EFFECTS OF ZOOPLANKTON SIZE AND CONCENTRATION AND LIGHT-INTENSITY ON THE FEEDING-BEHAVIOR OF ATLANTIC MACKEREL SCOMBER-SCOMBRUS

Atlantic mackerel Scomber scombrus had low clearance rates when fed ol der stage copepodites of the copepod Calanus finmarchicus at high conc entrations and high clearance rates at low concentrations. These rates were consistent with filter feeding at high concentrations and partic ulate feeding at low concentrations. Intermediate and small copepods p resented together at high concentrations were cleared at lower rates t han the large C. finmarchicus, suggesting lower filtration efficiencie s. Intermediate and small copepods were presented over a range of ligh t intensities (8.2 x 10(-8) to 1.6 x 10(0) mu E m(-2) s(-1)). Feeding rate did not change significantly between 1.6 x 10(0) and 2.0 x 10(-6) mu E m(-2) s(-1) but decreased to nearly zero at 8.2 x 10(-8) mu E m( -2) s(-1), indicating a light intensity threshold for feeding of about 10(-7) mu E m(-2) s(-1). This threshold enables mackerel to feed thro ughout the night near the ocean surface. Swimming speed decreased to a lesser degree than feeding rate at the lowest Light intensity, indica ting that the change in filter-feeding rate is only partially due to t he change in speed. The school dispersed in both low and high light le vels, but spacing between fish did not appear to be related to feeding rate.