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Changes in length-based indices (LBI) of longtail tuna (Thunnus tonggol Bleeker, 1851) during two time periods in the northern waters of the Oman Sea (Sistan and Baluchestan Province)
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Seyed Ahmadreza Hashemi *   , Mastooreh Doustdar , Rahimeh Rahmati |
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Abstract: (7 Views) |
Longtail tuna fish is one of the most important tuna and economic fish species in the south of the country, and the length-based index (LBI) model was used to describe the status of the Thunnus tonggol tuna stock in the northern areas of the Oman Sea. During 1994 and 2024, 3599 fish were biometrically measured (2010 in 1994 and 1589 in 2024), and the average length (range) of T. tonggol tuna fish in 1994 was 71±22 (125-38) cm and in 2024 it was 68±7 (122-28) cm. The average weight (range) of the longtail tuna fish in 1994 was 4231±2031 (1300-13000) grams and in 2024 it was 5629±2581 (700-22800) grams. Considering the calculated maturity length of this species (68 and 65 cm for the years under study), the percentage of specimens below maturity length in 1994 and 2024 was 32 percent and 48 percent, respectively, and the percentage of specimens below maturity length in 2024 was significantly different from that in 1994 (P<0.05).
The relationship between the fork length and body weight of T. tonggol tuna fish was obtained for the year 1994 W = 0.067 L^ 2.64 (R2=0.80, N=550) and for the year 2024 W = 0.069 L^ 2.59 (R2=0.89, N=450) and the b value of the length-weight relationship indicating allometric (asymmetric) growth was negative. The average infinite length for the years studied 1994 and 2024 was 128 (105-133) and 125 (108-129) cm, respectively and the maximum length in 1994 was 125 cm and in 2024 was 122 cm. The optimal catch length range (Lopt) for the years 1994 and 2024 of the T. tonggol tuna fish in the northern waters of the Sea of Oman was 73 (63-75) and 71 (59-74) cm, respectively, and the ratio of the mean length to the optimal length index (Lmean/Lopt) for the years under study was 0.82 and 0.79, respectively. The range of the mish size (MS) of the T. tonggol fish in the northern waters of the Sea of Oman in the years under study 1994 and 2024 was 14.6 (13-15) and 14.2 (13-15) cm, respectively. The decrease in the average length, the increase in the percentage below the maturity length and the ratio of the average length to the optimal length can be evidence of the existence of overfishing of the longtail tuna fish in the northern waters of the Sea of Oman (Sistan and Baluchestan Province).
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| Keywords: Sustainability, stock status, Thunnus tonggol |
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Type of Study: Research |
Subject:
Special
Received: 2025/09/17 | Accepted: 2025/12/28 | Published: 2025/12/28
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