Armstrong: Distribution and abundance of Pleuronectes putnami and Pleuronectes americanus 
425 
( 1966) found that winter flounder, acclimated to 21°C, 
had an upper lethal temperature of 27°C. Pearcy 
(1962) found an upper lethal temperature of 30°C 
for flounder collected during the summer in Mystic 
River Estuary. Olla et al. (1969) observed that win- 
ter flounder exposed to temperatures above 22.2°C. 
buried themselves in sediment and ceased to feed. 
Although comparable data do not exist for smooth 
flounder, Huntsman and Sparks ( 1924) reported that 
upper lethal temperatures for smooth flounder were 
2-4°C higher than those for winter flounder. In Great 
Bay Estuary, temperature may be a factor in deter- 
mining the relative distribution of the two species in 
late summer when water temperatures at site 5 
reached 22-24. 2°C but would not be a factor during 
most of the year. The low abundance of winter floun- 
der at site 5 persisted during times of the year when 
temperature would not seem to be limiting. 
Subtrate preference may play a role 
in excluding winter flounder from inter- 
tidal flats in Great Bay Estuary. Al- 
though the bottom type appeared simi- 
lar (silty mud) at all three sites along 
the depth gradient (Table 1), this simi- 
larity was based on gross examination 
of core samples. No detailed sediment 
size analysis was conducted for this 
study (nor in Fried [1973] or Targett and 
McCleave [1974]), and therefore differ- 
ences in sediment structure may have 
been present between sites but not 
noted on a gross scale. Sogard (1992) 
found that growth of winter flounder 
was negatively correlated with percent 
silt; faster growth occurred in sandier 
sediments. Bigelow and Schroeder 
( 1953) found that winter flounder were 
more abundant on coarser sediments, in 
comparison with smooth flounder which 
were more abundant in muddier sedi- 
ments. Thus, if the channel areas of 
Great Bay Estuary have coarser sedi- 
ments than the intertidal flats, perhaps 
the coarser sediment may explain the 
difference in distribution along the 
depth gradient. 
Summary 
Smooth and winter flounder are par- 
tially segregated as species along salin- 
ity and depth gradients in upper Great 
Bay Estuary. It appears that this is due 
to differential responses to the physical 
and chemical regime, but the effects of 
seasonal changes in biotic interactions 
cannot be excluded. Smooth and winter 
flounder feed on similar prey items in 
Great Bay Estuary (Laszlo, 1972; 
Armstrong, 1995). Competition or move- 
ments related to prey abundance may 
influence their respective distributions. 
There are many instances where com- 
0 ) 
-Q 
£ 
Sampling date 
Figure 9 
Mean number of smooth flounder, Pleuronectes putnami , caught per ten 
minute tow at three sites along a depth gradient in Great Bay Estuary, 
New Hampshire. Mean depth was greatest at site 3 (mean=6.2 m) fol- 
lowed by site 4 (2.1 m) and then site 5 ( 1.5 m). Error bars are one standard 
error of the mean. 
