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Yearly tuna and tuna-like catches in Iran are of the order of 163,991 t, close to 40 of which are longtail tuna. Fork length was measured on a total of 4313 longtail tuna by the technicians of the research centers of the IFRO at a number of sample sites along the north part of the Persian Gulf and Oman Sea from Oct 2007 to Sept 2008.Total fresh weight was recorded for a sub-sample of individuals.Monthly length -frequency distributions grouped in three centimeters class intervals.
The Von Bertalanffy parameters were then estimated by the software of FISAT.The growth parameters of Lâˆž and K were computed 133.7 cm and 0.35 Y respectively. The length -weight relationship was estimated as: W=0.00002 L 2.83.Total mortality (Z) was estimated by using the Powell-Wetherall plot as 1.82 per year. Natural mortality was obtained by Pauly equation (M=0.44).Fishing mortality (F) then estimated from Z-M=1.38.
Keywords: Population dynamic, longtail tuna, Persian Gulf, Oman Sea
Tunas form an important part of the marine fishery production of Iran, and are traditionally caught by artisanal fishermen in the Persian Gulf and Oman Sea (Figure 1).Most of the catches are made using drifting gillnets and trolling.
The main tuna species caught in Iranian waters are:
Longtail tuna (Thunnus tonggol),Yellowfin tuna (Thunnus albacares), Skipjack tuna(Katsuwonus pelamis),Kawakawa (Euthynnus affinis), Frigate tuna (Auxis thazard).
Yearly tuna & tuna - like species catch in Iran are of the order of 163991 t, close to 40 % of which are longtail tuna. Table 1 shows the estimated catches of tuna species in Iranian waters in 2010.
Table 1: Tuna & tuna like species catch in Iranian waters (2010)( IFO, 2011)
Species Catch (t)
Total catch 163,991
Few studies have been undertaken on longtail tuna, such as estimation of growth parameters and mortality rates of T.tonggol and determination of its exploitation pattern in coastal waters of Hormuzgan province (Davarpanah, 2007). In this regard, some biological and population dynamic parameters studies of tuna and tuna like species were carried out in the Persian Gulf & Oman Sea by using fishery- dependent data (kaymaram, 2009).
2. MATERIALS AND METHODS
Length and weight data were collected randomly from five traditional fish-landing sites: Chabahar,Jask, Bandar Abbas, Bandar Lengeh and Parsian in the Persian Gulf and Oman Sea coastal waters (Figure.1) from October 2007 to September 2008.
Fig 1.Landing sites for sampling of T.tonggol in the north Persian Gulf and Oman Sea
The fishes were simply measured and weighed to the nearest cm (fork length) and 50 g. respectively. The length frequencies were grouped in 3 cm intervals. The total samples size for population dynamic parameters was 4313.
Length - weight relationships were studied withthe equation W = aFLb, in order to verify if calculated b was significantly different from 3, the Student's t-test was employed (Zar, 1996).
Predicted maximum length was obtained by extreme value theory (Formacion et al., 1991).
Growth parameters (K, Lâˆž) were estimated by FISAT II software (FAO-ICLARM STOCK ASSESSMENT TOOLS) (Gayanilo et al., 1996) with using Shepherd,s method for scan of K value , Lâˆž andZ/K was estimated by Powell-Wetherall plot (Wetherall et al., 1987).
Natural mortality coefficient (M) was calculated with the equation of Pauly (1980) (multiplied by 0.8 as recommended by Pauly for pelagic species.
Log M= 0.0066-0.279 Log Lâˆž + 0.6543 Log K+ 0.4634 Log T
Where T is the mean temperature of surface water, which was considered in this study as 26.50C.
Fishing mortality (F) was then estimated (F=Z-M) (Sparre&Venema, 1998).
The parameters of the length-weight relationship W = a Lbwere calculated after linear transformation and regression analysis. The relationship between W (Kg) and FL (Cm) is:
W=0.00002 L 2.83
Length weight relationship of T.tonggol caught by gillnet is shown in figure 2.
Figure 2.Length-weight relationship of longtail tuna in the Persian Gulf & Oman Sea
The monthly length frequency distributions used in the analysis are presented in figure 3.
Figure 3.Length frequency distribution of longtail tuna in the Persian Gulf & Oman Sea
The average length of the fish was 74 cm. The smallest fish measured was 26-29 Cm and the largest one was 125-128 Cm.Most of the individuals was found between 74-77 Cm.
The observed extreme fork length was 125 Cm. The range of 95 % confidence interval for extreme fork length was 122.11-142.49 Cm (Fig 4).Estimated length infinity was 133.7 Cm which was based upon the largest fork length in each sampled month.
Figure4.Predicted maximum length of T.tonggol based on extreme value theory (2007-08).
The growth coefficient (K) was calculated by scan of K value as 0.35 per year (Fig.5).
Figure.5. Scan of K value of T.tonggol (2007-08).
Z/K and length infinity were estimated by Powell-Wetherall plot 5.2 and 133.79 Cm respectively. Coefficient correlation was r=-0.9.
The natural mortality coefficient "M" was estimated at 0.55 by employing the equation of Pauly, where mean sea surface temperature considered in this study as 26.5oc. As the pelagic species grows to a large size very fast, the "M" value may be an over-estimation. Hence the value was multiplied by 0.8 to get a revised estimate of "M" as 0.44.
The parameters of the length-weight relationship were estimated as:
Iranian gillnet catches of longtail tuna in Hormozgan province are seasonal and showed some differences in the size composition between the Iranian and Omani fisheries on the northern and southern shores of the Oman Sea which could be the result of a size-related migration or of differences in the mesh size of the gillnets used (Khorshidian& Carrara,1993).
The results of different studies about length-weight relationship parameters were presented in Table 2.
Table 2.Summary of length- weight relationship parameters on longtail tuna.
Base of length
James et al.,1993
Griffiths et al.,2011
Darvishi et al.,2003
Estimated length infinity and growth coefficient appears to vary markedly between studies using different estimation techniques and among regions. This is probably a result of very different maximum sizes of fish present in each study region or differences in the size selectivity of the sampling methods (i.e. gillnets). These factors would therefore affect estimates of Lâˆž and the instantaneous growth rate (K). A comparison of the growth curves and growth model parameter estimates in each study from various regions is provided in Table 3.
Table 3. Summary of growth studies on longtail tuna
Gulf of Papua
Silas et al.,1985
The natural mortality of longtail tuna (M) of 0.44 is in line with estimate obtained in 1992-93 as 0.49 in the same area (Khorshidian&Carrara, 1993). These two estimates through the Pauly,s formula appears to be very low when compared to natural mortality of 0.8 per year obtained in Indian waters by James et al.,1993.
The total mortality obtained in this study seems to be under estimated in comparison with other studies such as 3.84 reported in Gulf of Thailand by Supongpan&Saikliang, 1987 and 3.13 by Khorshidian&Carrara, 1993.
Morphometric studies of longtail tuna indicated that differences exist between sub populations of this species throughout its range of distribution (Yesaki, 1991). According to different definitions of unit stock such as Gulland in Sparre&Venema, 1998, further studies to identifying stock and its distribution should be carried out in different areas of Indian Ocean. It should be emphasized that whole areas such as Australia,India,Thailand,Oman,Iran and other countries situated in the Indian Ocean should be covered in this study.