Juvenile weakfish Cynoscion regalis distribution in relation to diel-cycling dissolved oxygen in an estuarine tributary

Abstract

Shallow estuarine waters that serve as nurseries for fishes along the Atlantic and Gulf coasts of the USA can undergo wide diel dissolved oxygen (DO) fluctuations (<2 to ~20 mg O2 l–1) during summer. In this study, the distribution of juvenile weakfish Cynoscion regalis was investigated in relation to diel-cycling DO during summer 2001 in a mesohaline tributary of Indian River Bay, Delaware, USA. Weakfish were collected at 3 sites (upper, middle, and lower) along the ~5 km length of Pepper Creek on 15 d using an otter trawl. Near-bottom DO was monitored continuously over the summer, every 15 min, using multi-parameter sondes. Peak abundance of weakfish coincided with the greatest frequency, intensity, and spatial extent of severe diel-cycling hypoxia events (<2 mg O2 l–1). Severe hypoxia first occurred in early June in the upper creek and recurred there almost daily for periods of 1 to 4 h until early September. Whenever bottom DO was >2.0 mg O2 l–1, weakfish were more abundant at the upper site than at the middle and lower sites, which also experienced severe hypoxia but at much lower frequency. However, under all environmental conditions they were absent from the upper site whenever bottom DO was <2 mg O2 l–1, and returned within 2 h of DO exceeding 2 mg O2 l–1. Daily up- and down-creek movement occurred over a distance of ~1 km. These findings indicate an avoidance threshold of ~2.0 mg O2 l–1 for juvenile weakfish and demonstrate very temporally dynamic DO-related movement. Their rapid return to these areas as DO conditions improve, and relatively high density in tidal tributary headwaters, suggests that these relatively small areas provide important habitat for fishes.