Fishing terms, defined

The clear, calm, cold day right behind a front. Looks perfect, fishes slowly.

The day that arrives once a cold front has passed: clear sky, light wind, air colder than yesterday, and a barometer that has stopped falling and started to climb. It is the pleasantest day of the week to be outside and frequently the slowest to fish. The sunshine is not the cause. Harsh light reaches deeper, a slick surface gives fish nothing to hide under, the water sheds heat overnight under a clear sky, and fish that roamed shallow ahead of the weather pull back to cover. We label these days in the forecast so a good-looking day scoring a C is not a mystery.

See also cold front, pre-front window. Read the full guide

The boundary where colder air shoves in, usually with a sharp pressure swing.

The leading edge of a colder air mass. Pressure falls sharply as it approaches, then reverses and climbs behind it, wind swings round, and temperature drops. Fishing in front of one is often excellent and fishing behind it is often poor, which is why the forecast treats the two sides separately rather than calling the whole event good or bad.

See also bluebird day, pre-front window, barometric pressure. Read the full guide

The hours before a front arrives, when fish often feed hardest.

The falling-barometer period ahead of an advancing front, typically with building cloud and a freshening southerly breeze. It is widely held to be among the best fishing of any week, and the forecast labels it when it detects one. Treat it as a reason to move a trip forward rather than back.

See also cold front, bluebird day, barometric pressure. Read the full guide

Air pressure. The trend matters; the absolute number mostly does not.

The weight of the atmosphere, usually reported in hectopascals or inches of mercury. Angling lore treats it as decisive and the peer-reviewed work does not: controlled studies repeatedly fail to find a direct effect on catch rates, which is why it carries only 7% of our score and is read as a rate of change rather than a level. The likeliest explanation is that pressure stands in for everything travelling with it, namely light, cloud, wind, chop and the temperature change behind a front, and fish respond to those.

See also pre-front window, cold front. Read the full guide

The autumn weeks when a lake mixes top to bottom and fishing falls apart.

When autumn cooling brings the surface layer down to roughly the density of the water beneath it, wind can stir the whole basin and the summer layering collapses. The water discolours, oxygen and settled material spread through the column, and fish scatter and suspend instead of holding on structure. It usually lasts two to three weeks. Deep clear lakes turn over sharply, shallow lakes and rivers barely at all, and a big lake often turns in sections rather than all at once.

See also thermocline, water temperature. Read the full guide

The narrow band where summer water temperature drops away fast.

In a stratified lake, the layer separating warm oxygenated surface water from the cold, often oxygen-poor water below. It behaves as a floor: through summer most gamefish stay above it because that is where the oxygen is. It breaks down at turnover, which is what opens the whole water column back up in autumn.

See also turnover, dissolved oxygen.

The heaviest single input to the grade, and the one most worth checking.

Surface water temperature, measured by a gauge where one is near enough and modelled from recent air temperatures otherwise. It carries more weight than any other factor in our score because it governs both where fish are and whether they are feeding at all. We read it against the species you have selected rather than against a single universal optimum, since 55°F is a good day for a trout and a poor one for a bass.

See also water temperature ceiling, turnover. Read the full guide

How much oxygen the water holds. Warm water holds less of it.

Oxygen available in the water, in milligrams per litre. It falls as water warms and during turnover, and fish stop feeding well before levels become dangerous. Cold-water species such as trout need more of it than warm-water species, so our thresholds are set per species rather than universally.

See also turnover, thermocline.

How cloudy the water is. A little is good; a lot is not.

A measure of suspended material in the water, in nephelometric turbidity units. A moderate stain is generally helpful, since it gives fish confidence to feed in shallower water without feeling exposed. Very clear water makes fish line-shy and very muddy water stops sight-feeders finding anything, so the relationship is a hump rather than a straight line.

See also turnover.

Ice cover, estimated from accumulated freezing degree days.

We estimate ice cover from how much freezing weather the location has accumulated over the past week, with a thaw override, rather than from a single cold night. Ice does not lower the grade by itself, because plenty of species are caught through it, but it changes the advice completely and the forecast shows ice guidance whenever it is likely.

See also water temperature.

The staging period before spawning, and often the best fishing of the year.

The weeks when fish move toward spawning areas and feed heavily before they begin. It is frequently the most productive stretch of the year, and it is the reason our water-temperature ceiling is lifted for it: pre-spawn fish genuinely do feed in water colder than their normal comfortable range, so capping the grade there would be wrong.

See also spawn lockout, water temperature ceiling. Read the full guide

Fish on beds are defending, not feeding.

During spawning itself, fish are focused on the bed rather than on eating, and catch rates usually fall. The size of the effect differs a lot by species, which is why our seasonal model carries a per-species figure rather than one number. It is also why the calendar matters less than the water temperature: a spawn month with cold water is not a spawn.

See also pre-spawn.

The autumn feeding binge before winter.

The period when cooling water pushes fish to feed hard ahead of winter. It usually follows turnover on lakes that stratify, and it is one of the two annual peaks alongside pre-spawn. Falling water temperature in autumn is treated as a feeding trigger in our score rather than as a problem.

See also turnover, pre-spawn. Read the full guide

A cap on the grade when the water is outside your species comfortable range.

When the water sits outside the range your chosen species can comfortably feed in, the grade is capped regardless of how good everything else looks, because a bass in 45°F water is not going to have a good day because the wind dropped. The cap tightens the further outside the range the water is, and it is lifted during the pre-spawn stage-up, when fish genuinely do feed in water colder than their usual comfort. When a ceiling is what set your grade the forecast says so, and tells you what the day would otherwise have scored.

See also water temperature, pre-spawn.

Moon-driven feeding windows. Real, and much smaller than the tables suggest.

Major and minor periods derived from when the moon is overhead or underfoot. Major periods run roughly two hours, minors about one. The effect is genuine but modest, which is why solunar carries 3% of our score rather than being the whole of it, as a printed almanac would have it. It is worth using to place a trip within a day, not to decide whether the day is worth fishing.

See also moon phase. Read the full guide

New and full moons score highest; quarters score lowest.

The lunar cycle, scored highest at new and full moons and lowest at the quarters. It carries 1% of our score. Quigley and colleagues found no significant lunar effect on trout catch rates in 2023, and we weight it accordingly rather than dropping it entirely, since the tidal interaction on salt water is real.

See also solunar period.