Assessment of Fish Abundance, Biodiversity and Movement Periodicity Changes in a Large River over a 20-Year Period

Abstract

A river is an ecosystem where fish fauna represents an important structural element. To re-establish connectivity, it is imperative to allow movement between functional habitats. Due to the hydromorphological complexity of large anthropized rivers and the lack of study techniques that can be used in such environments, relevant data with regard to fish ecology are scarce. On the River Meuse, Belgium, at a point 323 km upstream from the North Sea, the Lixhe hydroelectric dam is equipped with two fishways. Both were continuously monitored using capture traps for 20 consecutive years (from 1999 to 2018), representing 4151 monitoring events. The objectives of the present study were to describe the overall abundance and movement indicators of mainly holobiotic potamodromous fish species and to analyse their temporal evolution. We captured 388,631 individuals (n = 35 fish species) during the 20 years of fishway monitoring; 22.7% were adults (>75% of which were cyprinids), and 83.3% juveniles (>90% cyprinids). From 1999 to 2018, the results showed a drastic reduction in yearly captures for some native species as well as the apparent emergence of non-native (e.g., Silurus glanis) and reintroduced species (e.g., Salmo salar). The annual capture periodicities associated with environmental factors were clearly defined and were mostly related to the spring spawning migration of the adult stage. This long-term monitoring demonstrated how the fishways are used by the whole fish community and allowed a better understanding of their movement ecology in a large lowland anthropized river. The appearance of non-native species and the drastic decline in abundance of some common and widespread European fish should prompt river managers to adopt conservation measures.

Movement behaviour and fishway performance for endemic and exotic species in a large anthropized river

“…Among the eurytopic and the limnophilic species, roach, pike, Esox lucius, perch, Perca fluviatilis, tench, Tinca tinca and carp, Cyprinus carpio did not cross the fishway, but only a few were tagged and/or approached M 1 . These species were already captured in M 0 during a long-term monitoring period (Benitez et al, 2022), suggesting their ability to use a vertical slot fishway. The eel reached a passage efficiency of 49.7%.…”

“…The exotic catfish and asp demonstrated a very good capacity to cross the fish pass (53% and 95% of adjusted passage efficiency, respectively). Three species of exotic gobies are also present in the Meuse and already use a fishway located further downstream (Benitez et al, 2022). Unfortunately, selectively blocking exotic species in a multi-species fishway is very difficult to set up, especially if their size range and swimming capacities contrast.…”

“…According to physicochemical requirements, the water quality is good in the Meuse (Public Service of Wallonia -AQUABIO). The total number of fish species in the Belgian Meuse is 36 (75% are cyprinids), they belong to different ecological categories: eurytopic, limnophilic and rheophilic species (Schiemer and Spindler, 1989;Benitez et al, 2022). The exotic species (the Asp and the catfish) are represented by the suffix 'exo' (Table 1).…”

“…Over the study period, the trap was monitored 2-5 times a week, depending on the capture intensity. Captured fish were anesthetised in a solution of 4-allyl-2methoxyphenol (Eugenol 0.1 ml L− 1), identified, measured ( ± 1 mm fork length) and weighed ( ± 1 g) (Benitez et al, 2022). A total of 1065 adult individuals (representing 14 large species, including cyprinidae, salmonidae, esocidae, percidae, siluridae and anguillidae) were individually tagged (Table 2) using biocompatible RFID tags (Radio Frequency Identification, Texas Instruments, HDX, 134.2 kHz; 32 × 3 mm and 0.9 g in weight).…”

“…In the lower course of the Meuse in Belgium, large fishways have been progressively installed over the last 20 years, strongly stimulated by the Salmon program, aiming to reintroduce this flagship species into Belgium after its extinction in the beginning of the 20th century (Renardy et al, 2022). These fishways were primarily designed for large rheophilic species but it transpires that they are also intensively used by eel, eurytopic and limnophilic species, as well as by exotic species at different times of the year and day (Nzau Matondo et al, 2017;Benitez et al, 2018Benitez et al, , 2022. However, since their installation, the passage performance of the fishway of the main course of the Meuse was never quantified at the multispecies level.…”