Population assessment and yield per recruit of longtail tuna (Thunnus tonggol) in Northern of the Persian Gulf and Oman Sea (Iran, Hormozgan Province)

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 (Flimit) 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.