How to have a tribe measure the tides while living under the sea?

Question

I'm dealing with a primitive tribe (of a species that can breathe underwater) that lives deep under the sea, and has little in the way of human technology. Resources include things naturally found in the sea, and things salvaged from shipwrecks and human waste (i.e. plastics) (though they do not know how to use human technology).

While the species can breathe underwater, they can't poke any part of their bodies above water - to do so would violate some kind of sacred law. Getting close to the surface is frowned upon culturally. They typically stay far enough away from the surface that they don't receive a lot of light.

That said, they need to be able to track lunar phases, tides, and passage of day and night for their religious calendar system. Without looking directly at the moon or venturing close to the surface/shore, what would be an effective way to measure these cycles?

EDIT: In light of the first answer, and to open up any further ideas that they might give you, I should mention the species possesses natural sonar, infrared vision, and a shark-like "electroception" sense. They're also likely skilled at gauging water pressure to avoid "the bends", as divers call it.

Answer 1

Besides a convenient creature that follows a cycle close to the things needed to be measured a simple reverse Fish line would do. They release something buoyant and then reel it in back down to note the time of day. So something that reacts to light would do. ALso the length of the line would differ depending on high tide or low tide.

And the distance between sea bottom and surface can also tell you about the spring tides. Also current change with tide so those can all be measured using buoyant object attached to lines without leaving the sea surface.

Answer 2

For day-night cycles:

Sonar operators, using the newly developed sonar technology during World War II, were puzzled by what appeared to be a false sea floor 300–500 metres deep at day, and less deep at night. This turned out to be due to millions of marine organisms, most particularly small mesopelagic fish, with swimbladders that reflected the sonar. These organisms migrate up into shallower water at dusk to feed on plankton. The layer is deeper when the moon is out, and can become shallower when clouds pass over the moon. This phenomenon has come to be known as the deep scattering layer. ~ Deep Sea Fish - Wikipedia

For Tides

Observe the motion of shoreline species like certain types of starfish, coral snakes and turtles. When the tide is high, they would be miles further towards the shore and when the tide falls, they would return to the deep water line.

For Moon Phases

Observe the behavior of fish. Moon phases have an important effect on the reproduction of certain types of fish. For more details, read this article.

Answer 3

Passage of day/night

you just mentioned that they stay at a depths where 'they don't receive a lot of light'. Doesn't mean they don't receive any. And staying down there just seems a thing of customs and religion, not dictated by biology (some deep sea creatures can't ascend higher than 2-300m because they can't survive in the low pressure environment up top). So, even if the little light you get at your living altitude is not enough to properly determine day/night, you are capable of sending up a swimmer to 1m to report periodically.

Tides

The flow of water during a tide doesn't suddenly stop 2-3m beneath the surface. Careful observation of currents close to the coast (especially in coast lines where the land falls down steeply) should also give you results at a depths of 30m. Not to mention that in places where the difference between ebb and flood is several meters, you might be able to notice a difference in water pressure (each additional meter of water adds 1/10 bar).

Lunar Cycle

When you are able to measure the strength of tides (smaller/larger), you have a way of determining the moon phases, since full/new moon give you larger tides than quarter or three quarters:

Because the tides are influenced by both the Moon and the Sun, it's easy to see that when the Sun lines up with the Moon and the Earth, as during a New Moon or Full Moon (a configuration also called "syzygy"), the tidal effect is increased. These are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal. moonconnection.com

Combined with photosensitivity (assuming they can just visually observe light intensity), it is quite possible to determine whether you've got a full moon (=observable light even 1m beneath the surface) or a new moon.

Answer 4

They could just cut a long piece of submarine optical fiber cable and poke it above water (in a protected well, probably). I guess it would be easy to tell the difference between day and night, and possible tell the difference between full moon and no moon. If it's cloudy, they could use clock and moon calendar to fill the gaps.

Answer 5

The 1978 novel "Aquarius Mission" by Martin Caidin asked this question. In that case, IIRC, the undersea race marked the months by their female's menstrual cycles.

Do your people have regular sleep cycles or anything else that follows a circadian rhythm? Are there fish or undersea creatures that strictly follow a daily, weekly, or monthly pattern? (EDIT: Sorry, someone already mentioned the repeated behavior of deep-sea fish.)