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Making a Living in a Highly Dynamic Environment

The Marine B52 - A Complex System Based on Three Layers

From the Gulf to the Estuary, the St. Lawrence is divided into a three-layer-water system which appears seasonally. The surface layer (depth 20-30 m, temperature 2-5°C, salinity 18 – 31 ‰) is formed due to freshwater run-off into the St. Lawrence and freshwater outflow from the Saguenay Fjord. Below, the cold intermediate layer (CIL) consists of a denser and heavier water mass than the surface layer (depth 150m, temperature -1-2°C, salinity 32 ‰) and is the result of the cold water flow from the Gulf of St. Lawrence during the winter period.

The third water mass stretches throughout the Estuary and Gulf along the sea floor and is partially fed by the Labrador Current. Twice a day, strong flooding tides which hit the Laurentian Channel head create strong upwellings of the CIL and bottom waters flow over the sills resulting in cold surface waters between 2 – 7 °C.

Nothing Lasts for Long

These forces, the strong estuarine two-layer circulation, intense tidally driven upwellings and other complex physical oceanographic processes such as internal tides, high frequency internal waves (Ingram 1978), fronts and very complex tidal current dynamics (Service hydrographique du Canada 1997) closely linked to local topographic features lead to a site of ‘quasi-permanent rich krill aggregation’ on a semi-diurnal time frame. This is probably one of the richest and persistent krill aggregations yet documented in the northwest Atlantic. They are exclusively composed of individuals of the oldest cohorts (2+) of the two species Thysanoessa raschi and Meganyctiphanes norvegica. They link to the major species of shoaling fish, capelin (Mallotus villosus), which is one of the main predators of krill besides whales.

A Main Feeding Ground for Rorqual Whales

As rorqual whales need high concentrations of food during their feeding seasons, it is not surprising that the St. Lawrence Estuary is very attractive for several species of whales, seals and seabirds. Of all whale species, only the beluga whale (Delphinapterus leucas) lives in these waters year round.

All other species, the harbour porpoise (Phocena phocena), minke whale (Balaenoptera acutorostrata), finback whale (Balaenoptera physalus), and less frequently the blue whale (Balaenoptera musculus), humpback whale (Megaptera novaeangliae), and occasionally sperm whale (Physeter macrocephalus) migrate into this rich feeding ground during the summer months from April to October, with most sightings in June to September.

Of all rorquals, minke and finback whales are most abundant and both concentrate along the slopes of the Laurentian Channel head, thus in the area of dense krill and/or capelin aggregations.

Zones of Upwellings are the Key to High Foraging Success

In such areas of strong upwellings and along tidal and internal fronts, krill are frequently visible at the surface during daylight hours and often fish feeding on krill can be seen just below the surface. This contradicts the general downward migration of krill in order to avoid intense light. It seems that zooplankton often can’t withstand the strong vertical currents occurring in the study area thus becoming more accessible for whales. A yet unpublished study revealed that such surface and near-surface prey distributions are more often the targets of foraging rorqual whales than prey at depths of greater than 50 m.

Despite the high concentration of prey, strong intra- and inter-annual fluctuations of whale sightings have been recorded. Since 2000, numbers of minke whales and especially finback whales decreased continuously with lowest sighting numbers in 2005. In summer 2006 however, minke, finback and humpback whales were more abundant than ever.

Since studies on prey abundance are not carried out each year, the spatial and temporal distribution of whales, especially of the most abundant minke whale, might serve as indicators for prey fluctuations and long-term changes within the ecosystem. It is essential to study, understand and monitor this complex ecosystem and to provide crucial information for the management and conservation of these waters but also of other areas in the world where conservation and management measures are put in place.