Water Temperature does affect fishing.
From my River, records were I have taken water temperature readings: -
Out of 296 Barbel caught only 2 were caught when the water temperature was below 6 Deg C i.e. 0.675%
Out of 1156 Chub caught 203 were caught when the water temperature was below 6 Deg C i.e. 17.56%
In the summer Barbel can be put under high stress if a prolonged fight is carried out when the water temperature is above 20 Deg C
All species of fish have a temperature window when they will breed, if this is very short and or it is accompanied by high water levels (floods), then a complete year class can be disappear. If you do not take note of these conditions then you will not be aware of the consequences in 16 plus years time when these missing year classes would be your targeted fish.
Water temperature determines the amount of oxygen in the water, the lower the temperature the more oxygen, hence the problem with stressed out Barbel in the summer.
Although fish are cold blooded and do not use up energy as we do keeping warm, the cold does affect digestive systems and mussel efficiency. Some species become docile in cold water. Under cold water conditions you need to drop your bait on their noses, they are note going come searching for it. This requires you to locate the fish or shoal and not just rely on drawing in your fish by taste (smell). The reluctance to move generally means the fish will be in deeper and slower moving water (why waste energy swimming about).
And so it goes on!!
Water temperature dose affect fishing and also the insect and plant life upon which they exist.