Date of Award

Spring 1995

Project Type

Dissertation

Program or Major

Zoology

Degree Name

Doctor of Philosophy

First Advisor

W Huntting Howell

Abstract

Smooth flounder, Pleuronectes putnami, and winter flounder, Pleuronectes americanus, co-occur in estuaries along the east coast of North America from Labrador to Massachusetts. Results of a three year sampling program indicated that the two species were partially segregated along salinity and depth gradients in upper Great Bay Estuary, New Hampshire. Along the salinity gradient, smooth flounder were most abundant at the oligo-mesohaline riverine habitat while winter flounder were most abundant at the meso-polyhaline open bay habitat. Both species exhibited a generalized up-river movement with seasonally increasing salinity. Smooth flounder showed ontogenetic changes in distribution along the depth gradient, with the smallest individuals occupying the shallowest depths. Intertidal mudflats were an important nursery area for young-of-year smooth flounder but not for young-of-year winter flounder.

Laboratory and field experiments demonstrated that the distribution of smooth and winter flounder in Great Bay Estuary was based primarily, although not completely, on physiological constraints related to salinity. For smooth flounder, growth and survival were best in 12$\perthous$ and 22$\perthous$, while for winter flounder, growth and survival were best at 22$\perthous$ and 32$\perthous$. Both flounder species occupied sites along the salinity gradient that were metabolically least costly.

Seasonal changes in resource use were examined at several estuarine sites. Smooth and winter flounder had similar diets and showed greater overlap in diet than in habitat use. Important prey types included the polychaetes Streblospio benedicti and Scolecolepides viridis, siphons of the bivalve Macoma balthica, and gammarid amphipods. Prey abundance changed seasonally and appeared to play a role in affecting the distribution of smooth and winter flounder. Differences in diet among estuarine sites reflected differences in the benthic fauna at these sites. Even though their diets were similar, there were no consistent patterns in niche metrics to suggest that food was limiting to smooth and winter flounder in upper Great Bay Estuary.

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