Guide
Water temperature and salinity: where fish hold and why
Water temperature sets a fish's metabolism, so it decides which species are active and how deep they hold. Salinity, measured in parts per thousand, decides which species can be there at all — open ocean runs near 35 ppt, estuaries far lower. Together they explain most of where fish sit on any given day.
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Air temperature tells you what to wear. Water temperature tells you what is going to happen.
Why temperature matters so much
Fish are ectothermic: their body temperature follows the water. Metabolism, digestion rate and oxygen demand all rise and fall with it, which means the water temperature effectively sets how much a fish needs to eat and how hard it is willing to work for it.
Every species has a band it prefers. Inside that band fish feed actively and range widely. Outside it they slow down, hold deeper or move somewhere else entirely. Nothing else on a catch record explains as much behaviour.
What temperature changes
Depth. In summer many waters stratify, with warm surface water sitting over cooler water below and a sharp transition — the thermocline — between them. Fish frequently sit at or just below it, which is why the same mark can be empty at three metres and stacked at eight.
Timing. When surface water gets too warm, feeding concentrates into dawn and dusk, and the middle of the day becomes close to pointless in shallow water.
Season. Spawning runs, migrations and the shutdown at either end of the year are triggered largely by temperature crossing a threshold, not by the calendar.
Rate of change. A rapid drop after a cold front slows fish for a day or two even when the resulting temperature is still perfectly comfortable. Fish respond to change, not just to level.
Salinity, and why estuary fishing is its own puzzle
Salinity is the dissolved salt content, in parts per thousand (ppt). Open ocean sits near 35 ppt. Fresh water is close to zero. Everything between is brackish, and that is where most of the interesting fishing happens.
| Water | Approximate salinity | What it means |
|---|---|---|
| Fresh | Under 0.5 ppt | Freshwater species only |
| Brackish | 0.5–30 ppt | Estuarine and euryhaline species; the band moves with rain and tide |
| Ocean | Around 35 ppt | Fully marine species |
In an estuary that boundary is not fixed. Denser salt water pushes upstream along the bottom as a wedge while fresher water runs out over the top, and the whole arrangement shifts with the tide and with how much rain has fallen upstream.
Species that tolerate a wide range — snook, redfish, striped bass, bull sharks — follow that boundary around. This is why heavy rain can empty a mark that fished perfectly a week earlier: the fish have not stopped feeding, the salinity they want has simply moved downstream and they went with it.
How to use both
Learn your species' band. Once you know the range your target actually feeds in, water temperature stops being trivia and becomes a go-or-don't-go decision.
Hunt edges, not averages. Temperature breaks and salinity boundaries concentrate bait, and everything that eats bait follows.
Fish the rain, deliberately. After heavy rain, work downstream of where you would normally start. The productive water has usually moved that way.
Log both, every time. The relationship between a mark and its conditions only becomes visible across a season of records.
Global Fishing League records water temperature in °F and salinity in ppt on every catch, along with dissolved oxygen — which is what stops fish using otherwise perfect water. Seeevery reading recorded with a catch.