Population dynamics of the green tiger prawn, Penaeus semisulcatus de Haan, 1844 in the Saudi coast of the Arabian Gulf

The extensive coastal areas of the Gulf Council Countries (GCC) region have supported commercial and artificial fisheries for many decades (Al-Abdulrazzak et al., 2015; FAO, 2012). P. semisulcatus is one of the most important commercial crustacean species in the Arabian Gulf because of the significant contribution to the country’s economy (Van Zalinge, 1984; Hosny, 2007; Rabaoui et al., 2017). The total marine landing of shrimp of Saudi Arabia over the last decades have been varied considerably, reaching 4039t in 1988 before declining significantly to 1084t in 1991 (Siddeek et al., 1999) because of the Gulf War, thereafter increased to 8314t in 2014 (FAO, 2012 and Ministry of Environment Water and Agriculture, 2014). Such production variations could be the reason of high market price in the local as well as foreign markets.

A variety of studies has been conducted on the P. semisulcatus biology and ecology in the Arabian Gulf (Siddeek, 1991; Siddeek et al., 1999; Abdulqader and Naylor, 1995; Ben Hasan et al., 2018). This shrimp species occurs mostly in shallow, soft-sediment and coastal habitats with early life stages preferring the sheltered productive habitats of the Arabian Gulf (Abdulqader, 1999; Bishop and Khan, 1999; Al-Husaini et al., 2015; Rabaoui et al., 2017). Nursery grounds of penaeid shrimp larvae was recently studied by Rabaoui et al., (2017) which enabled to identify all coastal nursery areas of the green tiger shrimp along the Saudi waters of the Arabian Gulf. Numerous studies have been conducted so far on the biology and ecology of green tiger prawn in the Arabian Gulf of Kuwait (Siddeek, 1991 and Ben Hasan et al., 2018) and Bahrain (Abdulqader and Naylor, 1995 and Abdulqader, 1999) as well as in the Red Sea coast of Yemen (Abdul-Wahab, 2014) and Saudi Arabia (Alsolami and Jastania, 2017) but no such studies has been conducted in the Arabian Gulf of Saudi Arabia. Recently, Rabaoui et al., (2017) reported the biology and ecology of Penaeus semisulcatus juveniles in the coast of Saudi Arabia. However, it is still needed to observe biology and ecology of adult P. semisulcatus for better management of this species in future. Therefore, the present study was conducted within this regard and it aims to enrich the knowledge on the life history traits and populations dynamics of P. semisulcatus in the western Arabian Gulf. The specific objectives of this work are therefore i) to describe the size structure and sex-ratio of the green tiger prawn population occurring in Dareen area, ii) to examine the gender variations of growth parameters of this crustacean species and iii) to determine the other major parameters of population dynamics such as total mortality, natural mortality and fishing mortality. These parameters will certainly enable to evaluate the shrimp fisheries in Dareen area and help to make necessary recommendations for the protection and sustainable management of this valuable marine resource. 

The green tiger prawns were collected on monthly basis between May 2013 and April 2014, three sampling months were not observed because of the severe weather conditions at the specified days of monthly sampling. Prawn samples were collected from a fishing trawler using an otter trawl gear (cod end mesh size 1.25 cm) from two stations off Dareen (N 26°32' 600''E 50°04' 424'' and N 26°28' 883'' E 50°16' 818'', Fig 1). Three hauls were carried out in each station with a total duration of 30 min per haul. Parallel to sampling, sea surface temperature was measured in the sampling stations using a mercury thermometer. Collected samples were immediately transported in ice chests to the laboratory where they were sexed and kept for further analyses.



The carapace length (CL) and total weight (W) of all individuals collected were measured to the nearest 0.1 mm and 0.1 g using digital Vernier calipers and digital balance, respectively. CL was taken from the posterior edge of the orbit to the dorsal posterior edge of the carapace. The length frequency data of related samples were classified separately into 2-mm intervals. The length-weight relationship of Penaeus semisulcatus was determined for both males and females using the commonly used equation of King (2007):
 

W = a L^b

 
Where,
W= Weight (g).
LC= Carapace length (mm).
a= Intercept.
b= Slope.

The growth parameters of the green tiger shrimp, asymptotic carapace length CL_¥ and growth rate K, were determined from the carapace length-frequency data using FiSAT II and based on the von Bertalanffy growth function (VBGF):
 

CL_t = CL_¥ [1- exp-k (t-t₀)]
 

Where,
CL_t= Carapace length at age t.
CL_¥= Asymptotic maximum carapace length.
K= Growth rate.
t₀= Age when carapace length is zero (King, 2007).                                   

Both total (Z) and natural (M) mortalities were estimated as well. Total mortality rate (Z) was estimated using the length-converted catch curve method, which was involved by the integration between ELEFAN II and Beverton and Holt’s (1956) formulas. M was estimated using Pauly’s (1980) empirical equation incorporated in FiSAT II:
 

Log M = -0.0066-0.279 Log CL_¥+0.6543 Log K+0.4634 Log T

 
The estimation of M and related parameters requires the growth parameters CL_¥ and K obtained from VBGF. Since seawater temperature can have an effect on the abundance of shrimps and hence on the catch, the annual average sea temperature T was taken into consideration to assess the natural mortality; in this study T = 25°C. Fishing mortality F was computed according to the equation:
 

F = Z - M

 
Where as shrimp exploited rate E was estimated as follows:
 

E = F/Z

(Gulland, 1971).  

The probability of the mean length of the first capture Lc was calculated from length-converted catch curve using Pauly’s method (1984).

A total amount of 13.145 kg (1134 individuals) of P. semisulcatus was used for this study. The biggest sample catch was found in July 2013 (160 females and 212 males) whereas the smallest sample (18 females and 16 males) was in winter (December 2013) with summer catch was accounted for over 48%. Similar studies indicated also that summer catch is more than other seasons (Hettler and Chester, 1982; Costa et al., 2005; Castilho et al., 2008; Luchmann et al., 2008). The average CL was 38 mm for females and 36.7 mm for males, with highest values were found in July and the lowest in December 2013. These findings were also found in most of the fisheries studies in tropical and subtropical waters (Farmer, 1986; Ye et al., 2003; Amin et al., 2009; Mehanna et al., 2012b; Abdul-Wahab, 2014). The overall sex ratio was estimated at 1:0.85 for males and females with 10% more males comparing to females along the study period. The monthly variations in sex ratio were not significant (P>0.05; Fig 2). Taking into account the length groups, males dominated the catch of the range 38-46 mm, while females dominated the large individual sizes (>48 mm). This indicates that females of the green tiger prawn are more exposed to commercial catch because of their bigger size compared to males. This observation is concordant with those of Hosny (2007) on the same species and Mehanna et al., (2012a) on the Indian prawn, Fenneropenaeus indicus. Some researchers observed variations of length-weight relationship between the sexes for green tiger shrimp (Maheswarudu et al., 2011) while other did not for green tiger shrimp (Mohamed and El-Aiatt, 2012), chub (Benzer, 2013), dark sleeper (Haihua and Yanping, 2018) and sergestid shrimp Amin et al. (2009) which is in accordance with our findings (Fig 3).

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Table 1 summarizes most of the biological parameters of P. semisulcatus reported so far from the Arabian Gulf and some adjacent areas. The estimated growth parameters for both sexes were calculated and the asymptotic carapace lengths CL_¥ were found (female = 62 mm and male 51.50 mm). Growth rates K attained 1.10 yr^-1 for females and 1.77 yr^-1 for males (Fig 4). These findings were close to most of the previous studies on the life history of P. semisulcatus in the Arabian Gulf and adjacent areas (Mohammed et al., 1996; Hosny, 2007; Mehanna et al., 2012b; Mohammed and El-Aiatt, 2012; Rabaoui et al., 2017). A significant difference for both sexes was found with the only in the study conducted in Dareen by Ye et al., (2003) who found that CL_¥ and K for males and females were found to be 27.8 mm and 38.32 mm and 2.53 y^-1 and 3.27 y^-1, respectively. It appears that the difference of the growth parameters of the same research area might be attributed to some changes of trophic and environment conditions occurred in ten years period.  



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Total mortality coefficients Z were close for both sexes; 3.37 yr^-1 and 4.65 yr^-1 for females and males respectively (Fig 5 and Table 1). Estimations of natural mortality M were also calculated for females (1.47 yr^-1) and males (2.12 yr^-1). The range of Z values recorded in the previous studies was (3.24-7.84 y^-1) for males and (4.7-9.67 y^-1) for females while the range values for M were (1.05-2.5 y^-1) for males and (1.1-2.4 y^-1) for females. This study records Z values were found close to the lowest values of the Z range whereas M values illustrated consistency with most of the values reported by other authors in Table 1. Both fishing mortality (F) and exploitation rate (E) were calculated and results showed a marginal variations between males and females for F values but not for E values (Fig 5). F values calculated for both sexes were found to be the lowest values compared to previous records, female was 1.90 yr^-1 and male was 2.53 yr^-1. Exploitation rate ‘E’ were estimated at 0.56 yr^-1 and 0.54 yr^-1 for females and males respectively. In contrast with our findings, most of the studies on the population dynamics of P. semisulcatus in the Arabian Gulf recorded higher exploitation rate (E) values but similarities was showed for fishing mortality (Table 1). The lengths at first capture Lc (CL₅₀) were estimated at 42.2 mm for females and 41.52 mm for males (Fig 6). These findings of high Lc contradict (Mehanna et al., 2012a, Mehanna et al., 2012b and Mohammed and El-Aiatt, 2012).

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Knowledge of biological studies is important for adequate management and maintenance of fish stocks (Dirican et al., 2012). It can be inferred from this study that the exploitation rate of both males and females of P. semisulcatus in Dareen area is slightly higher than the optimum level of exploitation (E H» 0.5). In terms of fishery manager, these results indicate that fisheries managers should take necessary action to reduce the total fishing efforts. Actions should be also taken within extended periods, towards increasing the mesh size of the trawl net to catch larger individuals, which can guide to reduce the exploitation rates for both sexes.

We would like to thank the deanship of scientific research in King Faisal University for the fund specified for this work (No: 140004). We are also grateful to Mr. Jehad Matar who helped in the sampling surveys, to the technicians who helped in the Lab work (Mr. Rashed Alnjada and Mr. Mohammed Alawad) and to the colleagues who revised the draft manuscript and contributed to improve its quality before its submission (Dr Lotfi Rabaoui). Thanks should also be addressed to Dr. Abdul Raheem Saleh for providing the map of the study area.