Stock assessment of threadfin bream Nemipterus zysron (Bleeker, 1857) in the Sea of Oman, Sultanate of Oman

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1 Stock assessment of threadfin bream Nemipterus zysron (Bleeker, 1857) in the Sea of Oman, Sultanate of Oman Sahar F. Mehanna 1, Juma Al-Mamry 2, Iman Al-Kharusi 2, Dawood Al-Mamry 2 1 National Institute of Oceanography and Fisheries, Egypt 2 Marine Science and Fisheries Centre, Sultanate of Oman sahar_mehanna@yahoo.com Abstract The assessment of threadfin bream (Nemipterus zysron) stock were conducted based on the samples collected from Oman Sea during the demersal survey covered a period of three years from and updated during 2012 from commercial catch. Population parameters of threadfin bream were studied using 650 specimens. Age was estimated by examining sagittal otoliths and it was found out that the age composition varied from 0 to IV age classes for both sexes. Mean lengths and weights were calculated as 24.9±4.18 cm; ±77.52 g for males and 23.63±4.68 cm; ±78.67 g for females. Average length and weight for sexes combined were calculated as 24.11±4.16 cm and 146.2±72.1 g. Although males were observed as bigger in size in comparison to females, there was no gender dependent statistical difference in von Bertalanffy growth parameters and the growth curves were not significantly different between the observed and calculated length (p > 0.05). Von Bertalanffy growth equation for both sexes was derived as L t = (1-e (t+0.58) ). Instantaneous total mortality, natural mortality and fishing mortality rates were also estimated. Based on exploitation rate and yield per recruit analysis, the fisheries status of N. zysron at the Sea of Oman was appraised and came to conclude that the present fishing effort in the Sea of Oman can be increased to achieve a higher catch of threadfin bream. Under the present conditions, such an increase in fishing effort may adversely affect the other commercial stocks in the area so a multispecies stock assessment is highly recommended. Keywords: Sea of Oman, Nemipteridae, population dynamics, stock assessment, management. 1. Introduction The threadfin breams (Nemipteridae) are small to medium-sized marine perciform fishes widespread in the tropical and subtropical Indo-West Pacific region, but absent in the eastern Pacific and Atlantic oceans (Russell, 1990). They represent a significant component of coastal demersal fish communities and are important to fisheries throughout the Indo-West Pacific (Pauly & Martosubroto, 1980; Russell, 1990; Harris & Poiner, 1990; Sharum, 2002; The et al., 2005). Family Nemipteridae comprises 67 species belong to 5 genera, from which 10 species are found in Oman waters. Slender threadfin bream, Nemipterus zysron (Bleeker, 1856) is a small demersal species distributed in Indo-West Pacific: Red Sea and East Africa to the Indo-Malay Archipelago, north to Japan, south to northwestern Australia. This species has generally been treated under the name Nemipterus metopias by most previous authors. Page 510

2 It is benthic found on sand bottoms near rocks (Sommer et al., 1996) and swims in groups and feeds on benthic organisms. Oman Sea is one of the fisheries resources in Oman yielding about 62.2 thousand ton annually forming 36% of the total Oman fish production and achieve about 64.5 million Omani Rial per year (Fishery statistics book, 2012). Oman sea characterized by a large variety of fish species from which threadfin breams are common species and up to 11 species were recorded in Oman coasts (Al- Abdessalaam,1995). Although threadfin breams are common and of commercial importance in Oman waters, there is no records by species or any information about their biology and dynamics. The present study is the first to estimate the population parameters of N. zysron in Oman Sea. These parameters are used to assess the threadfin bream stock and to propose some reference points to exploit it rationally in Oman waters. 2. Material and methods The length and weight measurements were collected during the trap surveys in the Sea of Oman during the period from November 2008 to July 2011 and updated by collection and measuring more samples from the commercial catches along the Sea of Oman landing sites during the period from January 2012 to December In Oman, 18 trapping stations have been assigned to be along the three stratums in un-trawlable ground extended from Musandam in the north to Ras-Al Hadd in the south of the Sea of Oman (Fig. 1).The trap used were of 205 cm diameter, 130 cm high, one opening funnel 100x80cm and a mesh of hexagonal galvanized wire with a mesh size of 4x5cm. At least 10 traps were set at each station. The traps were baited with suitable fish bait used locally in each area and season, and have been left for 2-3 days. Data on total length of N. zysron (measured from tip of snout to tip of lower caudal lobe), weight and sex and stage of maturation were had from specimens while they were still fresh. A total of 482 specimens were collected and sexed of which 217 were males and 235 females while 30 fishes couldn t sexed. The length data obtained on each observation day were raised to the day's catch and these were further raised to get monthly length composition of the catch. The sagittal otoliths were removed from all specimens, washed, dried and stored in labeled envelopes for further age readings. Annual rings on whole otolith were identified and counted using optical system consisting of Zeiss research microscope at 4 and 10 magnifications connected to AxioCam HRC and Ziess KL 1500 LCD using transmitted light. The total radius of otolith and the radius of each annulus were measured to the nearest mm. Regression analyses of otolith maximum radius - total length was calculated by the method of least squares. Back-calculated lengths-at-age were computed by using the Lee method (Lagler, 1956). Also, the length frequency was resolved into normally distributed cohort components using Bhattacharya (1967) method. Bertalanffy growth equations were used to determine the age-length and age-weight relationships of the population (Bertalanffy, 1938; Ricker, 1975). The parameters of growth were estimated using the von Bertalantfy equation: L t = L (l-e -K(t-to) ), where L is the asymptotic length; K the growth coefficient, to theoretical age when length is zero and L t the length at age t. The L was estimated from the Ford- Walford plot (Ford 1933; Walford, 1946) of L t+1 against L t on the basis of lengths attained through the survey period. Phi prime (Ø ) was used to evaluate the reliability of L and K and calculated as Ø = logk + 2 log L (Munro and Pauly, 1983). The significance of the differences for the female and male groups and values for length within the same age group were tested by student t-test with the probability of P= Page 511

3 Total mortality rate (Z) was estimated using two different methods: the linearized length-converted catch curve method using the growth parameters from the Von Bertalanffy plot as input data (Pauly, 1984) and Ricker (1975) method. Annual mortality rate (A) was determined as A= 1-S (Gulland, 1971; Ricker, 1975) where S is the survival rate and equal to EXP (-Z). Figure 1: Sea of Oman map showing the trap stations and main landing sites Instantaneous natural mortality rate (M) was derived as the geometric mean of three different formulae. Pauly s (1983) empirical formula as: log M= log L log K log T where L and K are the VBG parameters, and T is the mean water temperature (26 C). The equation derived by Ursin (1967) based on the mean total length, where M = W -(1/b) (W: mean total length; b: constant of length- weight relationship). As well as, the formula of Taylor's (1960). Fishing mortality rate (F) was estimated by F = Z-M (Gulland, 1971), while the exploitation ratio (E) of threadfin bream was calculated as E = F/Z (Ricker, 1975). The rate of exploitation (U) for different years was estimated from the equation U = F/Z (1-e -Z ) (Beverton and Holt 1957, Ricker 1975). Microsoft Excel, FiSAT II, and SPSS 18 were used for the calculations, graphics and statistical tests where necessary. The length at first capture was estimated based on the equation of Beverton and Holt (1957) as Z = K*(L -L m )/(L m -L c ) where L m is the average total length of the entire catch, while L c is the length at which 50% of the fish entering the gear are retained (Sparre and Venema, 1998). The relative yield per recruit (Y /R) was calculated from the formula of Beverton and Holt (1966) as modified by Pauly and Soriano (1986) as follows: Y ' EU R Y ' B' R R F where 1 E m K M Z K Lc U 1 L M K 2 3 3U 3U U 1 1 m 1 2m 1 3m Page 512

4 % occurrence INOC - IIUM- International Conference on " Oceanography & Sustainable Marine Production: A Challenge of 3. Results and discussion Age and growth A total of 482 otoliths of N. zysron (Fig. 2) were examined, of which 420 (87.1%) were clearly readable and used for age determination. The total length range of males was cm (mean 24.9±4.18 cm), while that of females was cm TL ( 23.63±4.68 cm TL) and there is no significant differences in the age composition between sexes. Also, differences between the lengths of threadfin bream for different ages of each sex were found to be not statistically significant (P>0.05). The linear ration between fish total length (TL) in cm and otolith radius (R) in mm was estimated by least squares and found to be: Figure 2: Otolith of Nemipterus zysron from the Sea of Oman, Oman. Age of threadfin bream samples varied between 0 and 4 years in 420 specimens (Fig. 3) and the mean lengths of the four groups respectively were 20.17, 26.71, and cm TL. This species was dominant in age groups 1 (42.19%) and followed by the age groups 2 (27.34%), age group 3 (18.75%), then 0 (10.16%) and finally age group 4 (1.56%) Age group Figure 3: Age composition of Nemipterus zysron from the Sea of Oman. Also, plotting of the histogram and obtaining the modes was done using the method of Bhattacharya (1967). Four discernible cohorts appear to be present in the sample (Fig. 4). The mean lengths of the four cohorts were 19.8, 25.9, 30.1 and 32.8 cm TL. The back-calculated lengths from otolith readings were used for the subsequent calculations. Annual absolute increments in length were calculated as 20.17, 6.54, 4.11 and 2.33 cm while the annual absolute increments in weight were 76.44, , and g respectively (Figs. 5&6). The highest relative growth rate in length was recorded at the first year of life while that in weight was Page 513

5 Weight g INOC - IIUM- International Conference on " Oceanography & Sustainable Marine Production: A Challenge of observed in the second year of life. Growth is fast until the age class II, so this species should be caught in the second year of its life span. Figure 4: Bhattacharya analysis for length frequency of Nemipterus zysron from the Sea of Oman. Figurre 5: Length by age and growth increment of Nemipterus zysron from the Sea of Oman Increment Wt Age yr Figure 6: Weight by age and growth increment of Nemipterus zysron from the Sea of Oman. A likelihood ratio test (LRT) showed no significant difference between males and females von Bertalanffy growth curves. Back-calculated lengths from otolith readings of pooled data were applied according to Ford (1933)-Walford (1946) plot to estimate the growth parameters (L and K). The obtained equations were: Page 514

6 For growth in length L t = (1 - e (t ) ) For growth in weight W t = (1 - e (t ) ) based on L-Wt parameters obtained from Al-Mamry et al. (2013). Based on the obtained growth parameters, the growth performance index was calculated as Ø =2.83. The only previous work dealing with the growth of N. zysron is that of El-Ganainy and Mehanna (2003) who estimated the growth parameters for N. zysron in the Gulf of Suez as L = 28.1 cm TL, K=0.58 yr -1, and t o = year. The difference in the two sets of parameters was attributed to the difference in maximum length observed in the fishery. These results are agreeing with those reported for other nemipterid species. For N. Japonicus, the values of L and K along India coasts varied from 30.5 to 35.1 cm and from 0.40 to 0.70 / year respectively (Krishnamoorthi, 1973; Murty et al., 1992). The growth parameters of N. japonicus exploited from the Arabian Sea off Karntaka were estimated as L = 33.0 cm and k = 1.0 yr -1 for specimens of maximum size of 28 cm total length (Gulati et al., 2007). For N. mesoprion, Murty (1981) estimated L and K as 21.9 cm and 0.83 year -1 respectively at Kakinada, while Vivekanandan (1991) estimated L as 20.7 cm and K as 1.08 at Chennai. Using the ELEFAN programme Murty et al. (1992) estimated L and K off Cochin as cm and year -1. At Mumbai, N. mesoprion grows to 14.7, 21.5 and 25.2 cm at the end of I-III years of life and the estimated L was 27.4 cm and K was 0.76 year -1 (Chakraborty, 2002), while in Cochin, it reaches 15.8, 22.4 and 25.3 cm at the end of first, second and third year respectively and the length obtained by LFSA package was 15.9, 21.8, 24.4 and 25.4 cm at the end of I-IV years (Joshi, 2005). The same author gave L value as 27.4 cm and K as 0.85 per year. The growth parameters L and K for the pooled sample of N. mesoprion were estimated as 29.9 cm and 0.79 y -1 respectively off Malabar coast (Manojkumar, 2007). The growth parameters of the N. randalli in the Arabian sea coast of Oman were estimated as L = cm and the curvature constant K was 0.64 per year (Al-Kiyumi et al., 2013). Mortality and Exploitation rates Figure 7: Mortality estimates for Nemipterus zysron from the Sea of Oman. Instantaneous total mortality rate Z was estimated at 1.77 yr 1 from converted catch curve (Fig. 7) and 1.31 yr 1 from Ricker method with an geometric mean of 1.52 yr 1. The annual instantaneous rate of natural mortality (M) was estimated at 1.03 yr 1 from Pauly s equation, 0.57 yr 1 from Taylor s formula and 0.4 yr 1 from Ursin s formula. By taking the geometric mean, the M used in the calculations was 0.62 yr 1. Using the estimates of both Z and M, an estimate of fishing mortality F was Page 515

7 obtained at 0.90 yr 1 The exploitation ratio was estimated at 0.59, while the exploitation rate was estimated as 0.46 indicating that this species operates around its optimum status. Survival rate and annual mortality rate were calculated as follows: S = 0.22 yr -1 and A = 0.78 yr -1. The only available mortality and exploitation rates estimates for N. zysron were Z = 2.33 yr -1, M = yr -1, F= yr -1 and E = (El-Ganainy and Mehanna, 2003). Relative yield per recruit The relative yield per recruit (Y /R) calculated at the present natural mortality rate and length at first capture that calculated from Beverton and Holt (1956) equation against various exploitation rates ranging from 0 to 1.0 are shown in Figure 8. Y /R in terms of weight increased with increasing E, suggesting that the yield might reach an asymptote and decline only at a very high E value. The length at first capture of N. zysron off Oman coast of Oman Sea was presently 23.2 cm, which meant that the fish was 1.15 year old at the time of first capture. At the existing rate F = 0.9, M = 0.8 and L c of 23.2 cm, the (Y /R) = 0.04 g (Fig. 8). Based on the obtained results, the present fishing effort along the Sea of Oman coast can be increased, so that higher catch could be achieved. But, under the present conditions, such an increase in fishing effort may not adversely affect the stock of threadfin bream in Oman but will affect the other commercial stocks of demersals in the area. So, it could be said that, presently, the stocks of N. zysron are underexploited and one possible management policy towards obtaining better catches might be to suggest to increase the size of the mesh so as to increase the age at first capture from 1.15 year to 2 year (from 23.2 to 26.7 cm TL). Figure 8: Relative yield per recruit analysis of Nemipterus zysron from the Sea of Oman. 4. Conclusion Nemipterus zysron appears to be exploited under its optimum level and there is a scope for a considerable extension of the fishery. But any recommendation to increase the effort in a multi-species gear should consider all the components of the fishery by that gear. Also, before any advice to increase the fishing effort, a more precise assessment would be required and a sampling programme for the commercial landings should be established to provide total catches by size groups. Also, an investigation for the impact of the fishery independent factors like predation and other changes in environmental parameters on the growth and recruitment should be done. Finally, improving the catch statistics recording system to cover all species caught from Oman coasts should be considered. Page 516

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