The effects of rain varies. It is dependent on season, temperature, location and species of fish. Here are some examples from my experience.
Early spring rain promotes the migration of Rainbow Trout (Steelhead) and Sucker species. The fishing tends to improve after a decent amount of rain increases water level and decreases water temperature. However the same rain could be detrimental to Sunfish species, Common Carp, Bowfin and Northern Pike (late March) as the rain decreases water temperature for these warmer water species.
Sometimes, the air temperature and ground temperature is warmer than the water temperature at a lake or river. In that case, a spring rain can have the opposite effect by displacing the cold shallow water with warmer run off water. This is rare, but it does happen.
A midday, mid-summer rain can promote feeding activity, especially in the shallows, when the rain run off cools the water to a comfortable temperature range that prompts feeding activity. Not limited to rain alone, the cloud cover and the often accompanying wind also contribute to the temperature drop.
Late fall rain could have an opposite effect. Certainly, late fall rain usher Trout species and any straggling Salmon to spawn. However, the flush of colder run off into the system could push fish away from the shallows and out toward deeper areas where conditions are more stable, especially for warmer water fish such as Sunfish species, Common Carp, Bass species, and sometimes Pike and Muskies.
In winter, a rare flush of rain can often create run off into the lake through auger holes or shoreline edges. Depending on the time of winter, this effect could vary. In late winter, run off from rain can often introduce well oxygenated water into a pond or lake that has experienced oxygen drop over the winter (especially during an atypically cold winter with ice that is unusually thick). This flush of oxygen can stir up feeding activity when fish had stopped feeding to slow metabolism during low oxygen periods. However, in some lakes with well established thermocline, this flush of water can disrupt the nearshore thermocline and can disperse fish or shut down feeding.
The last example is less relevant to Ontario, but still interesting to consider. Run off from rain flushing from freshwater river into saltwater bays and even offshore lagoons can shut off a bite. The rain introduces water that is different in temperature, higher in turbidity and lower in salinity. All of this disrupts the saltwater environment and some large rivers can have lasting effects (days and even months) on the offshore bite during rainy seasons.
I can say that I've had some of my best and some of my worst fishing in the rain. I've had rain shut down a Common Carp bite in May. I've had rain jump start a Steelhead bite in May. I've had mid-summer torrential rain produce the best Pike bite I've ever experienced (over 2 dozen in 2 hours). I've had torrential rain completely shut down a Redhorse bite in mid-summer. I've had a mid-March rain (and thaw) stirred the best Lake Whitefish bite I've experienced. I've had a mid-March rain stopped the best Lake Trout bite that had been going on for weeks.
And you definitely don't want to go swimming or surfing at a river mouth in Hawaii after a heavy rain...unless you're after a Tiger Shark.
So the effects of rain depends on the species, the season and the temperature. On top of that, you have to consider barometric pressure changes.
