6. BIOLOGY OF TUNA (Contd.)

6.1.7. Associations of tuna schools with other large pelagic animals or various floating objects

6.1.7.1. Introduction

Tuna fishing associated with a “system” constituted, either of one or several pelagic animals, or of floating debris, have been known for a long time.

In the eastern Pacific Ocean, tuna fishing is very often conducted on schools associated with dolphins (Sund et al., 1981). In the western Pacific Ocean (Saccchi, 1986) as well as in the Indian Ocean, tuna fishing is conducted very frequently with the help of floating debris met by chance or placed intentionally.

In the tropical Atlantic this form of fishing has also been exploited to a lesser extent (around 38%, table 6.14) for around 15 years or less (Bard et al., 1985); purse seiners sometimes fish schools of tuna associated with whale sharks, dolphins and whales, or with floating debris, (designated as “fish aggregation gear” or FAD), with “carcasses” of deat marine mammal (whale, sperm whale, etc…), or with the tuna vessels themselves.

The FADs are driftwood (logs, tree trunks) or even simple piles of grass uprooted from river banks during floods; this type of FAD is called “tas de paille” (straw heaps) by French fishermen. FADs can also be hawsers, old net floats, crates or various debris.

Table 6.13 Comparison between tonnage caught (thousands of tonnes) by purse seiners of the FIS fleet from 1976 to 1982 and the tonnage caught by the purse seiners sampled.

6.1.7.2. Data and methods of analysis

From information collected from 1976 to 1982 in log books of the FIS tuna fleet (French, Côte d'Ivoire and Senegalese fleet) unloading in the port of Abidjan, we have coded for certain vessels all sets made during a trip, selecting trips according to the reliability with which the captain fills in his log book. This file does not contain landings at the port of Dakar (27% of catches of the period). Fishing in the north zones is in general poorly sampled. If one considers the catches carried out by the FIS fleet from 1976 to 1982, our sample represents 36.3% of yellowfin catches, 38.1% of skipjack catches and 42.4% of bigeye catches (table 6.13).

For every purse seine set, we have recorded the parameters concerning fishing itself (species, tonnage and size of the fish caught), as well as the ecological parameters (systems and/or species associated to the purse seine set), oceanographic conditions (surface temperature, speed and direction of the current and the color of the water) and classic meteorological conditions (speed and direction of the wind, state of the sky and sea).

Information used results solely from estimations made by the captains. Sampling specifically carried out during unloading have shown that, although the specific composition and estimated volume of catches of large individuals consigned by fishing masters in their log books were in general correct, the species composition estimated for small tuna was, on the other hand, most often biased.

This problem is analyzed in paragraph 8.1.3. These biases tend to confuse small yellowfin and small bigeye under the commercial name “skipjack”. From this fact, the notion of pure schools of skipjack or small yellowfin that will be mentioned following this paragraph is in general doubtful, and the conclusions about these schools should be interpreted with care. Bigeye which corresponds to around 6.7% of purse seine catches (1979–1982) is not considered in the present study because of its scarcity in log books.

In the same way, one understands that the identification of the specific composition of a null set is still more elusive; indeed, this set has failed and none of the fish from the school that the boat has tried to catch could be actually observed on deck. The specific composition of these null sets is only estimated from the fish of which the fishing master believed to know the species when they were visible at the water surface.

Keeping in mind these serious problems, for each type of school coded in the log books, the frequency of null sets and catches by set as a function of the fishing zone have been analyzed.

Table 6.14 Total number of sets, number of positive and null sets, percentage of null sets and percentage of occurrence on free schools and schools associated with natural FADs and with marine animals for all species combined.

Nbr. sets = Total number of sets.
Sets > 0 = Number of positive sets.
Sets < 0 = Number of null sets.
% nul.set. = Percentage of null sets.
% occur. = Percentage occurrence.

PERCENTAGE OF OCCURRENCE
OF FREE AND ASSOCIATED SCHOOLS
ALL SPECIES COMBINED

Figure 6.26 Percentages of occurrence of free and associated schools (all species combined).

Table 6.15 Total number of sets, number of positive and null sets, mean catch per set, percentage of null sets and percentage of occurrence on free schools and schools associated with FADs and with marine animals. (ns: not significant).

Nbr. sets = Total number of sets.
Sets > 0 = Number of positive sets.
Sets < 0 = Number of null sets.
% nul. set. = Percentage of null sets.
% occur. = Percentage occurrence.

6.1.7.3. Frequency of free and associated schools

In our file, 62% of tuna schools exploited by tuna boats were free schools and 38% were schools associated with aggregation systems (figure 6.26 and table 6.14). The observed frequency of associations of different types of schools (i.e. yellowfin, yellowfin + skipjack and skipjack) with various systems is given in table 6.15 and represented in figure 6.27. The frequency of positive and null sets for free and associated schools is given in table 6.15 for various types of associations observed.

6.1.7.4. Tuna and pelagic animals

(a) Association with whale sharks

The presence of a whale shark (or several as is often mentioned) will have a different effect whether or not it is associated with skipjack or yellowfin (Stretta and Slepoukha, 1986a). Globally, 8% of purse seine sets are carried out in the presence of a whale shark (table 6.14 and figure 6.26). The percentages of occurrence of each type of tuna school fished with whale sharks are presented in table 6.15 and figure 6.27; one notes a difference between monospecific and mixed schools of yellowfin and skipjack.

Figure 6.27 Percentages of occurrence of free and associated schools (by type of school).

If one compares the success rate of sets on free schools and on schools fished in the presence of a whale shark, the presence of the latter has no effect on the success of the set when the fished school is composed uniquely of “skipjack”. On the other hand, for “pure” yellowfin schools in the presence of a whale shark, the rate of null sets is higher and the average catch per set is lower (table 6.15). For mixed schools of yellowfin and skipjack, the rate of null sets is lower in relation to a free school, while the mean catch per set if not much different (table 6.15). Catches by size class of sets shows a preponderance of sets of small sizes (figures 6.28, 6.29 and 6.30).

The hydrological region where a fifth of the sets were made in association with a whale shark is the region designated as “Cape Lopez” (table 6.16 and figure 7.1); these sets are rare in other hydrologically comparable zones.

(b) Association with dolphins

Tuna fishing associated with dolphins is far from having the importance that this type of fishing has in the eastern Pacific. Sund et al., (1981) in their synthesis on the tuna environment in the central Pacific, labeled as vital the simultaneous presence of tuna and dolphins for the success of fishing. In the Atlantic, only 1% of tuna schools of the sample are fished with dolphins (table 6.14 and figure 6.26); the occurrence percentages for each type of tuna school fished with dolphins are very low (table 6.15 and figure 6.27). These numbers are in agreement with those of Levenez et al., (1980) for the period 1976–1979. According to Stretta and Slepoukha (1986a), dolphins do not have significant influence on the success of a set when they are associated with mixed schools, however, the average catch per set increases appreciably (table 6.15 and figure 6.29). It is not the same for fishing of “skipjack” schools where the rate of null sets becomes two times larger than for a free school; the average catch per set reaches 4.4 t (versus 11.8 t for a free school). For “yellowfin” schools, the rate of null sets increases also (table 6.15 and figures 6.28 and 6.30), while the catch per set remains about the same.

Catches by size class of sets presents for pure “yellowfin” and mixed schools, a non negligible proportion (2.5% and 10.0% respectively) of sets with a tonnage over 100 tonnes (figures 6.28, 6.29 and 6.30). In these sets, only yellowfin with a weight equal to or over 30 kg are represented.

Tuna fishing associated with dolphins is very unequally distributed. In the “Senegal” zone (figure 7.1), nearly 5% of the fishing is made with this type of association, while in the “Cape Lopez” and “Equator” region, less than 1% of the fishing is made with this type of association (table 6.16 and figure 6.31). All these results are to be considered with care, from the fact of the restrained number of observations of sets associated with dolphins.

Table 6.16 Total number of sets (positive and negative) and percentage of occurrence on free schools and schools associated with natural FADs and with marine animals in the different hydrological regions in the Atlantic.
First line: Number of sets.
Second line: Percentage of occurrence.

(c) Association with “whales”

The term “whale” employed by fishermen, is a generic term and includes baleen whales, sperm whales and more rarely killer whales. As with tuna fishing associated with whale sharks, globally, around 8% of the tuna schools of the sample are fished in association with whales (table 6.14 and figure 6.26). The percentages of occurrence of this type of association show that the “yellowfin” schools are two times more frequent than the “skipjack” schools (table 6.15 and figure 6.27).

In the presence of a whale, the null set rate is near that of a free school for “skipjack” fishing, while it is different for mixed and “yellowfin” school fishing (Stretta and Slepoukha, 1986a). For the latter type of school, the rate of null sets is double in the presence of a whale (table 6.15). On the other hand, catches per set are slightly different for the three types of schools, whether they are free or associated with a “whale” (figures 6.28, 6.29 and 6.30).

In the “Senegal” zone (figure 7.1) nearly 15% of tuna fishing in the sample is associated with whales, while in the other hydrological zones, this type of association represents only 7 to 9% of the sets (table 6.16 and figure 6.31). Note however that the catches in this sector are poorly represented in the sample.

Figure 6.28 Pure schools of skipjack. Catch per set by type of association.

Figure 6.29 Mixed schools of yellowfin and skipjack. Catch per set by type of association.

Figure 6.30 Pure schools of yellowfin. Catch per set by type of association.

Figure 6.31 Percentage of occurrence of different school types by hydrographic zone.

Figure 6.32 a) locations of flotsam in the eastern tropical Atlantic (in numbers). b) ratio of fishing with FAD/fishing total by zone of 2° latitude by 2° longitude (in percentage).

6.1.7.5. Tuna and natural debris

(a) Association with naturel fish aggregation devices

Percentages of tuna fishing associated with fish aggregation gears (FADs) in relation to total fishing by zone of 2° in latitude by 5° in longitude are presented in figures 6.32 a and b. There is a preponderance of purse seine sets on natural FADs in the zones between 5° E and 10° E and between 2° N and 4° S. This is linked to the fact that in these regions, the Congo river and numerous Gabonese rivers discharge in the south. In figures 6.32 a and b, one notes as well that in the zones off Liberia, the proportion of fishing on natural FADs is over 40% near the coast. One can also find that in the offshore areas in the “Equator” zone (figure 7.1), and in the “Senegal” region bordered by sahelian zones, the frequency of natural FADs is low (table 6.16 and figure 6.32).

In the eastern tropical Atlantic, nearly 20% of purse seine sets (table 6.14 and figure 6.26) and tonnages caught that constitute the sample, have been carried out as a result of natural FADs (Bard et al., 1985; Stretta and Slepoukha, 1986a). “Yellowfin” and “skipjack” do not associate in the same way under or in the proximity of a FAD. The bond with the floating object appears much stronger for “skipjack” (associated in mixed schools with small yellowfin and small bigeye) than for “yellowfin” (table 6.15). This group of small tuna (from 1.5 to 10 kg) is most often found in association with FADs. Furthermore, in the zones currently exploited in the Indian Ocean by purse seiners, the association of large yellowfin with FADs seems more frequent (Marsac, personal communication). For the Atlantic, in analyzing catches for average tonnages by positive set, for “skipjack” schools, the average catch by set in the presence of a FAD is 18 tonnes, compared to less than 12 tonnes per set without FAD. For mixed schools of yellowfin and skipjack, the average catch by set is very close to 25 tonnes, whether the schools are free or associated with a FAD. For yellowfin schools, the average catch by set is 8.7 tonnes for a school associated with a FAD and 30.5 tonnes for a school without FAD.

Figure 6.33 Sequences of tuna fishing associated with a whale carcass for four different tuna purse seiners.

Moreover, the natural FADs are actively sought by the tuna fishing masters for the low rate of null sets obtained because of them. In table 6.15 we present the different rates of null sets made in the presence of a FAD as a function of the three types of schools. For a school of “skipjack”, the null set rate in the sample, under 6%, is statistically lower than that of 30% observed for free schools. On the other hand for yellowfin schools, the null set rate is not statistically different from that of free schools. Finally for mixed schools the null set rate is under 1% is very much lower than the 14% observed for free schools. The low rate of null sets present in the sample could in fact be due to result a statistical bias. After discussion with the tuna fishing masters, the latter estimate the null set rate for a mixed school associated with a FAD, not at 0.6%, but at around 3%.

In regards to catches by size class of the set, for yellowfin schools, there is a peak that corresponds to sets from 1 to 10 tonnes (figure 6.30).

(b) Association with “carcasses”

A particularly spectacular type of fishing is tuna fishing in the presence of a whale or sperm whale corpse, or “carcass” drifting on the surface. Encountering a carcass is rare: less than 1% of catches happens with this type of association (table 6.14 and figure 6.26). We will present in table 6.15 and figure 6.27 the occurrence percentages of this type of association for the different types of schools.

For a matter of interest, we will mention the fact that in the presence of a “carcass”, the null set rate for different types of tuna schools is close or equal to zero (table 6.15). The largest catches per set by the FIS fleet have been on carcasses and catches per set of more than 200 tonnes are on record. As for catches by size class of sets, for “skipjack” schools, more than 70% of sets, are of an average tonnage over 20 tonnes (figure 6.28).

In addition to a low rate of null sets and large catch per set, the tuna boat will stay generally near “its carcass” and will set its purse seine every morning for several days until the decomposition of the carcass is complete. This type of fishing on a “carcass” demonstrates a fundamental point: the size of schools fished successively decrease sharply with the sequence of purse seine sets (figure 6.33). According to Bard et al., (1985), the fact that debris concentrates or does not concentrate tuna, depends only on the probability of encounter between the debris and a school passing the area. Debris “exhausts”, in some way, schools present in its neighborhood. These authors pose the question whether a floating object possesses “radius of action” that, as a function of density and size of tuna schools in the zone, would determine the actual power of attraction of the object in question.

(c) Association with the tuna boat

Another form of particularly spectacular fishing is that where the tuna boat plays the role of a FAD. It sometimes happens at sunrise, at the time where the tuna boat prepares to fish, that a tuna school is in close proximity to the hull of the vessel. This type of association is relatively rare; from our file, around 3% of the sets have been carried out in this fashion (table 6.14 and figure 6.26). However this type of association significantly favors a low rate of null sets (relative to a free school) for schools of “skipjack” and for mixed schools; for schools of “yellowfin”, the null set rate is the lowest (table 6.15), but is not statistically different from that for a free school. Average catches per set (table 6.15) and catches by size class of sets, the tuna boat acts as a natural FAD (figures 6.28, 6.29 and 6.30).

This form of fishing is exploited by pole and line boats that operate in the Dakar region. It very frequently happens that a pole and line boat fishes several subsequent days on the same tuna school “stuck” to its hull. While its holds are full, the pole and line boat “gives” his school to another pole and line boat, which will in turn fish the school to the end of its cruise. A single school of tuna can be exploited continually for several months by two relaying boats, the phenomenon of tuna aggregation under a boat concentrating more individuals than are captured.

6.1.7.6. Conclusions

The chapter shows the frequency and importance of associations between tuna schools and various objects and/or marine animals for tuna fishing in the region.

Several questions remain:

• why are tuna attracted to inert or living floating bodies?

• are there “good debris” and what are the causes of their existence?

• is there a relation between the size of a FAD and the quantity of fish caught?

• can the placing of artificial debris networks, floating or moored at great depths, increase the efficiency of tuna fleets in certain regions?

• what is the impact of the existence of this debris on tuna boat exploration strategies and therefore on effective fishing efforts of purse seiners?

Only the operation of new research oriented specifically on these themes can provide answers to these questions which remain unfortunately neglected in the research habitually requested for by ICCAT.

6.1.8. Parasites of major tuna (yellowfin, skipjack and bigeye)

6.1.8.1. Introduction

The description of the parasitic infestation of tuna is often used as a criteria for differentiation of stocks or groups of fish (Mackenzie, 1983); we will return in another chapter to this aspect of its use in the Atlantic Ocean (Baudin-Laurencin, 1974; Lardeux, 1980).

Nor will we give in this purely descriptive chapter an exhaustive list of parasites of the three principal species of tuna (yellowfin, skipjack and bigeye) captured in the eastern tropical Atlantic. A specific list concerning a restrained geographical zone would be extremely questionable for the following reasons:

• the systematics of the very numerous parasite species liable to infest tuna is still imperfect and the identification criteria of the species are variable from one author to the next; the same species of parasite found in different places may be noted under varied names and appear several times in this list.

• the amount of research work concerning yellowfin, skipjack and bigeye parasites in the Gulf of Guinea is very unequal for each one of these species and often limited; it would be necessary to borrow on works not concerning this zone, that is from descriptions made on tuna from other oceans (Pacific or Indian) so that this list is as exhaustive as possible.

As well as systematic uncertainties already mentioned, this borrowing amounts to an acknowledgment of extensive cosmopolitanism of parasite species; this supposed cosmopolitanism would be an apparently poorly justified hypothesis, since numerous works tend to postulate, on the contrary, the existence of parasites typical to certain regions (Baudin-Laurencin, 1974; Lardeux, 1980; Mackenzie, 1983; Lester et al., 1985) and to use these parasites as biological markers.

Regarding the designation of different parasites observed on yellowfin, skipjack or bigeye we will again refer the reader to specialized works on the subject: Silas, 1967; Silas and Ummerkutty, 1967; Bane, 1969; Baudin-Laurencin, 1971, 1972 and 1974; Bussieras, 1972; Watertor, 1973; Lardeux, 1980 …).

We will mention here only some very general aspects of the knowledge of the parasitic infestations of the three species as well as certain particularly widespread or easily observed parasites.

6.1.8.2. Yellowfin and bigeye

Works concerning parasitic infestation of these two species, specifically pertaining to the eastern tropical zones of the Atlantic, are relatively numerous compared to that which exists on skipjack in the Atlantic in general.

The first note on eastern tropical Atlantic yellowfin and bigeye parasites covers the period to 1962 (Rossignol and Repelin, 1962 and 1962a). Later, several documents describe parasitic infestations due to very distinct species of parasites (Bussieras and Aldrin, 1965 and 1967; Bussieras and Baudin Laurencin, 1970; Bussieras, 1972; Baudin-Laurencin, 1972 …). It is only from 1969 that a certain number of authors tried to review all of the known parasites infesting yellowfin on the coasts of West Africa (Bane, 1969; Baudin-Laurencin, 1971; Bussieras and Bausin-Laurencin, 1973; Baudin-Laurencin, 1974; Lardeux, 1980).

It is concluded from these works that yellowfin and bigeye parasites are often common to both species, with a well established exception that we will see further on. In total, about fifty parasite species have been more or less clearly identified. These species are either crustaceans (copepods), or by a great majority worms or helminths: monogenic and digenic trematodes, cestodes, menatodes, acanthocephalans. Numerous species are still poorly identified in particular because they have only been observed at larval or immature stages and also because of numerous uncertainties that still exist as for the systematics of these species in general.

All body parts or viscera are susceptible to parasites. The rate of infestation seems to increase with size of individuals, even though practically all species of parasites known in adults have also been observed on young yellowfin or bigeye (Baudin-Laurencin, 1971).

Although the hypothesis that the abundance of certain parasite species is characteristic of well defined zones is only imperfectly demonstrated, it seems established that the presence of the worm parasite Nasicola Klawei (monogenic capsalid) in the nasal fossa is very specific to yellowfin. This parasite and its potential utilization to distinguish young yellowfin from young bigeye was noted for the first time by Rossignol and Repelin (1962 and 1962a). At times described under the names Tristoma sp. (Rossignol and Repelin, op. cit.) and Caballerocotyla klawei (Bussieras and Aldrin, 1967), its designation as Nasicola klawei seems the only valid name according to Baudin-Laurencin, (pers. com.) and the works of Yamaguti (1968). This parasite is generally observable in pairs on mucous membranes in the nasal capsule of practically all yellowfin (from the smallest to the largest caught) and absent in those of bigeye. This explains how the presence of this parasite is utilized as a specific differentiation criterion between yellowfin and bigeye in certain determination keys of these species (Blache et al., 1970).

For other parasites often observed by fishermen, we will point out the copepod crustaceans of the genus Pennella and more precisely P. crassicornis identified by Baudin-Laurencin (1971). This parasite, situated toward the exterior in the dorsal muscles of yellowfin (and perhaps also in bigeye), has the appearance of a cylindrical tube, several centimeters in length and a few millimeters in diameter, which leads an uninformed observer to confuse it with a tag placed by scientists.

Finally we will point out that the gelatinous aspect taken on by the flesh of yellowfin has sometimes been attributed to the action of protozooan parasites (myxosporidian) of the genus Chloromyxum. However bacteria can sometimes be the source of this phenomenon (Baudin-Laurencin, pers. com.).

6.1.8.3. Skipjack

Research works on parasitic infestation of Atlantic skipjack in general are extremely rare, and all the more so on skipjack from the zone involved in this work (eastern tropical Atlantic). Also the observations that have been made are limited to well defined genera or families of parasites (Simmons, 1969) or which only partially involve skipjack. It is therefore inevitable to turn to works concerning the Pacific and Indian Oceans (Silas, 1967; Silas and Ummerkutty, 1967; Yamaguti, 1968 and 1970; Chen and Yang, 1973; Cressey and Cressey, 1980) in order to have an overall survey of species liable to infest skipjack in these oceans and by extension in the Atlantic ocean.

As in the case of yellowfin and bigeye, the number of species known as skipjack parasites is about 50; some of these species are also parasites on yellowfin and other tuna. These parasites belong to families already mentioned for yellowfin and bigeye, that is: crustaceans (copepods), helminths (trematodes, cestodes, nematodes, acanthocephalans) and can be observed on or in all skipjack body parts and viscera.

6.1.8.4. Conclusion

As well as numerous uncertainties in the systematics of parasites themselves, numerous points remain to be cleared up regarding the inventory of parasitic fauna of tuna and the effect of parasites on the biology (behavior, reproduction, growth, mortality …) of the different infested species. It seems that in certain conditions some tuna parasites are susceptible to infesting man, especially in the case of raw consumption, but these cases seem very rare.

6.1.9. Mixtures of species within tuna schools

6.1.9.1. Introduction

Analysis of log books reveals that mixtures of species are very frequent in schools exploited by surface tuna fisheries (pole and line boats and purse seiners, paragraph 6.1.7). Samples taken during landing show that information noted by fishing masters in their log books scarcely permits the mixture of these species in these cruises to be quantified; small yellowfin and bigeye are very often declared as skipjack.

The problem of the specific composition of schools is nevertheless important for matter of management because regulations on one species can influence other tuna species present and exploited in the same schools. The only reliable data for studying the specific composition of schools are those collected by scientists on board fishing boats as observers. This was done on two fleets: pole and line boats based at Tema, and French and Spanish purse seiners.

The first observations were published by Kume (1986); the second, unpublished to this date, correspond to all scientific observations made on board French and Spanish purse seiners from 1981 to 1983, principally in the context of the skipjack year.

Figure 6.34 Fishing zones sampled in 1980 and 1981 by Japanese pole and line vessels with observers aboard (after Kume, 1986).

Figure 6.35 Size-frequency distributions of yellowfin, skipjack and bigeye captured by Japanese pole and line vessels with observers aboard (after Kume, 1986).

Figure 6.36 Specific composition of the catches recorded by scientific observers aboard a) Japanese pole and line boats in 1980; b) Japanese pole and line boats in 1981; c) FIS and Spanish purse seiners from 1980 to 1982.

6.1.9.2. Pole and line boats of Tema

Results concerning this fleet (Kume, 1986) have been obtained from sampling the zone situated on both sides of the Greenwich meridian and between the equator and the African coast (figure 6.34). Tuna caught by pole and line boats of Tema are almost exclusively small tuna with a length under 60 cm (figure 6.35). The graphic representation of the frequency of exploited schools according to their specific composition (figure 6.36) shows that there is some variability in these frequencies with a dominance of plurispecific schools. Mixed schools including the three species are on the average more frequent (61%); next are mixed schools of skipjack and yellowfin (25%), then pure schools of skipjack (average 11.6%); schools having other compositions are rare (less than 3%).

6.1.9.3. Purse seiners

The only detailed information on specific composition of purse seine sets is furnished by scientific observers on French, Côte d'Ivoire and Spanish purse seiners from 1980 – 1983. The specific composition of a sample of 244 positive purse seine sets (including rejects) is known in detail. This sample corresponds to catch of nearly 4000 tonnes of tuna (45.1% yellowfin, 46.1% skipjack, 2.3% bigeye, 5.8% diverse) where the catch locations are indicated in figure 6.37. The summarized outcome of these observations (table 6.17) shows the high proportion of schools containing only large yellowfin (25% of the sample) or only skipjack (25%). Pure schools of small yellowfin (5.5%), of bigeye (only one school observed), or albacore (three schools observed) are, on the other hand rare; those containing various species in a mixture are more frequent, 41.3% of the observations.

Figure 6.37 Geographic distribution of purse seine sets sampled by observers aboard FIS and Spanish purse seiners from 1980 to 1982.

Table 6.17 Frequency of the types of schools observed by purse seiners.

6.1.9.4. Discussion of results

It seems there is a certain analogy between observations made on pole and line boats and purse seiners: the frequency of mixed schools is large for the two gears. If one considers the same size range of fish caught by pole and line boats and by purse seiners (excluding pure sets of large yellowfin carried out only by purse seiners), 89% of schools exploited by pole and line boats and 57.4% of those exploited by purse seiners in the sample are a mixture of various species. Pure skipjack schools are more frequent in the purse seine sample (33% excluding large yellowfin) than in that for pole and line boats (11.6%). Mixed schools comprising the three species are, on the other hand, more rare for purse seiners (12.7% of the observations excluding large yellowfin) than for pole and line boats (61% of the observations).

These differences in the proportions of school types can be interpreted in various ways:

• first of all the fishing zones of two gears are very different (figure 6.34 and 6.37).

• furthermore, specific compositions deduced by analysis of catches by pole and line boats or purse seiners, do not necessarily correspond to that actually existing in schools in the ocean; the selectivity of each fishing gear can come into play and cause a biased image of the actual specific composition of the schools.

• finally, the possibility cannot be excluded that the observers placed on purse seiners have at times poorly identified small bigeye by confusing them with yellowfin; the low percentage of bigeye (2.3%) noted by observers is well under that observed in landings (7.0% in 1981). This difference is all the more surprising since the areas fished during the presence of observers are on average those where bigeye are more abundant (see figure 4.12). Under this hypothesis of erroneous specific identification of bigeye, it is necessary to reconsider the present analysis taking into account the fact that small bigeye may be included under the name “small yellowfin”.

6.2. Small tuna and related species

6.2.1. Introduction

In the eastern tropical Atlantic, the biology of small tuna and related species is studied very little. This is due to the low importance generally accorded to these species and to difficulties in sampling landings of artisanal fisheries, the principal fisheries exploiting these species.

For the most of these species, the conclusions obtained come mainly from the work of Postel (1950). However, various aspects relative to the biology (reproduction, growth), could be complete notably for spotted tunny and to lesser degrees for the other species.

We will present here the conclusions of studies focusing on different components of reproduction, growth and ecology of each one of these species in the zone. However it appears that even in certain points where the analyses are the most advanced, major research remains to be done on these species which are more and more exploited in the region.

6.2.2. Spotted tunny (Euthynnus alleteratus)

6.2.2.1. Reproduction

6.2.2.1.1. Sexuality

The sexes are separated in the spotted tunny, male gonads have a higher relative weight than female gonads. Postel (1955), Diouf (1980).

6.2.2.1.2. Gonad maturation

Frade and Postel (1955) have made sections of the genital gland of spotted tunny of different sizes fished at Cape Verde. They note that spermatogenesis is very active in March in the males. It becomes more widespread from June to September. Sexual products are not released at a single time; spawning is fractional. The process of female gonad maturation develops with the increase in size of the ovocytes (figure 6.38); maturation extends from June to August.

Figure 6.38 Phases of ovocyte maturation in spotted tunny (Euthynnus alletteratus).

6.2.2.1.3. Size at first maturity

Size at first maturity of spotted tunny has often been defined as size of the smallest individual observed in spawning condition (Postel, 1955; Marchal, 1963), and also as the size at which 50% of individuals are capable of reproduction (Diouf, 1980). The results obtained in the eastern tropical Atlantic and in the Gulf of Guinea are presented in the table below. In this region the size at first maturity is around 42 cm for males and females.

6.2.2.1.4. Reproduction periods and zones

In Senegal, Diouf (1980), from GSR variations, fixes the reproduction period from May to November with two maxima: in June and September-October. The spawning period of young females (May-November) was more spread out than that of older individuals (fig. 6.39). In the Gulf of Guinea, reproduction extends from October to June (Marchal 1963; Kazanova. 1960; Alekseev and Alekseeva, 1979; Rudomiotkina, 1985). Frade (1955) and Da Costa and Frade (1958) find that the spotted tunny of Sao Thomé and Principe reproduce from October to December. In the south of the Gulf of Guinea, observations are still very fragmentary. However in the Congo and Angola, reproduction is observed from January to June (Chur, 1977; Rudomiotkina, 1985).

Spotted tunny reproduce during the period of the year when waters are warm and salty and it appears from different conclusions that the spawning period seems to be longer as the warm season is more extended. Furthermore, spawning takes place in coastal zones in the entire adult distribution zone (Conand, 1968); Caverivière et al., 1976 and 1980).

Figure 6.39 Changes of mean monthly GSR in male and female spotted tunny.

6.2.2.1.5. Fecundity

Individual partial fecundity, defined as the number of ovocytes of the last mode present in the ovary just before a spawning, varies from 70,000 to 2,200,000 eggs in the size interval sampled (300 to 785mm) of spotted tunny caught in Senegal (Diouf, 1980). The number of egg releases in a year is still unknown, also the total individual fecundity is undetermined. The relations between partial fecundity, length of females, weight of females and weight of ovaries from a sample of 28 individuals fished in Senegal have been established by Diouf (1980) (figure 6.40) [6.39 ??].

6.2.2.1.6. Sex-ratio

The sex-ratio of spotted tunny caught in Senegal shows that globally (all sizes included) the number of males is equal to the number of females no matter which month (Postel, 1955; Diouf, 1980) (figure 6.41). From samples collected in the eastern tropical Atlantic, Chur (1973) has estimated that there are more males than females but there was probably some imprecision in the determination of sexes, especially in young individuals. The sex-ratio as a function of size shows that the proportion of females decreases with size (Postel, 1955; Diouf, 1980). Above 810 mm all individuals examined are males (Diouf, 1980), figure 6.42. There is no information for the Gulf of Guinea.

6.2.2.2. Growth

6.2.2.2.1. Growth in length

Postel (1955) studied growth from the analysis of size frequency distributions of 906 individuals caught in Senegal. Cayre and Diouf (1980, 1983) determined the age and growth of spotted tunny caught in the same zone from transverse sections of the first dorsal fin. Vialov et al., (1985) studied, with the same technique, the growth of spotted tunny caught in the eastern tropical Atlantic (figure 6.43); these different results are presented in table 6.18. They are very comparable for ages between 1 and 3 years but are very different from those of Postel (1955) notably beyond one year. These divergences are due to methods employed. The Petersen method applied by Postel on samples from the Senegalese zone do not give good results because of the poor representation of medium size individuals in his samples.

The growth rate currently estimated for spotted tunny is around 8 cm/yr from 1 to 5 years; after this age the rate is only 3 cm/yr (Cayre and Diouf, 1983).

6.2.2.2.2. Longevity

The largest specimen measured in the eastern tropical Atlantic is 960 mm in fork length (Diouf, 1980). In the same zone, Cayre and Diouf (1983) have shown that an individual of 802 mm would be 8 years. The longevity of spotted tunny could be estimated at around ten years.

Figure 6.40 Spotted tunny (Euthynnus alletteratus) relationships: a) fecundity (F) - fork length (FL); b) fecundity (F) - body weight (P); c) fecundity (F) - ovary weight (Po).

6.2.2.3. Grouping in schools

Spotted tunny come together in schools of an elliptical form up to 30 meters in length along the main axis (Marchal, 1963) or form concentrations of more than 100 m in diameter (Chur, 1975). These schools can be pure or mixed; in the zone situated off Senegal, spotted tunny are often associated with Atlantic bonito in the cold season and with auxids and Spanish mackerel in the warm season (Diouf, 1980). The plurispecific schools of small tuna (yellowfin, skipjack, bigeye) of the coastal zone of the Gulf of Guinea contain spotted tunny, while in the exterior zone of the gulf and in the high seas, spotted tunny is absent. No precise analysis of school size has been made, but from purse seine sets in Senegal and from Marchal (1963), it seems that the larger the individuals in the school, the smaller the size of the school.

Figure 6.41 Monthly variations of sex ration in tunny.

Figure 6.42 Variation of percentage females as a function of size in spotted tunny.

6.2.2.4. Nutrition and feeding behavior

Spotted tunny have very varied diet and target in all prey presented to them, although with a certain preference for pelagic species (Postel, 1955; Marchal, 1963; Klawe, 1961; Diouf, 1980). However, it seems that the diet varies depending on the size of the spotted tunny. Chur (1975), Bullis (1967), Wicklund (1968) described techniques of approach and attack of prey for spotted tunny. They note that feeding is done during the day.

6.2.2.5. Predators

Predators of spotted tunny are sharks and large pelagics (large tuna, marlins and sail fish …). Larval and juvenile forms are found in stomachs of large yellowfin (Postel, 1954; Zavala Camin and Seckendorf, 1979), and skipjack (Klawe, 1961). Cases of cannibalism are also noted by Postel (1954) and Klawe (1961).

6.2.2.6. Parasites

Postel (1954) notes that spotted tunny has various copepods and trematodes as parasites localized in the body cavity and gills. He notes that it is the most parasitized species among tuna he has examined. Apart from these old and very incomplete observations, no detailed study of parasitic infestation of Atlantic spotted tunny has been made.

Figure 6.43 Growth curve of spotted tunny (Euthynnus alletteratus) obtained by different authors and methods (cf. text).

6.2.2.7. Physiology and responses to stimuli

Carey et al., (1974) have shown that tuna, thus spotted tunny, possess a system permitting them to conserve metabolic heat which explains why the core body temperature is higher than that of the surrounding water (cf. 6.1.6.). However, Sharp and Dizon (1978) have shown that the heat system is less developed in spotted tunny than in other large tropical tuna (yellowfin, skipjack and bigeye: paragraph 6.1.6.).

Spotted tunny shows positive phototaxis for moderate intensities and negative phototaxis for stronger intensities. The species reacts also to complex sounds of low frequency, this phenomenon could have effects on the formation of schools and the attraction of mates? (Roux, 1960; Bercy, 1985).

6.2.2.8. Association with flotsam

Wickham et al., (1973) have shown that spotted tunny are not attracted to flotsam on the sea but rather by the presence of organisms that they feed on that are attracted to this flotsam. Groupings of spotted tunny are observed around wrecks of boats situated several miles from the coast, off Dakar (Diouf, 1985).

6.2.3. Auxids (Auxis sp.)

6.2.3.1. Reproduction

6.2.3.1.1. Sexuality

As in all scombrids, the sexes are separate and there are no external characters that distinguishes males from females.

6.2.3.1.2. Gonad maturation

In Senegal, Postel (1950) and Frade and Postel (1954) have shown, from the study of maturity indexes (GSR) and gonad sections, that in December and April, the individuals examined are immature or in a sexual resting stage. In June, the spermiducts are filled with spermatozoids. They estimate that maturation occurs when the gonad weight reaches 2% of the total body weight.

6.2.3.1.3. Size at first maturity

Konstantinova and Chur (1976) have found in Eastern Atlantic that size at first maturity of A. thazard is 30 cm; for A. rochei, it is 20 cm according to Chur (1977).

6.2.3.1.4. Reproduction Periods and zones

In the Gulf of Guinea, Caveriviere et al., (1976, 1980), Alekseev et al., (1980), Rudumiotkina (1984) have shown that the intensity of reproduction of Auxis sp. is maximum during austral summer (May-June). It lasts from April to September in Sierra Leone, from September to March in the Congo and Angola. Frade and Postel (1955), Conand (1970) note that the spawning period in Senegal is from June to November. Spawning takes place in warm waters (t > 24° C) throughout the species range, near the coasts, on the edge and above the continental shelf.

6.2.3.1.5. Fecundity

Relative information on fecundity in the Eastern Atlantic comes from Russian authors (Chur, 1977; Konstantinova and Chur, 1976; Vyalov et al., 1979). They estimate that Auxis sp. has an average fecundity of some 600,000 eggs. Furthermore, Silas (1969) and Collette and Naunen (1983) found that it Auxis sp. can spawn more than a million eggs per year, but A. thazard was more fertile than A. rochei. Auxis spp. appear to be very fertile species which could explain the observations where auxid larvae were more abundant than all other scombridae larvae together (Chapman, 1960, Williams, 1960).

6.2.3.1.6. Sex-ratio

Sex-ratio has been studied very little in the eastern tropical Atlantic and in the Gulf of Guinea. Postel (1950) has estimated that in Senegal the sex-ratio is around 1.

6.2.3.2. Age and Growth

6.2.3.2.1. Growth in length

The growth of A. thazard has been studied in the equatorial Atlantic by Grudtsev and Korolevich (1985) from sections of fin rays of the first dorsal fin. They established an age-length correspondence table based on reading sections of spines and based on the Von Bertalanffy equation calculated on data from readings back calculated from these same spines.

The growth curve equation is:

L_t = 51.47(1 - e^{-0.32(t-0.83)})

Chur (1977) estimated that A. rochei shows slower growth than that of A. thazard. At one year, A. rochei reached a size of 17 cm. (Hotta, 1955).

6.2.3.2.2. Longevity

Morice (1953) notes that the largest individual measured in the eastern tropical Atlantic is 65 cm but individuals fished rarely exceed 50 cm. However, historical data collected on purse seiners operating in the zone show that a large range of sizes were exploited, from 35 to 60 cm in fork length. These observations linked to results on growth suggest that Auxis sp. may live around 5 years.

6.2.3.3. Schooling

Auxids group in monospecific schools composed of individuals of the same size. These schools are made up of 100 to 300 individuals according to Williams (1960) or more than 1,000 individuals according to Wheeler and Ommaney (1959). The schools can also be mixed, associated with other tuna of the same size. In the coastal zone, auxids are often associated with spotted tunny while in the high seas zones, they tend to form mixed schools with skipjack, yellowfin and juvenile bigeye (Stretta, pers. com.). Small fish from surface schools and move more rapidly than large fish (Morita, 1972).

6.2.3.4. Nutrition and feeding behavior

Very little data are available of the quality and quantity of food for auxids in the region. Postel (1950) found anchovies in the stomachs of Auxis sp. examined. Elsewhere, numerous observations show that Auxis sp. feed preferentially on small pelagics and also crustaceans, mollusks and cephalopods. Kumaran (1964), Uchihashi (1953) have shown that A. rochei find its food not only by sight but also by using its lateral line.

6.2.3.5. Predators

Studies done mainly in the Pacific, have shows that Auxis sp. at different stages of its development represent a large fraction of food for large tuna (Olson, 1982). In the eastern tropical Atlantic, Postel (1955) found auxids in yellowfin stomachs and spotted tunny caught off Dakar. In eastern Africa, Williams (1960) found specimens of A. thazard in marlin stomachs. Collette and Nauen (1983) point out frequent cases of cannibalism. Klawe (1963) notes that the sizes of auxids found in tuna stomachs varies from 60 to 125 mm and can reach more than 320 mm in those of large predators (Watanabe, 1964).

6.2.3.9. Natural mortality

No data is available in the eastern Atlantic. However, taking into account the large numbers of larvae found and the apparent abundance of adults in the zone, one can estimate that the natural mortality of larvae must be high, all the more so since several studies have shown that larvae and juveniles of Auxis are frequent prey for large pelagic predators.

6.2.4. Atlantic bonito (Sarda sarda)

6.2.4.1. Reproduction

6.2.4.1.1. Sexuality

In Atlantic bonito, the sexes are separate. However, a case of hermaphrodism has been observed in this species in the Mediterranean (Rey pers. com.).

6.2.4.1.2. Gonad maturation

Frade and Postel (1955) have shown from histological sections of gonads of individuals caught in Senegal, that spermatogenesis in males, widespread from February to April, is still active in June, the spermiducts being, for the most part, full of spermatozoids. In the females during spawning in February-March, the eggs show a filling in of empty follicles and a reabsorption of numerous atresic eggs of which only remains are found in April. Spawning is fractional and the interval of time between successive releases is still unknown. For Rey et al., (1983), there were two releases of eggs per year in the north-east Atlantic.

6.2.4.1.3. Size at first sexual maturity

Size at first maturity, defined as the smallest specimen observed in a spawning state, is 392 mm for males and 370 for females (Postel, 1955).

6.2.4.1.4. Reproduction periods and zones

Postel (1955) and Frade and Postel (1955) have shown, from maturity index change (GSR) and histological sections of Atlantic bonito ovaries, that the reproduction period in Senegal is from December to May; it is more active in January and April. Reproduction is earlier (November to May), and therefore longer, for older individuals. There is no available information For the Gulf of Guinea. Spawning takes place generally near the coast.

6.2.4.1.5. Fecundity

The fecundity of Sarda sarda has been studied very little in the zone. Postel (1955) estimated that it is 900,000 eggs for an individual of 60 cm fished in Senegal. These results are very close to those found by Rey et al., (1983) for individuals of the same size in the north-east Atlantic.

6.2.4.1.6. Sex-ratio

From samples collected off Senegal, Postel (1955) finds that the number of males is identical to that of females without regard to size. There are no data for the Gulf of Guinea.

6.2.4.2. Growth

6.2.4.2.1. Growth in length

Postel (1955) obtained the following age-size relation from the analysis of size frequencies of 852 Atlantic bonito caught in Senegal:

The growth, extremely rapid during the first year, slows for fish having reached size at first maturity (40 cm FL).

Rey et al., (1983) found comparable results in the north-east Atlantic, from sections of bony parts; the equation of their growth curve is the following:

L_t = 80.87(1 - e^{-0.352(t+1.7)})

6.2.4.2.2. Longevity

The largest specimen found in the eastern tropical Atlantic was 761 mm (Fowler, 1936). Individuals over 660 mm are mentioned by several authors (Cadenat, 1950; Postel, 1955 and Diouf, 1980). The most recent studies indicate that the age corresponding to these maximum sizes observed is 5 years.

6.2.4.3. Schooling

Atlantic bonito form schools of variable dimensions. The more coastal schools are comprised of small individuals. The schools are mixed, composed of spotted tunny and auxid, or monospecifically formed of Atlantic bonito of the same size. Schooling around flotsam is observed in Senegal (Diouf, 1985). The schools can break up and original individuals from the same school and age class could be found in 2 different schools 600 miles apart (Rey et al., 1983).

6.2.4.4. Nutrition and feeding behavior

Postel (1954) analyzed the stomach contents of 588 Atlantic bonito caught in the Atlantic and concluded that clupeids and ammodytids constitute the food base for the species. Atlantic bonito searches for its food early in the morning and evening, generally in the coastal zone.

6.2.4.5. Predators

No study has been done in the study zone but large pelagic predators, notably tuna, are certainly predators of Atlantic bonito (Zavala Camin and Sleckendorf, 1979). Cases of cannibalism have been observed in the Black Sea (Berg et al., 1949).

6.2.4.6. Parasites

Postel (1954), from autopsies of 588 Atlantic bonito caught in Senegal, has shown that the plerocercus larvae of Callitetrarhyncus gracilis (Rudolphi) localised in the general cavity and adults of Livoneca sp. in the gills are the known parasites of this species.

6.2.4.9. Natural Mortality

No data in the eastern tropical Atlantic, but Rey et al., (1983) has estimated natural mortality at 1.32 in the north-east Atlantic from tagging data. These authors conclude however that more careful studies are required to better determine this value.