Meristic differences between juveniles of two tuna species from the Western Mediterranean Sea.
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1
Oceanographic Center of Málaga, Spanish Institute of Oceanography (IEO), Spain
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2
Department of Animal Biology, Faculty of Science, Malaga University, Spain
Bluefin tuna (Thunnus thynnus Linnaeus 1758) and little tunny (Euthynnus alletteratus Rafinesque 1810) are highly migratory species that inhabit Atlantic Ocean and Mediterranean Sea. The Mediterranean Sea is a spawning area for both species (Rodriguez-Roda, 1964; Oray and Karakulak, 2005). Juveniles of tuna species can be found in mixed schools and catches. The very young juveniles of bluefin tuna and little tunny are difficult to differentiate. Differentiation of species is essential for biology, fishery management and conservation studies. Genetics is an accurate method to discriminate species but it is expensive, time consuming and results are not immediate. The objective of this study is to identify meristic characters to discriminate between juvenile of bluefin tuna and little tunny.
Juvenile of bluefin tuna and little tunny were caught between August and September from 2016 to 2018 from four localities throughout Western Mediterranean Sea. A total of 142 bluefin tuna (ranged from 121 to 357 mm) and 101 little tunny (ranged from 85 to 315 mm) were analysed (Table 1). The individuals were caught as bycatch by purse seine targeting small pelagic species (anchovy and sardine) and sport fishery. Juvenile of bluefin tuna (smaller than the minimum landing size) were collected under the provision of the International Commission of the Atlantic Tunas (ICCAT) Atlantic Wide Research Program for Bluefin Tuna (GBYP).
The specimens kept frozen at -20ºC until further analysis in the laboratory, except ten specimens of bluefin tuna that were analysed in fresh. For each individual fork length (FL) was measured with an ichtyometer to the nearest mm or a caliper to the nearest 0.01 mm. The excess of water was eliminated with a piece of paper and then, body weight (BW) was measured to the nearest 0.01 g (± 0.01).
Eight meristic characters were recorded: first and second dorsal fin rays, anal rays, pectoral rays, dorsal and anal finlets, left and right side gill rakers (table 2). A pin and a dissection needle were used to improve the count accuracy. Gills were taken out from the head to count the gill rakers.
The results of the analysis for bluefin tuna and little tunny are shown in the table 3. The number of first dorsal rays is the unique variable that does not overlap between both species (bluefin tuna 13-14 vs little tunny 15-17). The highest variability was found on the anal rays for bluefin tuna (CV = 6.79%) and on the second dorsal rays for little tunny (CV = 8.69%). The number of dorsal finlets range between 7 and 9 for both species. The counts of meristic characters are shown in table 4. Second dorsal rays on bluefin tuna has a probability of 96% of being included in the range 13-15 and little tunny has a probability of 94% of being included in the range 11-13 rays. The number of pectoral rays in bluefin tuna has a probability of 97% of being equal or more than 30 rays, and in little tunny the probability to be less than 30 rays is 100%. The anal rays in juvenile bluefin tuna has a probability of 98% of being included in the range of 12-15 rays and little tunny has a probability of 99% of being included in the range of 11-14 rays. The number of gill rakers was similar in both right and left sides. Bluefin tuna has between 35 and 43 gill rakers, and little tunny between 33 and 42; which indicates that gill rakers are not useful in identifying these two species.
The number of dorsal and anal finlets are shown in table 5. When comparing the counts of dorsal and anal side we found: a) one more finlet on the dorsal side than in the anal side (9 vs 8, 8 vs 7, 7 vs 6) in 73% of bluefin tuna and 93% of little tunny; b) the same number of finlets in both dorsal and anal side (9 vs 9, 8 vs 8, 7 vs 7) in 24% of bluefin tuna and 2% of little tunny and, c) two more finlets on the dorsal side than on the anal side (9 vs 7) in 2% of bluefin tuna and 5% of little tunny.
Our findings are in line with those of Pruzinsky (2018), who reported that pectoral rays in little tunny were less than 30 rays in 100% of individuals. Regarding genus Thunnus, Pruzinsky (2018) reported 100% of individuals with equal or greater than 30 pectoral fin rays, but our results showed 3% of bluefin tuna with less than 30 pectoral fin rays.
In conclusion, the number of first dorsal fin rays is the unique meristic character analysed in this study that can be used to discriminate juvenile of bluefin tuna and little tunny.
Acknowledgements
We are grateful to skippers and crews of the commercial and recreational fisheries. We also thank the effort of the scientific observers and collaborators, especially Maxalen Pauly and Jesús Torralba.
This project has been funded by the EU through EMFF within the National Program of collection, management and use of data in the fisheries sector and support for scientific advice regarding the Common Fisheries Policy.
References
Oray, I. K., & Karakulak, F. S. (2005). Further evidence of spawning of bluefin tuna (Thunnus thynnus L., 1758) and the tuna species (Auxis rochei Ris., 1810, Euthynnus alletteratus Raf., 1810) in the eastern Mediterranean Sea: preliminary results of TUNALEV larval survey in 2004. Journal of Applied Ichthyology, 21(3), 236-240.
Pruzinsky, N. (2018). Identification and spatiotemporal dynamics of tuna (Family: Scombridae; Tribe: Thunnini) early life stages in the oceanic Gulf of Mexico.
Rodríguez-Roda, J. (1964). Movimientos migratorios del atún, Thunnus thynnus (L.), deducidos por nuestras propias "marcaciones" en aguas españolas.
Keywords:
Meristic characters,
Taxonomy,
Thunnus thynnus,
Euthynnus alletteratus,
Juveniles
Conference:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019.
Presentation Type:
Poster Presentation
Topic:
Fisheries, Aquaculture and Biotechnology
Citation:
Castro-Gutiérrez
J,
Saber
S,
Ortiz De Urbina
J,
Gómez-Vives
MJ,
García-Barcelona
S and
Macías
D
(2019). Meristic differences between juveniles of two tuna species from the Western Mediterranean Sea..
Front. Mar. Sci.
Conference Abstract:
XX Iberian Symposium on Marine Biology Studies (SIEBM XX) .
doi: 10.3389/conf.fmars.2019.08.00126
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Received:
17 Jun 2019;
Published Online:
27 Sep 2019.
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Correspondence:
Mx. Jairo Castro-Gutiérrez, Oceanographic Center of Málaga, Spanish Institute of Oceanography (IEO), Málaga, Spain, jairocastrogtz@gmail.com