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[AOS15-02] Discovering hidden Lagrangian fronts favorable for sustainable fishery in the northwestern Pacific
Hydrological fronts can promote aggregation of nutrients attracting different marine organisms and fish for feeding. SST satellite images have been until recently the only tool to detect fronts remotely. However, cloudiness, low thermal contrasts in warm seasons and various kinds of noise may hinder detection of thermal fronts. The alternative approach has been developed recently by the author for capturing frontal features in near real time based on extraction of the so-called Lagrangian fronts (LF). The LFs are features, hidden in a chaotic flow, that can be identified by calculating horizontal gradients of relevant Lagrangian indicators, which are functions of trajectories of virtual particles advected in an altimetry-based or numerical-models velocity fields. The indicator values are coded by color on the geographic map of a study area on a fixed date. The LFs approximate locations of the current positions of strong hydrological fronts.
Inspecting the daily Lagrangian maps, we track different phases of the evolution of LFs: frontogenesis, frontolysis and disappearance. Available data on catch locations are superimposed on such maps in order to conclude which kinds of the LFs are favorable fishing grounds. Lagrangian methods are applied to study the connection of LFs with catch sites of commercial pelagic fish in the northwestern Pacific. The mechanism of accumulating food resources at LFs is the following. So-called unstable manifold of distinguished hyperbolic points ‘attract’ tracers. Fish follow the contracting food patch and aggregate along a strong LF that can be identified by large values of the relevant Lagrangian indicators, such as the finite-time Lyapunov exponent, displacement of virtual particles, etc.
The factors influencing the abundance and distribution of Pacific saury (Cololabis saira) are of considerable scientific and commercial interest. Using the database provided by Russian, Japanese and Korean vessels in the exclusive economic zone of Russia in 2004 – 2019, correlation of LF locations with saury catch sites has been studied. We distinguish strong LFs with accumulation of catches associated with the Soya Warm current, the Oyashio branches and Hokkaido quasi-stationary anticyclone (Fig. 1). We have computed daily gradient fields of the finite-time Lyapunov exponent within 10 km distance from the catch sites and from the points distributed randomly and repeated in 1000 trials. The comparison between the random trial and trials with catches, using distribution functions and a Kolmogorov–Smirnov statistical test, has shown a correlation of the catch sites with locations of the mesoscale LFs (p < 0.001).
The eastern Sea of Okhotsk is known to be one of the richest fishery in the world due to
nutrient-rich waters coming from the ocean. The altimetry-based data on current velocities and the Lagrangian approach have been used to detect the inflow of waters of Pacific origin into the sea and to describe their dispersal. Daily values of the indicators of the ‘age’ and latitude of entry of the ocean waters in the sea have been calculated for the long period of time. Using the fishing database during the pollock (Gadus chalcogrammus) fishing seasons in 1997-2021, it was shown that the catch sites were mainly concentrated in the intrusions of ‘young’ ocean water that entered the sea through the northern Kuril Straits. Statistically significant differences were found between the samples with true catch sites and randomly distributed points. The locations of LFs can be calculated in real time in any weather and transmitted to fishing vessels by e-mail for prompt forecasting of promising fishing sites.
Fig. 1. Maps on Oct. 1, 2004. a) The finite-time Lyapunov exponent Λ, day-1 and b) origin of water masses around the Hokkaido quasi-stationary anticyclone centered at 42.5° N and 147.4° E with the superimposed catch sites for two weeks shown by the orange points. b) The origin map. S, 1st O and 2nd O are acronyms for LFs associated with the Soya Warm Current, the 1st and 2nd Oyashio branches. The upward-oriented red and downward-oriented blue triangles are elliptic points indicating the centers of anticyclones and cyclones, and crosses are locations of the hyperbolic points.
The work was supported by the Russian Science Foundation (project no. 23-17-00068).
Inspecting the daily Lagrangian maps, we track different phases of the evolution of LFs: frontogenesis, frontolysis and disappearance. Available data on catch locations are superimposed on such maps in order to conclude which kinds of the LFs are favorable fishing grounds. Lagrangian methods are applied to study the connection of LFs with catch sites of commercial pelagic fish in the northwestern Pacific. The mechanism of accumulating food resources at LFs is the following. So-called unstable manifold of distinguished hyperbolic points ‘attract’ tracers. Fish follow the contracting food patch and aggregate along a strong LF that can be identified by large values of the relevant Lagrangian indicators, such as the finite-time Lyapunov exponent, displacement of virtual particles, etc.
The factors influencing the abundance and distribution of Pacific saury (Cololabis saira) are of considerable scientific and commercial interest. Using the database provided by Russian, Japanese and Korean vessels in the exclusive economic zone of Russia in 2004 – 2019, correlation of LF locations with saury catch sites has been studied. We distinguish strong LFs with accumulation of catches associated with the Soya Warm current, the Oyashio branches and Hokkaido quasi-stationary anticyclone (Fig. 1). We have computed daily gradient fields of the finite-time Lyapunov exponent within 10 km distance from the catch sites and from the points distributed randomly and repeated in 1000 trials. The comparison between the random trial and trials with catches, using distribution functions and a Kolmogorov–Smirnov statistical test, has shown a correlation of the catch sites with locations of the mesoscale LFs (p < 0.001).
The eastern Sea of Okhotsk is known to be one of the richest fishery in the world due to
nutrient-rich waters coming from the ocean. The altimetry-based data on current velocities and the Lagrangian approach have been used to detect the inflow of waters of Pacific origin into the sea and to describe their dispersal. Daily values of the indicators of the ‘age’ and latitude of entry of the ocean waters in the sea have been calculated for the long period of time. Using the fishing database during the pollock (Gadus chalcogrammus) fishing seasons in 1997-2021, it was shown that the catch sites were mainly concentrated in the intrusions of ‘young’ ocean water that entered the sea through the northern Kuril Straits. Statistically significant differences were found between the samples with true catch sites and randomly distributed points. The locations of LFs can be calculated in real time in any weather and transmitted to fishing vessels by e-mail for prompt forecasting of promising fishing sites.
Fig. 1. Maps on Oct. 1, 2004. a) The finite-time Lyapunov exponent Λ, day-1 and b) origin of water masses around the Hokkaido quasi-stationary anticyclone centered at 42.5° N and 147.4° E with the superimposed catch sites for two weeks shown by the orange points. b) The origin map. S, 1st O and 2nd O are acronyms for LFs associated with the Soya Warm Current, the 1st and 2nd Oyashio branches. The upward-oriented red and downward-oriented blue triangles are elliptic points indicating the centers of anticyclones and cyclones, and crosses are locations of the hyperbolic points.
The work was supported by the Russian Science Foundation (project no. 23-17-00068).