Comparing feeding niche, growth characteristics and exploitation level of the giraffe catfish Auchenoglanis occidentalis (Valenciennes, 1775) in the two largest artificial lakes of northern Ghana
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The stomach contents of the giraffe catfish, Auchenoglanis occidentalis, populations from Lake Bontanga and Lake Tono, two artificial lakes, were analysed, together with length frequency data collected from July 2016 to June 2017, to gain knowledge of the stock bioecology and exploitation status. The feeding characteristics of the giraffe catfish did not differ significantly between the lakes, as revealed by a Wilcoxon rank-sum test (p > 0.05). Insect larvae and algae dominated stomach content, with proportionate contributions of 43.8% and 14.2% in Lake Bontanga and 49.3% and 10.6% in Lake Tono, respectively. In the larger Lake Tono, the growth coefficient (K = 0.34 year) and asymptotic length (L∞ = 38.3 cm) were higher than in Lake Bontanga and the exploitation rate was comparatively low (E = 0.24). This lower exploitation level in Lake Tono agrees with a higher mean catch size of 27.6 cm and a high spawning stock biomass >0.4 of the unfished biomass, as well as a higher spawning stock biomass of 3.12 tonnes km−2, suggesting that there is scope for an intensification of the fishery. In the smaller Lake Bontanga, the species growth was lower (K = 0.31 yr−1 and L∞ = 28.9 cm) and the stock is fully exploited (E = 0.48). The mean catch size and spawning stock biomass were critically low; 17.2 cm and <0.4 of the unfished biomass, respectively. Accordingly, this stock requires close monitoring to prevent resource depletion.