Studying Gannets to better understand the feeding activities of whales

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.

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).

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.