Time-Lagged Responses of Purpleback Flying Squid Fishing-Ground Formation to Chlorophyll-a Variability in the Southern South China Sea

Fishing grounds of the purpleback flying squid (Sthenoteuthis oualaniensis) in the South China Sea are commonly related to chlorophyll-a (Chl-a), yet the temporal structure of this relationship is rarely tested explicitly. We examined whether fishing-ground formation responds to concurrent or preceding Chl-a conditions using monthly vessel monitoring system (VMS) records from 2014–2018 (50 valid months) aggregated to a 0.5° grid in the southern South China Sea, together with Copernicus Marine SST and 4 km gap-free Chl-a products. Four Chl-a windows (lag 0, lag 1, lag 2, and mean lag 1–2) were compared for mean log10 Chl-a and for the Chl-a spatial gradient using within-month Spearman correlations, centroid distances, Jaccard overlap, response curves, and cross-validated generalised additive models. The analyses showed a consistent separation between productivity level and productivity structure. Mean Chl-a associations remained small and did not produce a stable spatial lag, whereas fishing-ground centroids were substantially closer to high-gradient cells (median 166–181 km) than to high-Chl-a cells (280–295 km). The largest seasonal gradient correlation occurred in spring at lag 2 (ρ = 0.114), and median gradient overlap at the principal threshold was highest at lag 2 (0.034). Annual cross-validation, however, showed no consistent lag-2 advantage. Chl-a gradients therefore help characterise fishing-ground position, while the antecedent association depends on season and spatial metric rather than indicating a generally optimal two-month lag. This pattern is compatible with delayed food-web transmission and prey-field reorganisation, while its causal interpretation is considered in the Discussion.