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Understanding Tides

Talk to any saltwater old-timer who consistently brings in more fish than everyone else and you’ll find that they really understand how, where, and when to fish. The “when-to-fish” part of that old-timer’s equation depends almost completely on knowing the tides.

The tides are caused by the gravitational attraction of the earth’s waters by the moon and the sun. In the same way that iron filings are attracted to a magnet, the earth’s waters are attracted to the sun and the moon. The complicating factor is that the sun, moon and earth are constantly moving in relation to each other, so the tides are constantly changing.

Understanding TidesThe good news is that since the movement of the earth, moon and sun are highly predictable, tides are as well. Furthermore, since the movements of the earth, moon and sun only change slightly, from year to year, the annual changes in tides are very slight. That means, if you make the effort this year to understand the tides, you will be able to use that knowledge every year in the future. Learning a few tide terms will help to take the mystery out of any material you may read describing the tides.

TIDE STAGE

This is simply a description of what the tide is doing at a particular time. Low tide is the minimum water depth for a particular cycle. At low tide, the water is not moving at all. As it begins to move and depth begins to increase, the tide is rising, flooding or incoming. These three all mean the same thing.

Roughly six hours later, when the water has gotten as deep as it is going to get, high tide occurs. The brief period when there is no discernible water movement is called slack tide. Slack refers to both the high and low tides. Once the tide has topped out and depth begins to decrease, the tide is falling, ebbing or outgoing.

Understanding TidesTIDAL RANGE

The range is the difference between a high tide and a low tide. If a low tide is predicted at -0.1 feet and six hours later the high tide is predicted to be 2.2 feet, the tidal range will be 2.3 feet—the difference between -.1 and 2.2. Typically, since feeding times for fish often depend on bait being swept along in the moving water caused by a tide change, the bigger the range, the better the fishing. So if you are looking at a tide table, don’t just look at when high and low tides occur; look at the two tide heights and determine the range. This will tell you how much water will be moving during the course of that tide change. If the range is large, a lot of water will move and depths will change rapidly. If the range is small, change in depth may not be nearly as rapid or as evident.

CURRENT

This is the speed of the water as it moves from high to low, or vice versa. In general, maximum current occurs about halfway through both the incoming and the outgoing tides, and for an hour or two on either side of this midpoint, depending on the tide range. Currents diminish toward the full high or full low slack periods.

TIDE HEIGHT

This is the vertical distance the water rises, or falls, due to the tide. 
Tide height in the tide tables that follow is given in feet and tenths of
 feet. If, for example, tide height is predicted at 3.2 it means the water depth will be 3.2 feet above the reference plane used to construct the navigation charts used in that area.

Most charts were developed using the “mean low water” reference plane, or datum, defined as the average of all low tides. A predicted tide height of 3.2 feet means the water will be 3.2 feet higher than the depth indicated on the chart. However, since the reference plane is an average, some low tides will be lower (or higher) than the reference plane; the actual depth may be somewhat less, or more, than the charted depth.

SPRING TIDES

In general, when the moon and the sun are in line with the earth, the tides are strong. These are called spring tides, and the term has nothing to do with the season of the year. Anglers in Florida, for instance, have spring tides twice a month, when the sun and moon are aligned—during the full moon and the new moon.

Understanding Tides

NEAP TIDES

When the moon and the sun are at 90 degrees to each other, the tides are weak. These are the neap (pronounced “nip”) tides. These also occur every two weeks, on the first-quarter and last-quarter moons.

The question of time deserves some consideration as well. While clocks and calendars tell us a day is 24 hours long, a week seven days, and a month 28, 30, or 31 days long, a tidal day is 24 hours and 50 minutes long and the period during which the moon goes through all of its phases is 29.5 days long. Anglers quickly learn to “round off” these changes by assuming that each day the tide is “about an hour later” (actually 50 minutes). Another “round off” is to assume that quarter-moon tides come a week after spring tides. It’s actually 7.4 days but a week is close enough to predict what to expect, weekend to weekend.

Though tides are primarily affected by the location of the moon and sun, the shape of the basin in which the tide takes place also affects them. It’s one thing when the basin is the Atlantic Ocean and quite another when the basin is the relatively small, shallow Gulf of Mexico.

Semidiurnal simply means that there are two high and two low tides each 24 hours and 50 minutes (called a tidal day), with both highs about the same height and both lows about the same height. Mixed tides are tides that deliver two highs and two lows in a tidal day, but the two highs and/or the two lows may be at significantly different heights. Finally, diurnal tides have one high and one low during a tidal day.

Everything discussed thus far relates to the astronomical tides, that is, the tides that are predicted to occur due to the gravitational attraction of the earth’s waters by the moon and the sun. The actual tides, however, may be somewhat different from the predicted tides, due to meteorological conditions. Barometric pressure can cause the tides to fluctuate up to 6 inches. The tides are higher when the barometric pressure is low, and vice versa.

How Wind Affects Tides

A more important factor affecting the tides is the wind. As the wind blows it “pushes” the water in the direction the wind is blowing. The wind affects the rise and fall of the tide by retarding, or increasing, the tidal currents.

Wind direction, strength and the amount of time it blows are the main variables. A brisk wind of 20-30 knots may not seem to have any effect on the tide for a couple of hours, but let it blow for a half day or more and the tide times will be quite different from those predicted. If you are fishing along the lower Gulf Coast and the wind is blowing from the east, the effect is to blow water out of the backcountry and bays, into the Gulf. This causes extremely low tides and may hold back the incoming high tide, making it smaller and later than predicted.

The opposite effect occurs with an onshore wind. The wind tends to “pile up” water along the shore, making the tide higher than predicted. It also retards the outgoing tidal current to the point where the low tide may not be low at all. Thus the angler must balance the tide predictions in the tables against the weather—wind and barometer—in order to fine-tune a reasonable set of expectations for the tide on a particular fishing day.

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