The length frequency data of
Mastacembelus armatus was analyzed in FiSAT software using various methods to estimate the growth, mortality and recruitment. For commercially utilized fish species, growth parameters (L∞ and
k) have been estimated because these population parameters are important to describe the species and input data in several fishery production models (Hilborn, 1992). L∞ is the largest theoretical mean length that a species could attain in its habitat whereas k is related to the speed it grows towards their final size.
Bhattacharya’s plot
By using the Bhattacharya’s method in FiSAT,
M. armatus, one group or cohort at modal length 23.83±3.67 cm was obtained (Fig 2).
Growth parameters
A total of 255
M. armatus were examined and their length-mass relationships were computed as body mass = 0.004×body length
2.85 (R
2=0.90; p<0.05). Length-mass relationship indicated isometric growth with b= 2.85. The parameters of the Von Bertalanffy growth equation (VBGF) L∞and
k were estimated by running the program Shepherd’s method included in the FiSAT package. The monthly length-frequency distributions fitted with growth curves, are presented in Fig 3. This routine gave the L∞ = 58.8 cm and
k = 0.13 year
-1 (Table 1).
This value was found to be the best combination of L∞ and
k with the score being 1.0 (Fig 4). This value was further used to obtain the graph of von Bertalanffy Growth Function (VBGF). The VBGF of
Mastacembelus armatus illustrated in Fig 3 indicated that the origin of the growth curve starts in Middle and late April for the group of
M. armatus. On annual basis, the growth of
M. armatus was described by the following Von Bertalanffy growth equations:
L= 58.8 (1 - e- 013(t +1.83))
W = 436.0 (1 - e- 013( t +1.83))2.85
Pauly and Munro (1984) have indicated a method to compare the growth performance of various fish stock by computing a growth performance index (Ø¢) = log
k + 2 log L∞. Generally, the growth performance index (Ø¢) is a species specific parameters, which means that their values are usually similar within related taxa and have narrow normal distributions. The growth performance index for
M. armatus was 2.65.
Mortality coefficients
The Z, M and F of
M. armatus were estimated as 0.75 year
-1, 0.32 year
-1 and 0.43 year
-1 (Table 1), respectively, illustrated in Fig 5.
M. armatus in the Taojiang River, showed low mortality rates which relates to fishing mortality and natural mortality. The exploitation rate was estimated to be 0.57 year
-1.
Length at first capture (Lc)
The length at first capture, Lc of
M. armatus was estimated at 19.51 cm (Fig 6). The L
c was the length at which 50% of the fish are vulnerable to be captured by fishermen. This is the average size of fish vulnerable to fishing or enter the fishing ground, in the Taojiang River (Table 1).
Recruitment pattern
The recruitment patterns of
M. armatus suggested that there was one main pulse of annual recruitment, in agreement with the group separation using Bhattacharya’s Plot. The major pulse appeared in the end of May, or the beginning of June (Fig 7).
Relative yield-per recruit (Y¢/R) and relative biomass-per recruit (B¢/R)
The exploitation ratios were computed as Emax= 0.59, E10= 0.45, E50= 0.31(Fig 8). Here, Emax is the exploitation rate at which, maximum sustainable yield per recruit is obtained, E10 is the exploitation rate which was the marginal increase of relative yield per recruit was 1/10th and E50 is the value of E under which the stock had been reduced to 50% of its unexploited biomass.
Virtual population analysis
From virtual population analysis it was found that the maximum fishing mortality of
M.armatus occurred at the length range between 23 cm and 24 cm indicating low fishing mortality in the juvenile stage (Fig 9). Steady biomass also increased with length class until 36.0-36.9 cm and the total steady state biomass was found to be 19.12 tons.
The “b” value of
M. armatus was found 2.85. The “b” value was estimated to be slightly <3 which indicates relatively lesser body mass in relation to increment in body length (Kannan, 2016). Estimate of L∞ are similar whereas the k in current studies is substantially higher than their estimate. Sparre and Venema (1992) stated that the value of k=1.0 is fast growth, k=0.5 is medium growth and k=0.2 is slow growth. Hence, k=0.13, for
M. armatus obtained from this study is considered as slow growth (Wu, 2018). Mortality means the death of fish from the stock due to fishing mortality or natural mortality including predation, disease and old age. Fishing mortality assumed to be associated with physical injury or physiological stress from being captured in the gear used during capture. Natural mortality (M) and fishing mortality (F) were additive instantaneous rates that sum up to total mortality (Z). The total mortality coefficient, Z= M + F (Gulland, 1971). When comparing mortality rates to the total births or recruits to the population, we can determine if a population is increasing or decreasing.
M. armatus from study area had low total mortality 0.75 year
-1 and fishing mortality 0.43 year
-1. Gulland suggested that the optimum “E” should be 0.5, the exploitation rates 0.57 year
-1 indicated this species was over exploited mainly due to fishing in the study area. Epresent approximately equal E10 and Epresent > E50, indicating that, at the current rate of exploitation, there is the threat of over fishing as >50% of biomass-per recruit is fished and fishing intensity is appropriate for the economy. The maximum fishing mortality of
M. armatus occurred at the length range between 23 cm and 24 cm and the total steady state biomass was found to be 24.12 tons. The present study on this species is a preliminary study as it includes only one year of data. However, since no work about the population dynamics on this species has been done. This preliminary work covering growth, natural and fishing mortality and population parameters would help us understand the present status of species. Thus, all of this information would be the valuable sources for comparison in future, especially when the conservation and management of this fish stock is to be made (Hacer, 2018). The estimated exploitation rate (E) has slightly exceeded the optimum value of E indicating this species was over exploited mainly due to fishing in the study area. The stock has been over exploited mainly due to fishing. Considering recruitment and economic interest, it was suggested that capture intensity should be controlled in a timely manner to prevent the risk of resource collapse of
M. armatusin in the Taojiang River and its adjacent waters.
