Biological characterization of wild cobia, Rachycentron canadum (linnaeus, 1766) off Dungun coast, Terengganu, Malaysia for stock management and aquaculture

• Main Author: Ademola, Babatunde Taofik
• Format: Thesis
• Language: English
• Published: 2016
• Online Access: http://psasir.upm.edu.my/id/eprint/68671/1/fp%202016%2029%20ir.pdf; http://psasir.upm.edu.my/id/eprint/68671/
• Institution: Universiti Putra Malaysia

Investigation of some aspects of biology of a recreationally and commercially important fish, cobia, Rachycentron canadum in Dungun coast, Malaysia was conducted between April 2014 and May 2015. A total of 249 specimens were opportunistically collected from the catches of trawl net, gill net and by hooks and line. The total length ranged from 39.20 to 143.00 cm (88.29 ± 1.28 cm), fork length ranged from 37.50 to 124.00 cm (78.69 ± 1.09 cm) while the wet weight ranged from 0.50 to 20.40 kg (5.51 ± 0.26 kg). There was no significant difference in all the morphometric characters between sexes except for head width (HW) (p < 0.05). Ten (10) out of the seventeen (17) truss distances in the body landmarks of cobia showed significant differences between males and females. The equation of length-weight (LW) relationship for combined sex was W = 0.000002FL3.3204 (R² = 0.92). The growth of cobia in Dungun water followed positive allometric as the growth coefficient (b) was higher than 3. Observed length frequency of both sex followed a normal distribution pattern and there was a predominance of cobia in the medium size range from 65 - 85 cm fork length for the females and 55 - 90 cm for the males. The sex ratio of females to males was 1:1.18 and it was not significantly different from the expected 1:1 (X2 = 2.12 df = 1; p < 0.05). Growth parameters of von Bertalanffy growth formula for cobia estimated were L = 142.00 cm, K = 1.20 yr-1, while natural mortality rates (M), fishing mortality rate (F), total mortality (Z) and Exploitation rate (E) estimated were 1.34, 0.95, 2.29 and 0.41 respectively. The maximum allowable limit of exploitation (Emax) was estimated to be 0.56. The sizes attained by cobia at the end of age 1 (year) was 99.28 cm. The estimation of fork length at first maturity (Lmat) from the plot of gonadosomatic index (GSI) against fork length (FL) indicated early maturation of males compared to females with respective value of 70 cm and 72 cm. Males with mature gonads were encountered throughout the year, while the proportion of mature females was higher in March and November, and this may be taken as the peak of the spawning period, even though females with hydrated oocyte were also obtained in April, May, June, July and September. The mean batch fecundity (BF) for all matured females with hydrated oocytes was ranged from 54886 to 4316648, (11,37,317 ± 183712) while average eggs per gram of ovary were ranged from 2108 to 5413 (3427.032 ± 128.73) . Weak positive correlation (r2 = 0.48) was found between the BF and female fork length with equation BF = 38732.03FL-2200000. Similarly, BF was significantly correlated with ovary-free body weight (r2 = 0.56). Histological data support continuous spawning characteristics of cobia as oocyte size frequency distribution showed progressive batch development. Increase in GSI was observed from September to November and it reached the peak in March which corresponds to the period when the highest proportion of matured females was caught. Study on the stomach content of the cobia, R. canadum in Dungun coast, showed that the bony fish was the dominant group in the diet with crustacean and mollusc contributing less. The percentage index of relative important (% IRI) values of fish, crustacean and mollusc were 72.27%, 2.98% and 24.75% respectively. Out of the 231 cobia specimens examined in this study, 111 were found to contain food in their stomach, out of which 98 had at least one identifiable prey items. Gut content analysis revealed that fish was consumed by cobia all the year round with Hilsa sp., Alepes sp. and Carangides sp. being the frequently encountered species. Higher stomach fullness index (SFI) was recorded in the month of April, June and November with values of 2.88, 2.06 and 2.42 respectively while lower SFI was in the month of May and August with values of 0.67 and 0.86 respectively. The percentage of empty coefficient (EC) was higher in October, August and July with values of 80%, 70% and 69% respectively, intermediate in the remaining month as values were about 50%. This stomach fullness index SFI and EC suggest moderate feeding intensity of cobia in the waters of Dungun. The proximate and chemical composition of cobia species from Dungun and its seasonal changes was examined. The result showed that lipids content was significantly (p < 0.05) higher in females (6.38 ± 0.19) than males (5.44 ± 0.11). No significant (p < 0.05) difference was found in the moisture, ash, total cholesterol and malondialdehyde (MDA) based on season, sex and feeding regime. In the fatty acid (FA) composition of the muscle tissue, the unsaturated class was dominant followed by saturated FA followed by the monoenes. Similar trend was found in the liver. Total n-3 poly-unsaturated fatty acid (PUFA) was higher compared to the total PUFA (n-6) in the muscle, and the total PUFA (n-6) was significantly (p < 0.05) higher in the muscle during the monsoon. Other FA classes that showed significant (p < 0.05) seasonal differences in the muscle were pentadecanoic acid, palmitoleic acid and linoleic acid. However, in the liver, the total saturated, total monones, total PUFA (n-6) and total PUFA (n-3) showed significant (p < 0.05) seasonal variation. The FA composition of males and females cobia did not show significant (p < 0.05) differences. During starvation in cobia, a significant reduction in total PUFA n-6, pentadeconoic acid, and palmitoleic acid was observed in the muscle. The result of this work suggests adult cobia in Malaysia preferred bony fishes as food while it reproductive period was continuous and cobia lipid profiles contained appreciable quantity of PUFA such as docosahexaenoic, eicosapentaenoic and arachidonic acids and it can be promoted for human consumption.