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Water Temperature

This is part 3 of a series of articles on the theories of the different factors which produce good and bad fishing conditions. The theories aren't necessarily mine, although I've attempted to provide a thoughtful summary of information I have researched, with some modification based on personal experience.

I'm not entirely convinced that you should solely base your decision on whether to go fishing, on the measurement of the positive or negative factors based on theory. Although a number of the positive factors may indicate that the fishing will be good at your regular spot, I think to a large degree that the negative factors mean that you have to find and catch the fish in a different fashion than you would normally.

Water Temperature

Most fish species are cold blooded, which means they do not and can not control their internal body warmth. As a result, their metabolism is strongly influenced by the temperature of their surrounding environment. A few species, bluefin tuna as an example, are somewhat warm blooded in that they can control the temperature of some organs by using muscle movement to generate heat. But as far as I know, most sport fish are cold blooded.

Given their cold-blooded nature, fish need to move to stay within their temperature comfort zone. Generally they do not need to move too quickly, because water has a high heat capacity, so generally change in temperature happens slowly. Fish then are rarely trapped outside their comfort zone, unless exceptional changes happen, such as a fish being caught in a small landlocked pool on a very hot day, or when there is a large draw-down of a reservoir.

Fish generally try and find their thermal optimum, i.e. that temperature which is not too cold or not too hot. The thermal optimum varies for different fish species, and many feel that larger fish in a species have a better cold tolerance, and this may be a factor why larger fish are generally deeper than smaller fish.

For migratory species such as Striped Bass, water temperature generating movement of their preferred prey species is the key indicator. However when the predatory fish reaches the final practical limit of it's range, for example in a large tidal estuary, temperature will be a significant factor in the fish moving inland, with larger fish generally being able to move early and leave late, given their supposed greater tolerance for colder water.

Many species of fish feed or congregate where there are differences in water temperature. The warmer water accommodates a temporary increase in their metabolic processes, while the colder water is more oxygenated. Certainly the fish can move between zones depending on whether or not they are actively feeding, or are less active. Certainly if their preferred prey species at any given time is higher or lower than optimal, the fish are going to go where the meal is. When the water temperature finally goes below their desired range, a semi hibernation takes effect, and fish activity drops off almost altogether.

The following table attempts to summarize the temperature comfort zone and thermal optimum for many sport fish species.

Species Lower Limit F (C) Thermal Optimum F (C) Most Active F (C) Upper Limit F (C)
Largemouth Bass 50 (10) 73 (23) 62-75 (17-24) 85 (29)
Smallmouth Bass 58 (14) 68 (20) 58-73 (14-23) 85 (29)
Lake Trout 42 (6) 55 (13) 50-57 (10-14) 60 (16)
Musky 55 (13) 63 (17) 55-72 (13-22) 72 (22)
Pike 55 (13) 66 (19) 55-74 (13-23) 74 (23)
Walleye 50 (10) 67 (19) 55-74 (13-23) 76 (24)
Striped Bass 50 (10) 65 (19) 55-65 (13-23) 75 (24)

Draw your own conclusions on the effect of water temperature on fishing activity. However, this is one factor that I'd recommend you use in determining where and how deep to start fishing on a given day. Adjust as needed, but it's a good place to start.

By David Girdwood
Manager, Thefishingnut

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