close to be overfished and possibly overfished in various areas. This local overfishing was possibly in relation with an excessive use of FADs (it should be kept in mind that the seasonal moratorium on FAD fishing in the equatorial areas may have contributed to these reduced skipjack catches).
As a conclusion, skipjack tuna is clearly a species showing biological characteristics, large biomass and high productivity, that allow these stocks to be resistant to overfishing. However, the Atlantic example would tend to show that such strong stock can also be overfished at large geographical scales.
5.3 Yellowfin tuna
Yellowfin tuna is a typical tropical tuna. Its total catches world wide have been showing a steady increase during the last 50 years but catch trends (figure 7) and the situation of yellowfin stocks are highly variable between oceans.
In the west Pacific, it appears that the yellowfin stock is still in good shape but approaching its MSY. In the Indian Ocean, MSY was probably reached and possibly exceeded five years ago. On the opposite, it appears that in the Atlantic, yellowfin stock has been suffering a moderate growth overfishing during more than ten years.
In the Eastern Pacific yellowfin stock the scientific diagnosis has been that the stock was already growth overfished during the late sixties. It was subsequently managed actively by an IATTC quota system since 1966, but there is some controversy concerning the reality of this historical overfishing: in fact the yellowfin purse seine fishery was catching less than 100 000 tonnes during this early period, but mainly in coastal areas (see figure 5). As sustainable total catches of about 300 000 tonnes were taken during recent years in a much wider area of the eastern Pacific, there are strong reasons to consider that the total stock of yellowfin was not yet really overfished during the late sixties and seventies when the IATTC quota system was developed by the IATTC in the coastal areas (keeping in mind that the biological productivity of the stock may have been increased during recent years because of environmental reasons)(Polovina 1996). This stock provides another example, similar to the Atlantic and Indian Ocean cases, showing that the MSY of many tuna stocks can be reached only when large fraction of their geographical distribution is actively exploited by fisheries. When only a small fraction of the total stock is exploited by fishery, the apparent local MSY tend to be lower than the real productivity of the stock (Laloë 1989). This fact is probably related to the viscosity of tuna stocks.
5.4 Bigeye tuna
Bigeye tuna is a tropical species showing some similarities with temperate species, such as its wide habitat sometimes in temperate waters, and its affinity to inhabit deep and cold waters. Bigeye tuna stocks and fisheries have been showing similar patterns worldwide during recent years. Large increases of small bigeye catches were observed during recent years for the surface fisheries (in relation with the use of FADs), while large increases in large fish catches were also observed for longliners targeting large bigeye sold at high price to the sashimi market (mainly but not only in Japan). These increased catches were mainly due to a specific targeting of bigeye tunas using deep longline. The Atlantic bigeye, a stock well assessed by the ICCAT since the early eighties, can be taken as the typical bigeye scenario of bigeye growth overfishing The recent increase of catches has produced, during the mid nineties, catches that were well above the previously estimated MSY (figure 8). As this excess of catches was widely obtained on small individuals, there was a serious concerns based on all projections done by stock assessment models, that the stock would soon be facing a severe overfishing. After nearly 10 years of this regime of high catches, it appears that the MSY previously estimated by scientists were too low. In fact the present status of the Atlantic bigeye stock is not yet fully understood by scientists.
5.5 Bluefin tuna
World catches of bluefin tunas (northern and southern) have been fluctuating at less than 100 000 tonnes during the last 50 years (figure 9). Bluefin tuna is mainly sold at a very high value as raw fish on the sashimi market, mainly Japan, but also increasingly in other countries. The very high value of this species is worsening its pressure to be overfished as fisheries can be profitable even at very low cpue. As an example, during recent years, the average cpue of Japanese longliners was five times lower when targeting southern bluefin in the southern sea, compared to the cpue of the same boats fishing in the equatorial area (targeting a mixture of tropical tunas, mainly yellowfin and bigeye).
Among these potentially fragile species, southern bluefin provides the best example of a tuna stock recruitment overfishing. This stock was already overfished during the late sixties by large fleets of longline fisheries catching adults. This overfishing has later been worsened by an additional fishery of purse seiners catching juvenile bluefin. As a result, the stock has been suffering since the late eighties a long period of low spawning biomass and of low recruitment that is typical of recruitment overfishing. Despite more than 10 years of low quotas, its spawning biomass and recruitment remain low, and the prospects for a medium term recovery of this stock are still uncertain despite of the firm control of fisheries by CCSBT.
On the other hand, northern bluefin in the Atlantic has been heavily and increasingly exploited at all sizes (juveniles and adults) in the Atlantic and Mediterranean Sea (same stock). This stock seems to be under serious threat, following the uncontrolled and massive increase of its catches (for both juveniles and spawners). This tendency has been severely worsened during recent years by the increased value of all sizes of bluefin tuna that are sold alive to coastal farms where tunas are fed during several months (including fishes caught at illegal sizes, and growing at the farm to reach the ICCAT legal size). All the various management measures taken by the ICCAT since the early seventies (limiting fleet sizes, size limit and quotas) appear to have only limited or no effects on the fisheries. The present high catches on this stock during long periods, are in fact quite surprising for many scientists. Such high catches are against most previsions done by the previous ICCAT stock assessment models. However, there is probably a great risk that the present fishery will produce in the short or medium term a recruitment overfishing comparable to southern bluefin. It should also be noted that this risk may also depend on the environment variability and trend, because it has been shown that this Atlantic bluefin tuna stock has been showing long terms natural fluctuations as a function of environmental variability (Ravier et Fromentin 2001).
5.6 Strange cases of local extinction: shrinking areas?
It appears that while tuna stocks are quite hard to overfish at a global oceanic scale, they may be easily overfished at a local scale: the general rule of fishery science that too much local fishing effort tend to decrease the local biomass and cpue is also valid for all tunas resources. This quite logical rule has been observed for many tuna fisheries: local overfishing of fractions of tuna stock can be observed for a stock which is still in good shape at the oceanic level. In general, this local overfishing tends to disappear when the local fishing effort is reduced (following movements between neighbouring fractions of the same stock).
However, it appears that many strange local effects have been observed in various tuna fisheries, possibly but not necessarily in relation to stock overfishing. Among the best examples, one could cite three cases observed in the Atlantic:
Bluefin tuna vanishing from Brazilian and Norwegian coasts where they were abundant in the early sixties (Brazil) and fifties and sixties (North Sea, mainly Norway);
Yellowfin vanishing from Angola during the early seventies. (ICCAT Stat. Bull.) and from Senegal during the nineties (Hallier and Marec 1998);
Bigeye vanishing from Azores during the nineties (recent ICCAT SCRS reports).
All these examples are dealing with large tunas fished at the limit of their geographical distribution. Interestingly, none of these surprising changes has been thoroughly studied and they remain widely unexplained by scientists. They may indicate a shrinkage in the area of distribution, but the causes of such a change are not understood: as a function of environmental changes, of stock sizes, or due to other reasons?
6. POTENTIAL EFFECTS OF TUNA FISHERIES ON PELAGIC ECOSYSTEMS
The potential negative effects and disequilibrium exerted on pelagic ecosystems by tuna fisheries remain widely unknown, but they are a source of increasing concerns for most tuna Commissions and in several countries (Fonteneau and al 2002). Discards by tuna fisheries tend to be quite moderate at a worldwide scale, and they are scattered over the wide oceanic areas fished. They however, remain poorly estimated by scientists due to the lack of observers in most tuna fisheries. The worse uncertainties are probably faced for longliners (a gear showing large discards in some strata); while discards by purse seiners tend to be better sampled and quite moderate (most often less than five percent of tuna catches). Pelagic ecosystems worldwide are very large and deep, and they appear to be much less threatened by tuna fisheries than the continental shelves fished by bottom fisheries (at least until now).
However, two types of environmental risks can easily be identified for tuna fisheries.
The by-catches of sensitive and sometimes threatened species such as dolphins, turtles, birds and various species of sharks that are sometimes taken by various tuna fisheries. Tuna fisheries may accelerate the critical stage of these sensitive species. Good examples of this type of interactions have been faced by the Eastern Pacific purse seine fisheries killing too many dolphins in the mid seventies and recently by the Hawaiian longline fisheries because of turtles accidental mortality. In both cases the activities of these tuna fleets had to be temporarily stopped because of such accidental mortality of an undesired species (dolphin problems have been solved by the IATTC). Various species of sharks are now increasingly threatened by tuna fisheries. There is a serious concern that in the future this type of problem could be increasingly faced by tuna fisheries.
The major increase in the catches of oceanic top predators (such as tunas, sharks and billfishes) by the various tuna fisheries world wide has, during recent years, been producing a serious decrease of the summit of the oceanic food pyramid. The potential effect of such decline is not clear (keeping in mind the fact that this pyramid of pelagic biomass remains nearly unfished and virgin in all its lower components). All the potential top-down effects of this removal of predators on the food chain, especially the removal of some predators that are possibly playing a role of key stone species in the ecosystem (Paine 1966), remain widely or totally unknown. Similarly, the risks of changing interactions between the declining populations of the various species of top predators (including between tuna species) remain entirely unknown for scientists. As a whole, there are no doubts that these risks have been permanently increasing with the increase of the catches of tunas, billfishes and sharks world wide, and the probably large decline of their biomass. A precautionary management of tuna fisheries should take into account these risks.
After fifty years of increasing exploitation experienced worldwide by all major tuna stocks, it can be concluded by scientists that most of these stocks have well shown their resilience to overfishing, at least much more than most coastal resources. Many biological and behavioural reasons, as well as the great sizes of pelagic ecosystems, do help to understand this resilience. However, this historical overview is already tarnished by various tuna stocks that appear to be already growth overfished. Nowadays, there is probably little hope to see further significant increases of sustainable catches (for most tuna stocks world wide) in the future, as the entire habitat of most stocks is now more or less fully explored and fished by fishing fleets already highly modernized and very efficient. Taking into account the permanently increasing demand of the word tuna market for fresh, canned and sashimi tunas, as well as the structural difficulties to limit the activities of tuna fisheries, there is then a high risk to see increasingly large numbers overfished tuna stocks in the near future. There is also an unknown additional risk to face world wide in the future the still unknown top-down effects of this increased pressure on the pelagic ecosystems.
Figure 1: Yearly catches of major tunas (skipjack, yellowfin, bigeye, albacore and bluefin tuna) world wide
Figure 2: Map showing the average distribution of recent tuna catches (major species only)
Figure 3: A case study of tuna migrations: average historical catches of southern bluefin and movements between its spawning and feeding areas
Figure 4: Changes in the East Atlantic Yellowfin MSY estimated by production models during the period 1972-1984.
Figure 5: Average tuna catches by the purse seine fleets worldwide (yellowfin, skipjack and bigeye) during 3 periods: 1970-1973 (upper figure), 1982-1985 (middle) and 19941997 (lower figure. These maps well show the increase of fishing zones by purse seiners during the last 30 years.
Figure 6: Annual catches of skipjack tuna by area.
Figure 7: Annual catches of yellowfin tuna by area
Figure 8: Catch and effort relationship observed for Atlantic bigeye. The MSY and optimal effort estimated by ICCAT in 1992 and in 2002 are also shown. It can also be noted that large increases of juveniles catches have been observed since 1992 and they should have produced a lower yield per recruit. Similar events were observed in the Pacific and Indian Oceans
This paper has been widely improved with the support and advice given by Victor Restrepo and by Jean Jacques Maguire. I want to thank them for this valuable help.
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Paine, R.T. 1969. A note on trophic complexity and species diversity. American Naturalist 103: 91-93
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The multiple documents published yearly by the various tuna commissions and bodies (IATTC, ICCAT, IPTP, IOTTC, SCP, CCSBT) have been widely used to prepare this overview synthesis, but for practical reasons, they cannot be cited explicitly.
|  The views expressed
in this paper are solely those of the author, Alain Fonteneau, IRD, PO Box
570, Victoria, Seychelles, E Mail: firstname.lastname@example.org.|
 IATTC : Inter American Tropical Tuna Commission
 ICCAT : International Commission for the Conservation of Atlantic Tuna
 IOTC : Indian Ocean Tuna Commission
 CCSBT : Commission for the Conservation of Southern Bluefin Tuna