Why is sea fishing difficult after heavy rain

After rain, sea fishing often suffers. The main reason for this is the drop in salinity caused by the influx of large amounts of freshwater into the ocean. As you know, saltwater fish cannot survive in freshwater, so the drop in salinity forces them to migrate further afield.

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Now, let’s take a closer look at why the weather is not so good after a lot of rain.

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When heavy rain falls, freshwater containing sediment flows into the sea. This dilutes the salt concentration of the seawater near estuaries and lowers the water temperature, forcing fish to seek deeper waters. Therefore, after heavy rain, it’s best to avoid fishing in areas where the water appears murky due to sediment carried in with the rain. The effects of this rain can take two to three days to manifest in open waters.

In the previous post,  “What is the important tidal flat in sea fishing?”,  we learned that the point where tidal flats occur is rich in plankton, so many fish gather there and the tidal flats are usually formed in a diagonal shape. However, when there is a large amount of rainwater flowing in suddenly, two horizontal layers are usually formed, and the cold-blooded fish leave in search of a more stable water temperature layer, so the tidal flats are not good, unlike the point where tidal flats occur.

When fresh water flows into seawater, distinct layers are created due to differences in salinity and temperature. The terms commonly used in diving are halocline (a discontinuous layer where salinity changes vertically rapidly with depth) and thermocline (a layer where water temperature changes rapidly with depth).

※ The salinity and thermocline can be formed horizontally or vertically.

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In particular, when there is a large difference in salinity concentration, cuttlefish cannot survive in low-salinity conditions, so egging fishing in the coast cannot be expected to be successful. In addition, the specific gravity changes due to the decrease in salinity concentration, so the action of the squid appears different and the sinking speed is different from usual.

halocline

Liquids with higher specific gravity sink to the bottom, while those with lower specific gravity rise. Following this principle, after rain, fresh water forms between the surface and middle layers of seawater. In the absence of strong winds or waves, the seawater doesn’t mix well, resulting in a more distinct layering pattern.

thermocline

Just like air, water moves upward when it is hot and downward when it is cold. Similarly, when a large amount of rainwater flows into the ocean, it takes time for the seawater and fresh water to mix, so immediately after it rains, the surface and bottom water temperatures drop, and the middle layer actually shows a higher temperature.

Of course, this thermocline appears not only in rainwater but also seasonally depending on the temperature. For example, the reason why squid fishing in the summer shows good results between the middle and surface layers is due to the habits of young squid, but also because the physiology of squid dislikes low water temperatures.

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If you can accurately locate this thermocline, you can actually get a good catch, but that is almost impossible.

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If you search for “thermocline” on Google, you will find a lot of information about bass fishing. American bass fishing professionals target the thermocline as a target point during the pre-spawn and post-spawn periods around the time of spring spawning, which is a basic strategy for professionals.

※ spawn: (verb) fish, frogs, etc. lay eggs

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Source:  Discover Catfishing

And, in addition to the spring spawning season, strategies for targeting the thermocline of bass fishing include selecting points in the summer, using lake turnover in the fall, and targeting winter overwintering points.

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So far, we have looked at the reasons why the weather is not good after a lot of rain.