# Exploring the Global and Regional Factors Influencing the Density of Trachurus japonicus in the South China Sea

## TL;DR

This study identifies global and regional factors influencing the distribution of Trachurus japonicus in the South China Sea, revealing unexpected natural drivers like ozone and solar activity.

## Contribution

The study uncovers nine key causal factors, including Arctic ozone and solar activity, that influence the density of Trachurus japonicus in the South China Sea.

## Key findings

- Ozone concentrations in Greenland, solar radiation, sonar-detected ocean layering depths, and nitrate levels at 20 m depth show stable causal relationships with fish density.
- Natural factors like Arctic ozone changes have a stronger influence on tropical fish stocks than previously recognized.
- Human activities like shipping have unexpectedly weak causal impacts on fish distribution in this region.

## Abstract

Preventing the decline of Trachurus japonicus—a threatened fish vital to South China Sea fisheries—remains complex even as environmental pressures intensify. Using a machine learning analysis of oceanic and climate data (2014–2020), we identified nine key factors controlling the fish’s distribution, including atmospheric pressure, ozone levels, solar activity, and ocean nutrients. Significantly, we found consistent cause-and-effect links between four factors: ozone concentrations measured in Greenland, solar radiation levels, sonar-detected ocean layering depths, and nitrate concentrations 20 m below the surface. While these global patterns strongly drove fish movements (likely by altering light, nutrients, or navigation), human activities like shipping showed unexpectedly weak causal impacts—though their role remains critical elsewhere. We conclude that natural factors like Arctic ozone changes disproportionately affect tropical fish stocks, which may challenge local conservation efforts amid global environmental shifts.

In this cross-disciplinary investigation, we uncover a suite of previously unexamined factors and their intricate interplay that hold causal relationships with the distribution of Trachurus japonicus in the northern reaches of the South China Sea, thereby extending the existing research paradigms. Leveraging advanced machine learning algorithms and causal inference, our robust experimental design uncovered nine key global and regional factors affecting the distribution of T. japonicus density. A robust experimental design identified nine key factors significantly influencing this density: mean sea-level pressure (msl-0, msl-4), surface pressure (sp-0, sp-4), Summit ozone concentration (Ozone_sum), F10.7 solar flux index (F10.7_index), nitrate concentration at 20 m depth (N3M20), sonar-detected effective vertical range beneath the surface (Height), and survey month (Month). Crucially, stable causal relationships were identified among Ozone_sum, F10.7_index, Height, and N3M20. Variations in Ozone_sum likely impact surface UV radiation levels, influencing plankton dynamics (a primary food source) and potentially larval/juvenile fish survival. The F10.7_index, reflecting solar activity, may affect geomagnetic fields, potentially influencing the migration and orientation behavior of T. japonicus. N3M20 directly modulates primary productivity by limiting phytoplankton growth, thereby shaping the availability and distribution of prey organisms throughout the food web. Height defines the vertical habitat range acoustically detectable, intrinsically linking directly to the vertical distribution and availability of the fish stock itself. Surface pressures (msl-0/sp-0) and their lagged effects (msl-4/sp-4) significantly influence sea surface temperature profiles, ocean currents, and stratification, all critical determinants of suitable habitats and prey aggregation. The strong influence of Month predominantly reflects seasonal changes in water temperature, reproductive cycles, and associated shifts in nutrient supply and plankton blooms. Rigorous robustness checks (Data Subset and Random Common Cause Refutation) confirmed the reliability and consistency of these causal findings. This elucidation of the distinct biological and physical pathways linking these diverse factors leading to T. japonicus density provides a significantly improved foundation for predicting distribution patterns globally and offers concrete scientific insights for sustainable fishery management strategies.