Brown trout passage efficiency through a baffle series in a long culvert, assessed by fish translocation and by natural migration monitoring

The passage efficiency of brown trout through a 73 m long culvert equipped with 10 notched baffles, was studied using PIT telemetry and two approaches: (1) fish translocation from upstream to downstream of the culvert, (2) monitoring of the natural migration of downstream individuals. The migration rate of translocated individuals was higher (90%) than that of naturally migrating individuals (42%). The attraction efficiency was assessed at 94.4% and 93.1%, the passage efficiency at 94.7% and 83.3%, and the overall passage efficiency at 89.4% and 77.6% for translocated and naturally migrating individuals respectively. The majority of individuals passed through the culvert (overall passage time) fairly rapidly but the approach times were longer for translocated individuals, impacting their overall passage times (median value 24 h 40 min vs. 11 h 16 min for naturally migrating fish). For translocated fish, large individuals crossed the structure faster than small ones. For fish in natural migration, individuals that passed through the structure were smaller than those that did not. For translocated fish only, the approach time decreased with increasing water level. The majority of trout approached the culvert during the day and the luminosity changes at its entrance and exit did not seem to be a limiting factor for their passage.