In this study, I present a new approach to prey quantification in the context of the predator-prey relationship between an apex marine predator, the gray whale, (Eschrichtius robustus) and one of its primary prey, mysids (Family Mysidae) in a tertiary foraging area, in Clayoquot Sound, on the west coast of Vancouver, Island, British Columbia. With the use of a remote acoustical sounder, I first quantify mysid patches in the presence of foraging gray whales. Second, I address the biomass in those patches using a new approach to quantification. By creating an estimate of total biomass of mysids in the study area concurrently with the presence of gray whales, I demonstrate the effect that the foraging whales have on their prey. Tracking the changes of the number of whales and the number of patches, including the overall biomass of the patches provides evidence for this relationship. As the number of gray whales in the study area increases steadily from May to July, 2004, the number of mysid patches and biomass decrease during the same period. The number of foraging whales located in the study area ranged from only five in May to thirty-one on July 18th, in a region covering only 25 km of the southwest coast of Flores Island. After this period of foraging, the whales deserted the study area completely, and only one whale was seen during the month of August. The mysid population regroups slightly after the pressure of foraging whales is released, the number of patches and biomass increase a small amount, exhibiting the expected behavioral response of the prey to re-aggregate into tight swarms. The acoustic method for prey quantification is described in detail as this is a first application of this technology to the mysid-gray whale relationship. In this approach, l demonstrate that the Distorted Wave Borne Approximation model (DWBA) is useful to estimate mysid populations. This study provides a key piece in the progression of eight years of ongoing research on the foraging ecology of gray whales in Clayoquot Sound, British Columbia.