The natural flow of large rivers has been modified with dams throughout history to facilitate economic development. Dams provide stable water supplies, electricity (hydropower), navigation, flood risk reduction and recreation. Unfortunately, dams also reduce aquatic species diversity, diminish riverine fish abundance and alter water quality. A way of minimising the negative effect of dams is to enable migratory fish movement through the construction and operation of fishways.
rnrnRivers serve as the dynamic hydraulic connection between unique habitats throughout a river?s basin. Over time aquatic organisms have evolved to depend upon the geographically separated habitats within these systems. Changes to this system, whether human induced or natural, can block, delay, or alter migratory corridors and result in a substantial decline in biodiversity [Park et al., 2003]. The degree of impact depends on the adaptability of the organism, the availability of suitable alternative habitats and other factors such as fishing pressure, decreased water quality and alteration of sediment transport patterns. The regulation of flow in river systems has led to a decline in the abundance of long distance migratory fish species (e.g. salmons, sturgeons, shads, eels and lampreys).
rnrnUnderstanding fish passage in the context of a river?s ecological system leads to better decisions in managing rivers.
rnrnA conceptual model known as the ?River Continuum Concept? describes the physical, chemical and biological changes that occur on a longitudinal geographic scale in river systems. The River Continuum Concept helps illustrate how localised changes, either natural or man-made, have a ripple effect on the ecology of the entire river.
rnrnAccording to this concept, a natural river can be divided into three zones: upper reaches (or headwaters), mid-reaches and lower reaches. This longitudinal zonation of physical and hydrological conditions and biological productivity explains the wide diversity of habitats for fish found in river systems. These zones represent the spatial succession of aquatic organisms consisting of species that complete their life cycle within one single zone, or as is the case with migratory fish, species that require different zones at different life stages.
rnrnThis River Continuum Concept provides a conceptual model of the ecology of river systems. However, migratory fish are also affected by a variety of other temporal and physical factors, including the quantity and timing of flows; serial disconnectivity and dams; river landscape interactions; sediments and/or stream hydraulics and a blending of landscape, lotic and behavioural interactions, to name a few. A successful fish passage design considers these factors in juxtaposition with the project?s economic and social setting. rn
