Adriatic seabed, depth and habitat scores: why the wrong bottom blanks you

Folk-5 is the five-class seabed map from EMODnet Geology (mud to muddy sand, sand, coarse substrate, mixed sediment, rock and boulders) shown under every point in AlgoCatch. Class decides which fish hold there.

Updated 2026.

What are the five Folk-5 seabed classes?

Mud to muddy sand, sand, coarse substrate, mixed sediment, and rock and boulders: the Folk (1954) grain-size triangle collapsed into five groups, published by EMODnet Geology at 1:250,000 and 1:1,000,000 and drawn as a continuous class layer.

Folk-5 is a five-class sediment standard. Robert Folk's 1954 grain-size triangle sorted mud, sand and gravel by mix. The publisher collapses that triangle into five groups and maps them as a continuous polygon map at 1:250,000 and 1:1,000,000. Each polygon is one class for a stretch of seabed, not a single sounding.

The five words, in the order the product uses them, are mud to muddy sand, sand, coarse substrate, mixed sediment, and rock and boulders. Learn them once. Every place guide and the map legend use the same five. The layer does not name fish. It names the bottom those fish can use.

Grain size is the mechanism. Mud and muddy sand are fine. Sand is cleaner and mobile. Coarse substrate is gravel and shell. Mixed sediment sits between sand and rock. Rock and boulders are hard ground. The map draws those bins as polygons so you can see where one class stops and the next starts.

This page is the class list for fishing. How the substrate map is compiled sits on a child page. Your job at the pin is to read which of the five you are on, then sit the class edge, not the middle of the polygon. Depth without class is half a plan. Wrong bottom blanks you.

Which fish hold on which bottom?

Rock and boulders: ambush cover for dentex, seabass and scorpionfish. Sand and muddy sand: feeding ground for gilthead, pandora and flatfish. Mixed sediment and coarse substrate: transition zones patrolled by predators. Fish the edge between classes, not the middle.

Bottom class predicts which fish can hold there. Cover, food and transitions differ by sediment (T2, FishBase, FAO, GFCM). Two pins a hundred metres apart can be different classes. Sit the class edge, not the middle.

Class	What it is	Which fish	Which technique	Guide
Mud to muddy sand	Fine sediment; flatfish and pandora ground; dominant in the northern basin under Po sediment.	See meaning	Longline along the band	[sand and muddy sand: gilthead and pandora ground](/en/habitat/sand)
Sand	Feeding ground for gilthead seabream, pandora and flatfish; fish the edge to seagrass or mixed ground.	See meaning	Anchor on the class change; dawn surfcasting	Same sand guide
Coarse substrate	Gravel and shell ground forming transition zones patrolled by predators.	Predators on the change	Anchor on the edge; trolling as a contour search	mixed sediment and coarse substrate: the transition zones
Mixed sediment	Mixed bottom between sand and rock; transition zone for pandora and predators; anchor on the edge.	See meaning	Anchor on the edge	Same mixed guide
Rock and boulders	Ambush cover for common dentex, European seabass and scorpionfish; shore spinning and slow trolling ground.	See meaning	Shore spinning; slow trolling; anchor beside the rock	rock and boulders: the dentex and seabass class

Dated sits, habitat not catch: 14 June 2026 Savudrija sand edge; 3 May 2026 Rovinj rock wall; 22 August 2026 Poreč mixed; 8 September 2026 Murter coarse band; 11 May 2026 Slovenian-coast mud after flood colour sat on neighbouring sand. The hundred-metre miss at Savudrija: two pins 100 m apart, only the change was worth sitting.

How is Folk-5 different from a nautical chart's bottom notes?

A nautical chart marks bottom type as point abbreviations beside soundings (S sand, R rock, M mud) for anchoring. Folk-5 is a continuous polygon map of sediment class. Neither shows fish; the habitat score adds species logic.

Point notes and a class map answer different questions. A chart samples the bottom at soundings so you can judge holding ground under a marked anchorage. S, R, M are those abbreviations (T1, chart legend conventions). A Navionics or C-MAP user can see S in one corner of a bay and R on a headland and still not know whether the rest of the bay is sand or rock. Folk-5 maps the whole seabed as polygons. That is the continuous polygon map: one class drawn across a stretch, not a letter next to one sounding.

Neither product plots fish. The number you read after class and metres asks whether that bottom fits the species you picked. Full document contrast sits on fishing map vs nautical chart. This section only contrasts bottom notes with the class layer.

Use the chart note to judge holding ground under a marked anchorage. Use Folk-5 to find the class edge to fish. Do not treat either sheet as a fish map. The letter beside a sounding answers "will the hook hold". The polygon answers "what class is this whole patch". For what the habitat score means, the class is one input.

How do you read the seabed layer in the planner?

Tap the water: the free Inspect readout names the Folk-5 class and the depth. Read class first, then metres, then the habitat score. Find the class change and sit on it; if mute, move 10–30 m along it.

The method is class first, then metres, then score. The class is one tap away, and the Inspect readout is free. Tap the water. You get the class and the metres. That is the whole free seabed fact. Depth without class is half a plan. The multi-species layer built on Folk-5 is the VIP habitat layer. Same class underneath; paid access applies species logic on top.

Fishing layers are live for Trieste–Ancona, the Slovenian coast, Istria, Kvarner and Dalmatia. Status by coast sits on where the seabed layer is live. If you sit ten metres off the change, the number can collapse.

Metres come from the depth layer under the class. The figure you read after class and metres is habitat suitability, not a fish count. Tiers and inputs live on the score page, not here.

On 14 June 2026 I tapped that same sand-to-mixed seam, sat the change, and only then looked at the number. Before you cast or drop:

1. Tap the water.
2. Read the class.
3. Read the metres.
4. Read the number for the species and technique you will fish.
5. Sit the change; move 10–30 m if mute.

Do that on the free map before you leave the harbour wall. If the first tap lands in the middle of a polygon, do not fish that number. Slide the pin until the class changes, then sit the seam.

How do you fish Posidonia meadows without damaging them?

Posidonia oceanica is a priority habitat under the EU Habitats Directive. Fish the meadow edge over sand (a gilthead and seabass feeding lane) and avoid anchoring on it; anchoring on Posidonia is restricted in protected areas like Brijuni and Strunjan.

The meadow edge is the spot. The meadow is protected. Seagrass shelters prey; the sand edge is where those two inshore species feed. Annex I of the EU Habitats Directive lists the plant as a priority habitat. Park rules at the two named reserves restrict anchoring on it.

On 19 June 2026 I fished a meadow edge over sand on the Slovenian coast from the sand side, not from inside the blades. Fish the edge, protect the meadow. Anchor on the sand beside the change. Never drop in the meadow. Outside named parks the working rule is still avoid it; this page does not list every bay.

A meadow is class-adjacent: it is not a sixth Folk-5 bin, but it changes how you sit a sand polygon. If the sand you tapped borders a dark meadow, the feeding lane is the seam, not the middle of either patch. Check the park notice before you drop. If the notice forbids the hook, move to the sand side or leave.

Detail, species along the edge and the legal notes sit on Posidonia meadows and fishing. Here the rule is one line: sit the sand side of the edge, and leave the blades alone.

Where does the seabed data come from, and how accurate is it near shore?

From EMODnet Geology's seabed substrate map, harmonised to Folk classes at 1:250,000 and 1:1,000,000. Near shore the polygons generalise: a small rock insert can sit inside a sand polygon. Confirm with what the anchor or lure brings up.

The map is a harmonised survey product, not a survey of every bay. National sheets are brought to Folk classes at those two scales. At 1:250,000 and 1:1,000,000 a rock the size of a house can vanish inside a sand polygon. That is the near-shore caveat, not an accuracy percentage. None is published here.

On 3 May 2026 a west-Istria pin read sand on the sheet while the lead showed rock; the hard insert sat inside the sand polygon. Trust the class pattern. Confirm the exact bottom with what the anchor or lure brings up. If the lure comes back with weed and chip, you are not on the sand the polygon drew.

The habit is simple. Use the layer to choose the bay and the seam. Use the lead, the lure or the anchor to confirm the last ten metres. A generalised polygon is still the right planning tool. It is a bad excuse to ignore what the gear just told you.

How the data is compiled, and the longer accuracy note, sit on how the EMODnet Folk-5 data is made. This page only states the source, the scales and the confirm-on-the-water habit.

How does the seabed change from Trieste to Dalmatia?

Mud and sand dominate the northern basin under Po sediment; Istria is sand and mixed with rock at Rovinj; Kvarner and Dalmatia turn to rock walls, boulders and channels with sand pockets between islands. Same five classes, different proportions.

River sediment in the north and karst rock in the south sort the same five classes into different fisheries. Expect sand edges from Trieste through the Slovenian coast and northern Istria. Expect rock edges from Kvarner south, with sand pockets in the channels. After a Po flood, brown water can sit on sand that would otherwise fish; the class did not vanish, the day did. Wind and colour change the day, not the polygon.

The class did not change at the border. The licence did. Italy, Slovenia and Croatia stamp the same mud, sand and rock with different paper. A pin on the Italian side of the box is still one of the five classes. It is not a sixth national bottom.

That north-to-south shift is why a holiday week in the northern basin and a week in the southern channels need different sits: sand seams first in the north, rock seams first in the south. Same legend. Different proportions.

Coast-by-coast detail sits on Adriatic seabed types by coast. This paragraph is the basin pattern only. Town lists belong on that child and on the place guides.

Which technique fits each seabed class?

Anchor on the class change for gilthead and pandora; dawn surfcasting on sand; longline along a sand or mud band; shore spinning and slow trolling over rock for seabass and dentex; trolling as a contour search across mixed ground.

Bottom class narrows the technique. Gear has to sit on or over the class the target uses. The product names trolling, anchor, longline and shore spinning. This page names which class those modes sit on. Speeds, rigs and placement live elsewhere.

Sand and muddy sand: anchor on the class change; dawn surfcasting on sand; longline along the band. Rock and boulders: shore spinning; slow trolling; anchor beside the rock on the class change. Mixed sediment and coarse substrate: anchor on the edge; trolling as a contour search. Those sits match the table.

Pick the technique from the class row, then open which technique for this bay for the method. Switch when the class under the pin changes.

Seabed guides:

• rock and boulders: the dentex and seabass class
• sand and muddy sand: gilthead and pandora ground
• mixed sediment and coarse substrate: the transition zones
• Posidonia meadows and fishing
• how the EMODnet Folk-5 data is made
• Adriatic seabed types by coast

If the class under the pin is wrong for the fish you came for, change place before you burn the session. Then check a pin's class and score for free on the edge you will actually sit, not on the middle of the polygon.