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The purpose of this study was to find some important biological aspects of Thunnus tonggol in coastal waters of Hormozgan Province (Persian Gulf and Oman Sea). A total monthly data of 4383 individuals ranging from 25 to 124 cm fork length were collected from April 2015 to March 2016. The data were analyzed with FiSAT II software using the ELEFAN1 package to estimate the population parameters. The length-weight relationship was TW=0.00002 FL^2.87 (R^2 =0.97) showing an isometric growth for T. tonggol. Growth parameters were computed as L∞=129.6 cm, K=0.39 year^-1 and t0=−0.28 with the growth performance index, φ’ of 8.7. The total mortality (Z) was estimated 1.58 year^-1 using catch curve method. The natural (M) and fishing mortality (Fcurr) were obtained 0.49 and 1.09 year^-1, respectively. The exploitation ratio was 0.69. Length at first capture (Lc) was estimated as 60.2 cm fork length. The yield per recruit (Y/R) maximized in maximum fishing mortality rate 0.85 year^-1. The biomass per recruit decreased to 17.2% of unexploited biomass (virgin biomass) at Fcurr. The current fishing mortality exceeds optimum fishing mortality (Fopt) and limit fishing mortality (F limit) as biological reference points. The results indicated that population of T. tonggol is overexploited in the Persian Gulf and Oman Sea and there no further scope for improving the exploitation of this species.

The study of fluctuation of large pelagic stock (yellowfin tuna, skipjack tuna, longtail tuna, Narrow-barred Spanish mackerel ) in order to optimum exploitation in the Persian Gulf & Oman sea This study was carried out from 2005-07 in order to acquire some biological characteristics and population dynamic parameters for stock management and responsible fisheries. Thunnus albacares (yellowfin tuna), katsuwonus pelamis (skipjack tuna), Thunnus tonggol (longtail tuna) and scomberomorus commerson (Narrow- barred Spanish mackerel) were sampled. In total, length frequency of 9345 specimens of yellowfin tuna were analyzed. Size range was 37-172 cm. Teleost fishes were the most dominant prey species observed in this study (42%), Occurrence of Potunus Pelagicus was found to be the second (28%).Sthenoteuthis oualaniensis (22%), Natosquilla (5%) and octopus (3%) also were identified in the gut content of the yellowfin tuna. Length of maturity (50%) of yellow fin tuna was estimated 77.2 cm and spawning season was started from May. 8443 specimens of skipjack with size range of 32-90 cm were sampled. 48% of food items were teleost fishes. Squid and shrimp were also identified. Spawning season was begun from June. Growth parameters & fishing mortalities of yellow fin tuna and skipjack tuna were also estimated. Size range if longtail tuna was from 26-125cm Length infinity was estimated 132.3 cm with growth parameter of 0.35 per year. In total 10451 specimens of narrow- barred Spanish mackerel were sampled. Size range was from 20-164 cm. Teleost fishes were the most dominant prey species observed in the study (91.3%). Crustaceans (0.6%) and Indian squid (0.2%) also were identified in the specimens. Length maturity (50%) was estimated 83.6cm.

In order to come up with the responsible fishing pattern, there was a need to identify some of the biological characteristics and population dynamic parameters of yellowfin tuna, skipjack and longtail tuna in the Persian Gulf and Oman Sea. Occurrence of empty stomach was high in all the specimens obtained from the gill net fisheries. Purple back flying squid was the most dominant prey species observed in the yellowfin tuna study (61% in males and 57% in females). The length of maturity (Lm50%) for yellowfin tuna was estimated 76 cm, 62.5 cm skipjack and 62.5 cm longtail tuna. Length range of skipjack was from 31-100 cm and bony fishes were the most dominant prey species observed in the skipjack. The monthly gonad somatic index was driven higher values after June which could be indicated as spawning period. The catch per unit of effort of three purse seiners (Azadegan 1, 2, 3) was compared in 1998- 2000. The species composition was 47% yellowfin tuna, 51% long tail tuna and 2% skipjack.

Some investigation were performed on the industrial fisheries of tuna in Oman Sea in 1996. The result showed that the total fisheries of Spanish purse seiners, Montelape & Montealegre, were more than 8907 tons; yellow fin tuna (91%) and longtail tuna (9%) from March to September 1996. Their activity was in the Indian Ocean from September 1996 to March 1997. Using Jahadpozm longliner, 204 tons of tuna including yellow fin tuna (99.2%) and swordfish (< 1%) were fished in Oman Sea during a 9 month period in the years 1995 & 1996. The CPUE of these purse seiners were 40 tons per fishing day which was noticeable in comparison with 21 tons of purse seiners in the Indian Ocean and indicated the richness of EEZ of Oman Sea. Longtail tuna mostly were fished by Spanish vessels in western regions of 600 longitude and yellow fin tuna were caught from 57º to 62º longitudes in Oman Sea. Comparing the length distribution of yellow fin tuna caught by industrial and artisanal fisheries showed that the larger size of exploited fish by purse seiners were generally bigger than those caught by artisanal vessels.

The coast of the Hormozgan province has been divided into five strata. From west to east, the five geographical strata are, Lengeh, Qeshm Island, Bandar Abbas, Kolahi and Jask. The statistical procedure used in the study takes into consideration four boat type categories that are distinguished on the basis of carrying capacity, 1 ton boats, 1 to 20 tons, 20 to 50 tons more than 50 tons and four gear types; gillnet, trap, hooks, beach seine, with tunas being caught mainly by gillnetters and in minor proportion with lines. No data are available for the Bandar Abbas and Qeshm Island geographical strata. Longtail tuna is dominate species in the tuna catches of the Hormozgan province, followed by the two species of seerfish (Scomberomorus) and by yellowfin tuna. The three main strata; Lengeh, Kolahi, Jask, for which completes sets of catch data is available call be taken as being representative of the three main fishing zones of the Hormozgan province. A thazard is caught in all the three strata both inside and outside the Persian Gulf. Yellow tuna (T. albacares) is caught almost exclusively in the outer stratum of Jask between September and December. Longtail tuna (T. tonggol) shows marked differences in Seasonal trends between the open water stratum of Jask and the western stratum of length. Skipjack tuna (K. pelamis) is only caught in the area between September and March, showing approximately the same trend as yellow tuna. Kawakawa (E. affinis) is mainly caught inside the Persian Gulf. In Jask area the best catches of this species are obtained from June to August. The sudden switch between catches of longtail and Yellowfin in the outer stratum of Jask in August – October, could he partly due to a change in fishing strategy. The increase in tuna catches inside the Persian Gulf during the colder months could be the result or active migration by most of the neritic species toward this area in winter.