The biology and ecology of Oreochromis mossambicus (Peters, 1852)(Pisces: Cichlidae) in the Sepik River, Papua New Guinea : with reference to the management of the fishery
The introduced species Oreochromis mossambicus was studied to determine biological and ecological factors which may underly the rapid initial distribution of the species from its introduction in the 1950's to the relatively low catch rates experienced in recent years. Actual yield, determined from catch statistics for all species, was significantly lower than theoretical yield, based on formulae developed from other similar river systems. The importance of O. mossambicus to the fishery was indicated by the proportional contribution of this species to the catch (approximately 50%). Fluctuations in catch may be related to changes in water levels with a significant decrease in O. mossambicus catch as the water levels rose and this species moved onto the floodplain areas. Analysis of stomach contents revealed a predominance of diatom species, algae and flocculent detritus. There were some differences in diatom species consumed between sites and season although the general food quality was thought to be good based on monthly condition factors. Reproductive traits were similar to those found in other areas. Females incubated the fertilised eggs orally and behavioural differences were noted from changes in sex ratios and mean size caught each month. Condition of females was significantly lower during the dry season post spawning which could relate to the behavioural trait of mouthbrooding when females do not feed. Mean fecundity was low (475 per 100g body weight) and there was evidence of bimodal peaks in spawning activity which corresponded to rising and falling water levels. Growth throughout the year was not significantly affected by the potential changes in food availability created by the degree of floodplain inundation and an increase in allochthonous food sources. Scale rings were found to be fonned bi-annually and may correspond to increased feeding prior to spawning. CPUE varied between sites but was low overall and von Bertalanffy growth parameters indicated that males reached a larger maximum size than females over the same time period (L∞ = 35 and 26.56 respectively). Total mortality was relatively high (Z = 0.56/yr) but figures may relate to local conditions only since many habitats and environmental conditions would be represented in a river of the size of the Sepik. The potential impact of proposed introductions of fish to the river are discussed with regard to management issues.