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How to Read a Tide Chart

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Overview

One of the keys to shrimping is being able to identify the correct conditions for shrimping. Regardless of which method you use, the most important condition is always the tide. The tide determines if you can get your boat to the spot where the shrimp are or, if you can walk there. It also determines whether the shrimp are even going to be where you want to go. The tide can also determine whether or not it is even safe to be there.

In the course of this post, it is our hope that we are able to teach you how to use tide charts to identify the types of tides that you will want to shrimp instead of the types of tides that will result in you coming hope empty or all wet. As always, please be aware that due to the changing nature of tides, we advise you to judge the risks associated with venturing out into the water. It's your life, boat, property and time. Calculate your risks accordingly.

First of all, let's go over some basic definitions. These will help you talk about tides and understand them when we refer to them later in this post. Some of the information may seem simplistic or may be something that you already know. In that case, feel free to skip ahead to the next section.

The Three Types of Tide Cycles

Tides aren't just the waves that lap upon the shore. They are the slower rise and fall of the ocean surface over time. If you go to the beach and the water is far away from the shore line, you can judge that the tide is low; meaning that there isn't a lot of water near the shore. As the water comes closer to land, the water level rises and like a bath tub with the water on, the water level rises. To us, we think that the water is coming closer to shore but in reality, the water level is just rising.

A tide is measured by the point at which the water is at its lowest through the point at its highest and back to its lowest again. You can imagine a tide as a hump on a camel's back. The speed of these tides rise and fall is called the tidal period and can/is measured by the time that passes between the peaks or troughs of the tide. If you look at a graph of a tide, the tides that occur quickly look like a really pointy humps on a camels back and the tide that occur slowly look like a wide bump. The faster the tides occur, the more bumps or points occur in a given tidal day. A tidal day is roughly 25 hours but for the sake of simplicity, you can think of it as a normal day. The bigger the difference between a peak and trough, the bigger the tidal range. If there is a large tidal range and a quick tidal period, conditions exist where a strong and quick tide comes in or go out. If there is a large tidal range and a slow tidal period, conditions exist where a slow, long lasting tide comes in or out.

Regardless of the combination of tidal ranges, there are three types of tide cycles and just like a camel, the number of humps determines their type and yes, their names are just as obscure as dromedary and bactrian.


diurnaltide
Diurnal Tide (Courtesy Wikipedia)

Diurnal

A diurnal tide cycle is very simple. It is a tidal day in which only one tide occurs. This is a one humped camel. It can be a very slow tidal period (wide hump) or a very quick tidal period (pointy hump) but, there is only one that occurs in a day. Taking the definitions we learned above, a diurnal tide is when a one tidal period occurs during a tidal day.


Semi-diurnal Tide (Courtesy Wikipedia)
Semi-diurnal Tide (Courtesy Wikipedia)

Semi-Diurnal

Semidiurnal tides are when two high tides and two low tides occur in a single day. Taking the metaphor of the camel, this one would have two humps. With tides though, it is a little different because sometimes, the day starts in the middle of a hump or in the middle of a trough. So, we need to be a little flexible and realize that it is the total of the peaks and troughs during the day that is counted. Don't rely on the camel too much. In some diurnal tides, the camel might look like it had its hump cut in half or has a big gauge taken out of its back. In the following graphic, you can see just such a situation. The tidal day begins in the middle of a hump and ends in the middle of a hump. If you add the two half-humps together, you get a single hump.


Mixed Tide (Courtesy Wikipedia)
Mixed Tide (Courtesy Wikipedia)

Mixed

In a mixed tide cycle, the daily inequality between the two tides is significant. A mixed tide is a tidal cycle between a diurnal tide and a semi-diurnal tide. It normally exhibits two low and two high tides per lunar day, but the highs and lows are of unequal magnitude. This type of tide is common around the Pacific Ocean basin. If you examine the following graphic, you will see how the tide in the middle of the tidal day is massively smaller than the tides which surround it.

 


Tides vary in a pattern over the course of a season. Every tide that occurs is followed by another tide that is either larger or smaller in magnitude than the one before it. The difference may be minor but it always exists and is called the daily inequality. In tide tables, the daily inequality is identified by naming each tide either higher high water or lower high water. Likewise, one can look at the troughs and derive which ones are the lower low water and higher low water. In layman's terms, one low tide is lower than the other and one high tide is higher than the other.

How to Read a Tide Chart

So what have we learned by reading the above material. Well, first of all, tides are a lot more complicated and wordy than any of us would like. That aside, we learned a lot about how to read a tide chart and how to plan for shrimping.

Let's take a look at the following tide chart as an example and we can apply the lessons we learned in practice. The following tide chart is for a one day period in Gulfport, Florida; starting at midnight on May 5th, 2008. What can you derive about the tides during this day?

Gulfport Tide, May 5th, 2008 (Courtesy Harmonic Tide Predictor)
Gulfport, May 5th, 2008

Let's start by identifying the type of tide cycle that we are looking at. It this a diurnal, semidiurnal or mixed tide? We can tell right away that this isn't a diurnal tide because there are two humps in the picture. That narrows it down to either a semi-diurnal or mixed tide. Considering that this tide chart is for a place in Florida, we would all be underwater we would be underwater if this was a mixed tide. If you look at the left hand side of the graph, you can confirm this observation. The tidal range is only about a foot and a half between low and high tide. In a mixed tide, the difference, or daily inequality, between the two would be would be much more significant, most likely in the range of ten feet. Sometimes graphs are a little deceiving so it is always best to look at the scale on the left hand side and bottom to confirm the strength of the tides. In this case, we are looking at a semi-diurnal tide.

What else can we tell about this tide chart? In this case, the creator of the chart has been nice enough to color the flood period of the tide (when the water comes in) blue and the ebb period of the tide green. The first tide, peaking at about 2:00AM is relatively minor, cresting at about 1.5 feet above sea level and reaching its low at about 6:30AM at almost sea level. The second tide of the day reaches it's highest point at just about 12:00PM (noon) and proceeds to fall to its lowest point at about 8:20PM.

Based on this tidal information, what can we say about the strength of the tides? The first tide appears to be relatively weak. We can tell this based on its small tidal range (1.5 feet) and the fact that there is a relatively large amount of time between the peak and the trough (about 4 hours.) The second tide is another relatively weak tide but its length is long enough to make it significant. In this situation, the tidal range is about 3 feet. The amount of time between the peak and trough is nearly 8.5 hours, more than double the previous tide. In the second tide, we can expect much lower water than the first. The first tide is the lower high water tide and the second tide is the higher high water tide. They are separated by the higher low water trough and followed by the lower low water trough.

OK. We know all the technical babble about the tides. We know how strong the tides are, when they occur and what to expect. Now we need to apply this situation to the important question, when is the best time to shrimp?

When is the Best Tide to Shrimp?

Ah, the magic question. The simple answer is that it depends on how you are planning on shrimping.

If you are planning on using a chum ball and a cast net, it might only be important to you to find a time of day or night when there is enough water on the grass flats to get out there but not too much water to prevent you from seeing the bottom or preventing the shrimp from catching the outgoing tide. In this case, using the tide chart above, I would probably aim for the period between 1:00AM and 3:00AM. The tide is relatively small and therefore not likely to flush the shrimp out to sea yet it provides an ample amount of water to get out there.

If you are attempting to dip net your shrimp or use the Ozello Shrimper, you might be out of luck on this particular day. Dip netting and using the Ozello Shrimper requires a significant low tide that lasts long enough to walk out onto the flats, at night, and pick them up one by one. In this situation, the first tide never goes below a foot over sea level so the tide isn't very significant. You would basically be sloshing around in the grass flats, up to your waist in water. The second tide reaches its lowest around sunset and then begins to climb back upward afterward. If you went out there, you would probably have to wait a good half an hour for the afterglow of daytime to disappear and then, maybe, just maybe, you might get some shrimping in but, you would be forced closer and closer to shore as you did so, eventually being pushed back onto the beach, or your boat, by the tide.

If you want to shrimp using the Ozello Shrimper, you would optimally want a tide that was sitting at just about sea level and beginning to fall around sunset, ultimately reaching a point fairly well below sea level only to rise gradually. The tidal range would be large and the amount of time between the peak of the first tide and the peak of the second would be long, ideally around 8 hours. If you get really lucky, the first tide will be the largest, the higher high water and the second tide would be lower, or the lower high water. This will allow you to maximize your time on the flats.

For other types of shrimping, the specific tide you choose to go out on will vary. People who shrimp from boats using lights and long handled dip nets usually want strong outgoing or incoming tides. They like to be on the water in their boats when the water is flushing in or flushing out. They don't want to be on the water when it is slack, or not moving. If they do, they want it to be at the beginning or end of their shrimping trip. They also usually want to be out on the water at night. Most of the time, they can't see them without the aid of lights and therefore have no idea where to scoop the shrimp up. For those people who use regular nets that they drag behind their boats, they may choose strong tides if they are going to be in the deeper water or they may choose average or weaker tides if they intend on dragging the nets across grass flats. In these situations, the time of day really doesn't mater much to them except when it comes down to getting sleep.

Summary

So, there you have it. Now you can look at a tide chart and tell whether you should be out on the water shrimping based on the type of tide, the strength of the tide, what type of method you are using to shrimp and your personal style. We hope this description has been helpful to you!

For More Information:

Tide (2009, January 3). In Wikipedia, the free encyclopedia. Retrieved January 3, 2009, from http://en.wikipedia.org/wiki/Tide