Abstract
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.
Highlights
Abobi, Oyiadzo and Wolff a medium-sized anal fin
Food spectrum of Auchenoglanis occidentalis Lake Tono had more full and half-full stomachs than Lake Bontanga, whereas quarter-filled stomachs were more predominant in Lake Bontanga
Insect larvae and fish parts occurred in 30% and 3.3%, respectively, of the total stomachs examined at Lake Bontanga and in 35.6% and 3%, respectively, of those examined at Lake Tono (Figure 2)
Summary
Oyiadzo and Wolff a medium-sized anal fin. The mouth is supported dorsally by the premaxilla and part of the maxilla (Risch 1985). Lake Tono is a large lake formed by two water sources. It has dense aquatic vegetation in the littoral zones, which become inundated during the rainy season. The objectives of the study were to provide information on: (i) the food items ingested by the species and their relative abundance in the two lakes; (ii) the von Bertalanffy growth parameters (asymptotic length and growth coefficient) of the species; (iii) the population size (absolute and per area) and (iv) the fisheries exploitation level, biological reference points and length-based indicators for sustainable levels of exploitation. The first was based on an analysis of the length frequency data using the TropFishR software (Mildenberger et al 2017) in estimating growth parameters and exploitation rates from a catch-curve analysis. The second was based on the use of length-based indicators (Froese 2004; Cope and Punt 2009) in estimating the spawning potential of the species under the current exploitation regime
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