414 
Seasonal and ontogenetic 
changes in distribution and 
abundance of smooth flounder, 
Pleuronectes putnami, and 
winter flounder, Pleuronectes 
americanus, along estuarine depth 
and salinity gradients 
Michael R Armstrong 
Department of Zoology, University of New Hampshire 
Durham, New Hampshire 03824 
Present address: Massachusetts Division of Marine Fisheries 
Annisquam River Marine Fisheries Station 
30 Emerson Avenue, Gloucester, Massachusetts 01930 
E-mail address: michael.armstrong@state.ma.us 
Abstract .—The distribution and 
abundance of two potentially compet- 
ing flatfish species, smooth flounder, 
Pleuronectes putnami, and winter 
flounder, Pleuronectes americanus , 
were examined along salinity and depth 
gradients in upper Great Bay Estuary, 
New Hampshire. Both species were 
abundant in the estuary but exhibited 
differential use of habitats along both 
gradients. Smooth flounder were most 
abundant at the mesohaline, riverine 
habitat, whereas winter flounder were 
most abundant at the polyhaline, open- 
bay habitat. Both species exhibited a 
generalized up-river movement as sa- 
linity increased with the seasons. 
Smooth flounder showed ontogenetic 
changes in distribution along the depth 
gradient, with smallest individuals oc- 
cupying shallowest depths. Intertidal 
mudflats were an important nursery 
area for young-of-the-year smooth 
flounder. Winter flounder showed little 
separation by size along the depth gra- 
dient, and few were found in the inter- 
tidal mudflat habitat. The potential for 
competition between these two species 
is lessened by their partial segregation 
along the gradients examined. 
Manuscript accepted 14 January 1997. 
Fishery Bulletin 95:414-430 (1997). 
Smooth flounder, Pleuronectes 
putnami , and winter flounder, 
Pleuronectes americanus, are domi- 
nant members of fish communities 
in estuaries along the east coast of 
North America, co-occurring from 
Newfoundland, Canada, to Massa- 
chusetts Bay, USA. These morpho- 
logically similar species are sympa- 
tric over much of their geographic 
ranges. However, little is known of 
their spatial overlap within specific 
estuaries. Winter flounder use estu- 
aries primarily as nursery grounds, 
whereas adults spend most of their 
lives in coastal waters (Bigelow and 
Schroeder, 1953; Pearcy, 1962; Scott 
and Scott, 1988). In contrast, smooth 
flounder complete their entire life 
cycle within estuaries. 
Smooth flounder prefer softer bot- 
tom substrata than winter flounder 
(Bigelow and Schroeder, 1953), and 
Jackson (1922) noted they were 
most abundant in the low-salinity 
regions within Great Bay Estuary, 
New Hampshire. Little else is 
known of their intraestuarine habi- 
tat preferences. Several studies 
have examined movements and 
habitat use of juvenile winter floun- 
der in estuaries south of Cape Cod 
(e.g. Pearcy, 1962; Saucerman 
1991). However, because many 
northern estuaries differ consider- 
ably from those south of Cape Cod, 
most obviously in their temperature 
regimes, it is possible that juvenile 
winter flounder use northern estu- 
aries differently from ones to the 
south, as has been shown to be the 
case for adults (Hanson and Cour- 
tenay, 1996). 
The purpose of this study was to 
provide a quantitative comparison 
of the occurrence of smooth and win- 
ter flounder in various habitats in 
upper Great Bay Estuary, New 
Hampshire. The habitats comprised 
gradients defined by depth or salin- 
ity. Comparative studies along habi- 
tat gradients can define which habi- 
tats are important to a species, es- 
pecially in relation to different life 
history stages; such analyses can 
also be used to study the relative 
importance of physical and biotic 
factors in limiting species distribu- 
tions (Connor and Bowers, 1987). 
Examination of the shape of species- 
abundance curves along a gradient 
can provide inferences into whether 
competition or physiological limita- 
tions are important in setting dis- 
