Swimming behavior of krill in response to algal patches: A mesocosm study

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Abstract
An understanding of the response of zooplankton to phytoplankton patches is required to determine the relative importance of behavior vs. physical dynamics in generating and maintaining zooplankton aggregations. The swimming behavior of the krill Thysanoessa raschii was monitored with dual video cameras in a 117‐m3 tank before and after the introduction of a subsurface algal patch. Krill density within the patch was an order of magnitude higher than the control density within 0.5 h after algal introduction, suggesting krill have some means of detecting and remaining within a concentrated food source. Orientation of the swimming paths became more horizontal, swimming speed doubled, and sinking bouts were almost entirely eliminated inside the algal patch. The net‐to‐gross displacement ratio of the swimming paths indicated no increase in looping or spiraling. However, individual krill kept themselves within the patch by turning back at the boundary so that the angle of approach to and return from the edge was held roughly constant. The behavioral changes shown by T. raschii within the algal patch were maintained over the 24‐h period following its introduction. The ability to detect and remain within algal patches must be considered in explanations of euphausiid distribution and in the construction of more realistic energy budgets.