Acoustic Telemetry Project Provides Insight On Fish Movement Throughout Lake Pontchartrain
story by Ashley Ferguson, LDWF Biologist
If you are a juvenile bull shark, Goose Point in Lake Pontchartrain is the place to be. Located on the north shore of the lake about five miles south of Lacombe, Goose Point provides them with plenty to eat, warm water and little trouble from predators who like to bully the young sharks.
That is one of the findings of an acoustic telemetry project in which the Louisiana Department of Wildlife and Fisheries (LDWF) and LSU partnered beginning in 2012 to examine fish migration patterns and habitat usage of Lake Pontchartrain.
The confluence of fresh and salt water in Lake Pontchartrain makes it a water body where fresh water and marine species can thrive, including speckled trout, redfish and bull sharks. And because of the ebb and flow of the salinity levels in the lake, the temperature and turbidity, very few species are full-time residents.
Generally, there is a west-to-east, low-to-high salinity gradient in the lake. Freshwater enters Lake Pontchartrain through Lake Maurepas along with several rivers and bayous on the north shore, whereas saltwater creeps in through the Rigolets and Chef Pass.
While rainfall influences the amount of freshwater flowing into the lake, it is the wind rather than the lunar tide that drives the ebb and flow of salinity levels throughout the water body. A moderate to strong west wind for a week or so will push fresh water further into the main body of the lake, allowing a wider spread of freshwater species.
Likewise, a steady east wind will drive saltier water into the reaches of the lake. The many transient fish species with various salinity tolerances navigate through the lake, finding their preferred water parameters.
In an effort to better understand migration patterns and habitat usage by selected fish species, LDWF began its partnership with LSU to deploy an acoustic telemetry project in Lake Pontchartrain.
A total of 90 hydrophones, called receivers, were strategically placed throughout the lake, covering artificial reefs, shell pads, open water, passes, grass beds and dredge holes. After the receivers were deployed, tagging events were held where speckled trout, redfish and juvenile bull sharks were implanted with acoustic transmitter tags.
When a tagged fish passes within a half mile of the submerged receivers, a pinging noise, created by the tag, is translated into the date, time and unique fish identification number. The information is recorded and stored within the receiver until researchers visit and download the data, typically every six to eight weeks. An enormous amount of data is collected each time the receivers are downloaded. The data, coupled with water quality and weather information, begins to paint a picture of what areas of the lake fish inhabit and why. This data is extensive, and provides much more information about the activities of each tagged fish than traditional tagging techniques. Such traditional tagging programs provide location of the fish only during tagging and recapture, which can sometimes be very close to the same spot even though fish movement may have been extensive during the interim. Telemetry tagging programs can indicate more detailed fish movement and habitat usage information during that interim period. Unfortunately, the more useful data from telemetry tagging comes at a higher cost than traditional tagging making it prohibitively expensive at this time to expand the Lake Pontchartrain effort to other areas of the state.
Speckled Trout Movement
One question southeast Louisiana anglers who enjoy fishing for speckled trout in Lake Pontchartrain frequently ask is which habitat do specks prefer? Research by Noelle Bramer, who recently completed her masters research at LSU, has shown no one particular habitat is preferred over another. Instead, the key finding was that diversity of habitats was crucial.
Bramer conducted water quality testing throughout the lake. She coupled that data with the movement of 211 speckled trout tagged by LSU doctoral student Ashley Melancon to reach her conclusion. She also found that salinity is the strongest influence on speckled trout movement in the lake followed by temperature.
The telemetry project was also able to document detailed fish movement during last year’s Bonnet Carre’ Spillway opening. This spillway event diverted fresh water from the Mississippi River into the southwest portion of Lake Pontchartrainat 60 percent capacity on Jan. 10, 2016, for 22 days.
It was the most recent event-driven influence on the lake with a generous amount of sediment-loaded river water pouring into the lake. During the opening, 26 tagged speckled trout and 23 redfish were present. One week after the opening, tagged speckled trout and redfish that were located in the southeast portion of the lake near Seabrook began to move east toward the Lincoln Beach area, then north.
Two weeks after the opening, 100 percent of the tagged speckled trout and 90 percent of the redfish were located in the northeastern part of the lake, most near the St. Tammany East artificial reef, known as the Lacombe Reef. One week after the spillway closed, the speckled trout and redfish began to disperse to the middle of the lake again.
One redfish was detected leaving through the Rigolets and one speckled trout exited through Chef Pass, but both fish returned before Feb. 18, the cutoff for detection data.
Fish movements were overlaid with MODIS satellite images. MODIS is a moderate resolution imaging spectroradiometer found aboard NASA’s Terra and Aqua satellites. Speckled trout and redfish could be seen moving into pockets of cleaner water, although salinities in these areas were at or below two parts per thousand.
There were many variables that made the 2016 spillway opening unique. Since the structure’s completion in 1931, this was only the second time it was opened during the winter. It was open only nine days longer than the shortest recorded duration.
It’s during the summer, though, when juvenile bull sharks are seen in abundance in Lake Pontchartrain.
Temperature, protection from predators and abundant food appear to be why juvenile bull sharks flock to the lake during that time. The most current telemetry data shows that the juvenile bull shark movements into and out of the lake are cued by temperature changes. They begin entering the lake when the water temperature reaches at least 68 degrees, often around late March, and leave the lake when the temperature drops below 68, around mid-October.
The exact time and location of their birth is still a mystery. But we do know young bull sharks are instinctively born, or pupped, near low salinity water to give them the highest chances of survival.
At 2.5 feet in length and weighing about 10 pounds at birth, bull shark pups are very susceptible to predation by larger sharks of the same and different species. Unlike other shark species, bull sharks have adapted the ability to thrive in freshwater at a young age, allowing them to enter estuaries and rivers that other sharks cannot. In addition to protection from predators, Lake Pontchartrain is abundant with prey items, thus providing the two most vital components of a shark nursery.
While in the lake, the juvenile bull sharks spend the most time at Goose Point. This area has a natural marsh-lined shoreline, a shallow sandy bottom and abundant grass beds that are loaded with prey items. Sharks will cruise the shoreline on the edge of the submerged grass searching for fish and crabs.
Many sharks were also detected at deep holes, artificial reefs, shell pads and freshwater inputs around the lake. Overall, young sharks utilized the entirety of the lake, even moving as far west as Pass Manchac in July and August.
In total, 39 bull sharks have been tagged since the project’s inception. Twenty-one sharks have recently been tagged and are currently being tracked, and the data will be processed by spring of 2017.
The battery life of the tags is approximately three years, allowing several seasons of movement to be observed. If you happen to capture a shark with a blue dart tag or would like more information on the acoustic tagging project please email email@example.com. If you would like to watch the tracks of sharks that have been tagged, please go to: www.louisianafisheries.net/telemetry.
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