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Fishery Bulletin 108(4) 
also migrate to deeper waters during the winter to 
prevent their blood serum from freezing, which occurs 
between -1.25 and -1.38°C (Fletcher, 1977). Tempera- 
tures in the Plymouth estuary dropped to as low as 
-1.55°C during the winter of 2008, whereas the mini- 
mum recorded temperature in Plymouth Bay was 1.2°C. 
Therefore, the deeper waters of Plymouth Bay may act 
as a refuge from the extreme cold temperatures and 
ice formation that are present in the Plymouth estuary 
during the winter. 
Tagged winter flounder were most commonly present 
in the estuary during the late spring, when water tem- 
peratures ranged from 9° to 16°C, which is within the 
preferred temperature range (12-15°C) of this species 
(McCracken, 1963; Reynolds, 1977). Although winter 
flounder were common in the estuary during the late 
spring, the majority of the tagged winter flounder 
remained in coastal waters during this period. Howe 
and Coates (1975) also reported that winter flounder 
are concentrated in shoal areas during the spring 
months. Although we were unable to capture adult 
winter flounder in the estuary from March through 
early May 2008, we cannot conclude that winter floun- 
der were not present in the estuary during this time. 
Winter flounder in spawning condition often cease 
feeding or reduce their food intake (NMFS, 1999) and 
therefore it is less likely that we would have captured 
flounder using hook-and-line gear during the spawn- 
ing season. In addition, flounder that were buried in 
the sediment may not have been available to the small 
trawl net that we were using to capture flounder in 
the estuary. 
The emigration of winter flounder from the estu- 
ary coincided with rising temperatures at the onset 
of summer. Between 30 May and 19 June 2008 the 
majority of tagged winter flounder emigrated from 
the estuary as water temperature increased from 10° 
to 15°C (Fig. 6). These observations were consistent 
with previous reports that revealed that winter floun- 
der will migrate to cooler waters when temperatures 
exceed 15°C (Bigelow and Schroeder, 1953; McCracken, 
1963). Perlmutter (1947) found that the movements of 
winter flounder north of Cape Cod are typically local- 
ized, and Howe and Coates (1975) calculated that the 
average displacement of winter flounder tagged in 
Plymouth Bay was only 3 km. They also found that in 
the Gulf of Maine, winter flounder could find suitable 
temperatures (between 12° and 17°C) within 2 km 
of the shore zone during the summer. Weekly aver- 
aged temperatures in Plymouth Bay only exceeded 
15°C twice in 2008, and did not exceed 15°C in 2009. 
Therefore, winter flounder leaving the Plymouth estu- 
ary were not forced to migrate long distances to reach 
suitable temperatures. 
During the autumn months of 2008 five tagged indi- 
viduals returned to the estuary as water temperatures 
decreased below their seasonal maxima. Bigelow and 
Schroeder (1953) also noted that winter flounder will 
return to the Plymouth estuary during the autumn 
months when temperatures start to cool. 
Conclusions 
By collecting large amounts of high-resolution data, we 
are able to offer insight into the fine-scale behavior and 
distribution of winter flounder in the southern Gulf of 
Maine. Our results indicate that winter flounder exhibit 
divergent spawning behaviors which support inferences 
of contingent structure and that coastal waters may 
provide essential spawning grounds for this species. As 
such, coastal waters should merit consideration in the 
assignment of Essential Fish Habitat for this species in 
the Gulf of Maine. In this region of the Gulf of Maine, 
estuarine residence appears to be brief, and dependent 
upon water temperature. 
Acknowledgments 
The Massachusetts Marine Fisheries Institute provided 
the funding for this research. T. Grothues and two 
anonymous reviewers provided valuable comments on 
this manuscript. We would also like to thank M. Mather, 
G. Skomal, and G. Cowles for their guidance and advice 
during this experiment. We thank J. Manderson and M. 
Armstrong for allowing us to borrow acoustic receivers. 
We would also like to thank V. Malkoski, V. Manfredi, 
P. Milligan, B. Courchene, H. Bourbon, C. Sarro, and 
M. Marino for assistance in the field. Finally, we would 
like to thank the dozens of other people whose help at 
sea made this project possible. 
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