In-stream restoration (2): an assumption challenged

Posted on March 7, 2011

Environmental threats to aquatic species have prompted many attempts to improve instream habitats, often with the implicit assumption that the intervention will benefit most or all species to some extent.  Engineered instream structures such as lateral bays, current deflectors and islands are added to river reaches because they increase stream-bed complexity and therefore have the potential to improve habitat quality for fish at all life history stages.  However,  data on their effectiveness, especially in relation to non-salmonid species, are scarce.  This deficiency was addressed by sampling two critically endangered fish species (Arade chub and arched-mouth nase) in the Odelouca River, Portugal and using fish occurrence data to create curves of preference for differing stream depths, water velocities and substrate types.  A two-dimensional hydrodynamic / habitat model was then used to compare the existing area of usable habitat in a typical 250 m reach with that expected if instream structures were added.  The model indicated that placing three islands in mid-channel would increase the usable habitat area by over 100% for young nase and over 50% for young chub.  Adding two lateral  bays on opposite banks would also increase the amount of usable habitat, but to a lesser extent.  Islands provide depth diversity and help to create extra cover suitable for spawning and rearing, while lateral bays provide relatively still, shallow areas with high temperatures that promote growth.  In contrast, the model suggested that adding four alternating current deflectors would lead to only a minor increase in chub habitat and an 18-23% loss of habitat for juvenile and adult nase.  Current deflectors create areas of faster, sinuous flow, but such conditions don’t suit quiet-water species such as nase.  The results of this study show that river rehabilitation based on adding in-stream structures needs to take account of the varying needs of different species and life-history stages. 

Reference:  Boavida, I., Santos, J.M., Cortes, R.V., Pinheira, A.N.. & Ferreira, M.T.   2011. Assessment of instream structures for habitat improvement for two critically endangered fish speciesAquatic Ecology 45(1), 113-124.

Posted in: fish, restoration