Grey mullets (Family: Mugilidae) are extremely important fish, which are cultured in many countries due to their high quality flesh, superior growth and wide salinity and temperature tolerance. The grey mullets are the most important fish species inhabiting Bardawil lagoon. Their catches fluctuated between 733 and 1590 ton during the last 15 years [1] forming about 38% of the total lagoon production and 60% from fish production of the lagoon. In 2016, 1548 ton of grey mullets were landed forming a net profit of about 80 million Egyptian pounds. Mullet's catch is composed mainly from Mugil cephalus , L. ramada and L. aurata , while both Chelon labrosus and L. saliens are found in very small amounts and recorded under the “others” group. Although the mullets in Bardawil lagoon contribute greatly to the economy of Egypt, very limited studies concerning their dynamics and management are available [2-5]. On the other hand, the biology and dynamics of mugilid species were studied in the different water bodies in Egypt [5-13].

Effective management decision making requires knowledge of the state of the fishery relative to management targets or limits, and of the likely responses of the fishery to alternative management options. Providing such knowledge typically in a quantitative form is the aim of fisheries stock assessment [14-16]. To assess fish stock, many detailed information such as age and growth, mortality and exploitation rates should be available. The present paper provides an up to date data about the biology and dynamics of L. ramada in Bardawil lagoon required for proposing some targeted reference points for its management.

Sampling site

Bardawil lagoon (Figure 1) is one of the northern lakes in Egypt and it is a part of the Mediterranean coastal lands of Sinai. It is an important source of local and economic fishes in North Sinai, and it plays an essential role in the fish production in Egypt, where it produces very economically important species of fishes such as seabass, seabream, sole, grey mullet, eel, meager and white grouper. Bardawil lagoon is a shallow hypersaline lagoon. Its area is about 685 km2 extends for a distance of about 95 km, from a point 45 km east of Port Said and extending to a point 18 km west of El-Arish. Its maximum width is 22 km and The average depth is 135 cm with a maximum depth next to inlets 1 and 2, whereas, reach to (2-5) m, while in the main opening reach to (4-7) m. A long sand-bar, (200-1000) m wide maximum, separates the lagoon from the adjacent Mediterranean.

Figure 1: Bardawil lagoon.

Sampling and measurements

Monthly random samples of L. ramada were collected from the Bardawil lagoon during the period from May 2017 to December 2017 and from May 2018 to October 2018 (almost two fishing seasons). The length frequency for about 3000 L. ramada , 1080 males (12 cm to 42 cm TL) and 1920 females (11 cm to 47 cm TL), were grouped in 1 cm length groups.

For each fish data on total length (measured to the nearest millimeter), total weight (weighed to the nearest gram), and scale samples were collected. Age determination from scale readings was performed and only 2650 scale (88.3%) were accepted for age determination, the rest (11.7%) were regenerated scales and were rejected.

Length-weight relationship

The general power equation (W=a L^b) was applied to estimate the length-weight relationship, where a and b are constants whose values were estimated by the least square method. Confidence intervals of 95% were calculated for the slope (b) to see if these were statistically different from 3.

Population parameters

Age composition was constructed based on scales’ readings and the resulting mean lengths at age were used in the construction of growth models. The von Bertalanffy [17] growth parameters were estimated using the Ford et al. [18] version as described by FAO [19]. Total mortality rate Z was computed using three different methods; the semi-logarithmic regression method of Ricker [21] as Ln N_t=L_n No −Zt, this is a straight line with a slop equal to −Z, Jones et al. [22] as Ln(CN)=a+(Z/K)*Ln(L∞−L) where CN is the cumulative frequency, K is the growth coefficient and a and b are constants and Hoenig formula [23] as Z=1/(c1·(t_max−t_c)) and SE(Z)=(c₂·Z2)½ where the SE is the standard error of the estimated Z, c₁ and c₂ are table values and a function of N [24]. Natural mortality coefficient was calculated as the geometric mean of three different methods; Ursin [25] as M=W^−1/3, Rikhter et al. [26] as M=((1.52/t_mass)0.72)−0.16 and Pauly [27] as LogM=−0.0066−0.179 Log L∞+0.e43 Log K+0.4634 Log T. where T is the mean annual water temperature of the lagoon.

Critical lengths and ages

The length at first capture (L_c) was estimated by the analysis of catch curve using the method of Pauly [28]. The length at first sexual maturity Lm was estimated by the empirical formula of Froese et al. [29] as Log L_m=0.8979 Log L_∞−0.0782. While the corresponding age at first sexual maturity (T_m) was computed using the von Bertalanffy growth equation as T_m=t₀−(1/k*ln[1−(L_m/L_∞)]). Besides, the smallest individual in the catch was considered as the length at recruitment L_r.

Relative yield per recruit (Y’/R)

The model of Beverton et al. [30] as modified in Pauly et al. [31] was applied to analyze the relative yield per recruit (Y’/R) of L. ramada as follows:

(Y’/R)=EU(M/K) [1−3U/(1+m)+3U2/(1+2m)−U3/(1+3m)]

Where: M=1−E/(M/K)=K/Z and U=1–(Lc/L∞).

Resource status

Resource status was evaluated by comparing estimates of the fishing mortality rate with target (F_opt) and limit (F_limit) biological reference points (BRP) which were defined as: F_opt=0.5M and F_limit=2/3M [32].

The present study has established some target reference points for management the L. ramada stock which are exploited in Bardawil lagoon, Egypt. These regulations are suggested based on the study of population parameters and biological characteristics of this species including age and growth, length-weight relationship, mortality and exploitation rates and yield per recruit.

Age and growth

Scales were used for age determination of L. ramada collected from Bardawil lagoon. Scales have been proven as a reliable and valid method for ageing grey mullets [3,12,33-39]. The results showed that the maximum life span of L. ramada is five years for males and seven years for females and age group one was the most frequent group in the catch. The back-calculated lengths were (19.25, 28.08, 33.92, 38.71 and 40.94) cm for males for 1st, 2nd, 3rd, 4th and 5th year of life respectively and (20.15, 29.04, 35.19, 39.2, 42.39, 44.89 and 46.31) cm for 1st, 2nd, 3rd, 4th, 5th, 6th and 7th year of life respectively for females. The greatest incremental growth in length occurred during the first year of life and declined gradually thereafter (Figure 2). The same finding was recorded in the previous studies in different localities (Table 1) [3,12,13,33,34,36,37,39-41].

Figure 2: Growth in length and growth increment of L. ramada from Bardawil lagoon.

Table 1: Length at age (cm) of L. ramada given by different authors in the Egyptian water bodies.

Length-weight relationship

A total of 3000 individuals of L. ramada have been collected from the commercial landings in Bardawil lagoon. The total lengths of males varied from 12 cm to 42 cm, while their weights ranged between 20 g and 670 g. The total lengths of females varied between 11 cm and 47 cm TL and between 15.8 g and 790 g in weight. The length-weight relationship equations were: W=0.0173 L^2.7695 for males and W=0.0173 L^2.7687 for females (Figure 3). The allometry coefficient of the length-weight relationship indicates negative allometric growth. Also, there is no significant difference between sexes in length-weight relationship. The b-value is more or less similar to that obtained by the previous workers (Table 2).

Figure 3: Length - weight relationship of L. ramada from Bardawil lagoon.

The difference between estimated b-value between localities and in the same area in different periods is may be due to the fact that b-value could be an indicator of the physiological condition of the fish and vary seasonally in response to seasonal variations in environmental condition and changes in the fish wellbeing [42]. The length-weight relationship and the b value can also be influenced by fishing pressure that excessively catches the adults. This is can explain the difference in growth type in Bardawil lagoon in 2006 and in the present study.

Population parameters

Parameters of the Von Bertalanffy growth function were calculated for L. ramada population in Bardawil lagoon, the parameters of growth were L_∞=45.79 cm TL, K=0.40 per year and t₀=−0.36 year for male, L_∞=49.92 cm TL, K=0.35 per year and t₀=−0.41 year for female and for pooled data were L_∞=50.25 cm, K=0.34 per year and t₀=−0.42 year. Thomsonet et al. [43] mentioned that the average K value for this species is 0.15 per year and varies between 0.11 and 0.45 per year, which is in accordance to the findings of our study. The K-value indicates that this species undergoes a rapid growth. Also, the negative t₀ value worked out in the present study indicated faster growth rate at the juvenile stage [19,44-46].

As there is no sex specific fishing gear in Bardawil lagoon, the growth parameters for sexes combined were used for the subsequent calculations. Total mortality (Z) was estimated at 1.38 per year, natural mortality was estimated at 0.42 per year, while the fishing mortality was estimated at 0.96 per year. The exploitation ratio was computed at 69.6%. Based on the mortality data, the percentages of non-survivors were: From natural causes =1−e-M=34.3% and from fishing =1−e- F=61.7%.

Critical lengths

Length at first sexual maturity Lm is of great importance when determining the optimum mesh size and SSB. The Lm of L. ramada in Bardawil lagoon fishery was found to be 28.2 cm TL for sexes combined. The corresponding age was 1.9 yrs, which means that the exploited L. ramada must be protected till their second year of life in order to be able to spawn at least once. The estimated length at first capture L_c in the present study was 22.9 cm TL. Both the estimated Lc and the observed lengths of fish captured indicated growth and recruitment overfishing. In the light of these results, a minimum size limit should be implemented for L. ramada in Bardawil lagoon.

Reference points and management

The relative Y/R analysis (Figure 4) showed that the EMSY was 0.65 which is lower than the current one (0.696), so the current E should be reduced from 0.696 to 0.65. For the sustainability of this fishery resource, it should be work at E value that conserves 50% of the spawning stock. In the present study the E_0.5, the target reference point, was 0.32 and the current E should decrease from 0.696 to 0.32.

Figure 4: Relative yield per recruit analysis of L. ramada from Bardawil lagoon.

In conclusion, the obtained F (0.96) and E (0.696) values revealed that the thinlip grey mullet in Bardawil lagoon is overexploited. It is worth mentioning that the excessive fishing effort is not the only reason responsible for the declining of fishery resources in the lagoon, the illegal harvesting of fish fry and spawners, using destructive fishing methods as well as gazzering the sea grasses which represent the shelter and nursing areas for grey mullets are other important reasons. There are many efforts now to maintain the different fish stocks in the lagoon such as prohibition of destructive gears, dredging the boughazes (the connections with the sea) and monitoring the illegal fishing practices in the lagoon but more detailed study about the interaction between species and between species and the ecosystem as well as the multiple objectives of all stakeholders should be done.