# Diel Vertical Migration and Transport Pattern of Larvae and Juveniles of the Small Yellow Croaker (Larimichthys polyactis) in the Yangtze River Estuary

## TL;DR

Small yellow croaker larvae in the Yangtze River estuary use diel vertical migration to move against the current and enter their nursery grounds.

## Contribution

The study reveals how larvae and juveniles use tidal currents and vertical migration to successfully enter the estuary.

## Key findings

- Larvae and juveniles move upward during flood tides to faster currents and downward during ebb tides.
- Postflexion larvae and juveniles prefer the bottom and show low temperature responses.
- Mid-layer abundance is higher at night and during flood tides, aiding their estuarine ingress.

## Abstract

For successful lifecycle completion, some fish species must experience an inshore migration stage, gradually migrating from the open sea to the surf area to find a more suitable habitat during their early life stages. The Yangtze River estuary has always been an important nursery ground for the small yellow croaker, although recruitment into the estuarine nursery area is challenging, as there is a net flow of water from the estuary to the ocean and current speeds often exceed larval swimming speeds. Larvae and juveniles of the small yellow croaker recruiting into the estuary must therefore adopt strategies for successful ingress into the estuarine nursery ground. For this reason, a fixed station was used to examine the vertical distribution and explore the transport pattern of the larval small yellow croaker in the Yangtze River estuary. During every cruise, the highest and lowest abundances occurred at the bottom and on the surface, respectively. The larval small yellow croaker moved up to the surface and middle at night, with its abundance varying with periodic currents. The findings indicate that the larvae move up to the water strata in which the current speed is faster during flood tides and move down to the deeper water in which the current speed is slower during ebb tides. Depending on this pattern, the larvae and juveniles of the small yellow croaker can ingress into the Yangtze River estuary nursery ground.

In order to understand the diel vertical migration and transport pattern during their early life stages, the vertical distribution of larvae and juveniles of the small yellow croaker (Larimichthys polyactis) in the Yangtze River estuary was investigated. Four sampling cycles were carried out at the station (122°39′ E, 31° N) during May and June 2015. The samples were collected by vertically discrete plankton hauls in three strata of the water column: surface (0~2 m depth), middle (8~10 m depth), and bottom (18~20 m depth). The total number of small yellow croaker larvae was 5523, of which 65.2% and 25.8% were postflexion larvae and juveniles, respectively. Most of the individuals were collected in mid-May, and the sum of the quantity in May accounted for 94.8%. In all cruises, the highest abundance occurred at the bottom, while the surface maintained the lowest abundance. The flexion larvae, postflexion larvae and juveniles moved up to the surface and middle at night, but there was no significant difference between night and daytime for preflexion larvae in each stratum. The postflexion larvae and juveniles showed low temperature responses and a preference for the bottom habitat, respectively, while the preflexion and flexion larvae had weaker selectivity to the water layer and occurrence time. The abundance of larvae and juveniles in the middle layer was significantly higher at flood tide than at ebb tide, and it was more dominant at night than during the day. The study indicates that larvae and juveniles of the small yellow croaker change their located depth with diel vertical migration and utilize the faster speed of the mid-upper layer, which contributes to their ingress into the Yangtze River estuary nursery ground.