A text from Sonia Villalon
The monitoring of northern gannet populations (Morus bassanus) is of particular interest for understanding changes in whale diets in the Gulf of St. Lawrence. Gannets are seabirds with an exapansive feeding range that corresponds to that of several species of whales and a food-based diet similar to that of cetaceans (mackerel [50% of its diet], herring [33%], capelin, sand lance). Northern gannets provide information on prey abundance and the degree of contamination of the ecosystem.
The changing composition of water bodies entering the Gulf, climate change and pollution are leading to significant changes in prey communities. It is known that some species such as cod cannot tolerate the excessively low oxygen levels that have been recorded in recent years in the deep layers of the Estuary. Other more tolerant species such as shrimp concentrate in deep areas to avoid predators. Since techniques for tracking whales under water are limited, it is difficult to quantify and qualify any changes in their diet or the availability of their prey. Researchers are therefore conducting indirect analyses (such as examining the stomach content of carcasses or stable isotopes in muscles). Studying non-cetacean species such as gannets also provides occasional valuable information.
Gannets in the Estuary: good news?
In recent years, northern gannet observations in summer in the St. Lawrence Estuary have been more numerous than usual. Though observers might enjoy their aerial ballets, their presence is not necessarily good news. Northern gannets congregate in colonies in the Gulf during the summer breeding season, which historically has rather limited observations in the Estuary to non-breeding birds. Only in September will fledglings begin to fly to learn the rudiments of fishing and will the gannets leave their breeding grounds. However, since 2010, observers have been reporting more mature adults in the summer in the Estuary. Likewise, thanks to GPS tagging, scientists have noticed that mature gannets have been travelling unusually far (approx. 500 km) from their breeding sites to the Estuary or elsewhere. They are seeking critical food for the survival of their young, occasionally leaving them unattended in the nest for several days (up to 40 hours).
The scarcity of mackerel (Scomber scombrus) in the Gulf is believed to be the factor forcing them to travel such great distances (Chardine et al., 2013). This fat-rich fish is an essential component of the diet of young gannets. The decline is believed to stem from rising water temperatures in the North Atlantic: mackerel would have no choice but to live in deeper waters or to migrate farther north to remain in sufficiently cold waters.
Gannets feed by dive-bombing the water like torpedoes, penetrating up to 20 metres below the surface, which is not deep enough to reach their favourite prey. As a result, they are forced to travel farther to find their food, and sometimes even forgo mackerel or herring for less energy-intensive prey such as sand lance. As a direct result of this phenomenon, researchers are noticing a significant decline in the survival rate of young (Rail et al., 2013) and early departure from colonies by adults.
The northern gannet was therefore chosen as a bio-indicator species in the context of the St. Lawrence Action Plan 2011-2016. This plan to monitor the health of the St. Lawrence River was spearheaded by four government partners: Environment Canada, Fisheries and Oceans Canada, Parks Canada, and the Quebec Ministry of Sustainable Development, Environment and the Fight against Climate Change; as well as Stratégies Saint-Laurent, a non-governmental organization that is active in river communities. The information gathered will therefore be extremely valuable for the understanding and conservation of marine mammal populations that feed on the same fish in this sector.
- David Allen Sibley, mars 2014. The Sibley Guide to Birds – Second Edition, page 95.
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- Gilles Chapdelaine & Jean-François Rail, 2014. Direction de la conservation de l’environnement, Environnement Canada. ISBN 978-0-660-21433-7.
- J.W. Chardine et al, 2013. Population dynamics of Northern Gannets in North America, 1984–2009. Avian behaviour and ecology, Volume 84, Issue 2, Pages 187–192.
- Lesage, V. 2014. Trends in the trophic ecology of St. Lawrence beluga (Delphinapterus leucas) over the period 1988-2012, based on stable isotope analysis. DFO Can. Sci. Advis. Sec. Res. Doc. 2013/126. iv + 25 p.