How the Seaidx Fishing & Surf Index Works

The core of the fishing index is solunar theory: the long-standing angling tradition that fish feed more actively around the times the moon is overhead or underfoot. Solunar tables are folklore-adjacent rather than settled science, but they remain one of the most widely used planning tools in fishing because they give the day a concrete structure. Seaidx computes the moon's position for the spot's exact coordinates instead of reading a generic table: major periods run one hour either side of each moon transit, and minor periods run thirty minutes either side of moonrise and moonset.

Following the conventional solunar weighting, days near a new or full moon get a small quality boost and days near the quarter moons a small penalty. A further bonus is added when a feeding window overlaps the low-light hours around dawn or dusk — windows many anglers already treat as the most reliable of the day.

The solunar baseline is then adjusted for conditions with a clearer physical rationale. Strong wind costs points, and costs more above 12 m/s than above 8. A pressure fall of more than 1.5 hPa over three hours — often a sign of an approaching front — costs points too. On tidal coasts, the hours around a tide turn gain a small bonus. Each hour is scored 0–100, and the day's score is simply its best hour.

The surf rating reads marine-model swell and wind-wave data and asks three questions for every hour: how much energy is in the water, how clean the waves are, and how the wind sits against that particular beach.

Energy comes first. Because swell energy grows with the square of the height and roughly in line with the period, a 1.5 m swell at 14 seconds carries far more push than the same height at 7 seconds — the score keeps that intuition. Height is judged against the size range a beach works at: below 0.3 m the rating is Flat, and past the upper end of a spot's ideal range the score falls away again. Period quality is banded: groundswell of 12 seconds or longer counts in full, 9–12 seconds a little less, 6–9 seconds clearly less, and wind chop under 6 seconds much less.

Wind comes second, and it matters as much as swell. Below about 2 m/s the surface is rated glassy. Above that, the wind is classified by its angle to the direction the beach faces: offshore wind (blowing from land to sea) keeps the score highest, followed by cross-offshore, cross-shore and cross-onshore, with onshore wind — which crumbles the wave faces — costing the most. The stronger the wind, the heavier the penalty.

Third comes direction. Fifty-six spots — well-known beaches, points and fishing marks — have a curated profile recording which way the coast faces and which swell directions the break handles best, so the swell a spot is built for scores as the gift it is. Beaches without a curated profile are scored generically: the numbers stay honest, they are just not tuned to that specific shore.

Hourly and daily results use six words: Flat, Very Poor, Poor, Fair, Good and Epic. As with the fishing index, the scale compares hours and days at one spot. It is not a safety assessment — it knows nothing about rips, rocks, crowds or your ability.