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This document analyses some population parameters of Yellowfin Tuna (Thunnus albacares) taken by drift gillnet operation from southern part of Iranian coast in the Oman Sea during 2007 to 2009. A total of 9,345 specimens of T. albacares were sampled in the size range of 37 cm to 172 cm fork length (FL). The mean length was estimated to be 86.12 cm. There was influence from seasonal variation on the length distribution of individuals, and it showed a tendency of gradual increase in the model length with the fishing season. The temporal change in length distribution is more likely to be an indication of feeding migration of medium-sized Yellowfin Tuna from the western Indian Ocean into the Oman Sea during January to June. The statistical analysis of lengthweight relationship was achieved by sex. The electronic package "ELEFAN I" was chosen for describing the species' von Bertalanffy growth equation; parameters were L∞= 183.3 cm, K= 0.45 year-1, and t0= -0.184 year. Natural mortality (M) was 0.48 year-1, fishing mortality (F) 1.56 year-1, and total mortality (Z) 2.04 year-1. The exploitation ratio (E) was as high as 0.76. For sustainable exploitation of T.albacares, a decrease in fishing effort of the gillnet fishery would be alternatively an effective measure to decline the fishing pressure on the stock, and to prevent the probably overfishing events.

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.

The Yellowfin tuna was caught more than all other species in the southern waters of Iran (24000 tons in 1998). 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. This thesis was the first which covered the whole Yellowfin tuna distribution in the Oman Sea, included the fishing grounds of Berris, Ramin, Chabahar, Pozm and Jask. The data during 1998-99 from different fishing grounds were polled. Based on the exponential relationship between length and weight in the size range 38-173 Cm, the relationship (W=aL^ b) was calculated as W=0.000012L ^ 3.0831). The mean fork length,head length,girth and weight were calculated respectively 84.15 Cm, 23 Cm, 53 Cm, and 11828 g. Length infinity was estimated 189 Cm with growth parameters of 0.42 per year. Growth performance index was 4.18 which was in agreement with the findngs of the other studies in the Indian and Pacific Oceans. The mortality parameters and exploitation rate were estimated as below: Z = 1.75-1.85 M=0.6 F=1.25 E=0.68 Occurence of empty stomach was high (60%) in the speciemens obtained from the Oman Sea. Purpleback flying squid (Sthenoteuthis oualaniensis) was the most dominant prey species observed in the study (57% in females and 60% in males), occurrence of teleost fishes were found to be the second (38% in males and 42% in females). Crabs also were identified in the specimens(1-2%). The study on sex ratio indicated that males were predominant at all sizes above 120 Cm fork length. 50.82% of specimens were males and 49.18% females. The monthly gonadosomatic index was deriven higher values during January to June which could be indicated as spawning period. PhD Advisors:H. Emadi, B. Kiabi Counsellors:A. Keyvan, G. Vossoughi. The full text document is in Farsi only so it has not been possible for AC Editors to check the metadata input.