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Atlantic mackerel Scomber scombrus had low clearance rates when fed older stage copepodites of the copepod Calanus finmarchicus at high concentrations and high clearance rates at low concentrations. These rates were consistent with filter feeding at high concentrations and particulate feeding at low concentrations. Intermediate and small copepods presented together at high concentrations were cleared at lower rates than the large C. finmarchicus, suggesting lower filtration efficiencies. Intermediate and small copepods were presented over a range of light intensities (8.2 x 10-8 to 1.6 x 100 µE m-2 s-1). Feeding rate did not change significantly between 1.6 x 100 and 2.0 x 10-6 µE m-2 s-1, but decreased to nearly zero at 8.2 x 10-8 µE m-2 s-1, indicating a light intensity threshold for feeding of about 10-7 µE m-2 s-1. This threshold enables mackerel to feed throughout the night near the ocean surface. Swimming speed decreased to a lesser degree than feeding rate at the lowest light intensity, indicating that the change in filter-feeding rate is only partially due to the change in speed. The school dispersed in both low and high light levels, but spacing between fish did not appear to be related to feeding rate.