Complete Guide to Fish Biology and How it Impacts Your Catch Rate

Right, let's start with the big one that most anglers never think about: oxygen. Fish breathe dissolved oxygen through their gills, and how much is in the water changes all the time. Cold water holds loads more oxygen than warm water, moving water holds more than still water, and wind ruffling the surface mixes fresh oxygen in.

On top of that, weed and algae make oxygen in daylight but use it up at night, so on a hot, still summer lake the oxygen is often at its lowest just before dawn and at its highest late in the afternoon.

Why does that matter for your catch rate? Because digesting food costs a fish oxygen. When oxygen drops, fish cut back on feeding, slow down and move to wherever the water is fresher. When it's plentiful, they can feed hard and roam.

Low oxygen won't just slow bites — in extreme cases (hot spells, algae crashes, very low rivers) it stresses or kills fish, and that's when you should go easy or not fish at all.

Which fish feel it most? Trout and grayling are the most sensitive — they want cool, well-oxygenated water and go off the feed quickly in warm, still conditions. Barbel and chub on rivers are next; in a summer low they'll pack into the faster, broken water and weir pools.

Pike struggle in warm water and are easily stressed when caught then, which is why lots of pike anglers stop in summer. Carp, bream and roach sit in the middle — they'll keep feeding in warmer water but drift towards the fresher end of a lake.

Tench and crucian carp are the toughest of the lot and cope with low oxygen better than almost anything, which is part of why they thrive in weedy, silty little ponds.

So the practical rule is simple: on warm, still days, find the oxygen and you'll find the feeding fish. That usually means the windward bank, inlets, aerators, fountains, weir pools, faster runs, the edges of oxygen-rich weed in daylight and the deeper, cooler margins on the hottest afternoons.