Stehlik et al.; Distribution of 3 predatory fish at a salinity front in a small estuary 
147 
Table 1 
Total catch of striped bass (Morone saxatilis), bluefish {Pomatomus saltatrix), and weakfish {Cyjioscion 
regalis) by season, age class, and station (located at a channel [B] or salinity transition front [A]) for gill 
net sampling conducted in the Navesink River, New Jersey, in 2007. 
Species and 
age class 
Spring 
Channel 
Spring 
Front 
Summer 
Channel 
Summer 
Front 
Fall 
Channel 
Fall 
Front 
Age-1+ striped bass 
1 
6 
0 
0 
0 
0 
Age-0 bluefish 
0 
0 
46 
86 
147 
389 
Age-1+ bluefish 
1 
5 
21 
0 
3 
0 
Age-0 weakfish 
0 
0 
94 
4 
261 
52 
Age-1+ weakfish 
88 
3 
53 
7 
31 
7 
then released randomly throughout the river. In 2006, 
34 age-l+ striped bass (359-630 mm TL), 14 age-l+ 
bluefish (310-390 mm TL), 15 age-0 bluefish (175-270 
mm TL), and 15 age-l+ weakfish (224-535 mm TL) 
were released. In 2007, 12 age-l-i- striped bass (342- 
510 mm TL), 21 age-l+ bluefish (310-610 mm TL), 30 
age-0 bluefish (222-275 mm TL), and 27 age-l-i- weak¬ 
fish (304-480 mm TL) were released. 
The home range of an animal, where it spends 95% 
of its time during normal activities, was calculated 
for each species by using the “utilization distribution” 
method (Anderson, 1982; Tolimieri et al., 2009). Those 
fish that were ultrasonically detected 3 or more times 
on a given day and that were detected on a minimum 
of 6 days were included in our analysis. The adeha- 
bitatHR package (Calenge, 2006) in R, vers. 2.13.1 (R 
Core Team, 2011) was used to perform the analysis on 
the telemetry records. Signals were binned in 10-min 
intervals. Records were censored in instances when 
signals from more than one fish overlapped in a time 
bin. Because the Navesink River is relatively narrow 
in relation to the detection range of the receivers used 
in this study, mean daily positions in universal trans¬ 
verse coordinates were converted to distances upriver 
(in meters). Home ranges were generated for individu¬ 
al fish by using an analysis grid of squares with sides 
100 mxlOO m, limited to areas within the coastline 
boundary. Composite grids for all data from each spe¬ 
cies were then generated and plotted for each species, 
age class (in bluefish), and year. 
Results 
Hydrography 
When freshwater discharge was high, a well-defined sa¬ 
linity gradient was established in the upper Navesink 
River. At end of flood tide, this gradient was located at 
approximately river kilometers 10-11 between Jones 
Point and just east of the basin off Red Bank (Fig. 1). 
Usually, the salinity front shifted 0.5-1.5 km down¬ 
stream with ebb tide. In 2007, freshwater discharge 
was high although variable in July and August, and 
discharge was low in September and October (Mander- 
son et al., 2014), leading to a fully mixed salinity state 
in the river in fall. 
In a hydrographic profile of the upper portion of the 
Navesink River on 2 August 2007, during a period of 
high freshwater discharge, the salinity gradient was 
from near 17 at the surface to >22 near the channel 
bottom, at both tides (Fig. 2). The salinity front at high 
tide was located at the steepest vertical salinity gradi¬ 
ent at approximately river kilometer 11. The surface 
layer in the upper river at that time was 25.5°C and 
contained concentrations of chlorophyll-a >20 pg/L, at 
approximately river kilometers 7-11. West of the front 
near the surface was a zone of high turbidity, an area 
that typically extended into the Swimming River. 
In contrast, at a time of low discharge in late Sep¬ 
tember and October 2007, the estuary was well mixed 
and hydrographic profiles were much more uniform. No 
clearly delineated front was observed. The differences 
between the units of the contours of salinity and chlo¬ 
rophyll-a concentration in the upper river profiles were 
one-tenth the magnitude of the differences between the 
units in August, and turbidity was high only in the 
Swimming River. 
Predators, predator diets, and prey field 
During gill net sampling, 7 age-l-i- striped bass, 30 age- 
1-H bluefish, 648 age-0 bluefish, 189 age-1 weakfish, and 
411 age-0 weakfish were collected (Table 1). The sea¬ 
sonal arrival and egress of the species were discussed 
by Manderson et al. (2014). Catch at station A at the 
salinity front, as opposed to station B in the channel, 
was significantly different for all taxa and seasons 
(Mann-Whitney tests: P<0.01). Almost twice as many 
age-0 bluefish were collected at the front station than 
at the channel station in summer and fall. Age-l-i- blue¬ 
fish were collected rarely except in summer, and during 
