Abundances and biological traits of fish sampled by electrofishing (except specific abundance indices) on the River Oir since 1988 (France).

Study Extent

Oir river : Latitude ranges from 48.6840 to 48.5985; longitude ranges from -1.2949 to -1. 0994. The Oir River is located on the southern edge of Normandy. It is a tributary of the Sélune, a coastal river that flows into the Bay of Mont Saint-Michel. The Oir is 21 km long and has a mean gradient of 1.1%. Annual mean discharge is 1.1 m3/s and the drainage basin area is 85.4 km2, which is dominated by sedimentary schist and metamorphic hornfels with granite inclusions. Agriculture is the main human activity and land use. Migratory fish cannot access the upper Sélune because a dam located 15 km from the sea blocks them. The Oir, with a 16 kmlong section accessible to Atlantic salmon, is the main spawning ground and the most productive tributary of the Sélune hydrographic network.

Sampling

Since 1988, an annual campaign is conducted to quantify by electrofishing the abundance of fishes in the Oir basin. Over the years, sampling has been conducted at 49 sites spread along the mainstem and 53 situated on six tributaries. Originally, the main target species were salmon and trout. Beginning in 2000, eels were monitored in the same way as salmonids. The inventory of the entire fish community has been carried out on a network of seven sites. It is usually done in autumn. Two sites on two tributaries are also monitored in May for all species.

Method steps

1. Electrofishing surveys were part of long-term monitoring programmes on each river and followed consistent protocols with standardised effort. Fish capture was carried out with different electrofishing equipment using similar settings (direct current, 200-300V). When the target is only salmonids and eels, electrofishing is carried out on a set of sites spread over the entire river network of the Oir River. The capture efficiency is evaluated by carrying out 2 passages on several representative sites.
2. 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 (hereafter length to the nearest millimetre,mm), except for fish whose caudal fin has no fork and for which the total length has been measured. Fish are then weighed (0.2 g precision) and examined for evidence of sexual maturity (mature if it had a soft and distended underbelly from enlarged gonads [Frome] or produced milt when massaged gently. Scales and a piece of fin are taken from a subsample of individuals. Some of the salmon, trout and eels are identified individually with a pit tag. All the fish sampled are then released in their original site.
3. Age is verified from scale reading according to standard methods described by Baglinière & al (2020, 2022). Bagliniere, J.-L., Jeannot, N., Nevoux, M., Lamireau, L., Josset, Q., Marchand, F., 2022. Guide pour l’interprétation des écailles et l’estimation de l’âge chez le saumon atlantique (salmo salar) dans les populations françaises, Guides et protocoles. https://professionnels.ofb.fr/fr/node/1504 Bagliniere, J.-L., Hamelet, V., Guéraud, F., Aymes, J.-C., Goulon, C., Richard, A., Josset, Q., Marchand, F., 2020. Guide to the interpretation of the scales and the estimation of the age of brown trout (Salmo trutta) from the French populations. https://professionnels.ofb.fr/en/node/836

Freswater fish and crayfish

1. Alburnus alburnus (Linnaeus, 1758)
2. Anguilla anguilla (Linnaeus, 1758)
3. Austropotamobius pallipes (Lereboullet, 1858)
4. Barbatula barbatula (Linnaeus, 1758)
5. Carassius carassius (Linnaeus, 1758)
6. Cottus gobio (Linnaeus, 1758)
7. Cyprinus carpio (Linnaeus, 1758)
8. Esox lucius (Linnaeus, 1758)
9. Gobio gobio (Linnaeus, 1758)
10. Lampetra (Bonnaterre, 1788)
11. Lampetra fluviatilis (Linnaeus, 1758)
12. Lampetra planeri (Bloch, 1784)
13. Leuciscus leuciscus (Linnaeus, 1758)
14. Pacifastacus leniusculus (Dana, 1852)
15. Perca fluviatilis (Linnaeus, 1758)
16. Petromyzon marinus (Linnaeus, 1758)
17. Phoxinus phoxinus (Linnaeus, 1758)
18. Platichthys flesus (Linnaeus, 1758)
19. Rutilus rutilus (Linnaeus, 1758)
20. Salmo salar (Linnaeus, 1758)
21. Salmo trutta (Linnaeus, 1758)
22. Salmo trutta trutta (Linnaeus, 1758)
23. Squalius cephalus (Linnaeus, 1758)
24. Tinca tinca (Linnaeus, 1758)

Oir river in Normandy (France). Latitude ranges from 48.6840 to 48.5985; longitude ranges from -1.2949 to -1. 0994.