Fish Production and Ecology in African Small Water Bodies, With Emphasis on Tilapia

Abstract:  The thesis is a contribution to understanding the ecology of fish populations primarily in SubSaharan African small water bodies (SWBs). SWBs include ponds, small reservoirs, rivers and floodplains. These little studied, highly dynamic ecosystems have fish communities with adaptations to withstand variable and often adverse environmental conditions. The potential combined yield of enhanced or culture-based SWB fisheries may match or exceed the fisheries catch of the large inland waters of Africa. Enhanced SWB fisheries represent a promising approach to rural development.A standard method for surveying fish populations, the "Drottningholm method", which employs multimesh monofilament bottom and pelagic gillnets, was tested for use in SWBs. The number of nets (efforts) needed to i) catch all common species, and ii) attain a certain precision of catch per unit effort (cpue) estimates, was assessed. Because SWBs are shallow and have few pelagic fish species, it was concluded that pelagic nets should not be used.Gillnets are selective, i.e. a given mesh size catches primarily fish of a certain size. The selectivity of a gear is usually represented by a curve giving for each size of fish the proportion of the total population of that size which is caught by a unit operation of the gear. The selectivity of the multimesh gillnets to Oreochromis shiranus (Shire tilapia) was estimated directly in a population with numbers known by size class. The shapes of the selectivity curves were nearly normal, justifying standard indirect methods for selectivity estimates.Tag-recapture was tested as a way to obtain estimates of growth and mortality rates. Reduced recaptures of small (<13 cm TL) tagged O. shiranus suggested a possible source of bias. No significant effect of tagging was detected on growth rate, measured as marginal scale circuli spacing.Studies of the population dynamics of O. shiranus were carried out in two small reservoirs. Growth was estimated by fitting a seasonally oscillating von Bertalanffy growth function to tag-recapture data. Production was high (417 and 872 kg ha-1 year-1) in relation to mean biomass (84 and 64 kg ha-1), generating high production-biomass ratios of 5.Tilapia are phenotypically plastic regarding age and size at maturity. An existing life history model of optimal age and size at maturity in fish could not encompass tilapia or other fish with variable life histories, and was therefore modified. After incorporation of the observed negative correlation between the von Bertalanffy parameters K and L, the model predicted dome shaped reaction norms for age and size at maturity in fish.