New Acoustic Tags and Receivers Reveal Migration Secrets
Do the large striped bass found on Stellwagen Bank come into state waters or do they stay offshore, protected from being caught? Do Atlantic cod return to the same site to spawn every year? And if so, when and how long do they stay there?
These questions and many more are currently being answered by MarineFisheries through the use of newly improved tagging technology.
Acoustic telemetry is the method used to track animal movements with the use of acoustic transmitters (tags) and listening devices (receivers). It is extremely effective at allowing scientists to study an animal’s behavior, migration patterns, and responses to different environmental or man-made cues.
The technology for acoustic telemetry has been around for decades, but recent equipment advances have created new research opportunities. Historically, aquatic telemetry required researchers to manually track tagged fish with a hydrophone. This was labor intensive and not always possible. Now, battery-operated receivers, with internal memory, can be deployed in the field to store detection data until downloaded. Batteries in both the tags and receivers last over a year, and receivers currently have the ability to store over one million detections. Tag technology has also come a long way. Options of tag size and detection range have significantly increased, allowing scientists to study fish as small as a river herring or as big as a white shark. Tags now also have the ability to transmit sensor data such as depth, temperature, and swim speed directly to the receivers.
In 2008, MarineFisheries invested in an acoustic telemetry system manufactured by the Canadian company VEMCO. It was purchased to study striped bass and Atlantic cod; however, other species such as horseshoe crabs, sandtiger sharks, and white sharks are also being studied with the equipment. Due to the versatility of the telemetry system, it will afford MarineFisheries the ability to study many other species and expand on current studies well into the future.
Every summer large striped bass can be found gorging on schools of sand eels, mackerel, and herring in federal waters off of Massachusetts. Federal fisheries regulations prohibit the retention or targeting of striped bass by recreational and commercial fishermen outside of state waters, making all of federal waters a refuge for striped bass.
Tagging studies have documented the seasonal latitudinal (north-south) movements of striped bass along the eastern seaboard; however, the inshore-offshore (longitudinal) movements are not as well known. This information gap has an impact on regulations as managers must rely on public perception and anecdotal information to assess the effectiveness of regulations. In an effort to increase information about the longitudinal movements of striped bass off of Massachusetts, MarineFisheries initiated a study designed to monitor the movements of fish tagged on Stellwagen Bank and determine if and when the fish move into state waters.
Beginning in 2008, MarineFisheries deployed an array of 36 receivers that extended from the eastern tip of Cape Ann south to Scituate, plus 8 more receivers in southern locations off of the backside of Cape Cod and in the Cape Cod Canal (See Map). During the spring of 2008 and 2009, we caught 128 striped bass in federal waters on Stellwagen Bank and surgically implanted acoustic tags. The batteries in each tag will last for over three years, so the study is still ongoing, but data are being received and processed. While formal analysis will continue over the next couple of years, researchers are already finding interesting trends in migration.
If anyone has fished in the Cape Cod Canal, also known as “the ditch,” they know that during the spring and fall migration, schools of large bass can be observed riding the tide through the canal, freely eating plugs and bait that land in their path. The acoustic receivers in the canal have confirmed that it is a major conduit for bass migration. During the spring months, 74% of the tagged fish which were detected moving north, use the canal for northward migration. Comparatively, only 35% of the detected fish use the canal when moving south in the fall. In addition, approximately 58% of the fish tagged on Stellwagen Bank were detected entering state waters, indicating that these fish, at some point of their migration, are available to recreational anglers.
Over the past several years, MarineFisheries has been monitoring an actively spawning subset of Atlantic cod aggregating every spring in Massachusetts state waters. This formerly lesser-known aggregation was becoming general knowledge to local recreational fishermen. As reported catch rates and the average size of cod harvested from the aggregation began to decrease, it became clear that the fishing pressure was excessive and MarineFisheries took action to protect these fish. The concern was that if action was not taken, more recreational anglers would become aware of the location and exploitation would continue or expand. This would have resulted in further disruption and depletion of this discrete spawning aggregation.
To protect the spawning aggregation MarineFisheries delineated the area and prohibited the harvest of cod by both recreational and commercial fishermen. This closure, named the Spring Cod Conservation Zone (SCCZ), begins on April 16th and extends through July 21st. A similar closure was initiated in 2003, called the Winter Cod Conservation Zone (WCCZ) which runs from November 15th through January 31st. The SCCZ closure is much smaller (approximately 17 square miles) and located further north in Massachusetts Bay than the WCCZ (approximately 115 square miles).
The SCCZ has offered a unique opportunity to study the biological characteristics of a group of spawning Atlantic cod and their habitat. We have found that the aggregation is predictable in space and time each year. This is in contrast to other area closures, including the WCCZ, where the spawning aggregations vary in location from year to year.
To learn more about the aggregation, a 28-receiver array was deployed that allows tracking of tagged fish in 3D. Because the receivers are placed in a grid, rather than a line around the area, fish movement can be identified through triangulation and every movement of each tagged cod can be recorded and analysed.
Working in collaboration with Doug Zemeckis, a PhD candidate from University of Massachusetts, MarineFisheries staff caught and tagged 2000 fish between 2008 and 2011. Some of these fish are being studied using more traditional tagging technologies (t-bar anchor tags and electronic data storage tags), which need to be recovered and returned to retrieve the data, but the remainder (66 fish) were tagged with acoustic transmitters. The average size of fish caught was 12 pounds but ranged as big as 67 pounds!
In 2011, cod began showing up at the SCCZ spawning site in April and some remained as late as early August. The typical stay at the site was 37 days, but ranged as long as 100 days. When cod spawn the females and males pair up; the female broadcasts eggs high in the water column and the male fertilizes them. These buoyant fertile eggs are then swept away by currents, eventually hatching into larvae. In the SCCZ the spawning activity most often occurs over flat featureless mud bottom during the night. During the day, the fish leave the mud bottom and return to a gravel/cobble outcropping in the exact same location every day where they remain before the next evening spawning event.
Why are these cod findings important? Knowing cod behavior and movement patterns of the aggregation allows for the timing and boundaries of the area closures to be refined. The more that is known about the aggregation, the better protection that can be provided to the fish while spawning. It also will allow MarineFisheries to adjust the restrictions when the fish are not spawning, resulting in more opportunity for anglers to catch fish during a less vulnerable time.
By tapping into cutting-edge technology in the acoustic telemetry field, MarineFisheries is breaking new ground in the field of striped bass and Atlantic cod science. This information gathered will be used toward improving the recreational angling experience in Massachusetts, and will simultaneously improve the management of the stocks as well. Whether you are slinging eels in ‘the ditch’ or jigging the wrecks off Gloucester, anglers can only expect to find fish if the stocks are well regulated to ensure the sustainable existence of our favorite Massachusetts saltwater targets.
Regulations in red are new this year.
Purple text indicates an important note.