  Lagrangian fronts (LF) in the ocean delineate boundaries between surface
waters with different Lagrangian properties. They can be accurately detected in
a given velocity field by computing synoptic maps of the drift of synthetic
tracers and other Lagrangian indicators. Using Russian ship's catch and
location data for a number of commercial fishery seasons in the region of the
northwest Pacific with one of the richest fishery in the world, it is shown
statistically that the saury fishing grounds with maximal catches are not
randomly distributed over the region but located mainly along those LFs where
productive cold waters of the Oyashio Current, warmer waters of the southern
branch of the Soya Current, and waters of warm-core Kuroshio rings converge.
Computation of those fronts with the altimetric geostrophic velocity fields
both in the years with the First and Second Oyashio Intrusions shows that in
spite of different oceanographic conditions the LF locations may serve good
indicators of potential fishing grounds. Possible reasons for saury aggregation
near LFs are discussed. We propose a mechanism of effective export of nutrient
rich waters based on stretching of material lines in the vicinity of hyperbolic
objects in the ocean. The developed method, based on identifying LFs in any
velocity fields, is quite general and may be applied to forecast potential
fishing grounds for the other pelagic fishes in different seas and the oceans.
