Internal waves interact with topography over a small bank to produce a highly-focused feeding environment for fish, whales and birds

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A small (9x15 km) offshore bank in the Gulf of Maine (Platts Bank) is sometimes the focal point of intense feeding by herring, marine mammals and birds. Humpback whales (Megaptera noviangliae) use only a small portion of even this small bank as they feed on euphausiids (Meganyctiphanes norvegica). The euphausiids are similarly abundant on and off the bank, but form dense surface swarms near the crest of the bank. The swarms form abruptly and last only a few minutes or less before euphausiids descend below the surface, but they are quickly exploited by whales that appear to be well tuned to the brief but frequent appearances. The swarms appear to form in response to passing internal waves that are brought near the surface as the pycnocline shallows over the crest (internal tide) during periods of strong tidal flow. This only occurs when euphausiids are above the pycnocline and it may be reinforced by density-dependent (schooling) behavior, factors that may account for within- and between-year differences in feeding activity on the bank. While euphausiids are supplied to the crest from a large area related to the tidal ellipse and a residual flow field, the trophic transfer remains spatially very constrained.