Trekvis - Migratory fishes in the river Scheldt

Study Extent

The Long-Term Vision (LTV) for the Schelde estuary is a Dutch-Flemish managerial plan. Its ultimate goal is to reconcile the apparent conflicting multiple functions of the estuary in sustainable integrated management. The LTV sets quality targets for the estuary in the year 2030 and the management measures to achieve them. These goals and management are integrated from three central perspectives: accessibility, flood management and ecology. In this framework, in the Memorandum of Vlissingen (2002) agreements were made about a long-term monitoring and research program to support the cross-border cooperation of policy and management in the Schelde-estuary. In addition to the three main perspectives, the decree also pays attention to themes like fisheries and recreation. The present project 'Trekvis' is part of the theme 'fish and fisheries' and was contracted by the Maritime Access Division of the Department of Public Works of the Flemish government to the Research. Institute for Nature and Forest (INBO). This study aims to investigate the present status of diadromous fishes in the Schelde basin and to formulate measures for an integrated estuarine ecosystem management. Hereto, the causes for the local decline and extinction of the diadromous species were discussed and their present distribution in the Schelde basin was investigated in a field study. Based on this, a proposal for the monitoring of diadromous fishes and management actions are presented in order to come to a sustainable recovery of the diadromous fish populations in the Schelde basin. ## Distribution and status of diadromous fishes in the Schelde Catadromous species like European eel, flounder and thinlip mullet are quite abundant in the estuary. These species spawn at sea and use the estuary as a nursery or growing-up habitat. Their density is highest in summer. Thinlip mullets were caught in the freshwater zone for the first time and juvenile flounder arrives into the Zeeschelde from May on. The upstream sections of the river, above the tidal zone, are probably less important for these species. Eels, on the other hand, also colonize the upstream rivers, but their migration depends on the population density in the estuary. The status of the eel stock depends on the recruitment of glass eels. However, the recruitment of glass eels decreased dramatically during the last three decades and the eel stock is now in a critical state and well below safe biological limits. The occurrence of anadromous species in the estuary is related mainly to their pawning migration during winter and spring. In this period, mature smelts, river lampreys and threespined sticklebacks migrate upstream to reproduce in the freshwater zone. River lamprey was caught under the weir of Merelbeke as well as under the weir of Asper, indicating that their spawning grounds are further upstream. Both weirs/sluices are important migration barriers for this species. There are strong indications that the life cycle of river lamprey is complete in the Schelde, but the spawning sites and the larval habitats are unknown. Smelt was not caught upstream of the weir of Merelbeke. According to the literature, this species reproduces near the limit of tidal influence. Hence it is doubtful whether the riverine zone upstream of the barrier in Merelbeke is essential for the recovery of smelt in the Schelde. The catches of smelt near Rupelmonde during summer suggest that the Zeeschelde also functions as growing-up habitat for this species. Only a limited number of threespined sticklebacks (trachurus) were caught during the study. However, previous studies near the weir in Merelbeke showed that the upstream migration of sticklebacks occurs between January and April. The migration peak was possibly missed because of the limited temporal resolution of the sampling strategy. Only a few adult twaite shads were caught in the brackish part of the Zeeschelde. The low number of adults and the absence of 0+ individuals indicates that twaite shad does not yet reproduce in the Schelde. Our results clearly show that the main migration route of anadromous fishes in the Schelde is towards the Upper Schelde/River Leie. On the migration axis through the Rupel, on the other hand, almost no anadromous species were caught. From March 2007 on, the sewage treatment plant of Brussels became operational, which greatly improved the water quality in the River Rupel. For the moment, however, only flounder and eel seem to benefit from the increased oxygen concentration. Their numbers increased exponentially in the Rupel during the second half of 2008. Probably, the oxygen concentration is still too low for the other diadromous species or the stimulus for upstream migration is lacking (e.g. conspecific odours in river lampreys). A number of diadromous species does not occur anymore in the Schelde basin. Until the first half of the 19 th century, a stable and reproducing copulation of allis shad occurred in the Schelde basin, but they became extinct locally by the beginning of the 20 th century. For other species like Atlantic salmon, sea trout, Atlantic sturgeon and houting, we have less information about the historical status of their populations in the Schelde. Already in the Middle Ages the populations of these species were seriously threatened, which makes natural recovery unlikely to occur. Monitoring The three sampling methods that were applied in this project (cooling water, intertidal and subtidal fykes) proved to be complementary and each are useful to describe a different aspect of the migration of diadromous fishes. The volunteer network (intertidal fykes) has a high temporal and spatial resolution. Especially the catadromous species that use the estuary as a foraging and nursery area, were caught in the intertidal fyke nets. Subadult shads and downstream migrating juvenile lampreys were only caught in the cooling water of the powerplant of Doel. In addition, the fish monitoring of the cooling water in Doel started in 1991, which allows us to investigate long-term trends. The subtidal fyke nets that were deployed underneath the migration barriers proved to be efficient in catching the fishes during their spawning run. During their upstream migration, anadromous fishes accumulate underneath the barriers where they can be caught in large numbers. The proposed monitoring programme consists of a combination of the above sampling methods. We advise to continue the monthly sampling of the cooling water and to build on seven sampling locations of the volunteer network in the estuary. An extra volunteer sampling station near the Dutch-Belgian border and, if possible, a volunteer network in the Westerschelde should be set up in order to cover the complete estuarine gradient. In addition, the possibility should be investigated of fishing with stow nets at about three locations in the Zeeschelde. In this way, the Dutch and Belgian monitoring programmes can be harmonised and diadromous fishes that migrate through the main channel will be sampled more efficiently. Each fish barriers on the main migration routes that is being solved, should be fitted with a device for capturing migrating diadromous fishes. As long as those devices are in not place, the present fyke net monitoring should be continued at the barriers on the Upper Schelde, Leie and Kleine Nete. Finally, the glass eel monitoring by a volunteer in the Zeeschelde should be maintained and further supported. This monitoring proposal for diadromous fishes is part of the integrated monitoring plan for the Schelde estuary (MONEOS).

Sampling

The three sampling methods that were applied in this project (cooling water, intertidal and subtidal fykes) proved to be complementary and each are useful to describe a different aspect of the migration of diadromous fishes. The volunteer network (intertidal fykes) has a high temporal and spatial resolution. Especially the catadromous species that use the estuary as a foraging and nursery area, were caught in the intertidal fyke nets. Subadult shads and downstream migrating juvenile lampreys were only caught in the cooling water of the powerplant of Doel. In addition, the fish monitoring of the cooling water in Doel started in 1991, which allows us to investigate long-term trends. The subtidal fyke nets that were deployed underneath the migration barriers proved to be efficient in catching the fishes during their spawning run. During their upstream migration, anadromous fishes accumulate underneath the barriers where they can be caught in large numbers.

Quality Control

All records are validated.

Method steps

1. The proposed monitoring programme consists of a combination of the above sampling methods.

All species in this dataset are fishes (Actinopterygii, Cephalaspidomorphi), except for the Chinese mitten crab (Eriocheir sinensis), which is an invasive arthropod and the common littoral crab (Carcinus maenas). The top 3 recorded species are Rutilus rutilus (27%), Gobio gobio (10%), and Anguilla anguilla (7%).

1. Actinopterygii
   common name: ray-finned fishes rank: class
2. Abramis brama
   common name: common bream rank: species
3. Agonus cataphractus
   common name: pogge rank: species
4. Alburnus alburnus
   common name: common bleak rank: species
5. Ameiurus nebulosus
   common name: brown bullhead rank: species
6. Anguilla anguilla
   common name: European eel rank: species
7. Atherina presbyter
   common name: sand smelt rank: species
8. Barbatula barbatula
   common name: stone loach rank: species
9. Blicca bjoerkna
   common name: silver bream rank: species
10. Carassius gibelio
    common name: Prussian carp rank: species
11. Carcinus maenas
    rank: species
12. Ciliata mustela
    common name: fivebeard rockling rank: species
13. Clupea harengus
    common name: Atlantic herring rank: species
14. Cobitis taenia
    common name: spined loach rank: species
15. Cottus gobio
    common name: European bullhead rank: species
16. Cyprinus carpio
    common name: common carp rank: species
17. Dicentrarchus labrax
    common name: European seabass rank: species
18. Eriocheir sinensis
    common name: Chinese mitten crab rank: species
19. Esox lucius
    common name: northern pike rank: species
20. Gadus morhua
    common name: Atlantic cod rank: species
21. Gasterosteus aculeatus
    common name: three-spined stickleback rank: species
22. Gobio gobio
    common name: gudgeon rank: species
23. Gymnocephalus cernuus
    common name: Eurasian ruffe rank: species
24. Hyperoplus lanceolatus
    common name: greater sand eel rank: species
25. Lampetra fluviatilis
    common name: European river lamprey rank: species
26. Lampetra planeri
    common name: European brook lamprey rank: species
27. Lepomis gibbosus
    common name: pumpkinseed rank: species
28. Leuciscus cephalus
    common name: European chub rank: species
29. Leuciscus idus
    common name: ide rank: species
30. Leuciscus leuciscus
    common name: common dace rank: species
31. Liparis liparis
    common name: common seasnail rank: species
32. Liza ramada
    common name: thinlip mullet rank: species
33. Lota lota
    common name: burbot rank: species
34. Osmerus eperlanus
    common name: European smelt rank: species
35. Perca fluviatilis
    common name: European perch rank: species
36. Pimephales promelas
    common name: fathead minnow rank: species
37. Platichthys flesus
    common name: European flounder rank: species
38. Pomatoschistus microps
    common name: common goby rank: species
39. Pomatoschistus minutus
    common name: sand goby rank: species
40. Pseudorasbora parva
    common name: stone moroko rank: species
41. Pungitius pungitius
    common name: ninespine stickleback rank: species
42. Rhodeus amarus
    common name: European bitterling rank: species
43. Rutilus rutilus
    common name: common roach rank: species
44. Salmo trutta
    common name: brown trout rank: species
45. Sander lucioperca
    common name: zander rank: species
46. Scardinius erythrophthalmus
    common name: common rudd rank: species
47. Silurus glanis
    common name: wels catfish rank: species
48. Solea solea
    common name: common sole rank: species
49. Syngnathus acus
    common name: greater pipefish rank: species
50. Tinca tinca
    common name: tench rank: species
51. Trisopterus luscus
    common name: pouting rank: species
52. Umbra pygmaea
    common name: eastern mudminnow rank: species
53. Cephalaspidomorphi
    rank: class
54. Malacostraca
    rank: class

The Scheldt (Dutch Schelde, French Escaut) is a 350 km long river in northern France, western Belgium and the southwestern part of the Netherlands. Its name is derived from an adjective corresponding to Old English sceald "shallow", Modern English shoal, Low German schol, Frisian skol, and Swedish skäll "thin".