Spawning dynamics, seasonal movements, & population structure of Atlantic cod

UMass Dartmouth's School for Marine Science & Technology associate professor and research associate shed light on cod population dynamics and address fisheries problems by using various tagging methods.

Steve Cadrin (l), Associate Professor at SMAST, and Doug Zemeckis (r), research associate, display cod with data storage tag.

Atlantic cod have been a principal target species in New England's ground fish fishery since the 17th century. In recent decades, cod have experienced substantial declines in abundance. Many historical spawning ‌components have been depleted, reducing cod stock productivity and stability.

Over the last seven years, Doug Zemeckis, who recently earned his PhD in Marine Science and Technology, has worked closely with his advisor Steve Cadrin, Associate Professor in the Department of Fisheries Oceanography at SMAST, on several studies concentrated in the Gulf of Maine. The objective of these projects is to provide a more holistic understanding of cod population dynamics and address fisheries problems using various tagging methods that track the journey and whereabouts of cod.

 “Initially we used conventional tags, moved into electronic tags that monitor things like temperature and depth, and then advanced to acoustic tags,” says Cadrin. Movements away from an inshore spawning site were studied using conventional tags, geolocation utilizing data from archival data storage tags, and acoustic telemetry, which documented fine-scale spawning site fidelity with respect to an inshore spawning site in the western Gulf of Maine.

“One of the strengths of telemetry is its fishery independence, so we don’t have to rely on fisherman and their fishing patterns to receive data back. That’s why depending on the study we use one of the three different types of tags,” says Zemeckis.

From conventional to advanced tagging methods

“With the conventional tags, we typically rely on fisherman to recapture and report. With the electronic archival tags, we are able to track fish so that we know their journeys and behavioral patterns, and, again, we rely on fisherman to recapture them,” explains Zemeckis. And then there is the acoustic tagging method, which uses electronic tags that emit a signal that is detected by hydrophones so researchers can track tagged fish. “The advantage of the acoustic tag is that we don’t need a fishery recapture,” says Cadrin. “Really it’s our hydrophones that collect the information on where and when fish were.”

Advanced application of this technology allowed researchers to identify when and where fish are as well as their behavior and distribution patterns. “For example,” says Zemeckis, who recently completed his PhD dissertation defense on Spawning Dynamics, Seasonal Movements, and Population Structure of Atlantic Cod (Gadus morhua) in the Gulf of Maine, “while researching spring spawning cod, we used a positioning system that allowed us to determine GPS latitude and longitude positons of the fish to investigate their fine scale spawning behavior. We have fine scale location as well as the depth, so we had the position in three dimensions. We know precisely where they were in space, which permitted description of their fine-scale movements.”

Inter-related projects on spawning dynamics and discard mortality

‌In addition to the spring spawning project, which was the first of three major projects that began in 2009 and was completed in collaboration with the Massachusetts Division of Marine Fisheries (who first funded the ‌project) , Cadrin and Zemeckis have also  served as co-principal investigators on other tagging projects in collaboration with researchers at various agencies. Their winter spawning cod project, now funded through a grant from the National Oceanic and Atmospheric Administration (NOAA), was initially funded through the Nature Conservatory and Massachusetts Division of Marine Fisheries. Research with winter spawning cod has built off previous experience with the spring spawners and focuses on mapping the spatial and tempor‌al distribution of cod spawning in Massachusetts Bay to inform fishery management decisions. The overarching objectives between both studies have been to improve our understanding of the spawning dynamics, seasonal movements, and population structure of cod in the Gulf of Maine.

The spring spawning research informed the timing and size of the Spring Cod Conservation Zone (SCCZ) in Massachusetts Bay, which is the closure in which the researchers were tagging spring spawning cod. Additionally, the evidence for disruption of spawning by fishing that was collected by Massachusetts Division of Marine Fisheries researchers in 2009 has been considered by managers in informing other cod protection measures in the Gulf of Maine. The findings are also expected to contribute to the development of future fishery management plans and stock assessment models that work towards achieving stock rebuilding.

The next project focused on cod discard mortality and became a spinoff of the team’s previous research on winter and spring spawning behavior. Discard mortality involves estimating the portion of fish that die after release and those  estimates have been used in the stock assessments and  fisheries management decisions for the Gulf of Maine recreational fisheries. The discard mortality project was competed in collaboration with by researchers from the New England Aquarium, Massachusetts Division of Marine Fisheries, and the University of New England.

‌‌Results from the studies investigating spring and winter spawning cod have proven valuable for informing fishery managers in the development of regulations to protect cod spawning aggregations. “The results of Doug’s and his colleagues’ work have direct relevance to the way we monitor these fisheries resources and the way these fisheries are managed when it comes to protecting spawning behavior and conserving different spawning components, and accurately modeling the population dynamics,” says Cadrin. “All of these projects have direct relevance to New England fisheries, particularly the acoustic tagging work, which has been a great example of the research results produced by the Massachusetts Marine Fisheries Institute – a partnership between SMAST and the Massachusetts Division of Marine Fisheries.”

Zemeckis agrees. “I can’t think of a better example of the strength of that partnership than these projects. There’s a co-benefit where we have successfully combined everyone’s complimentary areas of expertise, which has led to the development of very strong and productive collaborations,” says Zemeckis.

School for Marine Science and Technology