Rega · Płoty model of a corridor with a camera

Fish pass on the Rega · sequence from the corridor

Fish get past the dam Do not count until you can see the direction.

A camera in the fish pass corridor records an object, but the counter only makes sense once the direction has been established. The four frames below are a teaching model, not a recording from Płoty.

Small diagram of a pool-type fish pass Seven sections, from the entrance (stage 0) to the exit (stage 6), rise in steps from the lower water on the left to the upper water on the right. The orange dashed rectangle in the sixth of the seven sections (stage 5) marks the notional position of the corridor with the camera.
A diagram, not a map: the shot comes from the corridor in the sixth of the seven sections, that is stage 5 (position notional). The whole cross-section is below.

Event

Select two frames. A third click replaces the oldest choice, and changing the event keeps the selected pair.

  1. Frame 1, 0.0 s Fish at the entrance on the lower-water side, snout pointing right.

    Frame 10.0 s

    ← water flowupstream →

    start of pair

  2. Frame 2, 0.4 s Fish in the first third of the corridor, snout to the right.

    Frame 20.4 s

    ← water flowupstream →

  3. Frame 3, 0.8 s Fish in the second half of the corridor, snout to the right.

    Frame 30.8 s

    ← water flowupstream →

  4. Frame 4, 1.2 s Fish at the exit towards the upper water, snout to the right.

    Frame 41.2 s

    ← water flowupstream →

    end of pair

Teaching model: four frames of a model sequence drawn in code, not a recording from Płoty. Downstream on the left, upstream on the right, water flow shown by an arrow pointing left.

Which way?

Event 1 · straight movement

Selected frames: 1 → 4 · interval 1.2 s

Upstream

The position changed from 8% to 92% of the corridor length, that is by 84% in 1.2 s. The object moved towards the upper water. The intermediate frames (2, 3) take no part in this result.

← downstream, 0%100%, upstream →
1 · 0.0 s8%
2 · 0.4 snot selected
3 · 0.8 snot selected
4 · 1.2 s92%

One scale for all events: 0% is the entrance on the lower-water side, 100% is the exit towards the upper water. The values come from a model; we do not know the real length of the corridor. The hatched area means no data, not zero.

Rule 1. A single frame only tells you where the object is. Direction is the difference in position over time, so two frames are needed. Click another pair: in a straight movement every pair points the same way.

Only now the counter

Each of the three events leaves one registration in the raw counter. But only the first can become a candidate for an upstream passage, and only once a specialist has confirmed it. The summary shows all three at once.

1Straight movement

Registrations in the corridor
1
Upstream movement
1
Downstream movement
0
Candidate for an upstream passage
1to be confirmed by a specialist

For the register: upstream movement, to be verified.

2Turn-back

Registrations in the corridor
1
Upstream movement
1entry
Downstream movement
1return
Candidate for an upstream passage
0the object went back downstream

For the register: turn-back. One object and two movements, not two fish and not a passage.

3Unreadable

Registrations in the corridor
1
Upstream movement
no data
Downstream movement
no data
Candidate for an upstream passage
no data is not zero

For the register: unreadable. Neither a passage nor an established absence of passage.

Second level: the same arithmetic for many events (a model, not a forecast)

Suppose the counter has registered a certain number of events. If some of them are downstream movements and turn-backs, and some of the images are unreadable, there are fewer candidates for an upstream passage than registrations. You set the shares yourself: they are assumptions, not a measurement from the Rega. The input fields work only with JavaScript; without it you see just the example: 40 events, 15% downstream movements and turn-backs, 20% unreadable, that is 40 = 26 + 6 + 8.

  • 26candidates for an upstream passage
  • 6downstream movements and turn-backs
  • 8undetermined (unreadable)

40 = 26 + 6 + 8

Rounded to whole events; the three buckets always add up to the counter.

Sequence described in words (an alternative to the image)

Positions are given as a percentage of the corridor length: 0% is the entrance on the lower-water side, 100% is the exit towards the upper water. These are model values.

Event 1 · straight movement (image readable)

  1. 0.0 s: fish at the entrance, about 8%, snout to the right.
  2. 0.4 s: about 36%, snout to the right.
  3. 0.8 s: about 64%, snout to the right.
  4. 1.2 s: about 92%, at the exit towards the upper water.

Event 2 · turn-back (image readable)

  1. 0.0 s: about 12%, snout to the right.
  2. 0.4 s: about 46%, snout to the right.
  3. 0.8 s: about 64%, body twisted across the corridor, snout pointing down the image.
  4. 1.2 s: about 14%, snout to the left, fish near the entrance again.

Event 3 · unreadable (turbid water, bubbles)

  1. 0.0 s: blurred shadow, position only within the range 6–34%.
  2. 0.4 s: shadow within the range 20–52%.
  3. 0.8 s: shadow within the range 30–66%.
  4. 1.2 s: image obscured by bubbles, no shadow visible. The position is not in the data.

Where this shot comes from

According to the 2021 Wody Polskie procurement, Płoty is one of two locations for new fish monitoring equipment in fish passes on the Rega (the other is Łobez). The camera shot shows a single place along the whole route a fish takes through a series of pools.

← lower waterupper water →
Cross-section of a pool-type fish pass with a monitoring corridor Seven sections, from the entrance (stage 0) to the exit (stage 6), rise in steps from the lower water on the left to the upper water on the right. The orange dot is a notional fish at the chosen stage. In the sixth of the seven sections (stage 5) the orange dashed rectangle between two plates marks the corridor with the camera. A general diagram, not a design of the structure at Płoty.

The orange dot is a notional fish. The dashed rectangle is the corridor with the camera, in the sixth of the seven sections (stage 5); its position in the diagram is notional.

  1. Stage 0 · entrance

    First, find the way

    Fish have to find the entrance to the fish pass. What matters is its position and the stream of water that guides them to the passage.

  2. Stage 1 · pool 1

    The first short section

    A pool-type fish pass divides the difference in level into stages. A fish passes through an opening or a slot, depending on the design.

  3. Stage 2 · pool 2

    A place to rest

    The pool gives the fish a place to rest. It also dissipates the energy of the water so that it is not carried in full into the next chamber.

  4. Stage 3 · pool 3

    Not every fish swims the same way

    The conditions for passage have to be matched to the species and size of the fish. This diagram does not simulate their swimming ability.

  5. Stage 4 · pool 4

    The obstacle divided up

    Instead of one large difference in level there is a series of smaller steps. In a real structure, flow and keeping the pass unobstructed also matter.

  6. Stage 5 · corridor with the camera

    The optical recording corridor

    Before the exit to the upper water, the system registers objects between two plates. This field of view is what the four frames at the top of the page show: downstream on the left, upstream on the right. Back to the frames.

  7. Stage 6 · exit

    On the other side

    The notional fish has reached the point above the obstacle. This is the end of the diagram, not a measurement of the effectiveness of a real fish pass.

Limits and sources

What this page does not claim

  • The four frames are the author's own diagram. They are not a recording from Płoty or from any other fish pass, and the positions given as a percentage of the corridor length are model values.
  • We do not know the real length and geometry of the corridor, the speed of the fish or the resolution of the camera. The page does not identify species.
  • The selected events are not statistics on the effectiveness of a fish pass, nor a representative sample of migration. According to the FAO, the possibility of passage is a different thing from a quantitative assessment of effectiveness, which requires broader studies.
  • The interpretation of direction, repeats and identification should be verified by an ichthyologist. This version has not undergone such verification.
  • The fish pass diagram shows a general principle, not a design of the structure at Płoty. The counting model rests on shares set by the reader.
What changed relative to V2
  • The assessment form, the session log with text export and the comparison with a “correct answer” were removed. The result for a pair of frames appears at once, without scoring.
  • Six exercises A–F were replaced by three events. Drift downstream shows up as downstream movement in the turn-back, and an object that is not a fish falls into the “unreadable” category.
  • The duplicate calculator moved to the second level. V2 subtracted the percentage of repeats from the counter; here it is the “downstream and turn-backs” bucket, and unreadable is a separate bucket. The three buckets add up to the counter.
  • The seven-stage diagram stayed, but without movement playback and without the “no fish pass” variant, which fitted into a single sentence.
  • The photograph of the fish pass in Bydgoszcz (a different structure) is no longer embedded, so that the page works offline. The link to the file page remains in the sources.

Sources

The links were carried over from V2. In this version they have not been reopened or checked again, so they need verification before being treated as confirmed.