Size Structures, Length-Weight Relationships and Condition Factors of Synodontis obesus (Boulenger, 1898: Siluriformes, Mochokidae) in the Lower Cross River, Nigeria

Abstract

Size Structures, Length-Weight Relationships and Condition Factors of Synodontis obesus ((Boulenger, 1898: Siluiformes, Mochokidae) were carried out with standard methods in the Lower Cross River, Nigeria to assess the population dynamics of the rarely studied but commercially important species. A total of 241 specimens (124: wet season and 117: dry season) were collected at Ayadehe Head Bridge fishing port in Itu Local Government Area, Akwa Ibom State, Nigeria from August 2020 to January 2021. The overall abundance showed high size variability within months and between seasons. The total biomass of the specimens collected was heavier in the wet season (12020.33g =12.02Kg) than in the dry season (10709.3g =10.7Kg) with an overall biomass of (22729.63g =22.7Kg). The overall size structures were: 15.0 – 28.30 cm, mean: 21.52.44 cm TL; 12.0 – 20.5 cm, mean: 15.881.6 cm SL and 13.20 – 21.50 cm, mean: 16.881.64 cm FL. The total weight of the species varied from 42.30 – 166.0g with overall mean value of 94.31603g. The length and body weight of fish samples caught for both wet and dry season were significantly different (p<0.05).The length composition of the species exhibits unimodal, bimodal and trimodal growth patterns. The species had heterogeneous groups or cohorts (1+, 2+ and 3+) with variation in body weight. The length-at-first maturity of S. obesus was 20.85 cm mid length. Growth coefficient (b) varied between (2.497) for the wet season and (2.617) for the dry season respectively with the mean growth coefficient (b=2.532) in the LWR (W= aLb). The species exhibited negative allometric growth pattern with an increase in length resulting to increase in weight of the fish. The condition factor (K) decreased with increased in the size of fish and ranged from 2.17-2.57 (Mean: 2.35) in both wet and dry seasons. The average condition factor (K) (2.35±0.15) was 1.0 for both seasons revealing that the species was in good physiological state of well-being. The condition factor (K) of S. obesus by size category showed K-values for recruitssexually mature fishaged fish. Thus fish size is an exponential index of condition factor. The results of this study constitute valuable fisheries data that would enhance the availability, conservation, valorization, exploitation and sustainability of S. obesus in the Lower Cross River, Nigeria.