Life history variation in tilapia populations within the crater lakes of western Uganda: The role of size-selective predation.

摘要

Life history theory seeks to describe and explain the evolution of adaptive responses in fitness-related traits such as reproduction, survival, age and size at maturity, and growth as a result of environmental variation. Interpopulational variation in life history characters is well documented in a wide variety of fish taxa, both marine and freshwater. Variability in fish life history characters may be a result of phenotypic plasticity in response to environmental changes and/or genetic changes associated with evolutionary response to size-selective fishing. For effective fisheries management, it is important to consider existing knowledge about fish life history strategies, biotic and abiotic factors simultaneously in predicting population responses to changes in the environment. I examined effects of size-selective fishing and other environmental factors on key life history characters of tilapia populations in the crater lakes of western Uganda with the hope of providing critical information needed for management of the fisheries in these lakes. I used tilapia as a model species because life history variation in tilapiine fishes is well-documented in tropical freshwaters. Tilapia also display high levels of phenotypic plasticity and can tolerate a wide range of environmental conditions due to their ability to vary allocation of resources to reproduction and growth depending on the environmental condition. The occurrence of both unexploited and exploited tilapia populations in a large number of the lakes that also differ in extent of catchment deforestation and fishing pressure further provided suitable systems and the replication needed to explore life history variation across broad environmental gradients. I determined a range of environmental characters in 19 crater lakes and found a wide variation in environmental features among lakes. There was a strong negative relationship between water transparency and Chl-a concentration with deforested lakes having a lower transparency and higher Chl-a concentration compared to forested lakes. I also explored the effects of deforestation and fishing on the condition of two introduced species (Oreochromis leucostictus and Tilapia zillii) in 17 of the crater lakes and found O. leucostictus in severely deforested and heavily fished lakes were in a better condition compared to similar fish in lakes with low productivity and low to medium fishing. Differences in condition of T. zillii were only detectable between lakes with high and low fishing effort. I also developed an appropriate protocol for aging T. zillii in Lake Nkuruba and validated the periodicity and timing of opaque zone formation in otoliths of T. zillii using marginal-increment and edge analyses. Tilapia zillii in Lake Nkuruba deposited two opaque zones on their otoliths per year corresponding to bimodal rainfall that is characteristic of this equatorial region. Tilapia zillii grew rapidly in the first 2 years after which growth slowed down considerably. Gillnetted Tilapia zillii grew faster and attained a larger size-at-age than the trapped fish. Finally, I examined the effects of sizeselective predation and environmental factors on life history traits of T. zillii in the 8 crater lakes. I found striking differences in life history traits in T. zillii between and within lakes, suggesting that environmental factors, density-dependence, and size-selective fishing influence variation in key life history characters of T. zillii populations in the crater lakes of western Uganda. These results provide useful baseline data for the management of heavily exploited and unexploited T. zillii populations in the crater lakes of western Uganda.