Tidally Induced Temporal Variations in Growth of Young‐of‐the‐Year Pacific Cod in the Yellow Sea

Abstract

AbstractThe Yellow Sea is the southern limit of the distribution of Pacific cod (Gadus microcephalus), and the Yellow Sea Cold Water Mass (YSCWM) provides an appropriate over‐summering habitat for this species. Tidal current is the major component of the Yellow Sea hydrodynamic system, and its periodic variations influence the local ecological conditions and related ecosystems, particularly in the YSCWM, which is stratified in summer. To investigate the influence of tidal currents on Pacific cod growth, otolith daily increment in young‐of‐the‐year (YOY) Pacific cod in the Yellow Sea was analyzed in 2017. These data were integrated with a simulated tidal current using a multi‐scenario time‐delay analysis. This showed that: (1) the otolith daily increment exhibited a strong signal similar to the fortnightly tidal variation; (2) there was a lag of ∼8 days between the highest growth rate induced by the tidal current variation and monthly spring tide; and (3) the influence of the tidal current on otolith daily growth was stronger after migration of the cod into the YSCWM, particularly during the settlement stage. Given the stratified ocean structure during the over‐summering period, enhanced tidal mixing during the fortnightly spring tide would improve the exchange of nutrients and oxygen within the YSCWM, and lead to a delayed response of otolith growth due to feeding/digestion and subsequent growth. Therefore, variations in the tidal current have a marked positive ecological effect on the growth of YOY Pacific cod, and ultimately on the YSCWM ecosystem.