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Population Parameters of the Orange Mud Crab Scylla Olivacea (herbst, 1796) from the Sundarban Mangrove Forest in Bangladesh

Muhammad Abdur Rouf*
Fisheries and Marine Resource Technology Discipline, Khulna University, Khulna, 9208, Bangladesh

Sheik Istiak Md Shahriar
Department of Aquaculture, Hajee Mohammad Danesh Science and Technology University, Dinajpur, 5200, Bangladesh

Al-Hasan Antu
Fisheries and Marine Resource Technology Discipline, Khulna University, Khulna, 9208, Bangladesh

Md Noman Siddiqui
Fisheries and Marine Resource Technology Discipline, Khulna University, Khulna, 9208, Bangladesh

Abstract/ Executive Summary:

Population parameters of orange mud crab (Scylla olivacea) were estimated, aiming to determine sex ratios, carapace width-body weight (CW-BW) relationships, asymptotic width (CW∝), growth coefficient (K), mortality(Z, M, and F), recruitment and level of exploitation (E) in the Sundarban mangrove forest, located in the south-western part of Bangladesh. Year-round samples were collected using hook-lines and baited traps, the population parameters were measured from CW frequency data using FiSAT-II analyzer. The study showed that the overall male and female sex ratio was 1:0.66, revealing a male domination in the study area. The CW-BW relationship indicated that the increment rate in the BW of the male crabs (b=3.06, R2=0.98) was higher than that of female (b=2.62; R2=0.98)S. olivacea. The b value differed significantly (P<0.006) from isometric growth (b=3) where males exhibited positive and females exhibited negative growth allometry. Estimated CW∝for male and female were 164 mm and 152 mm along with K values 0.90 yr1and 0.76 yr1, respectively. Total mortality (Z)was 2.67 yr1and 1.57 yr1, natural mortality (M) was 0.98 yr1and 0.90 yr1andfishing mortality (F) was 1.69yr1and 0.67 yr1for male and female, accordingly. Recruitment of both sexes exhibited a bimodal recruitment pattern where young population occurs continuously throughout the year and a major peak of recruitment for males was observed from November to January and for female, it was from February to April. The estimated exploitation rate (E) for male (0.63) was higher than the female (0.43) where the E for male exceeded the maximum permissible limit (E=0.50). A remarkable range of fishing pressure at lower size classes was revealed in this study and thus framing minimum legal size is crucial for effective management of the population.

Keywords: Growth, Recruitment, Mortality, Exploitation, Scylla olivacea, Bangladesh

Location: six (6) sampling stations at Shyamnagar Upazila adjacent to Sundarban mangrove forest, Bangladesh

Start Date: 00-07-2017

End Date: 00-06-2018

Program Area: Resource Development and Management

Commodity: Crab

Objectives:

Therefore, there is a gap of information on the population parameters of mud crabs in the south-western part. Considering the above-mentioned importance, this study aimed to estimate the key population parameters including sex ratio, carapace width-weight (CW-BW) relationship, asymptotic carapace width (CW∝), growth coefficient (K), recruitment, mortality (Z,M and F), exploitation level (E) and virtual population analysis (VPA) of S. olivacea.These data are necessary for formulating proper management and conservation policies for mud crab resources in Bangladesh.

Methodology:

Study area and sampling: The study was carried out considering six (6) sampling stations at Shyamnagar Upazila adjacent to Sundarban mangrove forest, Bangladesh. Sampling was done fortnightly over a 12 months period from July 2017 to June 2018 using two types of gear. Hooks and lines are mostly used in this area for crab harvesting. A total of 15 hooks baited with small fish (Cuchia) were set at each station during flood and ebb tide. Baited basket traps were also placed with same bait at a distance of 25–30 m in shallow water approximately for 6 h.

Data collection: A total of 3191 specimens of S. olivacea, comprising1969 males and 1222 females, were collected during the study period. The species and its sex were identified based on the morphological characteristics following Keenan et al. (1998). The CW was measured using digital slide calipers to the nearest 0.05 mm and weighed using anelectronic balance close to 0.1 g.

Data analysis: The sex ratio was determined by dividing the total number of females by the total number of males of the respective size class. The CW-BW relationship if. olivacea was described for each gender using the method potency equation W=aLb(W=BW in g, L¼CW in mm, a and bare constants). The observed CW and BW values of individual crabs were log-transformed and subjected to linear regression analysis (log BW¼logaþb*log CW). The significance of coefficient b was tested using the student's t-test at a 5% significance level. One-way analysis of variance(ANOVA) was used to analyze the relationship between CW and BW.

Asymptotic carapace width (CW∝) and growth coefficient (K) of the von Bertalanffy equation for growth in terms of CW were estimated by means of Electronic Length Frequency Analysis (ELEFAN-I) incorporated in the FiSAT-II (FAO-ICLARM Stock Assessment Tools) using the formula.

Source of Information: Heliyon 7 (2021) e06223

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

To sum up, the CW-BW relationship of S. olivacea implied that males are quite heavier than that females whereas the overall growth performances were comparatively slower. Possible spawning season has been obliquely predicted using length-frequency distribution data; however, finding actual spawning season for the S. olivacea in Bangladesh depending on real-time data needs to be focused where this study can be evident as reference point. Besides that implementation of effective management strategies like catch size restriction, limiting number, efficiency and types of fishing units could be effective to reduce the enhanced fishing pressure on S. olivacea in the Sundarban mangrove of Bangladesh. In Bangladesh, crab fishing is officially prohibited in Sundarban and its adjacent areas from January to February. However, attention of national policymakers is necessary to consider the observations reported in this study in order to protect the population and facilitate the local agencies to implement responsible fishing and sustainable utilization of mud crabs in Bangladesh.

Knowledge Management: Journal