Fish condition in introduced tilapias of Ugandan crater lakes in relation to deforestation and fishing pressure |
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Authors: | Jackson Efitre Lauren J Chapman Debra J Murie |
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Institution: | (1) Department of Zoology, Makerere University, P.O. Box 7062, Kampala, Uganda;(2) Department of Biology, McGill University, 1205 Avenue Docteur Penfield, Montreal, QC, H3A 1B1, Canada;(3) Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, 7922 NW 71st Street, Gainesville, FL 32653, USA;(4) Wildlife Conservation Society, 2300 Southern Boulevard, Bronx, NY 10460, USA |
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Abstract: | This study identifies environmental predictors of the condition of two introduced tilapia species (Oreochromis leucostictus and Tilapia zillii) that are known to have divergent trophic niches (planktivore and herbivore, respectively) in 17 crater lakes in western
Uganda. We asked whether fish condition differs among lakes characterized by differences in fishing pressure and catchment
deforestation; and we related relative condition factor to gradients of environmental variation across lakes. Lakes characterized
by severe catchment deforestation tended to be lakes with high fishing pressure, so it was difficult to explore independent
and interactive effects. However, mean relative condition factor was higher in populations with high fishing pressure compared
to populations with low fishing pressure for both O. leucostictus and T. zillii. The condition of O. leucostictus populations was higher in lakes with severely deforested catchments; but mean relative condition factor of T. zillii did not differ between deforestation categories. Principal components analysis (PCA) was used to describe the major environmental
gradients of variation among the lakes; and PCA factor scores were regressed against relative fish condition. The association
between fish condition and environmental gradients was stronger for O. leucostictus than for T. zillii. For O. leucostictus, fish condition was related to PC1 (43% of the variance) and factors that loaded most heavily included Chl-a, water transparency,
lake area and depth, suggesting higher condition in lakes characterized by higher primary productivity and smaller size. For
T. zillii, PC3 (11%) was the only axis related to fish condition; and factors that loaded most heavily included lake area (positive),
and conductivity and total nitrogen (negative). Some of the larger lakes are characterized by higher availability of macrophytes
that may positively affect the food base for T. zillii. |
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Keywords: | Human disturbance Fishing pressure Environmental predictors Primary productivity Oreochromis leucostictus Tilapia zillii East Africa Length-weight relationships |
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