Linking kinematic characteristics and high concentrations of small pelagic fish in a coastal mesoscale eddy

The Santa Barbara Channel (SBC) eddy is a cyclonic mesoscale eddy and a dominant oceanographic feature of the SBC. In June 1998 and 1999, the SBC eddy was surveyed for fishes. In 1998, very high numbers of small pelagic fishes were observed within the eddy, but not in 1999 when the eddy was far less stable. The ocean conditions that contributed to the interannual differences of fish abundances inside the eddy are investigated with three-dimensional numerical modeling. The life history and kinematic characteristics of the SBC eddy, which includes its life span, size, trajectory, three-dimensional rotational structure, and isopycnal displacement, are evaluated using a three-dimensional ocean circulation model. The retention ability of the eddy is quantified using a three-dimensional particle tracking model, which is driven by the flow fields from the circulation model. The kinematic characteristics and particle retention of the SBC eddy differ significantly in 1998 and 1999. In 1998, when the SBC eddy is rotating at a steady rate in space and time with vertically coherent flow, the particle retention is high and the isopycnal uplift sustained. However, in 1999, when the SBC eddy is rotating unsteadily without vertically coherent flow, the particle retention is low and the isopycnal uplift fluctuates. We theorize that the steady vertically coherent rotation of the eddy in 1998 has two important impacts on the fish abundances. First, it provides a prolonged period of cold nutrient rich water uplifting into the euphotic zone, which stimulates primary productivity attracting zooplankton prey. Second, it allows the zooplankton prey to be retained inside the eddy, which attracts small pelagic fish. We conclude that biological productivity inside mesoscale eddies may be linked to the stability of their three-dimensional rotational structure and their consequent ability to retain particles.