414 
Fishery Bulletin 108(4) 
departures from the estuary occurred when bottom 
temperatures were >12°C. In 2008, 18 of the 23 (78%) 
winter flounder tagged within the estuary migrated to 
coastal waters over a three week period from 30 May to 
19 June (Fig. 6). This emigration coincided with a sharp 
increase in water temperature throughout the estuary. 
As of 19 June 2008, water temperatures exceeded 15°C 
in all but the deepest portion of the estuary. During 
July and August of 2008 when temperatures reached 
their seasonal maxima, no winter flounder were detect- 
ed in the upper reaches of the estuary, and six winter 
flounder were detected at the mouth of the estuary, 
where temperatures were the coolest. In July 2009, only 
one winter flounder was detected in the estuary, for a 
period of one day. 
Our observations on the autumn distribution of win- 
ter flounder in the Plymouth estuary were limited to 
2008. Results indicated that five tagged individuals re- 
turned to the estuary as water temperatures decreased 
below their seasonal maxima. 
For tagged winter flounder that were detected in the 
estuary, residence times within the estuary ranged from 
2 to 91 days (mean=29 days). The estuarine residence 
times of fish tagged in Plymouth Bay (mean =40 days) 
were significantly greater than the residence times for 
winter flounder tagged in the estuary (mean=22 days) 
(single-factor ANOVA; P=0.025). No re- 
lationship was found between fish size 
and estuarine residence time (P=0.88). 
For winter flounder tagged within the 
Plymouth estuary in 2008, the observed 
residence time is almost certainly an 
underestimate because it is unknown 
how long these fish were present in the 
estuary before being tagged. 
Detections of tagged winter floun- 
der were not uniformly distributed 
throughout the study site. In the first 
year of the study, the inner gate of 
acoustic receivers typically detected 
more flounder and had more detections 
than receivers located within the estu- 
ary (Fig. 7, A and B). Tagged winter 
flounder were detected throughout the 
estuary, and seven of the nine receiv- 
ers inside the estuary detected at least 
one tagged flounder. In the second year 
of the study, tagged winter flounder 
were most commonly detected in the 
coastal waters of Plymouth Bay. The 
outer gate of receivers in Plymouth Bay 
typically detected the greatest number 
of winter flounder and had the larg- 
est number of detections (Fig. 7, C 
and D). Two tagged winter flounder 
(Tags 53876 and 53886) remained at 
the inner gate for long periods of time 
within the detection radii of receivers 
G and K, which accounted for the large 
number of detections by these receiv- 
ers (Fig. 7D). In 2009, tagged winter 
flounder were detected at only three of 
the 12 receivers inside the estuary, and 
the low number of detections (<1000 ) 
at these three receivers indicates that 
residence within the estuary was brief. 
The bimodal distribution of receiver 
detections (e.g., Fig. 70 supports the 
inference of divergent habitat use, 
spawning behaviors, and contingent 
structure. 
Two tagged winter flounder were 
detected at receivers used by other 
Total number of detections per day 
Vi 6 X? X? X? aC? X? aC? ^ x? ^ x? x? ^ 
T> T> 1 ' " vv -->■ 
Number of fish detected per day 
■o 
nj 
2 ^ XS° a<5° VT aO 3 n X5° XT’ r S> J .XT aO 
Percentage of receivers with detections, per day 
Figure 4 
Temporal patterns of winter flounder ( Pseudopleuronectes americanus ) 
detections obtained by using passive acoustic telemetry in the Plym- 
outh Estuary and Plymouth Bay, MA, between December 2008 and July 
2009. (A) The total number of detections each day. (B) The number of 
tagged winter flounder that were detected each day. (C) The percentage 
of acoustic receivers that detected a tagged winter flounder during each 
day of the study. 
