Armstrong: Distribution and abundance of Pleuronectes putnami and Pleuronectes americanus 
42! 
50 - 
40 
30 - 
20 - 
1 o — 
May 
50 — 
40 - 
30 - 
June 
1 o - 

50 - 
40 - 
30 - 
July 
20 - 
> 1 o - 
c 
0 ° 
<D = ° - 
LL -40 
3 O - 
20 - 
1 O 
August 
50 
-40 
30 - 
20 - 
-i o - 
September 
50 - 
-40 - 
October 
30 - 
20 - 
1 o - 
bAu. . 
O 50 1 OO 1 50 200 250 300 
Total length (mm) 
Figure 5 
Monthly length frequencies (5-mm size classes) of smooth 
flounder, Pleuronectes putnami , at site 1, pooled over 1989-91. 
choose habitats that are of least cost bioenergetically. 
Several estuarine fish species have been found to be 
most abundant along a salinity gradient where their 
metabolic costs of osmoregulation were minimal, in- 
cluding Ambassis spp. (Martin, 1990), Leiostomus 
xanthurus and Micropogonias undulatus (Moser and 
Gerry, 1989), and Paralichthys spp. (Peters, 1971). 
Conversely, Peters and Boyd (1972) found that 
hogchokers, Trinectes maculatus, underwent move- 
ments that appeared physiologically disadvanta- 
geous. They concluded that other factors, in addi- 
tion to salinity, must be considered. Salinity may 
provide a broad abiotic framework (Menge and 
Olson, 1990) within which biotic interactions, such 
as competition, predation, and prey abundance, 
can act to modify distributions. 
Depth gradient 
Smooth flounder showed clear segregation by size 
along the depth gradient. Larger (>100 mm TL) 
smooth flounder occurred primarily at the deep- 
water station (site 3). They were abundant only 
during April-June, before migrating upriver as 
salinity increased. Small numbers remained at site 
4 throughout the summer and autumn. The tidal 
flats (site 5) and shallow bay (site 4) were impor- 
tant nursery areas for smooth flounder. Young-of- 
the-year smooth flounder did not show a dramatic 
decrease in abundance during the summer, as seen 
in the larger individuals, and did not appear to 
make a pronounced seasonal up-estuary move- 
ment. Their inferior swimming ability, compared 
with that of larger individuals, or their inability 
to osmoregulate efficiently in lower salinity areas 
may underlie their relatively stationary habits. 
The tendency for smooth flounder to segregate by 
size, with the smaller individuals occurring in the 
intertidal and shallow subtidal areas, has been 
found in several other flatfish species including 
English sole, Parophrys vetulus (Toole, 1980), and 
European plaice, Pleuronectes platessa (Gibson, 
1973; Kuipers, 1973). Segregation by size may re- 
duce intraspecific competition. The intertidal zone 
may also function as a refuge from predators for 
small flatfish or as an abundant source of appro- 
priate-size prey items (Toole, 1980). Ruiz et al. 
(1993) found that shallow water functioned as a 
refuge from size-selective predation on juveniles 
of several species of fish and crustaceans in Chesa- 
peake Bay. Van der Veer and Bergmann (1986) 
found that young-of-the-year European plaice used 
tidal flats as a refuge from predators rather than 
for feeding purposes. Potential predators on 
smooth flounder in Great Bay Estuary include 
sand shrimp ( Crangon septemspinosus), grubbies 
(Myoxocephalus aeneus), bluefish (Pomatomus 
saltatrix ), striped bass (Morone saxatilis), white perch 
( Morone americanus), great blue heron ( Ardea 
herodias), and double-crested cormorants ( Phala - 
crocorax auritus ). Predation by large piscine predators 
is probably reduced in shallow water, and avian preda- 
tion is likely increased. Sand shrimp were abundant in 
trawl samples from both channel and flats areas. The 
value of tidal flats as refugia from predation cannot be 
assessed without knowledge of the relative rates of pre- 
dation by these different predatory groups. 
