Drivers of recruitment dynamics in Japanese major fisheries resources: Effects of environmental conditions and spawner abundance

Identifying driving factors of recruitment dynamics is essential for understanding population dynamics of fisheries resources and managing them sustainably. Spawner abundance and environmental conditions have been assumed as driving factors of recruitment, and the relative influence of these two drivers in fish populations has been debated for a long time. We addressed this issue by applying cross-correlation analysis to the time series of recruitment and spawner abundance of 28 Japanese fisheries stocks. The analysis showed that spawner abundance was significantly related to recruitment in 18 of the 28 stocks, but in many stocks, particularly for small pelagic species, recruitment influenced the later spawner abundance more strongly, suggesting a strong influence of the environment. We also detected temporal shifts of recruitment levels corresponding to shifts of wide-area climatic and oceanographic conditions. These results indicate that both spawner abundance and environment might drive recruitment in many stocks, but the apparent effect of spawner abundance might be a by-product of long-term recruitment changes caused by environmental conditions in some cases. Considering our observations, efficient management strategies are needed that are robust to uncertainties of environmental impacts on fish dynamics and spawner-recruitment relationships and match life-history- characteristics of managed stocks.