Shellfishing and tides: when to head down at low tide

Guide · Tides Pro

In shellfishing, the clock doesn't set the schedule — the tide does. You can know the best rock on the coast, but if you arrive as the water is already coming back, there's nothing left to do. This guide answers the two questions that really matter: when to head down to gather, and — almost more important — when to turn around.

The shellfishing window: ninety minutes before low tide

The useful window opens roughly an hour and a half before low tide and closes at the low itself. The strategy is simple: go down with the water. Follow the retreating edge, because the most productive strip is always the last one to be uncovered — the ground that spends the fewest hours in the air and gets the least pressure.

• 90 minutes before: arrive, read the terrain, start on the higher ground.
• The last half hour: the sea uncovers the good ground; this is the prime time.
• At low tide: you're done. From here the water rises — don't go any lower.

Remember the tidal cycle on these coasts is semidiurnal, about 12 hours and 25 minutes: two lows a day, each arriving roughly 50 minutes later than the day before. Saturday's window is not Sunday's.

Spring tides: why the coefficient rules

The tidal coefficient (0–120 scale, IHM formula) sums up the day's range: below 40 is a neap tide, 40–79 average, 80 and above a spring tide. For shellfishing you want strong springs, coefficient 90 or higher: the low drops further than usual and exposes rock and sand that spend weeks underwater. That "new" ground is where the best shellfish usually are.

Spring tides follow the new and full moon by one or two days — the so-called "age of the tide". To plan your month, mark the days right after each new or full moon: that's where the high coefficients live.

Read the height of the low, not just the hour

Two lows at the same hour can be worlds apart. A low of −1.0 m uncovers metres of rock that a −0.3 m low never even touches. That's why the height of the minimum tells you more than the time: before picking a day, compare the week's lows and choose the lowest one, not the most convenient one.

Keep in mind that the same coefficient doesn't drop equally everywhere: the range depends on the port. The reliable move is to check the predicted height of the low at your specific port, not a rule of thumb.

Licences and closures: ask before you take anything

Shellfish gathering is regulated, and for good reason. In Galicia there are specific licences, seasonal closures, quotas and minimum sizes, with many banks managed by the local cofradías (fishermen's guilds); in Portugal, recreational gathering has its own rules and bivalve zones can be temporarily closed for biotoxins. Wherever you are, check the current regional regulations and ask locally what you may take, how much and where — before touching a single rock.

This guide helps you with the tide; the permit comes from the authorities, not from us.

Turn back before the tide turns

A trick veteran gatherers use: when you arrive, pick a "witness rock" near the waterline. The moment the sea covers it again, the tide has turned and your margin is running out.

Plan it in two minutes

The perfect shellfishing plan is three numbers: a coefficient of 90 or more, the lowest low of the week, and the exact time at your port. Tides Pro gives you all three at a glance for 90 ports, powered by the FES2022b harmonic model with a validated median accuracy of ±10 minutes on open coast against the official IHM tables.

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