Fish pass use by shads ( Alosa alosa L. and Alosa fallax [Lacépède, 1803]): Implications for monitoring and management

Abstract Fish pass monitoring is essential to ensure the device's effectiveness as a mitigation measure for river impoundment, guaranteeing the longitudinal continuity of rivers, which is particularly important for diadromous fish that depend on obligatory migrations to freshwater reproduction (anadromous) or feeding areas (catadromous) to complete their life cycle. The upstream migration of the anadromous allis (Alosa alosa L.) and twaite (Alosa fallax Lacépède, 1803) shad in the River Mondego, Central Portugal, is being monitored at the Coimbra Dam fish pass since 2013, using visual counts. Statistical models were used to evaluate shad passage and to identify the main environmental variables that seem to condition their behaviour. A total of 26,561 shad were recorded in this infrastructure during the study period (2013–2017), with 96.5% of the total counted fish being counted between April and June. Overall, water temperature and river flow are the environmental predictors that consistently influence the number of shad using the fish pass, although its individual contribution changed between years. The models (Boosted Regression Trees) obtained were robust with an average explained deviance of 0.79 (R2), despite the poorer results associated with the 2015 spawning season, that were possibly related with the low number of adult fish observed that year. Results from this study contribute to better understand the dynamics associated with fish pass use by Alosa sp. and can help the conservation and management of these species through the improvement of fish pass attractiveness and, consequently, the overall efficiency of fish pass devices targeting shads.