Ben Raines | braines@al.com
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We’ve got a new tool to understand what’s going on in Mobile Bay on an hour to hour basis, and it should be a boon to fishermen of all stripes.
The National Oceanic and Atmospheric Administration has released a new salinity modeling app that you can access online, even on your smart phone. It is called the Mobile Bay Salinity Nowcast. It shows an animated forecast of how saltwater moves from the Gulf up into Mobile Bay and the Mississippi Sound hour by hour, every day. It's based partly on complicated computer modeling over water movement conducted over the last decade by scientists with the Dauphin Island Sea Lab.
This is vital information for fishermen, both commercial and recreational, though many fishermen -- particularly those who don't catch a lot of fish -- may not realize it. Saltwater is one of the most important influences over where different species of fish can be found in the bay. For the first time, this new model gives fishermen an overview of where the saltiest waters can be found. That’s critical, whether you are hunting speckled trout and redfish, or largemouth bass and bluegill.
Ben Raines
This is an image from the animation for Monday, at lunchtime, which was right at high tide. From studying the map, I’d tell someone going trout fishing to head to the bottom of the bay or to the Mississippi Sound. See all the green and yellow pushed up in the bay. That’s saltwater, with yellow representing higher salinity water, and red, higher still. There's a scale on the right side of the image, showing salt in parts per thousand of water. Normal Gulf water is about 32 parts per thousand.
The top of the bay, which is purple on the map, representing the lowest salt levels, is not going to be particularly popular with our saltwater gamefish. Normally at this time of year, the upper bay should be really salty, but we are still seeing the effect of Hurricane Irma’s rain that fell across the north end of Alabama.
Ben Raines
Now compare this map from 7 p.m. Monday, which was low tide. You can see the saltiest water has retreated back into the Gulf. As you watch the animation move through a 24 hour cycle, you see the salt move up the bay and then back down, but you also see the areas that the saltwater isn't reaching.
Looking at this image, I'd tell my friends not to bother fishing the upper bay for reds and specks until that purple starts to turn blue, at least. That's sort of a minimal amount of salt I like to see for catching those species. For someone hoping to catch a cobia, which can sometimes be found in the mouth of the bay in late summer, I'd say head way offshore, to find those yellow and orange zones, at the very least.
This is where it starts to become important for fishermen.
Ben Raines | braines@al.com
Let’s talk a little about the role of salt. Mobile Bay, like all estuaries, is a brackish system, meaning the salinity level fluctuates constantly. That fluctuation, with the bay acting as a mixing zone for fresh and salt water, is vital to the life cycles of more than 90 percent of the area's commercial fish harvest. Young shrimp, crabs, and fish, even saltwater fish, spend part of their lives in brackish waters. And some creatures, such as shrimp, must have brackish waters when they are young. The rivers draining into the top of the bay from the Mobile-Tensaw Delta provide a constant push of freshwater, while the Gulf provides salt. There is a push and pull between the two, with saltwater dominating the system at certain times of the year, and freshwater at others.
Ben Raines | braines@al.com
To understand the system, and where to fish at different times of the year, you must understand that our estuary – like any estuary - has well-defined seasons, just as the terrestrial environment does. A calendar of sorts controls life underwater, similar to the natural calendar controlling when daffodils pop out of the ground. The two parameters that govern the aquatic calendar are salinity and temperature.
Every year, shrimp season opens around the first week of June, just about the time that summer afternoon thunderstorms become a daily event. Mullet school in the lower bay starting in September, right after the kids head back to class, in preparation for their annual migration offshore to spawn. Redfish are thickest around Fort Morgan as leaves begin to turn colors. These things are as predictable as the camellias blooming in the winter and the azaleas in the spring, or the birds migrating through every spring and fall.
One of the dominant factors controlling this calendar in the bay is salinity, because some creatures love having salt in their water, and some can't tolerate it. Salinity in the bay has always been subject to broad seasonal changes. Those changes are dictated by how much water is coming down the rivers that feed into the bay, and that is controlled by how much rainfall makes its way into the rivers.
We must mention another player controlling salinity here, and that is man. All of Alabama's major rivers are dammed. Thirty-two major dams hold back freshwater from the spring floods, storing it up to make hydro power for late summer air conditioning. As hot as Alabama is, the hydro power accounts for perhaps 5 percent of the power consumed in the state annually. But that small percentage of the state's electric power comes with an outsize price. It affects salinity levels in Mobile Bay and the Mobile-Tensaw Delta. The dams release water at the height of summer, just when the Delta and Mobile Bay should be at their saltiest. Some scientists believe the practice has harmed the white shrimp population in the bay. Ultimately, our use of the rivers for power affects where fish and shrimp and their babies can live and survive. It is an undeniable fact, and one we must pay attention to as we manage the health of Mobile Bay.
Most of the year, the bay is brackish - a little saltier than the rivers, but not nearly so salty as the Gulf of Mexico. As summer progresses and rainfall runoff decreases upstate, the bay becomes almost as salty as the Gulf.
Numerous creatures depend on these seasonal changes in salinity, with freshwater usually dominating the system in the late winter and early spring, and saltier water moving in during summer and fall. (This is where our use of the rivers for power is most problematic. Releasing water from the dams to make AC occurs in high summer, precisely when the rivers, delta, and Mobile Bay should be at their saltiest, with the lowest flows from the rivers of the year.)
We had a graphic display of the importance of seasonal changes 14 years ago, when Alabama experienced what were then the wettest May, June and July on record.
Ben Raines | braines@al.com
The bay remained predominantly fresh for months, from late spring through the summer, according to data from a Mobile Bay National Estuary Program monitoring station on Dauphin Island. While salinity levels sometimes rose for a few days, heavy rains in the central and northern parts of the state quickly turned the bay fresh again.
That year, it was impossible to catch a speckled trout or redfish in the bay, and for the first time on record, people were routinely catching largemouth bass and channel catfish on Dauphin Island.
The low salinity rendered much of the bay inhospitable for the larvae of several species of shrimp, crabs and fish, like you see above, during the crucial migration to and from offshore spawning grounds. In the image above, you are looking at tens of thousands of eggs and larvae, representing almost everything that swims in the Gulf. The shrimp you see is about the size of a grain of rice.
It's these littlest creatures that depend on minute fluctuations in salinity for survival. Too much fresh, or too much salt, and the larvae die.
Ben Raines | braines@al.com
Numerous studies have shown that the larvae of many species are far more sensitive to salinity levels and often require different salinity levels during each stage of their life cycles. For instance, shrimp larvae need water containing around 10 parts salt per 1,000 parts of water. Too far on either side of that mark, and the larvae will die. Those conditions only occur in the upper end of the bay, and only at certain times of the year. Coincidentally, those times of the year occur exactly when our shrimp larvae are carried by currents from offshore, where they were spawned, into the estuary.
Such salinity fluctuations aren’t a big deal for adult crabs, shrimp and speckled trout. Fully developed adults are tolerant of a broad range of salinity, and, many saltwater creatures can thrive in freshwater conditions for extended periods. Plus, they are able to hit the road and swim to where the salinity is to their liking.
Ben Raines | braines@al.com
In the image above, you see tiny mangrove snapper, which will grow to about 10 pounds, and tiny speckled trout, none longer than two inches. They survived the gauntlet of predators and changing salinity levels to escape the larval stage and become juveniles, each a perfect replica of their parents. The long skinny fish are pipefish, inhabitants of the grassbeds that serve as nurseries for young trout and snapper. The pipefish don't get any larger.
So, back to what this new model means for fishermen. Typically, the larvae of shrimp and crabs move inland as they develop during spring, following the brackish water. As the primary food items for the fish that fishermen like to catch, the location of the juvenile crabs, shrimp and menhaden, controls where you’ll find the big predators. Remember that the adults can tolerate anything from full salt in the Gulf, about 32 ppt (parts per thousand) to 0 ppt, a fully fresh river. They will follow their prey as it follows the salt.
Mobile Bay is visited by both salinity extremes annually. Remember that it is one of the three largest river deltas in the nation by volume. Four states drain into Mobile Bay, (including the rainiest city in the country, Mobile, at 72 inches compared to Seattle's almost desert-like 55.)
In late summer, meaning August, even upper Mobile Bay will be noticeably salty to the taste. But during the annual spring floods, you might not detect a salty taste until you were almost at the mouth of the bay, 20 miles to the south.
What that means is that sea creatures are often abundant well north of the Causeway that spans top of the bay. Reports of juvenile shrimp and crabs deep in the Delta are common by mid-summer, when salinity levels begin to hit their peak. During those same periods, you’ll find big redfish, jack crevalle, trout, tarpon, and even bull sharks up in the Delta. During the recent drought years, biologists noted shrimp as far inland as Demopolis Lock and Dam, over 100 miles inland up the Tombigbee River.
With this new salinity map, fishermen have gained an advantage over their prey, a tool that provides precise clues about where to launch your boat depending what you want to catch.
Ben Raines | braines@al.com
You can follow Ben Raines on Facebook, Twitter at BenHRaines, and on Instagram. You can reach him via email at braines@al.com.
You can watch Ben's documentary, the The Underwater Forest, co-produced by This is Alabama here on Youtube, or on This is Alabama's Roku and Apple TV apps.
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