Abstract

Ramah J. Marvis

The natural diversity of most aquatic systems has witnessed various changes in stock diversity and
abundance resulting from structural changes in habitat, food composition and uncontrolled exploitation.
These have altered the ecology of the fish resources with the disappearance of some species and
dominance of others. This study seeks to identify the ecological implication of the stock diversity and
abundance in Lake Asejire. Lake Asejire is a 525 ha man-made lake on river Osun one of the major rivers
that drain the western part of Nigeria. Fish samples were collected for a period of 24 months using gillnets
of between 25 and 187.5 mm mesh sizes. Data obtained were analyzed using inferential statistics and
descriptive analysis of means, frequencies and percentages. Seventeen families identified and grouped into
trophic levels. Herbivores were 63.03 and 60.35%; carnivore 31.05 and 30.65% and omnivores were 5.91 and
8.99% by biomass in the first and second year respectively. The herbivores were dominated by the cichlids,
heterotis and cyprinids; carnivores by bagrids, characids, channids and Lates while mormyrids, synodonts
and clarids constitute the omnivores. Seasons had no effect on the population and number of the
carnivores. The carnivore to herbivore ratio (F/C) was 1.33 and 0.9 by number and 1.7 and 1.33 by biomass;
this is of concern as this could pose a threat to ecological balance. It is recommended among others that
fishing pressure should be increased on carnivores and more herbivores be injected to the environment to
prevent the collapse of the fisheries.