The biology of naturalized rainbow trout, Oncorhynchus mykiss (Walbaum), in Kenya cold water streams and implications for future management

Ngugi, Charles Chege (1999) The biology of naturalized rainbow trout, Oncorhynchus mykiss (Walbaum), in Kenya cold water streams and implications for future management. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this study I present information on the presence, current status and historical distribution of rainbow trout, Oncorhynchus mykiss (Walbaum), in three third order streams on the southwestern slope of Mt. Kenya, namely the Naro Moru, Sagana and Thego. The composition, distribution, and status of fish species within the study streams are different than they were in the 1950s. Although there are still self-sustaining rainbow trout populations their distribution range is reduced and populations are in decline. -- Multiple regression analyses indicated that altitudinal and seasonal variables had little influence on rainbow trout length specific condition among streams. Rainbow trout are growing about as fast as they did during the 1930s to 1950s - perhaps even faster – but population levels appear to be much lower and most of the fish are small with few reaching more than 2 years of age. Brown trout (Salmo trutta), another salmonid with a similar history in Kenya as rainbow trout, has also been affected and is presently found only in the upper reaches of the Sagana indicating that its distributional range has decreased faster than that of rainbow trout. Compared with the past, mountain catfish (Amphilius urcmoscoptis) and other native species have expanded their distributional range upstream from where they were in the 1930s to 1950s. Rainbow trout, brown trout and catfish exhibited overlap in their diet suggesting the competitive interactions between the species may be a factor in their changing distributional patterns. In the three streams, composition of benthic invertebrate fauna showed altitudinal and seasonal variation. There were more dipterans in the upstream stations and more ephemeropterans in the downstream stations. There was no marked difference between fauna composition in the present study and that reported for one of the study stream (the Sagana) by Van Someren (1952) in the 1950s. -- Factors associated with the variation in rainbow trout distribution among sites included temperature changes, and other environmental parameters, overlap with other species and over-exploitation. The main reasons appear to be environmental changes influenced by anthropogenic effects and fish over-exploitation i.e. heavy fishing pressure and high vulnerability of rainbow trout to anglers. -- Because much of the trout habitat has been altered, areas supporting rainbow trout populations are now critical for conservation management. Management strategies should incorporate societal values and recognize that good trout habitat is a reflection of better managed watersheds. Where land management has degraded stream habitats, acquisition of riparian corridors and instream management are necessary to rehabilitate and provide recreational fisheries. Enforcement of regulations on the trout fisheries appear to have become more liberal and more stringent legislation may now be required. Rebuilding trout populations is necessary to re-introduce recreational fisheries and by extension "pristine" environment. This study supports the hypothesis that rainbow trout distributional range will continue to decline unless corrective measures on catches and harvesting, as well as watershed management are considered.

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