The relationship between fishing, wind, water depth, and water temperature

In the previous  article , “The Relationship Between Fishing and Wind,” we looked at the basics of how wind changes water temperature. Today, we will look a little more deeply at the relationship between wind, water depth, and water temperature.

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Some anglers carry a device that can measure water temperature, but even though it can only measure the surface depth, knowing this much can be a huge help in catching fish.

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Also,  as we learned in “Why sea fishing is difficult after a lot of rain,” the thermocline can appear seasonally, and if you can accurately target this thermocline, you can get very good results.

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Typically, the temperature of the water at the surface and bottom of a lake or ocean differs. This area of ​​temperature difference, known as the thermocline, is ideal for fish because it’s both warm and rich in oxygen.

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On the other hand, as we enter fall and winter, the surface and bottom waters mix together and maintain a relatively similar temperature for a long time. In bass fishing, this is called a turnover, and when it occurs in fall, it is called a “fall turnover” and is even described as the enemy of bass fishing.

The turnover phenomenon, where the water in the lake mixes, occurs when cold water and dead organisms and fallen leaves sink to the bottom due to their heavy specific gravity, but as the temperature of the surface water begins to drop in the fall, convection causes them to sink to the depths, and as a result, the water in the entire lake turns over, making the water color cloudy and making it difficult to fish.

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To understand the relationship between wind, water depth, and water temperature in more detail, let’s look at data from Lake Biwa in Japan, which is easily available on the Internet.

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If you look at the picture below, you can see that the water temperature changes rapidly around 10 to 20 meters deep. The reason that a thermocline, where the temperature changes rapidly at depths deeper than the surface, can be formed is because of the wind.

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For example, if you boil water in a kettle to make coffee while fishing, the kettle is heated from the bottom. As the warm water rises, it continuously mixes, maintaining a constant temperature inside. However, in a lake, the surface water, heated by the sun, is prevented from sinking further due to convection.

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This is easy to understand if you think about how when you turn on the hot water in a sauna, the upper body feels hot, but the heat doesn’t reach the lower legs. Therefore, if the water in a lake isn’t stirred by wind, just as you stir the water in a bathtub to mix its temperature, the surface temperature of the lake will inevitably continue to rise.

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In this way, the water on the surface of the lake is mixed by the wind, and just as we blow on hot water to lower the heat of vaporization when we drink it, the surface water that has lost its heat of vaporization due to the wind promoting evaporation becomes heavier and sinks downward. This phenomenon is clearly shown in the figure below.

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Looking at the picture, the water temperature in the middle layer from the surface to a depth of about 40 meters shows a clear difference between spring and fall. As the temperature of the surface layer begins to rise in spring and summer approaches, the difference (the gap in the graph) increases, and as the temperature of the surface layer decreases in fall, the difference gradually decreases.

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Those who are sharp may already know that the thermocline, where the water temperature varies depending on the change in water depth, is mainly formed in the summer, and after September, the water temperature does not change much even as the water depth increases. This is the so-called turnover phenomenon, where the water temperature is maintained at a constant level due to the influence of convection.

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Assuming we’re fishing in Bihwa Lake, the graph shows us when and at what depth to target. In other words, while water temperature remains relatively constant between 0 and 5 meters in July, we should target the thermocline, which begins to change from 5 meters down to 10 meters.

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Additionally, in October, it is good to target waters with a constant water temperature and a depth of less than 15 meters, which is not a turnover point.

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The above content is based on lake data for example, but in addition to the things listed above, it is also important to understand the currents such as cold and warm currents that affect the fishing results in sea fishing. In the previous  article , “If you are targeting flatfish with surf lures, understanding the currents is essential!”, I emphasized that flatfish move in the same direction as the current flow.

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Although the season for fishing is difficult, I think that learning about the various factors that affect fishing performance one by one in a warm room is another fun part of fishing.