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Amount of light in the water

This theory has been used to support both sides- that fishing is worse, or better, due to the light in the water.

 

The "better fishing" side claims that fish need a bit of light to feed and that the light of the full moon might mimic the productive twilight hours. I'm not convinced this would have much of an effect. Fishing is good at twilight because this is when the insects are most active (read more on the impacts of insects below), not because of the amount of light in the water.

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But this raises the question: does moonlight provide more light for fish? The answer seems to be no. Moonlight penetrates water at a rate that is about one million times less than sunlight. Actual light penetration depends on water clarity, but since sunlight penetrates to a maximum of 1000 meters and only rarely penetrates beyond 200 meters (and moonlight would penetrate one millionth as deep), I think any impact of moonlight on the light levels in the water is minimal at best.

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But, even if we believe that moonlight in the water improves fishing, this also means you are fishing by the light of the full moon- that is to say, at night. It's worth noting that night fishing is sometimes against regulations.

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Insect activity

There is some evidence that insects are less active on the full moon. I've seen this argument used to justify both better fishing ("the fish are hungrier") and worse fishing ("the fish only feed with the insects") at the full moon.

 

If we accept that insects are less active at the full moon, I tend to believe that the fish would also be less active. We know that fish are more active when insect hatches are plentiful, and I would think the same pattern would apply to other cycles of insect activity.

 

Spawning activity

Some say that fish are more likely to spawn around the full moon. This seems more relevant to Australia than BC, so keep it in mind if you travel downunder.

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There is some evidence that salmon prefer to travel by the light of the full moon, perhaps taking advantage of greater visibility in the light water. Migrating chum have been found to swim faster and in shallower water at the full moon, and Atlantic salmon are more likely to enter spawning rivers. Pacific salmon smolts may be more likely to head downstream on the full moon. So if your goal is to catch some spawning salmon, or some river predators such as bull trout who might be chasing salmon smolts, fishing by the light of the full moon might help- if you fancy standing in your hip waders at night.

 

There doesn’t appear to be any evidence that kokanee spawn on the full moon- and, even if they did, I'm not sure how this would help your kokanee fishing.

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Stronger tides

The Farmer’s Almanac also states that fishing is better at the full moon, when the tide is higher than average. This theory relies on well-documented information that fish move in an out with the tides, feeding off the material that is churned up with the flow of water. While we know that the movement of the moon controls the tides, it’s not entirely clear how the moon cycles may further impact fishing- some say that because a full moon means a higher-than-average tide, it may open up new feeding grounds that are normally inaccessible.  Others say that the higher ride results in a stronger current as a greater volume of water flows in and out, which churns up more food than average. Others say that this rapid flow of water pushes fish further into the shallows.

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High tide theory: Tides are mostly controlled by the moon, and the full moon does cause higher-than-average tides. Tide depth is impacted by a number of factors, including where in the world you are located.

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I have created a table that shows the tide cycles for the Vancouver area for the first few months of 2020 using data from the federal government. The 'too long, didn't read' version is this: in January 2020, the lowest high tide in the Vancouver area, called the "neap tide", was between January 5 - 7, when the high tide was +4.3 metres. The highest high tide, or "spring tide," was between January 12 - 16, at +4.9 metres. This means that, between January 12 - 16, the tide was 0.6 metres higher than it was on the neap tide-  which very well could open up new feeding grounds. This impact would be most noticeable in parts of the world that see bigger tide fluctuations.

 

But just wait. If you looked at the table, you would have noticed that the full moon was on January 9. The tide was impacted by the full moon up to a week later, approaching the third-quarter moon. By the time April rolls around, the highest tide starts just before the full moon. When the impacts of the full moon are felt appears to change slightly month by month. This is because tides are also impacted by the sun, the earth's distance from the sun and moon, storms, and more. You can check out the tide tables yourself to find the highest tides for any month.

 

You also might have noticed that the tides were almost as high following the new moon, when the moon is not visible in the sky, from January 23 - 28. This is because the new moon impacts the tides nearly as much as the full moon.

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Stronger current theory: When looking at the table, you might have noticed that not only are high tides impacted by the full moon, but low tides, too. The lowest tides in the whole month happened in the days immediately following the full moon, when the low tide was just +0.3 metres. In comparison, the low tide at the quarter moon, on January 2, was +2.0 metres. This means that the greatest fluctation in tide levels happens on January 12 (shortly after the full moon on January 9) when the water level increased 4.6 metres between low and high tide. This is a significant amount of water rushing in, and it would make sense that this would cause a stronger current, which might churn up more food. A similar but less pronounced pattern also happens around the new moon, when water levels changed by about 4 metres.

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If you are trying to time the current, the rule of thumb appears to be this: the greatest fluctuation in tide depth happens in the third and fourth hour after low tide, with about half of the total increase occurring during these two hours.

 

It is important to know whether your believe in the higher tides or the stronger current theory, as it will impact whether you aim to fish shallow water with the high tide (looking for newly accessible feeding grounds), or when tides are moving in or out, also known as ‘running tide’ or ‘mid-tide’ (when the fish might be cruising the stronger currents). Either way, the best fishing might not be right at the full moon, but a few days before or after, depending on when the tides impacted- and also at the new moon.

 

The ‘high tide’ theory makes a number of assumptions, including:

• You are fishing tidal waters. Smaller- and medium-sized lakes experience little, if any, tide-like fluctuations with the moon.

• You are able to time your fishing excursion with the tidal cycles- either high tide or mid-tide, depending on which theory you believe. The tides cycle every 12 hours or so, and so your chosen tide might not align with the time you can make it out on the water. 

 

I am willing to concede that there may be truth to the stronger tide theory- but it’s not relevant to lake fishing.

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Unknown reasons

This theory basically says that, we don't know why it's true, but it is. For whatever reason, fishing is better (or worse!) at the full moon. 

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The government of New South Wales, Australia, has found that, in their research, tuna, swordfish and other large predator fish have moved into deeper water at the full moon, and are less accessible to catch. This also means that some of their prey fish become easier to catch during the full moon.

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But unless you have some magic tuna-fishing hole here in the Pacific Northwest, I’m not sure this info is relevant to us kokanee fishers.