How the KLEWA Bite Index Works

What the index is — and what it is not

The index answers one question: how favorable is this hour for the species you picked? A score of 80–100 means excellent conditions, 60–79 good, 40–59 fair, and 0–39 poor. An ordinary day at the best hour, in water the species likes, scores about 72. Good weather lifts it toward 100; above 80 the scale tightens, so the best hours of a good day still differ from each other.

It is not a calibrated probability of catching fish. How strong each factor is was our expert choice: from research where a study measured the effect, from angler experience where none did. We will recalibrate the strengths against real catch logs; the formula is built so that a catch-per-hour regression can set them directly.

The formula

index = 100 × 0.72 × water × time of day × spawning × (1 + k₁·e₁) × (1 + k₂·e₂) × …

Each weather or calendar factor has an effect e from −1 to +1, read from the species' own curve, and a strength k: how far the factor can move the score. A strength of 0.08 means up to ±8 percent. Ordinary conditions give an effect near zero and change nothing.

Step 1 — Gates: water, time of day, spawning

• Water temperature — the main gate. Every species has a range where it feeds normally. Above it the gate falls steeply: to half where heat stress starts, and to almost nothing where the fish stops feeding (trout in 28 °C water score 2 to 3). Below it the gate eases down, but not below the share of summer catches that anglers still take under ice: 0.55–0.58 for bluegill and crappie and 0.33 for walleye (Sass et al. 2023); we set 0.65 for pike and 0.33 for perch, roach, bream and trout by analogy. In winter (under ice, or December to February) the water is compared with typical water for the month, so an ordinary winter day scores like an ordinary summer day.
• Time of day. A continuous curve from real sunrise and sunset: a morning peak just after sunrise, an evening peak at sunset, and night from about 3 hours after sunset until 3 hours before sunrise, so the change takes two or three hours. The shape depends on the species and the water temperature. How deep the midday or night dip goes follows walleye, a little softened: at midday anglers caught them about half as often as at dusk (Shaw et al. 2021). A summer night hour gets about 54 percent of the best hour for pike.
• Spawning. When the water is in the species' spawning range during its spawning months, the hour is cut: ×0.4 for pike, which barely feed then (Diana 1979), and ×0.85–0.9 for perch, zander, roach, bream, carp, catfish and trout. Crucian carp, bass, crappie and bluegill have no cut.

Step 2 — Weather, season and the moon

Each factor below works by the species' own rules. The strengths given are for pike; other species have their own.

• Season (month), up to ±12 %. Angler calendars: feeding before winter and after spawning, the summer lull when natural food is plentiful. Water temperature already carries cold and heat, so the month only adds what the water does not. In the southern hemisphere the months shift by half a year.
• Pressure level, up to ±4 %. Sea-level pressure close to the value the species likes (pike about 1008 hPa, crucian carp about 1018 hPa). In a laboratory test perch did not react to pressure (VanderWeyst 2014), so the strength is small.
• Pressure change over 24 and 72 hours, up to ±12 %. Predators like a slow fall before bad weather, peaceful fish like steady pressure; a sharp rise after a cold front lowers the score for all of them. In walleye catches the pressure trend was not significant (p = 0.12, Shaw et al. 2021): this one is mostly angler experience.
• Cloud cover and its change over a day, up to ±6 % (zander ±10 %). Zander see well in dim light and bite less in bright sun; crucian carp and bluegill like sun. It counts only in daylight.
• Wind speed, up to ±6 %. A moderate ripple helps predators, a dead calm or a gale does not; fish of small ponds like it calm. Wind direction, up to ±5 % (zander ±8 %): in the northern hemisphere a south-west wind is best and a north-east wind worst; in the southern hemisphere north and south swap. Walleye catches did depend on wind direction (p = 0.011, Shaw et al. 2021). Under ice wind does not count.
• Moon phase, up to ±8 % for pike. In a German lake pike were caught about a quarter more per hour around full and new moon (Kuparinen et al. 2010; one lake, not repeated). For the other species the phase tables come from angler calendars and are weaker (±3–6 %). For trout the moon does not count: no effect was found (Quigley 2023).
• Solunar periods, up to ±4 % for pike (zander ±7 %). The hour around the moon overhead or underfoot (major periods) and around moonrise and moonset (minor periods), from the moon's real position above your spot. The evidence is mixed: walleye bit better with the moon underfoot but not overhead (Shaw et al. 2021), and trout bit worse in the minor periods (Quigley 2023), so trout get zero here too.
• Rain, up to ±5 %. Drizzle helps trout and pike, a downpour lowers the score for all; heavy rain over three days muddies the water, which hurts fish that hunt by sight and suits catfish and bream. In walleye catches rain was not significant (p = 0.67). Thunderstorms and downpours also show as a warning under the forecast.
• Air temperature and its change over 3 days, up to ±8 %. Warming helps while the water is below the species' range, cooling helps when it is above; a sharp cooling (a cold front) lowers the score; heat above 28 °C lowers the daylight hours.
• Snow, up to ±6 %. Light snow is an old sign for pike and perch; a heavy snowfall lowers the score. Angler experience only.

What the index does not count yet

• Geomagnetic storms. They are popular in some apps, but no catch study has shown an effect.
• Water clarity, river level and fishing pressure. They matter, but we do not have reliable data for them yet.

From hourly scores to your forecast

The daily index takes the best 8 hours of the day and combines them robustly: 60% median + 40% mean. Golden windows sit on the peaks of the day: the peak hour and the hours next to it that score within 10 points of it, up to 4 hours in a row (the inconvenient 01:00–03:00 night hours are skipped).

Hourly weather comes from Open-Meteo (CC BY 4.0), with OpenWeatherMap as the backup; water temperature, ice and three days of weather history come from Open-Meteo too; sunrise, sunset and the moon's position are computed for your exact coordinates. Each of the 12 species has its own temperature range, day curve, spawning rule, factor strengths and moon-phase and month tables.

How we write these articles

The KLEWA team publishes this blog. Every article starts from our own forecast model: the code and the research described on this page.

We read other fishing sites to see what anglers ask about. We never copy their words, examples or numbers.

• Every number is checked against our code or a published study.
• Each language is written for its own anglers: local fish, local waters, metric units.
• The “updated” date changes only when the text really changes.
• Found a mistake? Tell us with the feedback button on any page. We'll fix it.