144 
Fishery Bulletin 115(2) 
(Morone saxatilis), and weakfish (Cynoscion regalis). 
In estuaries, these 3 species are predators of fish and 
invertebrate species in varying proportions depending 
on season and availability (Lankford and Targett, 1994; 
Buckel and Conover, 1997; Collette and Klein-MacPhee, 
2002; Nemerson and Able, 2004; Rudershausen et al., 
2005; Ferry and Mather, 2012). They are frequently 
sympatric and are competitors for the same prey (Hart¬ 
man and Brandt, 1995; Uphoff, 2003). In a study in 
which gill nets were used in the Navesink River in 
1998 and 1999 (Scharf et ah, 2004), abundance of blue- 
fish was greatest at a station in the upper river in the 
Red Bank basin and was significantly correlated with 
areas of fine sediment. 
From May through October in 2006 and 2007, Man- 
derson et al. (2014) conducted a weekly hydrograph¬ 
ic study of the Navesink River from river kilometers 
1-12. On 12 of those weeks, they also conducted a hy¬ 
drographic study limited to the area around the sa¬ 
linity front in the upper or western end of the river, 
together with fish collection and diet analysis. Con¬ 
currently, acoustically tagged bluefish (age 0 and age 
1+), weakfish, and striped bass were monitored by us¬ 
ing receivers throughout the river to determine days 
of continuous occupation and movements by individu¬ 
als of these 3 species (Manderson et al., 2014). Medi¬ 
an residence times in 2007 were 8 d for striped bass, 
29 d for age-l-i- bluefish, 29 d for age-0 bluefish, and 
47 d for weakfish. Manderson et al. (2014) concluded 
that the seasonal residencies of these predators in 
the Navesink River were affected by 2 direct factors: 
variation in day length and temperature. Freshwater 
discharge also affected predator residence times indi¬ 
rectly, possibly through prey availability (Manderson 
et al., 2014). 
On the basis of the analyses reported in Manderson 
et al. (2014), we hypothesized that, when freshwater 
discharge was moderate to high, biophysical mecha¬ 
nisms supporting the salinity transition zone and con¬ 
centrating food resources would be maintained and 
predators would chiefly reside there. We hypothesized 
that, when discharge was low, the salinity transition 
zone would be disrupted, resulting in a reduction in 
food resources and emigration of predators from the 
zone or the entire estuary. Manderson et al. (2014) 
did not examine evidence extensively for testing this 
hypothesis, including examining within-estuary move¬ 
ments of predators, occurrence and distributions of 
prey, and diets, or the potential relationship of the 
biota to hydrographic features in the estuary. 
The objective for this study was to test the hypoth¬ 
esis that striped bass, bluefish, and weakfish are more 
abundant in the vicinity of the main salinity transition 
zone or front of the Navesink River than away from 
it. We used data from hydrographic surveys, gill net 
collections, predator diets, and telemetry to evaluate 
the available evidence that supports or disproves our 
hypothesis. The telemetry data were used to generate 
daily and composite home ranges of the 3 predator spe¬ 
cies in this river. 
Materials and methods 
Study area 
The Navesink River is approximately 12 km long, <1.5 
km wide, and around 10 km^ in area (Fig. 1); it flows 
eastward into the Shrewsbury River, then north to 
Sandy Hook Bay and Raritan Bay. It is a flood-domi¬ 
nated estuary, with a 1.4-m average tidal range (Chant 
and Stoner, 2001). The salinity front is near this upper 
or western end of the river (Fugate and Chant, 2005), 
and practical salinity ranges from ~1 in the upper 
Navesink River during spring freshets to ~28 at the 
mouth of this river (senior author and J. Manderson, 
unpubl. data). To the west, the Swimming River is the 
primary freshwater source. The upper river depth for 
the Navesink River averages ~2 m at high tide, and 
substrates are flne sand and silt with high organic con¬ 
tent (Chant and Stoner, 2001; Stoner et al., 2001; Meise 
and Stehlik, 2003). The lower Navesink River is char¬ 
acterized by shallow sandbars and channels (depths 
up to 4 m). High-velocity tidal currents and coarse to 
medium sands are found in the channels. Shallows 
and embayments in the summer and fall are vegetated 
with sea lettuce (Ulva lactuca) and other macroalgae. 
For our studies, we designated the confluence of the 
Navesink and the Shrewsbury rivers as river kilometer 
0 . 
Hydrographic measurements and station locations 
Weekly hydrographic surveys were made the length of 
the river from April through October, in 2007, along 
transects that intersected with an array of ultrasonic 
receivers (see Telemetry section). An SBE 25 Sealogger^ 
conductivity, temperature, and depth recorder (Sea-Bird 
Electronics Inc., Bellevue, WA) was cast at the location 
of each receiver, to measure temperature, conductivity, 
pressure, dissolved oxygen, photosynthetically active 
radiation, turbidity, and concentration of chlorophyll- 
a. Similar work was completed in 2006 with the same 
equipment and methods (see Manderson et al.^). 
Hydrographic mapping and gill net sampling at the 
upstream end of the Navesink River, near the salinity 
front, were conducted during 12 weeks in May through 
October 2007. Hydrographic mapping took place twice 
a day during daylight hours at the end of flood and 
at the end of ebb tides, once a week on 4 consecutive 
weeks during each of 3 periods: spring (May), summer 
(late July-early August), and fall (late September-ear¬ 
ly October). We integrated data from a global position¬ 
ing system (GPS), the SBE 25 Sealogger, and a Hy¬ 
drolab datasonde (OTT Hydromet, Kempten, Germany) 
that measured temperature and salinity at 1-s inter¬ 
vals 0.5 m below the surface. After the salinity front 
was located by using the SBE 25 Sealogger, the site of 
^ Mention of trade names or commercial companies is for iden¬ 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
