The Moon and the Glow

The sky over the bay is dark when two things are true at the same time: the sun is down, and the moon is not up. Almost everyone tracks the first and the phase of the moon, but almost nobody tracks the third thing that matters, what time the moon rises and sets.

The moon comes up about fifty minutes later each night. That single fact produces two different kinds of good night, and they sit on opposite sides of the new moon.

In the nights after a new moon, the moon is already in the sky at sunset but it sets early, and once it goes down the rest of the night is dark. In the nights leading up to a new moon, the opposite happens: the moon has not risen yet when darkness falls, and it does not come up until well after most tours are finished. Both instances give you a dark night at the bay. Only the new moon is on everybody’s calendar.

The standard advice gives you four nights before the new moon, the new moon itself, and four nights after. Call it nine nights a month, and that is what most visitors plan around.

In practice there are another eight to ten nights in most months when the moon rises late enough that the bay is dark for the entire time you are in the water. Those nights are not on any bio bay calendar I have seen, but they are on mine. Not because someone told me, but because I worked it out myself, and because I am out on the bay most those nights.

Experience has shown me there is a part no rule of thumb can handle. And you do not have to guess at it, because I have already solved it for you: darkness does not arrive at the same hour all year.

In winter the sun goes down around 6:30 and the bay is fully dark by 7:00. In summer it can be close to 8:00 before you have real darkness. So a moon that rises at 8:30 gives you an hour and a half of true dark in December and barely forty-five minutes in June. Same moon, same phase, completely different night.

That is why I do not responsibly publish a rule. I publish a calendar for the entire year, built from actual sunset and moonrise times, marking every night the bay will be dark while you are in it.

See the viewing calendar

I have to admit I have no way to count dinoflagellates the way specialized scientists do. The reason is simple: I do not have the equipment for it, and honestly I do not need it. So I will not publish a number I cannot measure. What I do have is a fixed camera, my eyes and my own body, and I have been using all three to gauge this phenomenon the same way for years.

Being serious about this natural wonder requires commitment, transform every part of your life to put every minute of spare time you got to be in service of the bioluminescence and the bay. In that process sometimes I felt imprisoned, but at the end I realized this is the place I find my peace.

The glow become my teacher, and research validated that I already had noticed. And through research and special opportunities to interact with marine life filmmakers I realized I had to shoot on Nat Geo level gear. And that's why I got a camera with a 12.1-megapixel Exmor R CMOS full-frame sensor with back-illuminated technology, the most advanced low light equipment commercially available. I also learned the lens were equally important and I use the largest usable aperture lens I was able get, a SIRUI Night Walker 16mm S35 cine lens at T1.2, wide open, on manual focus, because autofocus is useless out there. The lens never finds its focus, and without focus there is no way to film.

This is the tool of my trade. A huge investment for an operator of my size, to be put at the service of the one place I feel time stops and I am one with the universe. But at the same time a small one compared to the gift I get from the bay through it, because I fully committed to her.

When I film the shutter stays at 1/30 and I record twenty-four frames, and both remain locked. The lens is mechanical, so the aperture cannot drift. The focus is set by hand and stays set. That leaves ISO as the only thing that moves, which in practical terms means the ISO is my reading of how bright the water is that night.

In my experience I run it from 80,000 on the best nights up to 240,000 on the worst. It is not a static number but a continuous reading. Most nights I end up somewhere in between, 160,000, 200,000, depending on what the water gives us on any given night.

For comparison, at Vieques on a peak night I filmed at 80,000, and on less bright nights I used up to 140,000. At La Parguera on a peak night I also used 80,000. That is the same instrument reading in both bays, and it is the reason I am comfortable saying that at its peak, La Parguera practically matches Vieques. Those who disagree have never been at both at their peak like I have.

But filming is not the only thing that has taught me how to read this bay. Swimming in the glowing water has taught me I do not need a camera at all. There are two things any of us can do with our own body.

While you are in the water, look at the light around your arm. On an average night that halo runs about six to ten inches out from your skin, with several hundred separate sparkles in it, spaced roughly a quarter inch apart. On a peak night the halo reaches close to twenty-four inches, at twice the density, and it stops being points altogether. It goes milky and continuous, like dissolved powder, and you can no longer see any separation between the particles. This is something no scientist can tell you, because they may know far more about the biology of dinoflagellates than I do, but they have not spent the thousands of hours I have documenting their behaviour and the changes in the glow, not only filming it, but immersed in the bay, swimming with them.

That is not the only way to read how bright the bay is on a given night. When I am in the water I lift my arm slowly out of it and watch. Some of them stay on my skin and keep blinking there. On an average night I count thirty to forty points blinking on the top of my arm. On a peak night, hundreds. Lift it fast and they run off before you can see them, so remember: do not go fast. Slowly does it.

Armed with that, only one question is needed to tell you what kind of night you are in: can you see separate points of light, or has it gone milky?

Every photograph and every video you have seen of this, including mine, is electric blue. Nobody tells you that is not what your eyes are going to see, and I would rather tell you now than have you disappointed after finding out in the water.

What you will see is whitish. Maybe a very pale green. Only at peak does it take on any hint of blue at all.

Nobody is deceiving you, but nobody takes the time to explain what you will see and why. In near-total darkness our eyes stop using the cone cells that read colour and switch over to the rods, which only read light. Rods are far more sensitive, which is why we can see the glow at all, but they do not report colour. The camera sensor has no such limitation. It records the actual wavelength, and the actual wavelength is blue. So the camera says blue and our eye says white, and both of them are right.

Come expecting the blue from the pictures and you will spend the night comparing. Come knowing what your eyes actually do in the dark, and you get something no camera can hand you, the scale of it, all around your body, in every direction, answering everything you do.

Another thing experience has taught me is to be careful with my words, because I am comparing any given night against more than three hundred of them, from extreme peak glow down to whatever we get on a particular night. You are comparing against nothing you have ever experienced.

Think about what a few fireflies do to a dark night. A handful of them, out in the air, at a distance, and the whole night changes.

Now put yourself in the water instead. It is night, you are floating, and every time you move your arm five hundred flashes answer you at once, wrapped around your skin. Not out there somewhere. On you. Reacting to everything you do. And around each foot too, if you are moving them.

That is what I am calling an average night.

People who come out on those nights are astonished. They have nothing to measure it against, and there is nothing in ordinary life that looks like it. I can honestly tell you that even on the least optimal nights I am still the first one in the water, and I find myself doing exactly what I am telling you to do, because any night is a blessed night at the bioluminescent bay. None of this is here to talk you out of a night. It is here so you know exactly what you are experiencing while you are in it.

Every tour in Vieques and Laguna Grande goes out in kayaks, and in La Parguera they go in regular boats. We go in a pontoon, and the pontoon changes the game completely.

A pontoon rides on two aluminum tubes, twenty-four inches each, and they only sink four to six inches. That means the whole platform sits in the air, and underneath it there is a space about six feet wide, twenty feet long and eighteen inches high. Better still, the aluminum ribs that hold the floor up run across it and work like monkey bars, so you are not treading water the whole time, you can hang there and play with the water. That, together with the bay being saltier than the open sea outside it, means you float more easily than you might expect.

But all of that is just a description of the space. The real advantage is its darkness. Under the platform there is no moon. The dinoflagellates are far more evident under there than out in the open water beside it. People say it is twice as bright.

So we go out even on nights at eighty percent moon, and on those nights we are usually the only boat on the bay, because from a kayak or a regular boat there is not much to see. Those are not optimal conditions and I will not tell you they are. But it is always evident under our pontoon, and in three hundred nights I have never had a group come up disappointed.

Rain does one specific thing, and it is not what people assume.

The worst night I have had came after a very rare day in the south that put down about four inches of rain in six hours, and to make it worse the moon was at ninety percent. Fresh water is lighter than salt water, so most of that rain sat on top of the surface and made a layer where there was almost nothing to see. From the surface the bay looked dead.

Underneath the pontoon, the sparkles were there. The light had not gone anywhere. It was below the fresh water, where the salt water is, and you had to get under it to find it. That is worth knowing, because in my experience a bay that looks dark from six feet above the surface is not necessarily a bay that is dark at all.

Another little known fact is that the dinoflagellates charge during the day. Not knowing it leaves you without the tools to understand how the bay evolves.

They charge through photosynthesis, and they only discharge when something moves the water. So the state of the bay on any given night depends on the days and nights before it, not only on the weather that day.

After several overcast days with people going in and moving the water every night, they have been spending energy without fully recharging, and the whole bay reads dimmer. Somebody who goes once a month will never see the changes that somebody who goes seventy-five times a year, sometimes ten straight days, sees and understands. That is what my three hundred nights are about. It is not the clouds overhead that night. It is the accumulated draw of the days before.

It also means a quiet bay after three cloudy days behaves differently from a busy one. Nobody talks about that, because nobody sees the bay from my perspective, and nobody has the volume of documented and filmed visits to this natural wonder.

There are undoubtedly better nights to come, and that is why we provide our Bioluminescence in Puerto Rico calendar. It is built for the whole year and accounts for the season, the sunset and the moonrise together, which no other calendar or phase chart has done before.

But even our calendar cannot predict how the glow will be, and after three hundred nights that is the one thing I can tell you honestly. If I was out there yesterday and it was at peak, chances are it will be very bright for the next couple of days, and the same if it was at fifty percent. But it is truly impossible to predict from a calendar alone. A peak glow on the fourth of July in 2023 became a fifty percent night on that same date in 2025. That is my honest experience.

And I am not alone in seeing that. A six-year study at Laguna Grande, published in 2024, counted cell densities in that one bay ranging from 4,388 per litre on its worst month up to 2,190,000 on its best. That is a five-hundred-fold swing inside a single bay, and the researchers described their own counts as fluctuating widely enough to surprise their monitoring team. The published figures you see quoted everywhere for these bays are presented as if they were fixed properties of the place. They are not. They are one reading, on one night.

The first one explains something I had been seeing for years without understanding it.

There is a dip in the middle of summer, and it is the Sahara. Every year, roughly June through August, strong easterly winds carry dust plumes across the Atlantic to us. That dust cuts the sunlight reaching the water, and less sun means less charge. I had a fourth of July come in noticeably low and I could not account for it at the time. Now I can. If you are planning around mid-summer, know that the dust is a real factor and nobody tells you about it.

The second is that the science and my calendar agree on the season, and they got there by different roads. That same study found the highest densities of the year in October and November, and the lowest in April and May. I arrived at October to early December from the other direction entirely, longer nights, wider dark windows, fewer people. It is a good feeling when the cell counts and the clock point at the same months.

That study is Laguna Grande, not my bay, and I want to be careful with that distinction. But it lets me say something I have never seen written down. At Laguna Grande the bottom falls out. Their lowest monthly readings drop to a few thousand cells per litre, which is effectively off, and that is where the bioluminescent blackouts you may have read about come from.

In three hundred nights at La Parguera I have never once seen it off. It goes down, absolutely. It has slow stretches. But it has a floor, and it does not fall through it. My worst readings still sit at 240,000 ISO, and 240,000 is a reading, not a blank screen.

Why that is, I cannot tell you for certain, because nobody has studied it. My own guess is geography. Laguna Grande sits up on the northeast corner where the Atlantic and the Caribbean meet, and it depends on the currents pushing lower-salinity Caribbean water up to it. If that push does not come, it does not get fed. La Parguera is on the southwest coast, in the Caribbean already, not waiting for anything to be delivered. It is the same reason the sargassum current runs past us to the south and never enters our bay. But I want to be clear that this is my inference from three hundred nights, not a finding. It is a question I would love to see somebody actually study.

If your dates are fixed and the calendar gives you a bright night, take it. If your dates are fixed and the moon is against you, come anyway, but come with a platform that makes its own darkness. And if you are choosing your dates around this, choose a dark night between October and early December. The nights are longer, the window is wider, and you will have the bay to yourself.