Foraging ecology of humpback whales off Newfoundland

Bredin, Katherine Alberta (1985) Foraging ecology of humpback whales off Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Off the coast of Newfoundland humpback whales fee primarily on capelin which form schools of varying size, depth and age class. Observations were made on humpbacks foraging in six different prey situations, in each of which the distribution, abundance and behaviour of whales was related to bait characteristics. Whales were followed by boat and prey school information was provided by a paper printout depth sounder. -- When whales were exploiting large deep immature or post-spawning capelin schools, dive times were positively correlated with depths to the top of the prey school and to the ocean floor in the immediate dive vicinity. Time spent at depth (filtering time) appears to be constant (t = 2.3 min) while dive time is proportional to prey depth. Surface times were positively correlated with dive times and with prey school and water depth and appear to function as a recovery period from foraging dives. When preying upon small concentrated schools of post-spawning capelin, water depth and vertical extent of prey school did not significantly predict either dive time or surface time. The time spent and the large amounts of directional change that occurred while whales were at the surface may function to reposition whales close to the prey school. When the concentrated post-spawning capelin schools dispersed into very small widely scattered schools, whales spent variable amounts of time at the surface travelling primarily in straight lines from school to school, diving only once into each pocket of bait. Surface and dive times were not correlated with vertical extent of prey or with water depth. Humpbacks were also observed to exploit small surface swarms of krill, using a foraging strategy distinct from that seen when feeding on capelin: a series of lateral lunges at the surface. -- A dominant feature of humpback whale foraging is the formation of highly coordinated groups. It is hypothesized that coordination functions to minimize the disruptive effects that repeated dives of foraging whales may have on prey schools. When prey schools were large, whales foraged almost continuously and formed larger groups. Whales left small prey schools after 0.t -2.0 h exploitation to travel and to rest, possibly letting the school regroup before returning to forage. Group sizes were larger when whales were foraging than when not foraging. Local whale populations were greater and whales stayed longer where prey was more abundant.

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