A most simplistic way to describe the vertical structure of the ocean is to consider that the ocean is made of a deep layer of cold water topped by a warmer surface layer. The thermocline is the boundary between these two layers.
Warm eddies, seen as positive sea level anomalies (bumps) in altimetric maps, are often associated with surface water convergence. This induces local accumulation of warm surface water, down-welling of this surface water and hence a deeper thermocline.
On the opposite, cold eddies seen as negative sea level anomalies (holes) in altimetric maps, are associated with the surface divergence. In such regions, upwelling from the deeper layer occurs to compensate for the surface water loss and the thermocline moves up. With this upwelling, nutrients and phytoplankton are brought to the surface where they are exposed to solar radiation. This activates photosynthesis. The fronts between warm and cold structures (i.e. significant boundary between bumps and holes) are thus naturally areas with a higher probability of fish presence.
For this raison, altmetric maps and associated geostrophic currents are important information to locate more easily favorable locations with high probability of increased primary production.
With upwelling, nutrients and phytoplankton are brought to the surface where they are exposed to solar radiation.
Cold eddies are thus favorable locations for increased primary production.