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BIOLOGY AND POPULATION DYNAMIC OF BLUE SWIMMING CRAB Portunus pelagicus IN BARDAWILL LAGOON, NORTH SINAI, EGYPT

Crabs are considered non-native crustacean species in Bardawill lagoon which require more studies. Length frequency analysis were used to estimate size distribution, growth, maturity, mortality and yield per recruit of Blue swimming crab, Portunus pelagicus in Bardawill lagoon during the fishing season 2016. Samples were obtained from trammel fishing vessels. Carapace width of 1068 samples ranged from 4.7 to 13.4 cm and total weight from 10.4 to 198.3 g. Size distribution showed the major portion of the catch ranged of 8 to 10.9 cm carapace width. FiSAT II computer program was used to estimate the growth parameters as L∞ = 14.3 cm and K = 1.1 yr. The growth performance index (ø’) equal 2.358. Recruitment was extent from June to October. Sex ratio between male and female were 1.09: 1. The size at first maturity (CWm50) was estimated at 9.1cm. Total, natural and fishing mortality rates were 4.25 yr, 1.6 yr and 2.65 yr, respectively. Size at first capture (CWc50) was 8.6 cm. So, stock of Blue crab in Bardawill lagoon should be controlled by increased the fishing effort and adding unconventional fishing methods such as traps with traditional fishing such as trammel, trawlers and gillnetters, as this species may be become dangerous on native species in region.


INTRODUCTION
Bardawill lagoon is not native region for crabs. Blue swimming crab Portunus pelagicus occurred in the Eastern Mediterranean Sea as a Lessepsian migrant (Smith, 1982;Foka et al., 2004). Bardawill lagoon is characterized by its catches of economic fishes as Bream, Bass and Sole fish. Oftentimes, crabs are seen as an annoying species in fishers' gears. In past few years, crabs have increased dramatically in catches, which have almost dominated in productivity. Crabs was accounting for 41.9% of the total landing in the fishing season, 2015 (GAFRD, 2017). Despite the socio-economic importance of crab fishery in lagoon, many aspects have not been comprehensively studied. Some studies have been conducted of this species in the lagoon such as population biology (Abdel-Razek et al., 2006); Ecological and population dynamics (Emam, 2010) and age determination (Kilada and Ibrahim, 2016 The relationship between invasive species and decline of native species is widely accepted by scientists. Crabs have not yet been determined are they wanted or unwanted species in Bardawill lagoon as invasive species may be having negative effects on fisheries as recorded by Walton et al. (2002) and Gurevitch and Padilla (2004). Growth, distribution, recruitment and reproductive strategy of this species may be makes a threat to lagoon in future. Biology and fishery of crab need of much an evaluation in relation to fishers' interests compared to native species and will be necessary to provide some information in an attempt to develop or control it in the future if it has any negative impact on the main catch in the lagoon. Therefore, this research was done of crab fishery.

MATERIALS AND METHODS
This research was done in Bardawill lagoon ( Fig. 1), eastern Mediterranean Sea.
Sampling was collected bimonthly (twice a month) for one fishing season from May to December, 2016. The research was cover three landing sites; west, central and east lagoon. Samples were collected from trammel fishing vessels with mesh sizes of 5.5 (square mesh ≈27 mm; the distance between two sequential knots) and 6.5 cm (square mesh ≈33 mm). Samples were iced and taken to the laboratory for analysis. By U-shaped or V-shaped abdomen was determined the male and female individuals. Length frequency analyses of 1068 individuals (557 males and 511 females) were used to assess of blue swimming crab P. pelagicus stock. Carapace width (CW) base of the largest lateral carapace spines were measured to nearest 0.1 cm. weight was recorded to the nearest 1 gram using a digital balance (one gram). Samples classified into one cm class intervals based on their carapace width (CW).
ELEFAN-routine in FiSAT II computer program was run to estimate the growth parameters (L∞ and K) for fish stock assessment (Gayanilo et al., 2003).
von Bertalanffy growth equation was used to describe the growth as: CW t is the carapace width at time t; CW ∞ is the mean carapace width of very old crabs; K is the von Bertalanffy growth coefficient; t 0 is the time at which carapace width is theoretically zero.
FiSAT II was used to obtain the monthly proportion of crab recruitment. The length-converted catch curve method (Pauly, 1983) was used to estimate the instantaneous rate of total mortality (Z) by using the FiSAT program. Two methods were used to calculate the natural mortality (M); Jensen (1996) by M=1.5×K and Hewitt and Hoenig (2005) as M= 4.22/t max (K the body growth coefficient and t max is the oldest fish). The fishing mortality (F) calculated as F = Z -M and the exploitation rate (E = F/Z). The probability of capture was estimated from length-converted catch curve, using the running average technique to determine L 50 .

RESULTS AND DISCUSSION
Carapace width of 1068 individuals of Blue swimming crab, Portunus pelagicus ranged from 4.7 to 13.4 cm (Fig. 2)  Monthly distribution of average sizes are presented in Fig. 3. The smallest size (4.2 cm) was recorded in May and the largest size (13.5 cm) was reported in November.
Bertalanffy growth equation has the form of CW t = 14.3× (1-e -1.1(t-(-0.177)) ) in this study according to Rugolo et al. (1997) as resulted that, the Bertalanffy model adequately described blue crab widths at ages. The mean length distribution using a modal progression analysis was given cohort through its monthly growth progression (Fig. 4). Recruitment of crabs in Bardawill lagoon takes throughout the fishing season with one peak which was extent from June to October (Fig. 5). Crab recruitment is influenced by many factors as water characteristics, fishing, sex ratio, predators, and disease (Sugilar et al., 2012;Green et al., 2014).
Sex ratio between male and female were 1.09: 1. This ratio may be related to the age and population dynamics. The total instantaneous mortality (Z) equaled 4.25 (Fig. 6). The instantaneous mortalities; natural (M) and fishing (F) equaled 1.6 and 2.65, respectively. Mortalities are an important factor in constructor of stocks. The absence of predators may be reflecting the lower of natural mortality of crab, that resulting abundance this species. Development of Sea bass stock in the lagoon is one of the important measures to control crab stocks. Invasive species are frequently associated with the decline of predator populations (Gurevitch and Padilla, 2004).
The size at first capture (CWc 50 ) of crab in this study equal 8.6 cm (Fig. 7). The study concluded that, the stock of blue crab was characterized by lower of growth parameters, domination of small sizes and earlier

Length-Converted catch curve
Relative age (years-to) sexual maturity. These are indicators of the low economic importance of the species. Therefore, lower quality and price was observed compared with other species in the study area. The study recommended that, Blue crab in Bardawill lagoon may be become dangerous on native species; therefore, stock of this species in region should be controlled by: Increase the current fishing effort. Adding unconventional fishing methods such as traps with traditional fishing such as trammel trawlers and gillnetters. Development of predator stocks such as bass fish.