Decreased stock entering the Belgian Meuse is associated with the loss of colonisation behaviour in yellow-phase European eels

Abstract

The upstream migratory behaviour of yellow-phase European eels was investigated in regulated inland rivers (>320 km upstream the sea), where the stock is in drastic decline. From 2010 to 2015, eels entering the Belgian Meuse River (n = 1357; total length, 231–755 mm) were caught in fish passes, tagged with a pit-tag and released. Their upstream movements were tracked during the next six consecutive years, using three detection stations installed in vertical-slot fish passes of the Meuse and its Ourthe tributary. Among the 1357 eels tagged, 27.6% (n = 374 individuals) were detected at one or more of the three upstream detection stations. Only 6.6% (n = 89) of tagged eels were detected at the two subsequent stations. In this last group, most of the detected eels continued to move upstream through the Meuse rather than leaving it for the Ourthe. Water temperature >13 °C, river flow 24–226 m3/s, dark time 00:00–05:00 h and the spring–summer seasons were the most important cues for upstream migration. Temperatures and flows at detection did not differ between size classes of ascending eels, while the detection period was earlier and daily speed was faster in large (>450 mm) eels. However, small (≤300 mm) eels moved further upstream at slow speeds because they alternated between short periods of movement and long stationary periods. This behaviour suggests the existence of a few nomad individuals and probably more home range dwellers in the entering population. Small eels were better suited to colonise upper rivers.

What happens to glass eels after restocking in upland rivers? A long‐term study on their dispersal and behavioural traits

“…Negative relationships were observed between body size at tagging and mobility parameters, even though this relationship was not significant for the home range; this clearly indicates that the smaller‐sized eels were more mobile. Nzau Matondo and Ovidio () reported similar observations during the colonization phase of yellow eels, showing that small eels (≤300 mm) moved further upstream by alternating short periods of movement with long stationary periods. The ability of eels to breathe outside the water, and their forward and backward undulatory swimming could contribute to the greater movement performance of the small eels travelling further and climbing numerous physical obstacles in the study area (Baudoin et al, ; D'Août & Aerts, ).…”

“…This detection success exceeded that of detections reported in many studies using conventional mark–recapture methods (0–18.5%, Naismith & Knights, ; White & Knights, ; 2.1%, Baras et al, ; 6.8%, Nzau Matondo et al, ). The detection rate was also higher than that of RFID telemetry studies of wild yellow eels using fixed detection stations (37.5%, Nzau Matondo et al, ; 27.6%, Nzau Matondo & Ovidio, ). The high detection performance in this study could be attributed to the portability of the system, which allowed active searching for individuals during the seasons when the species is more active (from spring to summer) and less active (from autumn to winter), and to the shallow environment of the selected study area that facilitated detections.…”

“…As this study is the first on this early life stage, it cannot be compared with other studies, but the findings contrast with the long distances that the species travels during its upstream and downstream migration phases in rivers and during its spawning migration in the ocean. In the Meuse River, Nzau Matondo and Ovidio () reported a daily ascending distance of 0.317 km (41.5 km annually) in yellow eels (mean TL = 412 mm). White and Knights () estimated the daily distance travelled by elvers and juvenile eels of 80–320 mm to be approximately 0.64 km in the non‐tidal Severn and Avon rivers; however, our observation is consistent with the restricted mobility reported for wild yellow eels during mark–recapture (eels, mean TL = 313 mm: Laffaille et al, ) or tracking studies (mean TL = 591 mm: Baras et al, ; 689 mm: Ovidio et al, ).…”

“…Old resident eels (total length, TL: mean = 666 mm; range = 515–910 mm) were present, but their quantity was very low (density: Mosbeux = 0.003 eels m −1 ; Vesdre = 0.005 eels m −1 ). Similarly, these eels disappeared with time and were not replaced by new naturally recruited eels because of the shutdown of the natural immigration of wild eels from the Meuse River (Nzau Matondo & Ovidio, ; Nzau Matondo & Ovidio, ).…”

“…The estimated eel stock in the lower part of the Meuse River in Belgium, more than 320 km upstream from the North Sea, dropped from 445 000 individuals in 1993 (Baras, Philippart, & Salmon, ) to 7200 in 2013 (Nzau Matondo, Benitez, Dierckx, Philippart, & Ovidio, ). Similarly, the number of new eels entering the Belgian Meuse River has drastically declined by 95.5% in 23 years, and their body size has increased by 4 mm per year since 1992, with the inhibition of colonization linked to density (Nzau Matondo & Ovidio, ; Nzau Matondo & Ovidio, ). In the Meuse basin, the reason for this drastic decline in the local stock of eels is clearly riverine recruitment failure from the North Sea.…”