Proposals for optimizing sea lamprey passage through a vertical-slot fishway

Movement of sea lamprey through a vertical-slot fishway (built at the Coimbra dam, in central Portugal) was continuously monitored between 2013 and 2016 by a video recording system. Visual count data were used to quantify the overall successful movements and identify seasonal and circadian patterns of fishway use. Explicative models (Boosted Regression Trees) were used to study the relationship between frequency of successful movements during the species migration peak and environmental variables. The aim of the study was to identify predictors that may be related with successful sea lamprey upstream migration through the fishway. Collected information was used to support further management recommendations for optimizing the fishway performance. During the 4-year study, near 50,000 lampreys successfully negotiated the Coimbra dam fishway reaching the upper stretch of river Mondego. Migratory peak occurred between April and May with an increase in passages during the night period (between 11p.m. and 6a.m.). It was observed an increase in use frequency that was related with the combined effect of flow and temperature. Higher passages were achieved when Coimbra dam discharge flows are lower than 100m(3)/s and temperature between 15 and 19 degrees C. Flow discharges higher than 150m(3)/s seemed to inhibit successful sea lamprey passage. In dry years, when flows are almost constantly lower than 50m(3)/s, temperature was the most important factor influencing fishway use.