Decline in glass eel catch and by-catch and changes in species composition in the River Minho estuary compared to 40 years ago

The glass eel fishing by-catch in the Minho River was evaluated by assessing its composition during the 2021–2022 fishing season and comparing results with the first data collected during the 1981-1982 season. Results showed that by-catch composition was mainly dominated by fish and crustaceans. Large reductions in glass eel and by-catch yields were observed, with decreases in both fish and invertebrate yields. The largest reductions were recorded for Anguilla anguilla (yellow eel), Atherina boyeri, Saduriella losadai and Palaemon longirostris. On the other hand, a few species showed an increase in both number of catches and yield, such as Crangon crangon and Gammarus spp. Decreases in glass eel and by-catch yields were less pronounced upstream. By-catch is mainly composed of estuarine species, with higher catches of marine species downstream. Most of the marine invertebrate species caught were correlated with the presence of marine red, green and brown algae. Small fish and juveniles of larger species were most commonly caught, with the most common species being Atherina boyeri, Pomatoschistus microps, Syngnathus abaster and Ammodytes tobianus. Similarly, invertebrate species such as Crangon crangon, Eurydice pulchra, Lekanesphaera rugicauda and Gastrosaccus spinifer were also important in the composition of the by-catch. High levels of marine fish stragglers and marine invertebrates associated with marine algae were also observed, in contrast to data collected in the 1980s, suggesting a possible shift in the composition of the lower estuarine fauna over the past 40 years.