Efficiencies of two types of bypass, a surface and a bottom sluice, were tested for the natural downstream migration of silver eels Anguilla anguilla at a small hydroelectric power plant at Halsou, on the River Nive in France. Naturally migrating eels were caught after their passage through either bypass. A total of 637 eels were trapped during the three-year study. Total efficiency for both bypasses, evaluated on the basis of downstream movement of radiotagged eels, ranged from 56% to 64%. Given a bias due to hydrological conditions at the time of the runs, the precise efficiency of each separate bypass was not calculated. However, preferred passage through the bottom bypass for both tagged and untagged eels was confirmed by telemetry, as three to four times as many eels transited through the bottom bypass compared to the surface one. The behaviour of 74 individuals released in the forebay was observed by radiotelemetry. Close to half of the radiotagged eels returned up the headrace after their release, and most eventually migrated downstream over the dam with appropriate environmental conditions. Upon arrival at the power plant, eels displayed foraging behaviour in the forebay with frequent displacement interrupted by long resting periods in zones with low current. The repulsive effect of the trashrack located in front of the turbine intake increased with increasing turbined discharge. The study indicated that a trashrack with a smaller bar-spacing (around 20 mm), associated with an appropriate bypass, could deflect a large proportion of the female eels from the turbines. However, this solution needs to be tested on site to quantify the risk of mortality due to impingement on the trashrack.

Gosset C, Rives J, Labonne J. Effect of habitat fragmentation on spawning migration of brown trout (Salmo trutta L.).Abstract -Human-induced habitat alteration is one of the main causes of the decline of freshwater fish populations. The watershed of the River Bidasoa (Spain) is an example of heavily fragmented habitat. The local brown trout (Salmo trutta L.) population is disturbed, with evidence of poor recruitment as well as low adult densities in the main stem. Forty male and female adult migratory trout were tagged with external or internal radio transmitters and released. Fixed stations with data loggers and mobile antennae were used with daily surveys to track fish movements during the migration and spawning period (3 months). Migration distances did not exceed 10 km, and half of the fish never entered a tributary in the study area. Fragmentation because of weirs on the main stem apparently prevented fish from reaching their spawning destination. Fish that entered the tributaries were first confronted with an accessibility problem because of low discharge. However, each fish chose one tributary, without making attempts to run up in other tributaries. Once in the tributary, fish were restrained in their upstream movements by dams. The study area appeared to be isolated from the vast upper part of the watershed. Within the study area, upper parts of tributaries also seemed strongly disconnected from the main stem. This study illustrates the negative impact of river fragmentation on S. trutta migration pattern. Population sustainability can be directly affected through the low availability of spawning grounds for migratory fish. Long-term effects of fragmentation may cause reproductive isolation within watersheds, which in the case of trout also means isolated phenotypic population units.

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