How Does Water pH Level Affect Fish & Impact Your Chances Of Catching Fish?

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Fish respond to their environment, and as their environment changes, so does their behaviour. 

Understanding these changes and how fish react to them is something every angler should be aware of if they want their fishing success to increase. 

One of the most critical factors in fish behaviour and their health is water quality, as you probably know, but what about the pH of the water? 

The pH of lakes and rivers can change subtly without any obvious indicators that the naked eye can pick up. In fact, it changes daily through normal chemical reactions between all the organisms, and it changes fish behaviour. 

This begs the question, how does water pH level affect fish and catching fish? This is the case for both freshwater and saltwater scenarios. And this is the question I am going to answer below. 

By the end of the article, you’ll understand what pH levels fish prefer, how pH levels change, and how fish respond to them so you can use it to your advantage and catch more fish. 

What pH Do Fish Prefer?

pH is a scale that measures how acidic or alkaline an aqueous substance is. pH 7 is neutral, and anything lower than this, pH 1 to 6 is acidic, 1 being the most acidic. pH 8 to 14 is alkaline, 14 being the most alkaline.  The lowercase p stands for power, and the H stands for Hydrogen ion.

Fish have a blood pH of around 7.4 in most cases, so any pH close to 7.4 is the ideal level for fish to thrive in. However, fish can survive just fine in a pH range of 6.5 to 9, and any river or lake with a pH of  6.5 to 9 is deemed acceptable. 

Fish do not do so well when the level drops to 4 to 6.5 or increases to 9 to 11. They become very stressed, their growth decreases to nearly nothing, and they stop having offspring, as fish fry usually die at a pH of less than 5. 

Any mature fish living in water with a pH of 4 or less or a pH of 11 or higher will certainly die. 

Take note that aquarium owners will control the pH levels in their tanks to these criteria of around 6.8 to 7.8 to ensure their fish will survive and be healthy. 

So, to protect fish, we want them in water with a pH range of between 6.5 and 9, and the ecological systems in most bodies of water do maintain this unless an event like a toxic spill changes them. 

Let’s look at the natural changes in water pH to understand how fish react to them.

How Does The pH Of Water Change Day to Day? 

The reason water pH changes throughout the day is due to the fluctuations of carbon dioxide concentrations in the water. 

But what causes these fluctuations? It is primarily phytoplankton, but plants and other small invertebrates also play a part. 

Phytoplankton are microorganisms that live in the water and are crucial building blocks for all life in the water. 

During the day, phytoplankton and plants photosynthesise – using sunlight to create energy, taking carbon dioxide out of the water and releasing oxygen in the process. 

By taking carbon dioxide out of the water and releasing oxygen, they increase the pH of the water and slowly make it alkaline. 

At night, phytoplankton and plants respire, taking oxygen out of the water to breathe and releasing carbon dioxide into the water, a reverse of the daylight scenario.  

The carbon dioxide then reacts with water to create carbonic acid and thus lowers the pH of the water, making it more acidic. 

This means that pH and oxygen levels are usually lowest at dawn and highest at dusk. 

What Else Affects The pH Of Lakes & Rivers? 

There are several other factors that have a large effect on the pH of the waters you are fishing in. The main factors that affect pH are the weather and the abundance of phytoplankton. 

Hot Weather 

Hot, sunny weather causes water temperatures to increase, which in turn increases the respiration of almost all the organisms in the water. This is to regulate their internal temperatures (both fish & invertebrates) or to grow at an accelerated rate (plants). 

Respiration = more carbon dioxide in the water and, therefore, more carbonic acid, lowering the pH. 

Luckily, warm water is usually held in a layer on the surface and does not penetrate the deep parts of a lake or river. 

Lots Of Rain

Rainfall has an average pH of 5; therefore, a large amount of rainfall can quickly increase the acidity of a lake, river, or surface. Rain can have a positive and negative impact on your fishing.

Phytoplankton Abundance 

The more phytoplankton a river or lake has, the more oxygen it uses at night, and this can bring the oxygen levels and pH of the river or lake to an extremely low level. 

If a river or lake does not have enough phytoplankton, there is not enough to remove carbon dioxide during the day and increase the pH or to produce the oxygen fish need during the day.  

You can measure phytoplankton abundance through visibility into the water. Visibility of 12 to 30 inches is the ideal amount of phytoplankton. Less than 12 inches means there is too much phytoplankton; over 30 inches, there is not enough. 

Cloudy Weather 

If it is cloudy for a long time, the phytoplankton struggle to create oxygen and remove carbon dioxide during the day. This reduces oxygen levels and decreases the pH. 

Eventually, the phytoplankton will die, and it will be decomposed. This process removes more oxygen and adds more carbon dioxide, further decreasing oxygen levels and pH. 

Alkaline Balance 

If a lake or river is naturally abundant in alkaline substances, it has the ability to balance the drops in pH. 

Alkaline substances such as borates, carbonates, phosphates, bicarbonates, and hydroxides react with acids and neutralize them, helping the PH stay around 7, which is ideal for fish. 

If a body of water has enough alkalinity, it will have a stable pH and will not fluctuate intensely, therefore mating a pH that fish are happy with.

How Does The pH Level Impact Fish? 

When the pH drops, fish become less active. This is because the acidity reduces their ability to absorb oxygen, and acidic waters often have less oxygen in them, too. 

This means the fish become lethargic and lazy, do not eat as much, and can not swim as fast. 

How Do Fish Behaviors Change With pH? 

Fish do not like low-pH water and don’t just accept it; they do something about it! Most low-pH water is in the surface layer where the phytoplankton lives or in shallow water where plants and phytoplankton live. 

So, the fish compromise and move into the thermocline instead of hanging out in shallow water or at the surface. 

What’s a thermocline? The thermocline is where the lower pH, more oxygenated surface water, and the colder, deeper water with a neutral pH mix. It has the ideal combination of temperature, oxygen, and pH for the fish. 

The depth of the thermocline changes throughout the year, but when you find it, you will find the fish!

How To Use pH Changes To Catch More Fish 

So, how can you use this vital knowledge we have just run through to catch more fish? 

Fishing At Dawn 

We know that pH levels are high at dawn and the oxygen level is low. Dawn is notoriously a prolific time to go fishing, but regarding the pH levels, you must also consider the surface water temperature. 

If the surface water temperature is ideal, the fish will be actively feeding below but close to the surface, as it has the right temperature and oxygen levels. The acidity may be an issue, which is why the fish won’t be on the top and generally prefer to be a few feet below the surface. 

If the surface temperature is too warm, this, combined with a low pH, will cause the fish to be in the thermocline. 

Fishing During The Day 

As the day continues, the pH increases towards neutral and more oxygen is being released into the water. This increases fish activity and means they will always be at their ideal temperature. 

Fish will be nearer the surface on cooler days and, on warmer days, move deeper in the thermocline. 

Fishing In The Late Afternoon/Evening 

By the late afternoon, the pH will be as close to neutral as it will get and have as much oxygen as it will have. This is the perfect time for fish to be active; during this time, the water cools as the sun drops. 

Perhaps this is why fishing for the final two hours before sunset and into dusk is always a great time to be out there. 

The fish will feed actively near the surface, so long as it hasn’t been 35 degrees Celsius continuously for five days. 

Which pH Meter Is Best?

As fishermen, we don’t need to use a high-end and expensive pH meter to check the water we fish in. A simple to use and easy to carry, such as the Vivosun pH and TDS Meter Combo, is a great choice as it measures water temperature and pH value.

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Final Thoughts

Successful anglers require as much knowledge of the weather and water conditions as possible. Understanding these factors will significantly increase your chances of catching more fish and also those larger specimens that we all hope to catch.

You can read more on environmental factors such as the seasonal impact on freshwater fishing and the significant impact of barometric pressure changes and how they affect your fishing. 

Steve Fitzjohn