Responses of fish production to fishing and climate variability in the northern South China Sea

Abstract

To understand the responses of fish production to both fishing pressure and climate variability in the northern South China Sea (NSCS), catch time series are partitioned into interannual trends and variations. These are separately related to growth in fishing effort and physical variables. Catches of the fishery underwent dome-shaped trends corresponding to a monotonic growth in fishing effort, first in the inshore and then in the offshore demersal fishes, while total catch and catches of the low-trophic species showed a rising trend until 1990, and eventually experienced a decline under high fishing pressure. After accounting for the trends from the growth in fishing pressure, variations in catches were lag correlated with land precipitation, monsoon wind speeds, and a proposed index of tropical cyclone influence. The linkage suggests that river runoff, monsoon circulation and tropical cyclone impacts are the physical forcing factors dominating the catch variations, and the effects are largely through controlling the nutrient supply for biological production. Runoff provides nutrients to the inshore waters, while monsoons and tropical cyclones control the distribution and availability of nutrients. The winter monsoon increases primary production and hence fish production by offshore diffusion of the nitrogen-rich riverine water masses and by enhancing vertical mixing. In contrast, the summer monsoon reduces availability of nutrients for biological production by confining the distribution of the riverine water masses and strengthening water column stratification. Tropical cyclones are a strong forcing factor that has a positive effect on production of fishes. Tropical cyclones mobilize nutrient elements and enhance biological production by increasing water circulation, wind mixing and upwelling, and by inducing heavy rainfall and territory runoff. Based on the suggested physical forcing factors and their controlling mechanisms, the analysis predicts that increasing anthropogenic nutrient inputs and future climate changes are likely to result in an increase in fish production in the NSCS; this may be associated with even larger interannual and interdecadal fluctuations in fish production.