SCRS/2005/103 Col. Vol. Sci. Pap. ICCAT, 59(2): 579-589 (2006)
SOME BIOLOGICAL ASPECTS OF SMALL TUNAS (Euthynnus alletteratus, Sarda sarda & Auxis rochei) FROM THE SOUTH WESTERN SPANISH MEDITERRANEAN TRAPS
D. Macías, L. Lema, M.J. Gómez-Vives, J.M. Ortiz de Urbina & J.M. de la Serna 1
SUMMARY
Bullet tuna (Auxis rochei), Atlantic little tuna (Euthynnus alletteratus) and Atlantic bonito (Sarda sarda) are the most abundant small tuna species in the Mediterranean Sea. These species are commercially exploited in the Spanish coast by traditional fisheries. Nevertheless, the biological information about these species is currently scarce in Western Mediterranean. Size and weight data from a Spanish Mediterranean trap (la Azohía, Murcia) were collected during the three years period 2003-2005 in order to determine size-weight relationships and size distributions of the populations. Atlantic bonito collected ranges from 415 to 480 mm in fork length; bullet tuna from 334 to 470 mm; and Atlantic little tuna from 560 to 677 mm in fork length. Size distribution for bullet tuna shows a bimodal shape suggesting that the reproductive stock is mainly made up of two cohorts (two and three years old). Size distribution for Atlantic little tuna shows a multimodal shape suggesting that the reproductive stock is made up of different cohorts (age III and older). Atlantic bonito size distributions show a bimodal shape suggesting that the reproductive stock is mainly made up of two cohorts (one and two years old). These and other biological aspects will be discussed in this paper.
RÉSUMÉ
Le bonitou (Auxis rochei), la thonine (Euthynnus alletteratus) et la bonite à dos rayé (Sarda sarda) sont les espèces de petits thonidés les plus abondantes dans la mer Méditerranée. Ces espèces sont exploitées commercialement sur la côte espagnole par les pêcheries traditionnelles. On dispose néanmoins de peu d’informations sur la biologie de ces espèces dans la Méditerranée occidentale. Les données de taille et de poids d’une madrague méditerranéenne espagnole (la Azohía, Murcie) ont été recueillies au cours de la période triennale 2003-2005 afin de déterminer les rapports taille-poids et les distributions de taille des populations. La bonite à dos rayé échantillonnée mesure entre 415 et 480 mm de longueur à la fourche ; le bonitou entre 334 et 470 mm et la thonine entre 560 et 677 mm de longueur à la fourche. Pour le bonitou, la distribution des tailles montre une forme bimodale, ce qui laisse penser que le stock reproductif est principalement constitué de deux cohortes (âges 2 et trois). La distribution des tailles de la thonine signale une forme multimodale, laissant entendre que le stock reproductif est constitué de différentes cohortes (âges 3 et plus). La distribution des tailles de la bonite à dos rayé indique une forme bimodale, ce qui suggère que le stock reproductif est principalement constitué de deux cohortes (d’un an et de deux ans). Ces aspects, ainsi que d’autres, seront examinés dans le présent document.
RESUMEN
La melva (Auxis rochei), la bacoreta (Euthynnus alletteratus) y el bonito (Sarda sarda) son las especies de pequeños túnidos más abundantes en el mar Mediterráneo. Aunque estas especies son explotadas comercialmente en las aguas de la costa española por pesquerías tradicionales, su información reproductiva y biológica es escasa en el Mediterráneo occidental. Con el fin de definir las relaciones talla-peso y las distribuciones de tallas de las poblaciones, se recopilaron datos de talla y peso de una almadraba española del Mediterráneo (la Azohía, Murcia), durante un periodo de tres años (2003 a 2005). La talla de los ejemplares de bonito recogidos osciló entre 415 y 480 mm de longitud a la horquilla, la de la melva entre 334 y 470 mm y la de
1 Instituto Español de Oceanografía. C.O. de Málaga. Pto Pesquero s/n, 29640, Fuengirola, España. [email protected]
579 la bacoreta entre 560 y 677 mm de longitud a la horquilla. La distribución de tallas de la melva exhibe una forma bimodal que sugiere que el stock reproductivo está formado principalmente por dos cohortes (edades 2 y 3). La distribución de tallas para la bacoreta muestra una forma plurimodal que sugiere que el stock reproductivo está formado de diferentes cohortes (edad 3 y superiores). La distribución de tallas del bonito muestra una forma bimodal que sugiere que el stock reproductivo está formado principalmente por dos cohortes (edades 1 y 2). En este documento se discuten estos y otros aspectos biológicos.
KEYWORDS
Bullet tuna, Atlantic bonito, Atlantic little tuna, size-weight, size distribution, South Western Mediterranean.
1 Introduction
The genus Auxis distributes worldwide in tropical and subtropical waters. Bullet tuna (Auxis rochei) is an epi and meso-pelagic fish that choose a seasonal coastal distribution in temperate and tropical areas including the Mediterranean (Uchida, 1981; Collete, 1986)). This species is abundant in the Strait of Gibraltar, North coast of Africa and Spanish Mediterranean coast (Postel, 1973). The first maturity size has been stated in 35 cm (FL) when the fish is two years old (Rodriguez-Roda, 1983). Certain areas in the Mediterranean have been suggested as possible spawning sites of Auxis spp: Greece and Gulf of Catania (Bellot, 1954), Balearic Islands (Duclerc et al., 1974), and Tunisian and Algerian waters (Postel, 1964).
The Atlantic Bonito (Sarda sarda) inhabit temperate and tropical areas of both hemispheres of Atlantic Ocean, Black Sea and Mediterranean. It forms large mixed schools with other tuna species near the surface (Collete and Nauen, 1983; Anon ICCAT, 2003). The first maturity size has been stated in 38 cm (FL) when the fish is one year old (Rey et al., 1984; ICCAT, 2003). Several authors have observed that the species spawn in the Mediterranean although its spawning areas are better known in the eastern part of Mediterranean and Black Sea (Mayorova and Tkacheva, 1959; Demir, 1963).
The Atlantic little tuna (Euthynnus alletteratus) distributes in temperate and tropical areas of Atlantic Ocean, Mediterranean and Black Sea (Belloc, 1955). The size at first maturity is about 42 cm in Easter Tropical Atlantic (Anon, 2003).
Small tunas are abundant off Spanish Mediterranean coasts where has been commercially exploited by seasonal artisanal fisheries (Sabatés and Recasens, 2001; Demir, 1963). The species have been caught traditionally by Spanish seasonal coastal fisheries using several fishing gears as traps and other minor fixed gears, purse-seine and hand-line (Uchida, 1981; Rey et al., 1984). In spite of the importance to local economy of these fisheries, little is currently known about the biology of small tuna species.
The aim of this paper is to describe and discuss the results obtained about biological characteristics of small tuna caught off Spanish South Western Mediterranean coast.
2 Material and methods
A total of 458 bullet tunas, 217 Atlantic small tunas and 465 Atlantic bonitos were measured and weighed during three scientific surveys developed during the three years period 2003-2005, in “La Azohía” trap (Murcia) in the Spanish South Western Mediterranean coast. For each specimen, fork length (FL) to the nearest millimetre and round weight were measured.
These pairs of observations of length and round weight were used to fit length-weight relationships using a classical potential model. Weight = a Lengthb.
Equation parameters were calculated through the logarithmic transformation: Ln (weight) = Ln (a) + b Ln (length).
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3 Results and Discussion
3.1 Size distribution
Bullet tuna
Figure 1 shows the annual size distributions and the combined size distribution for the whole studied period. Descriptive statistics are summarized in Table 1.
All annual distributions showed a bimodal appearance. The highest modal value approximately corresponds to first maturity size: 35-26 cm (2 years old tunas). The second (minor) modal value was around 44 cm (3 years old specimens) except for last studied year where this modal value cannot be found (All sampled fishes were around two years old size). These results agree with previous papers about size at first maturity and also indicate that the bullet tuna reproductive stock is mainly compounded of age class II individuals. The second modal value could be compounded by 3 years old and older specimens.
Atlantic bonito
Figure 2 shows the annual size distributions and the total period distribution. Table 2 summarizes the descriptive statistics for each studied year and for the whole period.
All these size distributions showed a bimodal shape. The first and highest modal value was around 42-44 cm, and the second mode was around 55 cm. These modes correspond respectively to 1-2 and 2-3 age classes. The minimum size was 40 cm in fork length (age I) and the maximum was 61 cm (age IV-V). These data agree with those obtained by Rodriguez-Roda (1966) and Demir (1963). In conclusion, the reproductive stock of Atlantic bonito is mainly made up of 1-2 age class individuals. The second age class more abundant in catches was the 2- 3 one. There is no fish in the catches over the V year class.
Atlantic little tuna
Size distributions (annual and total) of Atlantic little tuna are summarized in Figure 3. Table 3 shows the annual total descriptive statistics for this species.
Size distribution of Euthynnus alletteratus shows multiple modal values. The first mode, around 62-63 cm, was the only found in 2003 sampling. In 2004 this year-class was found less abundant than the second one (around 71 cm in fork length). A third modal value can be found in 2004 around 74 cm and a fourth one around 79 cm. According previous papers (Rodriguez-Roda, 1979, Landau 1965) the first mode would correspond to 3 years old fishes. The second modal value in 2004 (71cm) would correspond to 4 years old individuals, the third mode to 5 years old (75 cm.) and the last one to 6 years old fishes. It must be taken into account that these measures were made using fork length and the previous one using total length. In 2005 the first mode (III year-class) was the most abundant in catches. The second mode corresponds to the older fishes around 79-81 cm (more than 6 years). These results allow us to conclude that the year-class that contributes most to reproductive stock is the III one. The oldest exemplars found in catches were over 7 years old.
3.2 Length-weight relationships Pairs of observations on length and round weight were used to obtain the following relationships:
Bullet tuna
Total weight = 5.5916E –3* FL 3.2923
Figure 4 shows the observed and expected values of bullet tuna weight. The regression statistics are summarized in Table 4. Residual analysis (Figure 5) shows some outliers that could be due to different condition factor or different reproductive situation of some individuals.
Atlantic bonito
Total weight = 9.4632E-3 * FL 3.1011
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The observed and expected values of Atlantic bonito length-weight pairs are showed in Figure 6. Regression statistics are summarized in Table 5. Residual analysis (Figure 7) indicates an optimal fit to the model assumptions.
Atlantic little tuna.
Total weight = 4.4098E-2 * FL 2.7549
The observed and expected values of Atlantic little tuna length-weight pairs are showed in Figure 8. The regression statistics are summarized in Table 6. Residual analysis (Figure 9) indicates an optimal fit to the model assumptions.
References
ICCAT. 2003. Executive Summary of Small Tunas report. Oct 2003. Madrid, Spain. ICCAT. Report for Biennial Period, 2002-03, Part I, 2003. (Vol. 2): 111-113. BELLOC, G. 1955. Les thons de la Méditerranée. Deuxième note: Thonine et Bonite. FAO Proc. Gen. Fish. Counc. Medit. nº 3 (52): 471-486. COLLETE, B.B. 1986. Scombridae. In: P.J.P. White head, M.-L. Bauchot, J.-C. Hureau, J. Nielsen, and E. Tortonese (eds.), Fishes of the North-eastern Atlantic and the Mediterranea. Vol II, pp. 981-997. UNESCO, Paris. COLLETE, B. B. and C.E. NAUEN. 1983. FAO species catalogue. Vol 2. Scombrids of the world. An annotated and illustrated catalogue of tunas, mackerels, bonitos and related species known to date. FAO Fish. Synop. , 125 (2). DEMIR, M. 1963. Synopsis of biological data on bonito, Sarda sarda (Bloch). FAO Fish. Rep., 6: 101-129. DUCLERC, J., J. Sacchi, C. Piccinetti, G. Piccinetti-Manfrin, A. Dicenta and J-M Barrois. 1974. Nouvelles données sur la reproduction du thon rouge, Thunnus thynnus (Linnaeus), et d’autres espèces de Thonidés en Méditerranée. Rev. Trav. Inst. Pêches maritimes, 37: 163-176. LANDAU, R. 1965. Determination of age and growth rate in Euthynnus alletteratus and E. affinis using vertebrae. Rapp. Proc.-Verb. Réun. Comm. Int. Explor. Sci. Mer Mediterr. XVIII (2): 241-244. MAYOROVA, A. and K.S. Tkacheva. 1959. Distribution and conditions of reproduction of pelamid, Sarda sarda (Bloch), in the Black Sea according to data for the period 1956-1957. Proc. Tech. Pap. GFCM, 5: 509-514. POSTEL, E. 1964. Les thonidés d’Afrique du Nord. Proc. Symp. Scombroid Fishes, Part I. Mar. Biol. Assoc. India, Symp. Ser. 1: 211-220. POSTEL, E. 1973. Thunnidae. Check-list of the fishes of the north-eastern Atlantic and of the Mediterranean. Clofnam I. Editors: J.C. Hureau and Th. Monod, Paris. UNESCO. REY, J. C., E. Alot and A. Ramos. 1984. Sinopsis biológica del bonito, Sarda sarda (Bloch), del Mediterráneo y Atlántico Este. Col. Vol. Sci. Pap. ICCAT, 20(2): 469-502. RODRÍGUEZ-RODA J. 1966. Estudio de la bacoreta, Euthynnus alletteratus (Raf.), bonito, Sarda sarda (Bloch) y melva Auxis thazard (Lac.), capturados por las almadrabas españolas. Invest. Pesq. 30: 247-92. RODRÍGUEZ-RODA J. 1979. Edad y crecimiento de la bacoreta, Euthynnus alletteratus (Raf.) de la costa sudatlántica de España. Invest. Pesq. 47 (3): 397-402. RODRÍGUEZ-RODA J. 1983. Edad y crecimiento de la melva, Auxis rochei (Risso), del Sur de España. Invest. Pesq. 43 (3): 591-599. UCHIDA, R.N. 1981. Synopsis of biological data on Frigate Tuna, Auxis thazard, and Bullet Tuna, Auxis rochei. FAO Fish. Synop., 124.
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Table 1. Descriptive statistics of length and weight of bullet tuna. Length in cm. and weight in gr. 2003 2004 2005 Total BLT Length Weight Length Weight Length Weight Length Weight Mean 38.1 950.5 37.3 876.0 35.7 786.2 37.3 886.3 Standard Error 0.3 24.5 0.3 24.3 0.4 31.1 0.2 17.4 Median 37.1 860.0 36.0 740.0 35.5 744.0 36.2 780.0 Mode 36.8 760.0 36.0 720.0 33.4 604.0 36.0 640.0 Standard Deviation 3.5 334.7 4.0 374.5 2.3 178.6 3.7 348.6 Variance 12.3 112028.9 15.8 140237.6 5.4 31896.8 14.0 121545.2 Range 13.6 1640.0 20.1 1520.0 8.0 596.0 21.1 1640.0 Minimum 33.4 180.0 25.9 280.0 31.5 522.0 25.9 180.0 Maximum 47.0 1820.0 46.0 1800.0 39.5 1118.0 47.0 1820.0 n 186.0 186.0 237.0 237.0 33.0 33.0 456.0 456.0 Confidence Interval (95.0%) 0.5 48.4 0.5 47.9 0.8 63.3 0.4 34.2
Table 2. Descriptive statistics of length and weight of Atlantic Bonito. Length in cm. and weight in gr. 2003 2004 2005 Total BON Length Weight Length Weight Length Weight Length Weight Mean 43.8 1148.5 46.7 1453.7 45.0 1364.6 46.5 1439.5 Standard Error 0.1 12.6 0.2 23.8 0.4 52.6 0.2 21.7 Median 43.5 1160.0 45.3 1300.0 44.1 1265.0 45.0 1300.0 Mode 44.0 1160.0 44.0 1160.0 43.0 1320.0 44.0 1160.0 Standard Deviation 1.4 150.5 4.3 470.8 3.5 452.4 4.3 468.6 Variance 1.9 22645.0 18.9 221657.6 12.4 204690.9 18.3 219574.4 Range 6.5 880.0 20.7 3067.8 19.2 2396.0 20.7 3371.8 Minimum 41.5 920.0 40.3 2.2 40.9 978.0 40.3 2.2 Maximum 48.0 1800.0 61.0 3070.0 60.1 3374.0 61.0 3374.0 n 143.0 143.0 391.0 391.0 74.0 74.0 465.0 465.0 Confidence Interval (95.0%) 0.2 24.9 0.4 46.8 0.8 104.8 0.4 42.7
Table 3. Descriptive statistics of length and weight of Atlantic little tuna. Length in cm. and weight in gr. 2003 2004 2005 Total LTA Length Weight Length Weight Length Weight Length Weight Mean 61.3 3722.8 70.4 5548.3 67.6 5062.2 66.2 4733.1 Standard error 0.3 61.3 0.8 159.8 1.0 218.9 0.5 104.3 Median 61.5 3740.0 71.7 5827.0 64.9 4300.0 64.0 4120.0 Mode 62.0 4040.0 73.5 6200.0 67.5 3300.0 62.0 4000.0 Standard deviation 2.6 541.6 6.2 1317.6 8.1 1844.2 7.1 1535.8 Variance 6.9 293379.0 38.3 1736051.7 65.7 3401247.3 50.5 2358703.7 Range 11.7 2800.0 26.5 4814.0 26.3 5840.0 30.4 6400.0 Minimum 56.0 2600.0 59.9 3194.0 57.7 3160.0 56.0 2600.0 Maximum 67.7 5400.0 86.4 8008.0 84.0 9000.0 86.4 9000.0 n 78.0 78.0 68.0 68.0 71.0 71.0 217.0 217.0 Confidence Interval (95.0%) 0.6 122,1 1.5 318.9 1.9 436.5 1.0 205.5
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Table 4. Regression statistics of bullet tuna
Multiple correlation coefficient 0.859915263 R square 0.739454259 Adjusted R square 0.738801262 Standard Error 0.191288725 n 401
Table 5. Regression statistics of Atlantic bonito
Multiple correlation coefficient 0.944252542 R square 0.891612863 Adjusted R square 0.891378766 Standard Error 0.094758164 n 465
Table 6. Regression statistics of Atlantic little tuna
Multiple correlation coefficient 0.958456645 R square 0.918639140 Adjusted R square 0.918260717 Standard Error 0.085017170 n 217
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Total BLT BLT 2003
20 20
18 18
16 16
14 14
12 12
10 10
8 8
6 6
4 4
2 2
0 0 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Size Size
BLT 2004 BLT 2005
20 20
18 18
16 16
14 14
12 12
10 10
8 8
6 6
4 4
2 2
0 0 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Size Size
Figure 1. Total and annual size distribution of Bullet tuna.
Total BON BON 2003
30 30
27 27
24 24
21 21
18 18
15 15
12 12
9 9
6 6
3 3
0 0 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Size Size
BON 2004 BON 2005
30 30
27 27
24 24
21 21
18 18
15 15
12 12
9 9
6 6
3 3
0 0 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Size Size Figure 2. Total and annual size distribution of Atlantic Bonito
585
Total LTA LTA 2003
20 20
18 18
16 16
14 14
12 12
10 10 %
8 8
6 6
4 4
2 2
0 0 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87
Size Size
LTA 2004 LTA 2005
20 20
18 18
16 16
14 14
12 12
10 10
8 8
6 6
4 4
2 2
0 0 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87
Size Size
Figure 3. Total and annual size distribution of Atlantic little tuna.
BLT: Weigth = 5.5916E-03 * Size 3.2923
4000
3500
3000
2500
2000
Weigth (g) Weigth 1500
1000
500
0 20 25 30 35 40 45 50 Size (cm )
Observed w eigth Espected w eigth 95%Upper 95%Low er
Figure 4. Size-weight relationship of bullet tuna
586
6 401
385
369
4 353
337
321 2 305
289
0 273
257
50 250 450 650 850 1050 1250 1450 1650 1850 2050 241
-2 225
209
193
-4 177
161
Observations 145
-6 129 Standard residuals
113
97
-8 81
65
49
-10 33
17
1 -12 -12 -10 -8 -6 -4 -2 0 2 4 6 8 Weigth Standard residuals
25
20
15
10 Percentaje
5
0 -3 -2,75 -2,5 -2,25 -2 -1,75 -1,5 -1,25 -1 -0,75 -0,5 -0,25 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25 2,5 2,75 3 Standard residuals
Figure 5. Residual analysis graphics of bullet tuna.
BON: Weigth = 9.4632E-03 * Size 3.1011
6000
5000
4000
3000 Weigth(g) 2000
1000
0 50 55 60 65
Size (cm )
Observed w eigth Espected w eigth 95%Upper 95%Low er
Figure 6. Length Weight relationship of Atlantic Bonito.
587
6 451
433
415
397
4 379
361
343
325
2 307
289
271 0 253 235
50 550 1050 1550 2050 2550 3050 3550 217
199
-2 181 Observations
163
145
127 Standard residuals -4 109
91
73
55
-6 37
19
1
-8 -8 -6 -4 -2 0 2 4 6 8 Weigth Standard residuals
25
20
15
10 Percentaje
5
0 -3 -2,75 -2,5 -2,25 -2 -1,75 -1,5 -1,25 -1 -0,75 -0,5 -0,25 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25 2,5 2,75 3 Standard residuals Figure 7. Residual analysis graphics of Atlantic Bonito.
LTA: Weigth = 4.4098E-02 * Size 2.7549
18000
16000
14000
12000
10000
8000 Weigth (g) Weigth 6000
4000
2000
0 50 55 60 65 70 75 80 85 90 Size (cm )
Observed w eigth (g) Espected w eigth 95%Upper 95%Low er
Figure 8. Length Weight relationship of Atlantic little tuna.
588
211 6 197
183
4 169
155
141
2 127
113
99
0 85
71
50 2050 4050 6050 8050 10050 Observations 57 -2 43
29 Standard residuals -4 15
1
-6 -6 -4 -2 0 2 4 6 Weigth Standard residuals
25
20
15
10 Percentaje 5
0 -3 -2,8 -2,5 -2,3 -2 -1,8 -1,5 -1,3 -1 -0,8 -0,5 -0,3 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25 2,5 2,75 3 Standard residuals
Figure 9. Graphics of residual analysis of Atlantic little tuna.
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