IMIS

A V-shaped pool-and-weir fish pass was monitored on the River Grote Nete, Belgium, from 2006 to 2008 using a stationary passive integrated transponder (PIT) telemetry system. From April to May 2007 the PIT detection system was combined with fyke netting to evaluate migration activity in/through the fish pass. The PIT telemetry system consists of two fair sized antennae used in sequence. Antenna 1 is situated at the most downstream overflow in the bypass, antenna 2 upstream the upper fish pass overflow, both antennae adapted to river bottom and bank. The fyke net was installed 90cm upstream antenna 2, mouthing in downstream direction. 1554 fishes were tagged, 19 species (>9mm and ≥9g): chub, roach, eel, brown bullhead, pike, gudgeon, dace, rudd, perch, burbot, pumpkinseed, tench, carp, bream, ide, white bream, gibel carp, ruffe and pikeperch. Most of the tagged fish were cyprinids (chub and roach). Length of tagged fish varies between 9cm and 70cm. Minimum weight of tagged fish is 9g. In total 139 fish (11 species) migrate succesfull through the fish pass, this makes 25227 registrations. Especially small fish (12cm-16cm) are registered. Larger individuals are under represented in the PIT registrations. In the fyke net 823 fish were caught (April ttill June 2007) of 13 species: especially roach (N=564) and chub (N=114), 82,4% of the total fyke net catches. Lenght of the fishes in the fyke net peaks between 8cm en 10cm (N=399). When temperature rises up to 14°C, migration activity of the tagged fish increases. No relationship was found between number of registrations and water flow. The automatic PIT telemetry system, in combination with fyke net catches, has potential in examining migratory behaviour of lowland river fish through fish passes. Using only PIT-telemetry however restricts conclusions and leads to an underestimation of the effectiveness of the pass. Otherwise the fyke net catches do not represent a good estimate of the true numbers of ascending fishes, since the entering into the fyke net of PIT registered fish is very low and mostly small individuals are caught. Our results, combining both methods, suggest that most fish species in the River Grote Nete are able to successfully cross the pass. Additionally, as only few large individuals successfully ascended the pass, questions rise about their intention to swim upstream and their willingness to adapt their migration patterns to a changing environment due to defragmentation.