672 
Fishery Bulletin 95(4), 1997 
Clupeids and bay anchovy were also important prey 
items of YOY blue fish captured in the gill net. Diets 
of YOY bluefish captured in the surface trawl on 15- 
16 July 1993 were dominated by bay anchovy (56 %F, 
52 %W) and striped bass (7 %F, 20%W) (Table 3). 
Discussion 
Diei movements 
We found large differences in the CPUE of bluefish 
with the diel cycle in beach-seine, gill-net, and sur- 
face trawl collections. There are several mechanisms 
2 
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Date 
V 
A Beach seine (spring-spawned) 
H Beach seine (summer-spawned) 
® Surface trawl (spring-spawned) 
Figure 5 
Estimates of consumption rates versus date 
for spring-spawned (triangles) and summer- 
spawned (squares) bluefish, Pomatomus 
saltatrix, captured by beach seines in 1992 
(A) and for spring-spawned bluefish cap- 
tured by beach seine (triangles) and surface 
trawling (circles) in 1993 (B). Bars repre- 
sent 95% confidence intervals. 
that could account for these patterns. Rountree and 
Able (1993) distinguished two types of diel sampling 
bias: 1) direct avoidance of the gear or 2) a change in 
fish behavior. They further divided the second bias 
into diel movement between habitats (into or away 
from the gear sampling area) and diel changes in 
local activity (e.g. foraging). 
Catch-per-unit-of-effort of YOY bluefish (both 
spring- and summer-spawned cohorts) was higher 
during day beach-seine collections than during night 
collections in both 1992 and 1993 (Fig. 2, A-B). Fish 
would more likely detect and avoid beach-seine gear 
during the day than at night. Additionally, we used 
a boat to set the seine, which helped to standardize 
set time so that there were probably limited avoid- 
ance biases between diurnal and nocturnal collec- 
tions due to “operator” efficiency. We therefore rule 
out direct avoidance of the gear (bias one) and ac- 
cept a diel behavioral change (bias two) as an expla- 
nation for low night CPUE. 
The surface trawl CPUE in 1993 was also highest 
during daylight hours (Fig. 2D). Because the pattern 
of CPUE in 1993 was not that expected if fish were 
visually avoiding the gear (bias one), we propose that 
a behavioral change that increases the susceptibil- 
ity of bluefish to the surface trawl gear during the 
day is most likely responsible for the pattern. 
Bluefish CPUE with the gill net was highest at 
sunset and night sets in 1993 (Fig. 2C). The pattern 
of gill-net CPUE was the opposite of what we saw 
with the 1993 beach-seine and surface trawl CPUE 
data. During the gill-net and beach-seine “movement 
collection” on 11-12 August 1993, beach-seine catches 
were higher an hour before sunset than an hour af- 
ter (Fig. 2E). The opposite pattern was seen at sun- 
rise. On this date, gill-net catches were low during 
the day and increased to a midnight peak before de- 
clining to zero after sunrise (Fig. 2E). We attempted 
to determine the direction of bluefish movement from 
the orientation of individual bluefish in the gill net; 
however, data were inconclusive. 
According to beach-seine, surface trawl, and gill- 
net collections, bluefish occupy nearshore and sur- 
face waters during the day and then move offshore 
and below surface waters at night. Although we can- 
not rule out avoidance of gill-net gear (bias one) as a 
possible explanation of low day gill-net CPUE’s, con- 
comitant declines in beach-seine CPUE of bluefish 
in nearshore areas suggest that increased gill-net 
catches are at least partly a result of bluefish mov- 
ing offshore. Support for our findings comes from field 
collections in other estuaries. Pristas and Trent 
(1977) found significantly higher catches of adult 
bluefish at night with monofilament and multifila- 
ment gill nets in shallow-water (0.7-1. 1 m), mid- 
