Variability in home range size and habitat associations inwild Nile tilapia as potential responses to fishing pressure
University Of East Anglia & Mc Gill University
This study aimed to investigate the potential effects of stressors on habitat use in Niletilapia and determine whether deviations from the positive relationship of fish bodysize to home range size could be used as a proxy of ecological adaptation in Niletilapia. This may then contribute to an understanding of the effects of stressors onintroduced food fish in a world where domination by man and the need to fulfill theirbasic needs is a reality. Moreover, it may contribute to improved fisheriesmanagement in a region where ineffective and weakly-enforced regulation is commonplace.
We used radio telemetry (a powerful tool for studying freshwater fish movement) tocharacterize home range and movement patterns, and detect environmental predictorsof home range size in a heavily-exploited fish. A total of 19 Nile tilapia of varyingbody sizes were tagged with radio transmitters and relocated daily for 2 months andthen 2 weeks out of the months for the subsequent 2 months. At each relocation, werecorded environmental variables, including water depth, water temperature,dissolved oxygen (DO) concentration and water clarity. Home ranges were thenestimated for 10 fish for which sufficient and reliable data was available.
Larger Nile tilapia were associated with cooler, deeper waters with higher DOconcentration and mean home range size (using MCP estimation method) was 2,300,187.9 (± 1,137,417.28) m2(range = 1,252 – 10,216,694 m2). However, we observed no significant relationship between Nile tilapia body size (neither fish lengthnor weight) and home range size. Nile tilapia home ranges were larger in fish taggedin the habitat with lower fishing pressure than in the fish tagged in the habitat withhigher fishing pressure. DO concentration, water depth and clarity, and distanceoffshore were all significant predictors of Nile tilapia home range size.
The findings of this study suggest strong associations between habitat and home rangecharacteristics in Nile tilapia that are independent of body size; and hint at thepotential of an ecological response to fishing pressure in this species. Fishing pressurebetween distinct habitats should be equalized to avoid inducing ecological change thatmay ultimately be harmful to the target stock. These results may also be applicable tothe conservation of the endangered haplochromine cichlids of the Lake Victoria basinthat are heavily harvested. Fishing as an ecological and evolutionary force is thereforean equally important consideration for the conservation of these endangered cichlids.Consequently, the potential for shifts in home ranging behaviour and habitat use as aresponse to fishing pressure in haplochromine cichlids is a worthwhile considerationfor future studies