Abundances and biological traits of the juveniles trout sampled in the survey of trout abundance Indices (VIGITRUITE®) in the Bresle river (France)

Study Extent

Bresle river, CdSandre=G01-0400 Source latitude and longitude : 49.6909, 1.7498 Last confluence point latitude and longitude : 50.0644, 1.37 The Bresle is a small coastal river that flows into the English Channel, in the commune of Le Tréport. 68.1 km separate in a straight line its source, in the commune of Abancourt in Oise (60) from its mouth. However, the particular hydrology of limestone substratum rivers, as well as the multiplication throughout the history of mills and other structures along its course, more than doubles the length of the actual watercourse. The river drains a watershed of nearly 748 km², composed mainly of a chalky substrate dating back to the upper Cretaceous. The basin has historically been marked by the glass industry, but also by the paper industry, as well as by gravel extraction. Agricultural practices that were once mainly focused on dairy farming and grazing are now shifting towards the production of maize, protein crops for livestock, and potatoes. Migratory salmonids can now reach the municipality of Saint-Germain-sur-Bresle, just over 50 km from the sea.

Sampling

In the Bresle, sampling was carried out at 11 sites spread over several years along the main branch in the area of the river system. 8 stations will be surveyed throughout the Bresle valley as part of the permanent network set up from 2022

Method steps

1. At the selected station, the VIGITRUITE® protocol was designed to ensure that sampling is representative of the range of available habitat conditions, without any a priori bias towards the habitat preferences of the species. This condition is central to meet the objective of standardization, to allow comparison of results from different operators and different watercourses, and to authorize the use of density conversion abacuses per 100 m2. 1. After resetting the counter of the fishing tackle and the hand meter, the operator progresses from downstream to upstream in the watercourse. He is followed by the main operator equipped with the 2 landing nets, who must respect the travel instructions given to him by the operator, as well as by the second operator. 2. The operator follows a predefined zigzag path that takes him from one bank to the other along successive diagonals, at an angle of about 45° to the banks. Along its path, it makes electrode lines about 2 meters long, parallel to the current, and from upstream to downstream. After making the first line along the bank, the operator must visualise the next line before moving: it will be placed on the diagonal, its downstream limit always upstream of the previous line. At the end of the diagonal, if the space is too short for a line along the bank, the line will be placed on the next diagonal, always upstream of the previous line to avoid overlapping. 3. Once the location of the haul has been visualised, the operator indicates to the main fisher where to immerse his semi-circular net, i.e. upstream of the previous haul and offset on the diagonal. The fisher makes sure that the net is well placed on the bottom. If there is any difficulty in placing the net, the operator may decide to shift the position of the haul significantly. The haul is made, then the position of the next haul is displayed, and so on. The result is a sampling by regularly spaced tows from one bank to the other, covering a line of comparable watercourses regardless of the water width of the river. The two stakes marking the downstream and upstream boundaries of the fishing station are also set. 4. For each haul, the operator immerses the electrode about 2 metres upstream of the flat-bottomed landing net. The contact is made when the anode penetrates the water. The fisherman catches the visible fish with his small net, but does not move the large one. The second operator is ready to intervene in case of high abundance. During this operation, it is important that the fishermen catch only the juveniles of the year (0+) and of one year (1+). The presence of larger and older individuals (sub-adults or adults) must not cause a decrease in the team's concentration of juveniles. The hand-held competitor is incremented by one unit at the end of each haul. 5. At the end of the haul, the operator shall leave the anode at the entrance of the large landing net for 1 to 2 seconds before switching off the ignition and instructing the angler to simultaneously raise the flat-bottomed net. The haul made is counted by the hand-counter operated by the bucket holder. Once the collected fish are put in the bucket, the angler follows the operator's instructions to put the large net back in place for the next haul. 6. As the five minutes of fishing approaches, the time counter should be checked before each new haul. Station sampling shall be completed when the effective fishing time (cumulative time of electrical pulses in the water) reaches at least 5 minutes on the counter; no new electrode tows shall be started if the counter already displays 5 minutes. 7. Using the decameter, the width at the downstream stake is measured and its value noted. Then a new width measurement is made by progressing 10 meters upstream, and so on until the end of the station is approached. If necessary, the remaining distance to the upstream post (less than 10 m) is measured to complete the length measurement of the station. This work Biometric measurement of the fish are recorded after their anesthesia with benzocaïne. The fish length is measured from the tip of the mouth to the fork of the caudal fin (1 millimeter precision). Fish are then weighed (0.2 g precision) and maturity status is assessed by a gentle pressure on the belly that allows to get sperm in mature males. All the fish sampled are then released in their original site.
2. Age is verified from scale reading according to standard methods described by Baglinière & al (2022).

salmo trutta

1. Salmo trutta
   common name: Truite, Trout rank: Species

The Bresle river in Normandy (France). http://www.sandre.eaufrance.fr/geo/CoursEau/G01-0400