MEPS 325:255-266 (2006)  -  doi:10.3354/meps325255

ABSTRACT: Growth and feeding rates were measured in juvenile summer flounder Paralichthys dentatus and winter flounder Pseudopleuronectes americanus exposed to sub-lethal hypoxia (low dissolved oxygen, DO) over a range of temperatures, to determine its potential effects on nursery habitat quality for these 2 estuary-dependent flatfishes. Growth rates of both species were generally reduced as DO decreased, particularly at DO levels of 50 to 70% air saturation, and as temperature increased. Summer flounder were more tolerant of low DO than were winter flounder at both 20 and 25°C. At these temperatures, summer flounder growth was reduced by ~25% (compared to growth at normoxia [7.0 mg O₂ l^–1]) at 3.5 mg O₂ l^–1 and by 50 to 60% at 2.0 mg O₂ l^–1. In contrast, growth of winter flounder at 20°C was reduced by ~50% at both 3.5 and 5.0 mg O₂ l^–1, and growth was zero at 2.0 mg O₂ l^–1. At 25°C, winter flounder grew poorly in all DO treatments and lost weight at 2.0 mg O₂ l^–1. Summer flounder were also tested at 30°C. Growth was significantly reduced even at 5.0 mg O₂ l^–1, and was reduced by ~90% at 2.0 mg O₂ l^–1. A significant relationship between feeding rate and growth suggested reduced consumption to be a major cause of growth limitation under hypoxia. There was no evidence of growth acclimation for either species after 7 to 14 d exposure to hypoxia. The effect of hypoxia on growth of summer flounder was reduced at lower salinity (15 vs. 25‰) and was unaffected by the presence of a sand substrate. Similarity between modeled growth under hypoxic conditions, based on our laboratory results, and observed growth of summer flounder in a hypoxic estuarine tributary suggests growth limitation in the wild. These laboratory and field results demonstrate that even moderate hypoxia can adversely affect growth rates, and thus the quality of estuarine nursery habitats for juvenile flatfishes.

KEY WORDS: Hypoxia · Summer flounder · Winter flounder · Flatfish · Essential fish habitat · Nursery habitat · Growth limitation