More culturally important than flashy, and best treated as a core Belgian river rather than an afterthought behind the better-known East Belgium names.
- Mixed
Rules at a glance
- Permit required
- Fly only
Water temperature for Grayling
Cool — slow4°C est.ideal 4–14°C
0°14°28°
How to fish it · for Grayling
When
Midday window — grayling feed best from late morning into early afternoon.
Where
Work runs and glides — grayling shoal in the bottom third of the water column.
Method
Tight-line nymph the bottom third with weighted patterns.
Kit
10 ft #3 rod, floating line, long leader to 4 lb fluoro for nymphs.
Why this works
Good — water temperature is working for you, but insect activity is the limiting factor today. Grayling feeds actively in cold water — peak winter sport.
Hatch timeline · todayQuiet day
Hatch predictions
Supporters see the full hour-by-hour hatch timeline.
Start free trial14-day free trial. Cancel anytime.
Sub-surface · on the nymphunder the surface
Hatch predictions
Top nymph shown — see all 4 guilds as a supporter.
Start free trial14-day free trial. Cancel anytime.
Evidence
Survey-backed · regionalHigh confidence
Backed by regional invertebrate surveys; no sampling on this exact reach yet.
Through the year
0–3 scale · November highlighted
J
F
M
A
M
J
J
A
S
O
N
D
Trout seasonSeason
1
2
2
2
2
2
1
Grayling seasonSeason
2
1
1
2
2
2
2
2
2
Evening SedgeHatch
2
3
3
3
2
Large Dark OliveHatch
1
2
2
1
Iron BlueHatch
1
2
2
2
2
2
2
1
Blue Winged OliveHatch
1
2
2
2
2
1
Numbers are intensity 0 (none) to 3 (peak) — a guide, not a guarantee.
Ranked for today
Grayling fly box
A proper Ardennes river-trout pack for Wallonia, centred on clear to lightly coloured rivers with trout and some classic fly-fishing culture.
Hook 14–18 · Fly
in season
Permits & access
Permit required — see local rules.
- Walloon permit required, but some stretches may sit under additional society, forest-zone or local rights structures depending on section.
What's coming
Plan ahead
5-day outlook
Other water nearby · 3
Gallery · 2
File:Frahan JPG01.jpg Terrain map
About this water
Water quality (WFD)
- Ecology: Moderate
- Chemical: Failing to achieve good
What this classification means
WFD re-coded 2026-06-18: prior id BERW06_SM01R was a wrong-basin code (no row in EEA WISE). Re-mapped to BERW17_SC28R 'Semois III' near the venue coordinate (49.85,5.08, Bouillon area). Status: EEA WISE 2022 (3rd RBMP). Confidence: medium — VERIFY reach against a map (Semois has 4 reaches I–IV). Source: discodata.eea.europa.eu.
WFD classification · BERW17_SC28R
The full read · show the working · for Grayling · confidence 70%
How the 76 is built — score × weight = contribution
Temperature93 × 28%26.0
Flow80 × 22%17.6
Clarity85 × 18%15.3
Feeding Time60 × 13%7.8
Pressure80 × 7%5.6
Insect activity23 × 12%2.8
Conditions total= 75
Can you trust it?
Water temperatureair-to-water estimateestimated
Level / flowon-river gaugeobserved
Prey activitysurvey-backed invertebrate datasurvey
What would change the calculation
The dawn and dusk windows score higher than the midday lull.
Directions
Seasons & zones
- Trout 3rd Saturday of March → 30 September
- Grayling 1 June → 28 February
Related guides
Booking & contacts
Semois
One of Wallonia's defining southern rivers, winding through wooded valleys and long glides with a mixed trout-and-grayling identity depending on section.
Water temperature for Grayling
Cool — slow4°C est.ideal 4–14°C
0°14°28°
How to fish it · for Grayling
When
Midday window — grayling feed best from late morning into early afternoon.
Where
Work runs and glides — grayling shoal in the bottom third of the water column.
Method
Tight-line nymph the bottom third with weighted patterns.
Kit
10 ft #3 rod, floating line, long leader to 4 lb fluoro for nymphs.
Why this works
Good — water temperature is working for you, but insect activity is the limiting factor today. Grayling feeds actively in cold water — peak winter sport.
Hatch timeline · todayQuiet day
Hatch predictions
Supporters see the full hour-by-hour hatch timeline.
Start free trial14-day free trial. Cancel anytime.
Sub-surface · on the nymphunder the surface
Hatch predictions
Top nymph shown — see all 4 guilds as a supporter.
Start free trial14-day free trial. Cancel anytime.
Evidence
Survey-backed · regionalHigh confidence
Backed by regional invertebrate surveys; no sampling on this exact reach yet.
Through the year
0–3 scale · November highlighted
J
F
M
A
M
J
J
A
S
O
N
D
Trout seasonSeason
1
2
2
2
2
2
1
Grayling seasonSeason
2
1
1
2
2
2
2
2
2
Evening SedgeHatch
2
3
3
3
2
Large Dark OliveHatch
1
2
2
1
Iron BlueHatch
1
2
2
2
2
2
2
1
Blue Winged OliveHatch
1
2
2
2
2
1
Numbers are intensity 0 (none) to 3 (peak) — a guide, not a guarantee.
Ranked for today
Grayling fly box
A proper Ardennes river-trout pack for Wallonia, centred on clear to lightly coloured rivers with trout and some classic fly-fishing culture.
Hook 14–18 · Fly
in season
Permits & access
Permit required — see local rules.
- Walloon permit required, but some stretches may sit under additional society, forest-zone or local rights structures depending on section.
What's coming
Plan ahead
5-day outlook
Other water nearby · 3
Gallery · 2
File:Frahan JPG01.jpg Terrain map
About this water
More culturally important than flashy, and best treated as a core Belgian river rather than an afterthought behind the better-known East Belgium names.
- Mixed
Water quality (WFD)
- Ecology: Moderate
- Chemical: Failing to achieve good
What this classification means
WFD re-coded 2026-06-18: prior id BERW06_SM01R was a wrong-basin code (no row in EEA WISE). Re-mapped to BERW17_SC28R 'Semois III' near the venue coordinate (49.85,5.08, Bouillon area). Status: EEA WISE 2022 (3rd RBMP). Confidence: medium — VERIFY reach against a map (Semois has 4 reaches I–IV). Source: discodata.eea.europa.eu.
WFD classification · BERW17_SC28R
The full read · show the working · for Grayling · confidence 70%
How the 76 is built — score × weight = contribution
Temperature93 × 28%26.0
Flow80 × 22%17.6
Clarity85 × 18%15.3
Feeding Time60 × 13%7.8
Pressure80 × 7%5.6
Insect activity23 × 12%2.8
Conditions total= 75
Can you trust it?
Water temperatureair-to-water estimateestimated
Level / flowon-river gaugeobserved
Prey activitysurvey-backed invertebrate datasurvey
What would change the calculation
The dawn and dusk windows score higher than the midday lull.
Directions
Seasons & zones
- Trout 3rd Saturday of March → 30 September
- Grayling 1 June → 28 February
Related guides
Booking & contacts
At a glance
Semois is a freestone river in Wallonia / Luxembourg Province, Belgium. It sits at about 430 m above sea level. Our data lists brown trout and grayling here. The trout season is recorded as running from 3rd Saturday of March to 30 September. The grayling season is recorded as running from 1 June to 28 February. Best window recorded for anglers: trout: 3rd Saturday of March (Wallonia) – 30 September (Wallonia); grayling: 1 June (Wallonia) – 28 February (Wallonia). Core methods listed: Nymph fishing; Dry fly; Wet fly / soft hackle. Level and flow data for this water is linked from 2 gauges. Its Water Framework Directive ecological status is recorded as moderate. Nearby waters in our listings include Lesse at 31 km, Pisciculture de Vendresse at 35 km and Ourthe at 44 km.