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Britton JR, Harper DM, Oyugi DO. Is the fast growth of an equatorial Micropterus salmoides population explained by high water temperature? Ecology of Freshwater Fish 2010: 19: 228–238. © 2010 John Wiley & Sons A/S Abstract –  Marginal increment analysis of scales collected from the introduced Micropterus salmoides population of Lake Naivasha, Kenya revealed the formation of an annual growth check, validating their use to age individual fish. Subsequent analysis of scales from 372 fish collected between 2002 and 2009 revealed individuals were very fast growing compared with native populations in North America and other introduced populations in Europe, South America, Africa and Asia. This was likely to be as a result of the water temperatures in Lake Naivasha exceeding 20 °C throughout the year. This was corroborated by a meta‐analysis of the growth parameters asymptotic length L∞ and growth coefficient K from across their geographical range that revealed variance was explained by differences in mean annual air temperatures. At a break point of approximately 10 °C, there was a shift to reduced L∞ and increased K, suggesting a temperature driven trade‐off between growth rate and ultimate length. When adjusted for temperature and weighted for sample size, there were significant differences between the growth parameters of the North American and introduced populations, suggesting that other abiotic and biotic variables were also important determinants of the growth of individuals between the two ranges.

Micropterus salmoides is a North American piscivorous fish on the IUCN list of 100 of the world's worst invasive alien species. Introduced into Lake Naivasha (Kenya) in 1929, their current population abundance is significantly depressed in a lake that has recently become dominated by fishes of the Cyprinidae family; the introduced cyprinid Cyprinus carpio now dominates catches in the commercial fishery and Barbus paludinous is now numerically dominant in the fish community. Long-term diet studies of M. salmoides based on gut contents analysis (GCA) have defined their diet spectrum, feeding preferences and ontogenetic dietary shifts. Between 1987 and 1991, diet was size-structured; fish \260 mm were mainly insectivorous and fish [260 mm fed mainly on invasive crayfish Procambarus clarkia with B. paludinosus rarely taken. More recent GCA data revealed that up to 2003, these sizestructured trophic relationships were still evident, but there has been a subsequent shift to their feeding almost exclusively on small (\100 mm) B. paludinosus, coincident with a size-related functional switch whereby M. salmoides [120 mm were now piscivorous. However, a Bayesian stable isotope mixing model (SIAR) suggested M. salmoides diet actually remained relatively varied in 2006 and 2007; it indicated P. clarkii were still contributing more to their diet than B. paludinosus in fish \260 mm and provided only partial support for the functional shift. The consequence of the M. salmoides depressed population abundance is their predation pressure on prey fishes is limited and preventing top-down effects. This is in contrast to their invasive populations elsewhere in the world and the likely result of invasion meltdown processes in Naivasha involving the introduced C. carpio and P. clarkii that have produced sub-optimal foraging conditions for M. salmoides.

The tilapiine cichlids Oreochromis leucostictus and Tilapia zillii were introduced into Lake Naivasha, Kenya, in 1956. Previous studies on data collected to 1987 revealed they were persistent following establishment, despite environmental variability and exploitation. Recent data, however, suggest this persistence is under threat as data indicate some significant declines in aspects of their abundance since 1999. The influence of changes in lake level, allodiversity and fishing effort on this decline was tested and showed that a decline in lake level was a significant causal factor. The recent change in allodiversity, with the establishment and dominance of Cyprinus carpio in the fishery, was not significant on the catch per unit effort of O. leucostictus but was on T. zillii. Since 1999, catches of tilapiines in the fishery have been independent of fishing effort, contrary to between 1975 and 1987, suggesting their management through application of fishery models may no longer be applicable. As it was anthropogenic-mediated lake level changes that were mainly responsible for their decline, then lake management should focus on sustainable water utilization that maximizes lake levels in accordance with the basin-wide water balance.

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Habitat suitability indices indicate how fish species respond to different habitat types. We assessed effects of habitat characteristics on fish distribution in an equatorial lake, Lake Naivasha, Kenya, where habitats vary according to substrate, depth and turbidity. Using monthly data between 2008 and 2010 using multi-mesh gill nets, catch per unit effort was used as a relative abundance measure to identify how habitat variables drive fish distribution. The focus was on commercial fishes: two introduced species (Cyprinus carpio and Micropterus salmoides) and two naturalised species (Oreochromis leucostictus and Tilapia zillii). Analyses revealed distinct preferences for different habitat variables by all commercial species except for C. carpio. For example, O. leucostictus preferred shallow waters with silt-clay substrates whilst M. salmoides preferred deeper waters with sandy/rocky substrates. Conversely, C. carpio showed no specialised habitat requirements. Niche overlaps were significantly lower between O. leucostictus and its respective sympatric species than between other species, a likely result of its territorial behaviour. The continued environmental degradation of Lake Naivasha may imperil the preferred habitats of the niche restricted M. salmoides, O. leucostictus and T. zillii. By contrast, the ubiquity of C. carpio may facilitate their invasion, and consequently sustain their dominance in the lake's commercial fishery.

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