F.X. Bard, P. Cayré and T. Diouf
Tuna are frequently cited as examples of migrating fish. It has been known for many years that some of them undergo large displacements that carry certain species (bluefin, albacore) across the oceans. It is convenient at this time to specifically restrict the very general term migration, which normally designates any movement. In this chapter, we will mean by the term “migration”, horizontal movements executed with regular periodicity by a large fraction of individuals of a tuna stock or population.
Studies of migration consist partly to determine migratory paths, migration times and their periodicity and partly to try to understand how these migrations are determined and the factors, linked to the environment and the biology of the species, that provoke them.
Studies of migration provide information on:
This information is important for the assessment of stocks.
Various methods are used to determine migrations and attempt to explain them. These methods can be classed as two types: those permitting a direct objective observation of displacements; or those which deduce migration indirectly for analysis of different factors linked to the biology and exploitation of the species.
At present there is still no method that follows tuna displacement on an oceanic scale and for a long duration (months, years); this ideal method is however being studied (Hunter et al., 1986) and will make use of major technological means (transmitting or recording tags, satellites…).
The only direct method of migration evaluation currently in wide use is that consisting of individually marking fish. Atlantic tuna (yellowfin, skipjack and bigeye) have been tagged individually with a flexible plastic spaghetti shaped tag ending in a harpoon-like point. This tag, called a “dart tag”, is inserted in the dorsal muscles of tuna; the long part, brightly colored, has information necessary for its identification and protrudes from the back of the fish. When the tagged fish is recaptured by a fishing boat, the place and date of recapture are carefully noted; by comparison of place and date of tagging with those of recapture, the minimum rectilinear distance traveled by the fish and its time of travel are known. It is very obvious that the actual path covered by the fish between the locations of tagging and of recapture remain unknown; if large numbers are tagged in the course of multiple operations that are widely distributed in time and space, and if there are active fisheries liable to recapture the tagged fish in a large region of the ocean at all times, it is possible to have a good idea about the details of the migrations (courses, seasons, periodicity).
The great limitation in the analysis of tagged fish recaptures stems from the tagging method itself, that is, a tagged fish is only of interest if it has a chance to be recaptured. The probability of a marked tuna recapture is directly linked to the intensity of fishing effort displayed by tuna boats in different zones where this fish may pass. Different methods have been used to analyze recaptures of tagged fish as a function of fishing effort or to compare catches of tagged fish and those of untagged fish (Cayre et al., 1986) in order to better understand and interpret tagging results.
Other more material difficulties whether linked to the tagging of fish or to their recapture are:
it is difficult and therefore rare to be able to tag tuna with a weight above approximately 15 kg;
the recaptures of tagged fish in a time frame of more than a year are rare for tropical tuna because of the high mortality rate of these species;
information on recaptured fish (date, place …) is often incomplete.
These difficulties often make the analysis of migrations by older fish difficult. This is why the study of migrations also requires the use of indirect methods.
Numerous indirect methods permit a first evaluation of migrations. The first and the most usual consists of the analysis of fishery data to identify seasonal variations in abundance of individuals in various zones. Other methods based on known characteristics of the biology of species permit, in some cases, an understanding of how and why individuals displace and aggregate themselves in certain places; these methods can be based on anatomical, genetic and parasitic characteristics of the species.
Analysis of fishery data
If the fishery occurs over a very wide area and exploits all sizes of fish, it is possible to outline the major features of the migrations of captured species from the spatio-temporal distribution of catches by size or age. One must note however that variations in capturability of studied species can bias results derived from this simple examination of fisheries:
sizes of captured individuals are, in the case of tuna, a function of fishing gear (pole, purse seine or longline)
even if present in a given location, tuna are not always vulnerable to fishing gear for various reasons linked to behavior.
These potential biases must be kept in mind when one uses this method, even if they can be reduced by use of average pluriannual data.
If data on certain fundamental aspects of species biology such as reproduction or nutrition is available, these can aid in understanding migratory patterns deduced from the use of other methods.
Various other methods can help in the study of migrations; these methods all tend to try to identify individuals and their origin from different criteria that we will touch upon.
Blood analyses, from serum or other proteins by electrophoretic and immunogenetic techniques have been used frequently with varying success. They tend to identify the existence of more or less distinct genetic groups in populations and to estimate mixtures that occur according to areas considered.
Chemical analysis of bones seems to be a promising method that could permit the identification of the place of birth or source of individuals in certain cases. This method is now being developed and tested on bluefin and will enable distinction of individuals born in the Gulf of Mexico from those born in the Mediterranean.
Biometric analyses that consists of comparing the size of different parts of the body of individuals may also indicate certain information on the origin of individuals and mixtures of groups individuals with different characteristics.
Certain parasites that infest tuna (paragraph 6) can be theoretically used as natural tags characteristic of a given zone. Unfortunately in order for this type of method to be really usable for the study of migrations, it is necessary that the life cycle and complete biology of the parasites be known, which is not the case for most of those infesting tuna; this explains the biased nature and unreliability of this type of method at this time.
No single direct or indirect method is capable of resolving the problem of tuna migration. It is therefore by joint application of these methods and comparison of the results that it will be possible, at least in part, to determine and understand the migrations of tuna. The more logical approach will consist of initially examining fishing data, then analyzing tagging data and finally comparing the results obtained with that which is known of the biology of the species and with information eventually furnished by other indirect methods that we have touched on (genetic, biochemical analyses, parasites).
The yellowfin is a rapidly growing tuna that can reach large sizes (180 kg). Its habitat varies according to different phases of development, as a result of modifications to the hydrodynamic and hydrostatic capacities that occur during growth (Magnuson, 1973).
Different fishing gears, pole and line, purse seine and then the longline successively exploit tuna according to size. Surface gear (pole and line, purse seine) frequently catch, in certain locations and certain seasons, large quantities of young yellowfin that live in schools which are often mixed with skipjack and young bigeye. The preferred purse seine target is large yellowfin which they catch throughout the study area. Longliners catch only large fish, which include numerous yellowfin between 20°N and 15°S across the entire Atlantic. Small fisheries (Cape Verde, Canary Islands, Madiera, Azores), using gear such as the pole and line or hand line, also catch yellowfin of medium and large size, but in moderate quantities.
The different fishing techniques or strategies have very visible effects on size frequency distribution of the yellowfin catch. Small yellowfin (35 to 65 cm) are caught by pole and line boats and purse seiner while large yellowfin (size grater than 10 cm) are primarily taken by purse seiners and longliners. Yellowfin of medium size (65–110 cm) are less frequent in pole and line boat catches as well as in those of purse seiners.
Figure 5.1 Gross migratory trajectories of all yellowfin tagged from 1971 to 1984 in the Eastern Atlantic. Only displacements greater than 300 miles in a straight line for times at liberty greater than 30 days have been considered.
Biologically, the growth of yellowfin occurs in two phases: a slow growth to around 65 cm and a more rapid growth above this size (chapter 6). Moreover, the species reaches its sexual maturity at 110 cm.
This leads to a division in the study of yellowfin migrations according to three size categories:
Juvenile yellowfin from 5 to 65 cm, with two stages, pre- and post- recruit, separated at around 35 cm.
Preadult yellowfin from 65 to 110 cm.
Adult yellowfin from 110 to 170 cm.
For each one of these three phases, we will successively cover the geographic distribution of catches by FIS and Spanish purse seiners, tagging results and finally biological data.
Tagging, an essential tool for the study of migration, can not always be applied to this classification of yellowfin as a function of their size, whether due to lack of information on length at recapture (a frequent enough case) or because of growth of individuals from one category to the next. It is therefore useful to present a map of all the gross migratory trajectories of yellowfin recorded from 1970 to 1984 without regard to their size (figure 5.1).
The migrations extend from Angola to the Canary Islands, without passing 25°N latitude. Examination of distance covered as a function of time at liberty (table 5.1) reveals rather slow displacements with an average speed of 1.74 miles per day and relatively few long distance displacements. (As will be seen, this contrasts with skipjack and bigeye.)
Size of tagged yellowfin ranges from 35 to 130 cm, with a large majority of juvenile fish between 35 to 70 cm in size. In total, close to 13,000 yellowfin have been tagged and 975 recaptures recorded to date (Cayre et al., 1974; Fonteneau, 1982; Bard and Amon Kothias, 1986).
Table 5.1 Distances traveled by tagged yellowfin in 100 mile classes as a function of time at liberty (in 3 month classes). The gross average speed of displacement is 1.74 miles per day and the corresponding dispersion coefficient 2831 miles2 per day. Number of yellowfin originally marked = about 13000; Number of recaptures = 800
Table 5.2 Releases and recaptures of skipjack in the eastern Atlantic. (from Bard, 1983; Cayré, 1985; Gong and Lee, 1983; and Santos, 1982.)
|ZONE||YEAR||NUMBER MARKED||NUMBER RECAPT.||%|
|North Tropical Atlantic||70–73||700||2||0.29|
|North Subtropical Atlantic||1980||437||110||-2.58|
Fisheries for very small yellowfin (< 35 cm) are rare, as no specific fishing gears take an interest in them. However Albaret et al., (1976) cite occasional fishing for small yellowfin of less than 35 cm by pole and line boats in the Gulf of Guinea during most of the year. This zone corresponds to a specific nursery for yellowfin of the Eastern Atlantic. In open sea zones, there are no catches of these small fish, in spite of the equatorial zone from the coast of the Gulf of Guinea up to 25°W being the principal spawning zone of yellowfin in the Eastern Atlantic (paragraph 6.1). There is probably a migration of these very small yellowfin from the west to the east, but the details of these displacements are unknown.
The geographic distribution of juvenile yellowfin measuring between 35 and 65 cm recruited into the fisheries is well known. These juveniles are caught in large quantity in the Eastern Atlantic by purse seiners and pole and line boats. However the fishing grounds of pole and line boats are restricted, whereas those of purse seiners extends throughout the region. Purse seine catches are better indicators of the actual distribution of these individuals.
Figure 5.2 shows the average monthly distribution of juvenile catches by purse seiners from 1979 to 1983 (Map based on species sampling at landing, therefore exempt from biases on the specific composition). The more or less permanent presence of these juveniles in the coastal sectors is observed, and also occasionally in the open sea along the equator and in the zone west of Liberia. This distribution is very similar to that of skipjack of the same size and makes the specific periods and places of concentration of catches appear precise: Cape Lopez from May to September, in the open sea off Liberia in November-December, the Gulf of Guinea in January.
These juvenile yellowfin were subjected a major fishery when the pole and line fisheries, based mostly in Dakar, Abidjan, and Point Noire, were still active. This has permitted certain authors to examine the spatio-temporal distribution of young yellowfin catches (Postel, 1969; Zharov, 1967) at a very early date. Champagnat (1974) has proposed a partial model of migration putting forward a passage of young yellowfin along the continental shelf from the Côte d'Ivoire to Mauritania.
Figure 5.2 Monthly geographic distribution of juvenile (less than 65 cm) yellowfin catches based on samples collected from FIS and Spanish purse seiners from 1979 to 1983 in the Eastern Atlantic.
Figure 5.3 Migratory trajectories of juvenile yellowfin recovered at less than 65 cm for the historical period (1971–1978). Only displacements greater than 30 miles in a straight line for times at liberty greater than 30 days have been considered.
Fonteneau (1982) analyzed size frequencies of FIS purse seiner and pole and line boat yellowfin catches from 1969 to 1977 in a synthetic fashion. He was thus able to examine the size structure in the sectors of Dakar, Abidjan and Point Noire. His conclusion points out synchronous recruitment of young yellowfin the in the three sectors, then a certain isolation of the three groups thus constituted of a size up to 50 cm. The largest group called “the Point Noire group” was that of the inner Gulf of Guinea (between Cape Lopez and Cape Three Points). Above 50 cm, there were exchanges, fairly infrequent, between these three groups of individuals.
The ease of capture of young albacore by pole and line has made it possible to tag numerous juveniles between 1971 and 1984. The bulk of tagging locations is situated in the inner Gulf of Guinea.
Figure 5.3. shows migratory trajectories of juveniles tagged and recaptures during 1971–78, when fishing effort was exercised essentially in the coastal sectors. The majority of movements were within the interior of Gulf of Guinea, but where were also some long distance migrations toward Senegal and even the Canary Islands. Cayre et al. (1974), using the tagging results from 1971 to 1974, concluded a certain independence of yellowfin of the Cape Lopez zone; these had seasonal north-south displacements along the coast from Gabon to Angola. Long distance recaptures observed since, temper this conclusion. To this effect, it is interesting to examine tagging results carried out from 1980–1984 when fishing effort was distinctly greater and extended into the open sea zone.
Juvenile recaptures can be analyzed by separating them in two size classes (greater than and less than 50 cm. at recapture) in order to compare results with Fonteneau's conclusions (1982) (figures 5.4 and 5.5.). Large numbers of yellowfin of less than 50 cm were returned between Cape Lopez and Cape Three Points, which confirms the hypothesis of a fairly stationary population of juvenile yellowfin in the interior of the Gulf of Guinea (figure 5.4). Passages beyond the Abidjan sector were also observed and these individuals may then migrate to the Senegal fishing zone or into the equatorial zone, even with sizes as small as 50 cm.
For yellowfin from 50 to 65 cm, the pattern changes progressively; there is a certain sedentarity in the Gulf of Guinea, but long distance migrations are also observed. These are along the coast on an axis from the south-east to north-west (figure 5.5). It appears plausible that these migrations are responsible for the swasonal ruthm of the timing of the fishing grounds of Senegal, the Gulf of Guinea and the Angola zone.
|Figure 5.4||Migratory trajectories of juvenile yellowfin recovered at less than 50 cm for the recent period (1980 – 1984). Only displacements greater than 30 miles in a straight line for times at liberty greater than 30 days have been considered.||Figure 5.5||Migratory trajectories of juvenile yellowfin recovered between 51 and 65 cm for the recent period (1980 – 1984). Selection criteria identical to those for figure 5.4.|
The study of parasites of juvenile yellowfin of the three sectors (Baudin-Laurencin, 1971; Lardeux, 1982) shows isolation of individuals from 55 to 80 cm between the Dakar sector and interior of the Gulf of Guinea; this seems consistent with tagging and size analysis conclusions.
In conclusion, yellowfin during their juvenile phase (between 35 and 65 cm) seem to be moderately migratory. The migration tendency increases with the size, especially above 50 cm. Although the majority of these fish remains in the coastal sectors of the Gulf of Guinea, there are possibilities for passage between different coastal sectors from Angola waters to those of Senegal and the Canary Islands, with a sharp tendency to a migration oriented from the south-east toward the north-west, along the coast of Africa.
Supplementary analyses from size frequency distributions and tagged fish returns as a function of fishing effort, could clarify these migrations which present remarkable similarities to those of skipjack of similar sizes (paragraph 5.3).
Preadult yellowfin, from 65 to 110 cm, are poorly represented in pole and line boat and purse seine catches in the Eastern Atlantic. This lack of medium size yellowfin could be tied to the problem of purse seine vulnerability. It is possible that these fish form only small schools and do not justify a fishing effort by purse seiners. The pole and line boats of Tema have rigging too light to catch this size of fish; however, the pole and line boats of Dakar fished quite large quantities between 1965 and 1970, as has also been the case for pole and line boats based at Point Noire (Marcille and Poinsard, 1970; Pianet and Le Hir, 1971, 1972 and 1973).
Actual purse seine catches are not however negligible. Figure 5.6 shows the average monthly distribution (1979 – 1983) of catches of these yellowfin by purse seiners. Catches are carried out in all of the Gulf of Guinea up to 30° W, with some seasonal concentrations in favorable zones: Cape Lopez and Senegal in boreal summer, equator in boreal winter. There could therefore be migrations between these various concentrations.
The small Azores fishery shows the presence of this size range in summer (Pereira, 1986). Tagging of fish of this size is infrequent; one must therefore be content with information derived from long term returns of individuals tagged as juveniles in the Gulf of Guinea and recaptured at preadult size. These returns indicate a tendency to migrate in the direction of high latitudes with a south-east to north-west axis that extends those observed in juvenile yellowfin (figure 5.7).
There are also numerous preadult yellowfin recaptures after small displacements in the inner Gulf of Guinea. It would be proper to analyze these observation more precisely, notably with respect to fishing effort, in order to actually determine whether sedentary behavior or cyclic seasonal migration brings back individuals periodically to the same locations.
Studies of parasitic infestations already cited seem to indicate probable mixing of yellowfin of more than 90 cm between all the coastal sectors of the Gulf of Guinea.
Migrations of preadult yellowfin occur in all of the Gulf of Guinea in a similar manner to those of juvenile yellowfin but with a greater amplitude. The movements follow a south-east north-west axis, between Angola and Senegal and even the Canary Islands and the Azores. In tropical waters, these migrations are seasonal and could be tied to hydrological conditions or to the productivity of the environment.
Yellowfin from 30 to 150 kg (110cm to 160cm) are adult and are likely to begin genetic migrations (i.e. in relation to reproduction) as well as the feeding migrations common to all size categories. These large yellowfin are vulnerable to the longline and the purse seine. Japanese longline was the only gear to exploit them almost from 1957 to 1970. The log book records of these vessels have permitted the establishment of the geographical distribution of adults throughout the Atlantic (figure 6.23). The fine analysis of this data has permitted the establishment of the hypotheses on adult yellowfin migrations that were summarized and discussed by Bard and Cayre (1986).
Starting in 1975, catches of large yellowfin by purse seiners became progressively dominant while the portion for longliners declined regularly. The interpretation of their catches differs and complements the one originally proposed based only on the analysis of longline fisheries. It must be remembered however that the layers of water exploited by these two gears are very different: purse seiners exploit the layer of water between the surface and around 80 meters in depth, while longliners exploit the layer between 80 m and 200 m in depth and even 300 m in depth since the recent introduction of the “deep” longline.
Figure 5.9 shows the average monthly distribution of catches of large yellowfin by purse seiners during 1979 – 1983. There are seasonal concentrations, principally at the equator but also in the high seas off Cape Lopez and Senegal. The concentration at the equator lasts from November to March and extends from 5° E to 25° W. Those off Cape Lopez and Senegal are of a shorter duration and are occur respectively in May-June and August-September.
The few tag returns obtained on adult yellowfin (12 well identified) indicate only returns in the interior of the Gulf of Guinea and none in the equatorial concentration zone where the majority of catches of large yellowfin are made (figure5.8). This is however a troubling fact on which one can hardly base a conclusion primarily because of low numbers of recaptures. The absence of recaptures in longline catches (Fonteneau, 1982) must also be noted.
Figure 5.6 Monthly geographic distribution of preadult (from 66 to 110 cm) yellowfin catches based on samples collected from FIS and Spanish purse seiners from 1979 to 1983 in the Eastern Atlantic.
Figure 5.7 Migratory trajectories of preadult yellowfin recovered between 66 and 110 cm for all tagging conducted between 1971 and 1984 in the Eastern Atlantic. Selection criteria identical to those for figure 5.4.
Figure 5.8 Migratory trajectories of adult yellowfin recaptured at greater than 110 cm for all tagging conducted between 1971 and 1984 in the Eastern Atlantic.
Figure 5.9 Monthly geographic distribution of preadult (greater than 110 cm) yellowfin catches based on log books of FIS and Spanish purse seiners from 1979 to 1983 in the Eastern Atlantic.
It seems that the equatorial concentration of purse seine fishing is linked to the reproduction of yellowfin (paragraph 6.1.1) as this region is a reproduction zone notably in the first quarter. Bard and Cayre (1986) underline that the recently observed localization of fishing concentrations requires the extension of the limit of this reproduction zone to 25° W. Limited observations of gonads of yellowfin caught in these concentrations by purse seiners seem to indicate they are spawning concentrations but this conclusion requires confirmation.
If the observed concentration in the high seas off Cape Lopez in May and June can not be tied to reproductive behavior, the concentration exploited off Senegal in August-September could correspond at least partially to a reproductive concentration (Postel, 1955; Rossignol, 1968). The higher average temperature of the locations where these yellowfin are caught (chapter 7) also constitute a priori, a condition favorable to maturation and spawning of yellowfin.
The migration pattern of large yellowfin in the Eastern Atlantic is not perfectly established; tagging data in particular are insufficient and one can only make a few conclusions:
Large yellowfin are essentially circumscribed to a zonal band of the Atlantic between 20° N and 15° S.
They aggregate to spawn in the equatorial zone.
The few available tag returns indicate no long distance, particularly transatlantic, migration that one would expect from the models of Honma and Hisada (1971) and according to Hayashi (1974) and Yanez (1979).
Currently, it is the paradoxical conclusion that although it is known that large yellowfin migrate, no direct proof exists of true migration over very great distances and especially of the existence of transatlantic migrations.
It has been shown that tuna migrate either for food (trophic migrations) or for reproduction (genetic migrations).
Yellowfin from 35 to 110 cm, that is juveniles and preadults, are immature. A priori they only undertake trophic migrations. One of the facts emphasized in the description of the migration of these juvenile yellowfin is the similarity of their migrations with those of skipjack. Also juvenile yellowfin catches are made in mixed schools with skipjack and young bigeye. These permit the articulation of a hypothesis of a common migration, described in the following paragraph, which could be mediated by the successive appearances of different coastal upwellings and by the seasonal heating of boreal and austral tropical zones.
Young yellowfin of less than 50 cm are rather sedentary and stay in the inner Gulf of Guinea, while those of 50 to 65 cm follow the migratory route of skipjack.
Preadult yellowfin that are less frequently fished in mixed schools with skipjack, seem also to migrate in relation to a pattern analogous to that of yellowfin from 50 to 65 cm. The hypothesis of similar migrations of yellowfin, skipjack and bigeye in mixed (or separate) schools must be verified and analyzed in a more thorough manner.
Adult yellowfin are likely to undergo trophic and/or genetic migrations. With regard to trophic migrations of these large yellowfin in the Atlantic, there is little information, but the fishing concentrations of Cape Lopez, Ghana and Senegal already cited seem tied to major surface enrichments in neighboring coastal upwelling zones (chapter 7). Other yellowfin make migrations, probably trophic and at the surface following, the seasonal heating of tropical or subtropical waters. Thus in the Eastern Atlantic, Santos (1977) and Pereira (1983) respectively point out the presence of large yellowfin at the Canary Islands and the Azores in boreal summer. Genetic migrations are toward the equator from 5° E to 25° W in the Eastern Atlantic and toward Northern Brazil in the Western Atlantic. Because of the lack of sufficient tagging of large yellowfin, the relative importance of migration routes leading to these different zones remains unknown; the one at the equator seems however the most important.
Figure 5.10 Model migration pattern of Atlantic yellowfin.
The extension of longline fishing zones has provoked various authors to postulate at an early date the existence of transatlantic migrations of large yellowfin. Wise and Le Guen have introduced, in 1966, the hypothesis of the existence of two stocks of yellowfin situated on both sides of the Atlantic. Homna and Hisada (1971), using fishing maps taken from Japanese long liner log books and also various biological data (size frequencies, state of gonads and distribution of larvae) established the first coherent general pattern of yellowfin migrations in the Atlantic; this has been reconsidered and slightly modified by Hayashi (1974).
This pattern presents two concentrations of adult yellowfin on both sides of the Atlantic, separated during the boreal winter, but forming a continuous band in boreal summer. There are east to west and west to east genetic migrations of yellowfin originating from these two concentrations; the yellowfin combine in a central zone situated at around 30° W in summer. This zone could constitute a common spawning zone. After the reproduction period, yellowfin return to their specific feeding locations on both sides of the Atlantic (trophic migrations).
This description of deep adult yellowfin migrations between three zones, west, central and east, was developed by Yanez (1979 and 1981), who integrated in his analysis the increasing catches of large yellowfin fished at the surface by purse seiners from 1975.
One of the interpretations of such movements is to postulate the existence of a unique spawning zone situated in the central part of the Atlantic with a very protracted spawning period. This conception could apply to the distribution of yellowfin larvae gathered in the central Atlantic. Referring to the spawning locations and seasons deduced from the examination of gonads (paragraph 6.1.1) it can be concluded that there is some separation between yellowfin stocks exploited at the surface and those exploited by deep longliners, and for stocks exploited at the surface, between those exploited in the eastern Atlantic and those exploited in the west (Fonteneau, 1982).
The overall pattern of yellowfin migrations is presented in figure 5.10 and can be summarized as:
juvenile yellowfin were recruited to the fisheries at a size of 30 to 40 cm and in the interior part of the Gulf of Guinea. At these small sizes, they are quite sedentary, but, as they grow, manifest a tendency to migrate along the coast following a general axis from the south-east to the north-west.
in the next size range, from 50 to 65 cm, at the end of the juvenile stage, their migratory capacities appear greater permitting them to have seasonal cyclic migrations in mixed schools with skipjack and young bigeye as far as the zones of Angola and Senegal.
preadult yellowfin from 65 to 110 cm behave like juveniles, amplifying north-south trophic migrations. The existence of a sedentary population is however possible. In reality, the migrations of this phase remain poorly known.
Adult yellowfin, larger than 110 cm, generally move throughout the equatorial Atlantic and seasonally in the tropical and subtropical zones. Dispersive trophic displacements take place during most of the year, principally in accordance with a east-west axis, with a return to the equatorial zones by the deep yellowfin fished by longline. These displacements can be exchanges between eastern and western stocks.
Genetic migrations seem to exist that carry mature yellowfin to two major zones of reproduction, one situated in the east and the other in the west, in the first and third quarter respectively. It is not impossible that deep yellowfin and those of the surface have a different reproductive behavior (Fonteneau and Fontana, 1978; Yanez and Barbieri, 1980) and that there is some independence between these deep and surface individuals.
Skipjack are present throughout Atlantic Ocean in a vast zone approximately between the 18° C surface isotherms, a domain that extends near 40° N to 40° S. In the Eastern Atlantic, skipjack are fished in an area extending from 20° N to 20° S, limited to the west by the 25° W longitude. They are caught in large quantities all year in the equatorial zone and during summer seasons in north and south tropical zones. Gears used are pole and line and purse seine. More in the north, coastal fisheries (pole and line and hand line) of the Canary Islands, Madeira and The Azores catch skipjack for a few months during summer. Skipjack are even caught in particularly warm years in the Bay of Biscay and as far as the coasts of England. Longliners, which do not target skipjack, catch them sporadically throughout the Atlantic (figure 5.11).
For commercial reasons, skipjack is not a permanent target species for purse seiners and pole and line boats, except for freezer pole and line boats based at Tema. Fishing strategy must therefore be taken into account in the interpretation of skipjack catch data.
Biologically speaking, knowledge of Atlantic skipjack has progressed greatly thanks to the international research program created by the Commission for the Conservation of Atlantic Tuna (ICCAT) and executed from 1979 to 1983. This program has included numerous tagging experiments that are summarized in table 5.2. Skipjack migrations in the Eastern Atlantic are now quite well known. As with yellowfin, these migrations have been determined by analysis of fisheries data (taking into account biases due to the selectivity of fishing gears and strategies), and by tagging analysis. Indirect methods, taken from species biology support these analyses.
220.127.116.11.1. Presentation, method
The size range of skipjack fished by the two surface fishing gears (pole and line and purse seine) is remarkably constant without regard to fishing locations and seasons (chapter 4); it extends from 35 to 70 cm in fork length but the great majority of the catch is composed of individuals of size between 40 and 60 cm, corresponding to an interval of life of around two years.
Figure 5.11 Incidental catch of skipjack by Japanese longliners throughout the Atlantic.
The very reduced size range and brevity of exploitation is explained by a variation of the availability of skipjack in fishing zones of the tropical-east Atlantic. They are recruited progressively between 35 and 45 cm, remaining approximately two years in the fisheries and reaching a size of around 60 cm; above 60 cm skipjack seem to emigrate very rapidly outside of the surface fishing zones. In the north tropical zone, the pattern is identical, with the exception that skipjack might stay for an even briefer time in the fisheries, which seems to be linked to a more rapid growth (Cayre, 1985).
For these reasons it is preferable to divide the examination of the geographic distribution of catches into three fork length ranges:
Examination of catch by fishing gear demonstrates the preponderance of purse seine catches (chapter 4). The monthly maps of FIS and Spanish purse seine skipjack catches by size is represented in figures 5.12, 5.13 and 5.14.
Skipjack less than 45 cm
Catch locations of skipjack of less than 45 cm can be used to determine places where recruitment occurs (ie. entrance of young skipjack in the fisheries) (figure 5.12). There are three concentrations of skipjack less than 45 cm and three principal recruitment locations (in decreasing order of importance):
The recruitment of Cape Lopez is particularly large. Otherwise less regular recruitment may take place in the inner the Gulf of Guinea from November to January.
This map, which contains only the years 1979-83, does not take into account fishing carried out off Angola by purse seiners of various nationalities until 1977; the examination of historical fisheries (chapter 4) shows that the catches were made more or less all year long (with the exception of the period between the months of April and July), with a dominance of small skipjack less than 45 cm in the first quarter (ISRA - ORSTOM, 1976). The Angolese zone is thus without doubt also a recruitment location, which may be important as judged by the volume of catches during certain years, notably 1974).
Figure 5.12 Monthly geographic distribution of catch of skipjack less than 45 cm by FIS and Spanish purse seiners from 1979 to 1983 in the Eastern Atlantic.
Skipjack from 46 to 59 cm
Skipjack from 46 to 59 cm, the majority caught by purse seine, constitute the bulk of catches all year long without regard to fishing location (figure 5.13). These individuals are present continuously in the equatorial zone with well defined concentrations that are:
The catches in these three major concentrations are equivalent. The historical catches off Angola in the third and fourth quarters by American purse seiners must be added to these concentrations. Finally, there are occasionally large but sporadic catches at Cape Three Points in July-August and October-November.
These zones of large catches by purse seiners give a good indication of areas of high skipjack abundance. This does not exclude the presence of lesser quantities of skipjack in the interior of the Gulf of Guinea all year long. The quarterly map of skipjack catches by the Tema pole and line boats (figure 4.35) show very constant catches in a reduced zone of two 5° × 5° geographic squares. This confirms the presence of skipjack permanently off Cape Three points. Moreover in the third quarter, these pole and line boats exploit, as do the purse seiners, the concentration of Cape Lopez; an extension of their zone of activity toward the west in the fourth quarter is also observed.
Skipjack greater than 60 cm
Catches of large skipjack by purse seiners (figure 5.14), are well dispersed in time and space. Characteristic concentrations occur in the Liberia zone from November to February, at Cape Lopez from May to September, and off Senegal from April to June. These concentrations of large skipjack are situated in space-time stratum identical to those of medium skipjack; the quantities fished are generally low which may be interpreted as a lower availability of these fish.
The fact that the bulk of skipjack catches in well determined periods and locations has brought Fonteneau and Laloe (1986) to analyze the distribution of sizes caught in successive weeks in these concentration areas, assumed to be temporarily closed systems; two interesting conclusions stand out:
there is a strong identity of the size structure of skipjack caught first at Cape Lopez then off Cape Three Points.
off Senegal in the third quarter there is recruitment of small skipjack of less than 45 cm along with skipjack from 50 to 60 cm.
The analysis of fisheries data permits the creation of the following migratory scheme: skipjack are mostly recruited at Cape Lopez in the second quarter and in Angola, in some years, in the first quarter. This recruitment, as with those occurring all year long that have also been mentioned (Senegal, Liberia, Gulf of Guinea…), sustains a series of concentrations where the bulk of skipjack is fished which move toward the west from the second to the fourth quarter.
Figure 5.13 Monthly geographic distribution of catch of skipjack from 46 to 59 cm by FIS and Spanish purse seiners from 1979 to 1983 in the Eastern Atlantic.
Figure 5.14 Monthly geographic distribution of catch of skipjack 60 cm and greater by FIS and Spanish purse seiners from 1979 to 1983 in the Eastern Atlantic.
Table 5.3 Table of distance traveled by tagged skipjack in 100 mile classes as a function of time at liberty
(in 3 month classes). The average gross speed of displacement is 2.80 miles per day and the
corresponding dispersion coefficient 2088 miles2 per day.
Number of skipjack originally marked = 31,038. Number of recaptures = 2,760
Table 5.2 summarizes all skipjack tagging in the Eastern Atlantic and table 5.3 presents the distances covered as a function of time at liberty of the tagged fish. The proportion of long distance recaptures is clearly higher than that observed with yellowfin (table 5.1). The average migration speed is also higher than that of yellowfin: 2.80 miles/day compared to 1.74 miles/day. This emphasizes the migratory character of skipjack in the Eastern Atlantic; figure 5.15 represents long distance migrations of tagged skipjack.
Given the short duration of skipjack in surface fisheries of the Eastern Atlantic (between one and three years from which growth is known), the times at liberty of tagged fish are accumulated in intervals of 6 months; figures 5.16, 5.17, 5.18 represent straight line trajectories covered by tagged skipjack for different times at liberty: from 1 to 6 months, 7 to 12 months and more than a year.
We will successively analyze the tagged skipjack movements in the equatorial zone (off Ghana, the Côte d'Ivoire and Cape Lopez) and in the north (Senegal, Cape Verde Islands) and south (Congo, Angola) tropical zones.
Tagged skipjack in the equatorial zone:
The details of the apparent movement of tagged skipjack in the equatorial zone and size at tagging, indicated in figure 5.19 B, are described by Miyabe and Bard (1986); the principal conclusions are:
In the first six months after tagging, there is a vast movement following the contour of the coast that carries skipjack from Cape Lopez as far as Cape Three Points;
After 6 months, relatively large numbers of skipjack reach the north tropical zone of Senegal. (A skipjack tagged at Cape Three Points has even reached the Canary Islands).
After one year, skipjack have also reached the Liberia zone as well as that of Cape Lopez and Senegal, where they returned.
Tagged skipjack in tropical zones:
The size of tagged skipjack in the north tropical zone (the large majority of tagged individuals) is given in figure 5.19 A.
|Figure 5.15||Migratory trajectories of all skipjack tagged from 1970 to 1984 in the Eastern Atlantic. Only displacements greater 600 miles in a straight line for times at liberty greater than 30 days have been considered. The higher minimum distance was used to emphasize long distance movements.||Figure 5.16||Migratory trajectories of skipjack tagged from 1970 to 1984 in the Eastern Atlantic. Only displace-than ments greater than 600 miles in a straight line for times at liberty greater between 30 and 180 days have been considered. The higher minimum distance was used for the clarity of the figure.|
In the first six months following tagging (which occurred in the first quarter), skipjack tagged off Senegal and the Cape Verde Islands are clearly displaced toward the Liberia zone. After six months, there is practically no return of these skipjack (Cayre et al., 1986). An extremely interesting exception is that of a skipjack recaptured in the mid the Atlantic at 3° N, 32°W, two years after tagging. Noted also is the recapture off Senegal of a skipjack tagged at the Canary Islands. This long distance migration is the only one of a series of 510 skipjack recaptures that were tagged and returned near the Canary Islands (Santos and Torres, 1982).
The few tags released in south tropical zones (off Angola) show a clear migration from Angola up to the inner Gulf of Guinea, generally in less than six months.
As mentioned in the introduction of this chapter, it is necessary to analyze the gross tagging results as a function of variations of tuna availability to the fisheries in time and space. This has been done for the numerous skipjack recaptures observed in 1980 – 1982 when fishing effort on skipjack was at its peak owing to the shortage of yellowfin (Fonteneau, 1986). Tagged skipjack returns during this period are all the more significant.
Adjustment of skipjack recaptures as a function of quantities fished in the Gulf of Guinea has been done by Bard et al., (1986) using the weighting method of Bayliff (1979) and comparing catches in weight and numbers recaptured by large spatio-temporal strata. The results indicate a predominance of two cores of migration: one from Cape Three Points toward the Liberia zone, the other from Cape Three Points toward Cape Lopez.
The method consisting of comparing catches and recaptures has been improved by Cayre et al., (1986). These authors compare the changes over time of catches (in numbers) and recaptures of tagged skipjack in three zones, analyzing separately the fate of small (FL < 45cm) and large (FL > 45cm) skipjack. The three zones used are the equatorial zone and two areas situated respectively off Senegal and Cape Verde Islands.
|Figure 5.17||Migratory trajectories of skipjack tagged from 1970 to 1984 in the Eastern Atlantic. Only displacements greater than 600 miles in a straight line for times at liberty greater between 181 and 365 days have been considered.||Figure 5.18||Migratory trajectories of skipjack tagged from 1970 to 1984 in the Eastern Atlantic. Only displacements greater than 600 miles in a straight line for times at liberty greater greater then 365 days have been considered.|
In the equatorial zone these authors notice a remarkable similarity in the change (17 months) of total numbers of skipjack caught and recaptures of tagged skipjack in this zone. This indicates that the movements of tagged skipjack clearly represent the general movement of skipjack population exploited in the equatorial zone during a period of a year and a half. This analysis shows also an emigration by certain individuals of this population outside of the equatorial zone. The analysis of the change in numbers of tagged skipjack allows an estimation of this emigration (Bard, 1986). Cayré et al., (1986) point out however a difference between relative changes in the number of recaptures and catches of small and large skipjack; this difference indicates that the larger skipjack will emigrate earlier outside of the fishing zones (after about a year).
In the Senegal zone, although there is a good similarity between commercial catches and tagged fish recaptures during the 3 to 5 months of the fishing season that corresponding to that of tagging, there are only a few tagged skipjack that return in the same fishing zone in the following season. This indicates an emigration at the end of the season of the whole population and replenishment of the skipjack population in the north tropical zone from one season to another. At least one part of these new arrivals come from the equatorial zone. These facts can be established from the heterogeneity observed in the sizes of skipjack caught in the third quarter in the Senegal zone.
For the Cape Verde Islands zone, there is also a strong similarity between the number of tagged skipjack recaptures and catches at the end of the fishing season, but no recaptures the following season in spite of very large numbers tagged. It is thus a fishery through which a migratory flux of skipjack pass each year.
Figure 5.19 Size-frequency distributions of skipjack tagged in the Eastern Atlantic during the period 1979 – 1982. A - northern tropical zone in two distinct regions situated off Senegal and around the Cape Verde Islands. (Tagging conducted by Senegal and the Republic of Cape Verde). B - equatorial zone, in the Gulf of Guinea exclusively. (Tagging conducted by Japan).
The fate of all skipjack emigrating out of the fisheries of the Eastern Atlantic was analyzed at length during the final meeting of the international program of Atlantic skipjack research. In conclusion, several observations seem to confirm that there is, in the middle of the Atlantic, an unexploited population of large skipjack sustained at least partially by emigrations of individuals out of the fishing zones:
The recapture, already cited, of a skipjack tagged off Senegal at a size of 46 cm and recaptured in the middle of the Atlantic at 71 cm.
Indirect arguments taken from the examination of size frequencies of exploited skipjack by different fisheries; the availability of skipjack to surface fisheries decreases very rapidly above a size greater than 60 cm, while skipjack can reach 80 cm. Nevertheless these large skipjack exist and are caught occasionally by longliners in the central Atlantic (figure 5.10); the majority measure from 60 to 80 cm (Kume, 1977). Moreover, in the Azores and Canary Islands, very large skipjack appear in pole and line catches at the end of the fishing season (Santos and Torres, 1979; Pereira, 1983).
Finally, analyses of numbers of tag returns supports the hypothesis of an emigration toward the central Atlantic of fish of size larger than around 55 cm (Bard, 1986; Cayre et al., 1986; Kleiber et al., 1984); Bard estimates the average emigration rate to be around 60% per year for the equatorial skipjack population for all sizes combined.