Population dynamics and life-history styles of Nile tilapia,Oreochromis niloticus, in Ferguson's Gulf, Lake Turkana, Kenya

Abstract

SynopsisThree observed dynamic aspects of the Nile tilapia population around Ferguson's Gulf at Lake Turkana, Kenya are evaluated and discussed: the seasonality in catch rates, the enormous inter-annual abundance variations, and the large changes in median size at first maturity. A clear understanding of the regulating mechanisms behind these features has never been achieved, although seasonal changes in the hydrology of shallow sheltered refuges seems to play an important role. This paper suggests a further holistic approach taking the impacts and interrelationships of both the primary productivity and the various predators into account. A synthesizing ecological hypothesis is elaborated, which concludes that most observations on the tilapia dynamics can be explained from changes in the oxygen concentrations and size-specific mortality pressures. Variations in these two proximate factors can ultimately be explained by the floodplain-type fluctuations in the Ferguson's Gulf environment.

Anthropogenic impacts on the contextual morphological diversification and adaptation of Nile tilapia (Oreochromis niloticus, L. 1758) in East Africa

“…Nile tilapia (ISSION 1960;Green 2009), with some reports from the Local Government archives indicating restocking continued up to the 80s. L. Turkana (previously called Rudolf), located in the Great Eastern Rift Valley is the largest freshwater body in Kenya (Kolding 1992). The Lake lies in the arid North-Western part of Kenya at an altitude of 364 m above the sea level, much lower than most of the other East African freshwater bodies (Table 1).…”

“…Surprisingly, despite L. Turkana having much of its inlet from River Omo (originating from Ethiopian highlands), the Lake does not have any surface outlet and hence, the water balance is compensated for by evaporation, estimated at 2.3-2.8 m/yr. (Kolding 1992). Peripheries for most of these lakes are populous and user conflicts have amplified the demand for fisheries resources culminating into overfishing (Ogutu-Ohwayo et al 1997).…”

Revisión de los patrones de distribución de Crocodylus niloticus Laurenti 1768, en Etiopía, y estima de tamaño corporal de grandes ejemplares del lado Chamo

“…En perímetros lacustres con gran fluctuación del nivel hídrico la detección también es complicada, ya que no existen zonas predeterminadas de termorregulación o reproducción. Es especialmente ardua la tarea de localización de la especie en el sector etíope del lago Turkana, donde el nivel de las aguas ha descendido más de 10 m en las últimas décadas (Avery, 2010) y la principal fuente de alimentación de los cocodrilos, (tilapias Oreochromis niloticus, Linnaeus, 1758 y percas del Nilo Lates niloticus, Linnaeus, 1758) ha sido sobrexplotada (Kolding, 1993). En la actualidad los cocodrilos son perseguidos por los pescadores, que destruyen los lugares de nidificación incluso dentro de áreas protegidas como en el sector keniata del lago Turkana, (observ.…”

Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s ecosystem services

“…In a later step, the seasonal upwelling of deep anoxic waters can lead to large fish kills (Ochumba 1990, Gophen et al 1995. Reduced oxygen concentrations also have a chronic negative effect on species by influencing their metabolic processes and growth rates (Kolding 1993, Rutjes et al 2007). …”