Temporal patterns of the catadromous thinlip grey mullet migration in freshwater

Abstract The thinlip grey mullet (Chelon ramada Risso, 1827) is a catadromous fish that per forms massive migrations to freshwater habitats for feeding purposes that can assume a structuring role on riverine ecology due to the biomass involved in these movements. Seasonal movements of thinlip grey mullet through a vertical slot fish pass located in River Mondego (Portugal) were continuously monitored between 2013 and 2017. The extent of trophic migration, population size structure, biomass and the environmental triggers of bi-directional species' migratory activity were analysed. Between March and November of 2013/2014, 2 million and 1 million movements were respectively recorded. From a subsampling approach, the upstream movements between 2015 and 2017 were estimated. Annually, around five hundred thousand upstream movements can occur to provide species access to the upstream freshwater reaches. Movements are exclusively diurnal, and the population com posed by young adults in their first year of maturity, yet juveniles and larger fish were present (TL range: 90–540 mm). Upstream movements increased with temperatures above 15 C, reaching a peak at around 20 C coupled with a photoperiod of 15 h. Downstream movements attained the higher rates when temperature dropped from 22 C to 20 C and photoperiod to 13 h. However, under wetter hydrological condi tions (as in 2014), discharge flows have a higher influence. These finding provide unique information regarding species migration to freshwater habitats in the Atlantic coast, namely the extended periods spent in such environ ments, overlapping with the spawning migration period. Additionally, highlights the importance of species' trophic migration both for its life cycle and riverine food-web