Estimating Acoustic Abundance of Forage Fish in Rosario Strait, Washington

Lance and Jefferies determined that the seasonal change in the diet of harbor seals (Phoca vitulina) is related to the availability of pelagic forage near their haul-out. Hydroacoustic-trawl surveys were conducted in June of 2008 to assess pelagic forage abundance and composition in Rosario Strait and Burrows Bay, Washington.

Replicate surveys of each area were attempted during the second and third weeks of June. Mean acoustic density of pelagic forage varied between paired surveys: 0.0323 and .0128 kg/m2 in Northern Rosario, 0.0067 and 0.0082 kg/m2 in Burrows Bay. Pelagic forage species composition varied by area and survey date, but was dominated by juvenile Walleye Pollock (Theragra chalcogramma), (TL=60.9 mm) and pre-metamorphosed Pacific Herring (Clupea pallasii), (TL= 45.0 mm). From these results, 1,369 metric tons of pelagic forage, including 505 metric tons of juvenile pollock and 169 metric tons of pre-metamorphosed herring were estimated for the three areas combined.