New Research Provides Insight Into Shearwaters, Humpbacks, Their Prey, and Future Management Implications
By Anne Smrcina
New Insights on Shearwaters’ Winter Range
In spring months, humpback whale mothers bring their calves to Stellwagen Bank to wean them as they teach them to hunt schools of fish. Stellwagen Bank National Marine Sanctuary serves as their summer nursery. Researchers now believe that the bank, as well as the greater Gulf of Maine, serves as a nursery for another important but very different species – the great shearwater, an abundant seabird locally, and a global traveler.
A sanctuary-led research team found that this seabird species, which “summers” on islands in the southern South Atlantic and along the Patagonian shelf off Argentina December-February, uses the Gulf of Maine as a “winter” nursery during the months of approximately July-November. More mature birds extend their range to the Scotian Shelf off Nova Scotia and the Grand Banks off Newfoundland. These findings were published in a recent issue of Marine Ornithology.
Predators Follow their Prey
In another recently released paper in the journal Conservation Science and Practice from the Society for Conservation Biology, a sanctuary science team led by post-doctoral researcher Tammy Silva, analyzed six years of data related to great shearwater sighting locations. The data showed that shearwaters, as well as humpback whales, consistently co-occur in areas with sand lance. The strong co-occurrence between predators and their sand lance prey suggests that it may be important to identify other locations that sand lance inhabit. Uncovering more information about sand lance habitats may indicate other areas where shearwaters aggregate -- places that could deserve special management consideration.
Tracking a Marvelous Migration
Great shearwaters make one of the world’s longest migrations each year, more than 11,000 miles (17,000 km), going from northern feeding grounds in the Northwest Atlantic, then crossing over the North Atlantic towards Europe and Africa, before turning back to the South American coast. Mature birds continue on to the Tristan da Cunha Islands for breeding, while juvenile, non-breeding birds stay behind on the Patagonian shelf and older immatures, but still not breeding birds, roam the South Atlantic, as far as South Africa. When the shearwaters return north in the spring, it appears that juvenile birds, generally two years old or younger, remain in the Gulf of Maine into November.
Sanctuary science teams have tagged great shearwaters since 2013. The researchers use small, satellite- transmitters that are sewn onto the loose skin on the birds’ backs. Location data sent by the tags bounces from the satellites to a landside computer many times a day, allowing the researchers to monitor the birds’ movements. While Stellwagen Bank National Marine Sanctuary’s research project primarily focuses on understanding prey distribution, the long-lasting technology allowed the team to continue tracking many of the birds on their southward migration during our winter (the southern summer). The recent research paper is based on data from 58 birds from 2013 to 2018. Elsewhere, scientists from different institutions have tagged birds at the Tristan da Cunha Island archipelago in the South Atlantic to follow their northward paths.
Insights for Future Management
The researchers report that understanding where and why juvenile birds are now located may prove to be critically important for better management of the species and its prey now and into the future, especially as environmental conditions change.
“The birds are often a very visual indicator of changing prey patterns, water temperatures, and other ecosystem responses to climate change,” said Kevin Powers, lead author on the paper and a volunteer with the sanctuary’s seabird research program.
“Young humpback whales and great shearwaters both target sand lance fish for food and we often find them together” said Dr. David Wiley, the sanctuary’s research coordinator and co-author on both papers. “Throughout the Gulf of Maine, humpback whales and great shearwaters tend to be found over sand lance habitat, demonstrating the importance of this thin, lipid-rich (oily) fish to the sanctuary and beyond."
While feeding primarily on sand lance, the team used DNA from bird fecal samples to determine that they also fed on other lipid-rich fish -- mackerel and menhaden -- but to a lesser extent. The research provides further evidence of the importance of the Stellwagen Bank ecosystem to the local food web and to some of the area’s most iconic species in their critical juvenile stage.
The team included representatives from Stellwagen Bank National Marine Sanctuary, the University of Rhode Island-Graduate School of Oceanography, the University of New England Animal Behavior Program, the U.S. Fish and Wildlife Service, the U.S. Geological Survey, the University of Massachusetts/Dartmouth, and the Boston University Marine Program.
Anne Smrcina is the education coordinator at Stellwagen Bank National Marine Sanctuary.
Powers, K.D., Wiley, D.N., Robuck, A.R., Olson, Z.H., Welch, L.J., Thompson, M.A., and Kaufman, L. (2020). Spatiotemporal characterization of non-breeding great shearwaters Ardenna gravis within their wintering range. Marine Ornithology 48: 215–229.
Silva, T.L., Wiley, D.N., Thompson, M.A., Hong, P., Kaufman, L., Suca, J.A., Llopiz, J.K., Baumann, H., and Fay, G. (2020). High collocation of sand lance and protected top predators: implications for conservation and management. Conservation Science and Practice. https://doi.org/10.1111/csp2.274