THis is a reprint from my blog post of nine months ago and I will repost this blog at least once each year.
While millions of Americans purchase tidebooks each year, very few of them actually understand how the tidal sequences fit together. The easiest way to better understand tidal influence is to make sure you are looking at the four tides that occur each day (or roughly 24 hours) in chronological order. In that way, one can look at the difference in tidal height between adjacent tides. The tide times in a tide booklet are slack tide times. That means that the listed high tide is a slack tide – a period of little or no water movement due to tidal flows – where the water has stopped coming in and is about to start going out. Conversely, a low tide is also a slack tide where the water has stopped going out and is about to start coming in.
By putting the tides in chronological order, one can make an accurate prediction of how strong the incoming and outgoing tides will be. For instance, if the high tide is a big one with a height of 8.5 feet and the adjacent low tide is a very low one of 1.0 feet, someone reading a tide book will know that with a difference of 7.5 feet that there will be a lot of water movement and a strong outgoing tide. If the adjacent tides include a low high tide and a relatively high low tide, there will be greatly reduced water movement and a much weaker tidal current.
Most crabbers try to crab at times close to high tide and to a lesser degree, low tides, but few have given much thought to why they do. It is because crabs can move around much more freely during the periods of little or no water movement associated with slack tides. When there is a big height differential in adjacent tides, the time period surrounding the slack tide when crabs can move around freely is much reduced. Conversely, if there is little height difference – the crabs can move around seeking food for a much longer time – but they may not feed as intensely. Some skilled crabbers intentionally crab during periods of strong tidal flows because they have the skill and electronics to be able to drop their pots or traps into a depression in which crabs seek relief from the strong flows.
Fisherman can also benefit from a better knowledge of tidal effects. For instance, anglers fishing the lower tidewater areas of a muddy river can find somewhat clear water by fishing close to the ocean during high tides when the clearer ocean water will tend to dilute the muddy river water – a fact some crabbers also take advantage of. Both crabbers and anglers can also benefit from tidal knowlege in other ways. Salinity decreases as one moves upriver but varies between low and high tides. Tidal effect is also delayed as one moves upriver. The delay can vary depending upon river flows, but a rough estimate is that for each mile upriver from the ocean there will be a time delay of about ten minutes.
Since a river and the ocean it runs into are usually substantially different as to their water temperatures, skilled anglers can concentrate their fishing efforts in areas with water temperatures preferred by the fish species they are pursuing. Except for the winter months, the ocean is almost always much colder than the rivers entering it – so the water temperatures drop as high tide approaches. The temperature change is reduced the farther upriver one goes, but the temperatue change is usually noticeable for several miles. Of course, as the tide runs out, the river gradually becomes slightly warmer – at least during the period between late spring and early fall.
Bait anglers, such as sturgeon anglers, often concentrate their efforts during periods of high tide differential hoping that the strong currents limit the mobility of such bait stealers as sculpins and crabs. They also realize that the closer to the ocean they are fishing the greater the temperature differential is between surface water and the water near the river bed – because the cooler saltier water is slightly heavier than freshwater. Of course the difference will be more noticeable during an incoming tide than it will be during the outgoing tide where the salt and freshwater will have had several hours to mix.