Manderson et al.: Residence time and habitat duration for predators in a small mid-Atlantic estuary 
155 
are traps for autochthonous and allochthonous nutri- 
ents and organic matter from adjacent terrestrial and 
marine systems (MacCready and Geyer, 2010). Sta- 
ble isotope studies indicate that estuarine food webs 
are supported by inputs of freshwater and terrestri- 
al sources of nutrients and organic matter (Kostecki 
et ah, 2010). It is assumed that discharge effects on 
estuarine hydrodynamics and nutrient transport ulti- 
mately concentrate high secondary production of zoo- 
plankton in estuarine regions where fresher and saltier 
waters converge (North and Houde, 2006; Baptista et 
ah, 2010). These mechanisms are thought to produce a 
dome-shaped relationship between estuarine fish pro- 
duction and freshwater discharge (Dolbeth et ah, 2010; 
and references therein). 
In the Navesink River, tidal asymmetries produce 
a short-duration, high-velocity flood tide followed by a 
long, slow ebb (Chant and Stoner, 2001). During flood 
tides, particles are suspended and transported up- 
stream. When freshwater discharge from the Swimming 
River is sufficient, the water column in the Navesink 
River stratifies during the ebb and particles accumulate 
in the central and upper reaches of this river. Finer 
particles and flocculants can remain in suspension in 
the upper Navesink River (Fig. 1., between locations B 
and C), where a convergence zone is formed by the tur- 
bulent mixing of freshwater inflow from the Swimming 
River and tidal inflows of saltwater from Sandy Hook 
Bay and the Atlantic Ocean (Fugate and Chant, 2005). 
In this area, Shaheen et ah (2001) reported high con- 
centrations of the copepod Eurytemora affinis, an impor- 
tant constituent of estuarine food webs. We measured 
relatively sharp gradients in salinity and chlorophyll-a 
and, compared with levels observed in other areas in 
our study, higher abundances of small fish prey, includ- 
ing Atlantic Silverside and age-0 Atlantic Menhaden in 
combined hydrographic and gillnet surveys (L. Stehlik 
and senior author, unpubl. data). Additionally, most of 
the predators that we tagged established home ranges 
in this region for days to weeks when temperatures in 
the upper estuary remained below thresholds associated 
with egress (L. Stehlik and senior author, unpubl. data; 
see also Scharf et ah, 2004; Manderson et ah 2 ). 
Estuaries change from stratified to well-mixed 
states when freshwater discharge decreases and salin- 
ity stratification weakens to the point that estuarine 
Richardson numbers reach a range of 0.08-0.8 (Fischer, 
1979; MacCready and Geyer, 2010). This transition oc- 
curs in the Navesink River when freshwater discharge 
from the Swimming River falls to ~1 in 3 s _1 , (Chant 3 ), 
2 Manderson, J. P., J. Pessutti, J. E. Rosendale, and B. Phelan. 
2007. Estuarine habitat dynamics and telemetered move- 
ments of three pelagic fishes: Scale, complexity, behavioral 
flexibility and the development of an ecophysiological frame- 
work. ICES Council Meeting (C.M.) Documents 2007/G:02, 
36 p. 
3 Chant, R. 2004. Personal commun. Institute of Coastal 
and Marine Science, Rutgers Univ., 71 Dudley Rd., New 
Brunswick, NJ 08901. 
a discharge rate similar to the value at which all the 
predators we tagged were likely to leave this small es- 
tuary in New Jersey. We speculate that the relation- 
ship between predator egress and freshwater discharge 
reflects a shift from a partially mixed to a fully mixed 
estuarine state and the relaxation of physical mecha- 
nisms that control and concentrate the high primary 
and secondary productivity that supports the 3 studied 
predators in the upper reaches of this estuary. 
Conclusions 
Our analyses of residence time and egress of individual 
Striped Bass, Bluefish, and Weakfish in the Navesink 
River, New Jersey, indicate that small estuarine tribu- 
taries contain the habitat resources required to sustain 
juvenile and small adult stages of these 3 predators for 
relatively long periods of time but that the resources 
that regulate habitat suitability are ephemeral. Re- 
quired resources include temperature, which regulates 
metabolic demand and predatory capacity in cold-blood- 
ed fishes (Magnuson et al., 1979; Neill et al., 1994). 
Summer temperatures in the Navesink River appeared 
to support smaller predators for longer durations than 
they did for larger fishes presumably because prey re- 
quirements increase with body size and temperature 
and because the small tributary is dominated by small 
rather than large prey during the warmest summer 
months. 
Freshwater discharge also appeared to be a critical 
habitat resource that controlled residence time for ani- 
mals in this estuary. We believe this relationship re- 
flects the essential role that freshwater discharge plays 
in regulation of physical processes that both drive and 
concentrate the secondary productivity required to 
meet the prey resource requirements of the predators. 
Other factors that we were not able to measure effec- 
tively, particularly dissolved oxygen concentrations and 
human predation pressure, also may have influenced 
habitat suitability and the residence time and timing 
of egress of predators in this small estuarine system 
(Brady et al., 2009). 
Because estuaries occur at the land-sea boundary, 
high-frequency variability in atmospheric temperature, 
precipitation, and wind is rapidly translated into vari- 
ability in water temperature, freshwater discharge, 
dissolved oxygen, and other biophysical processes that 
determine estuarine habitat suitability. Changes in 
seasonal rates of warming, cooling, and precipitation 
that alter and reduce the persistence of suitable es- 
tuarine habitats should require animals to undertake 
more frequent, long-distance movements that are ener- 
getically costly. Conversely, long durations of suitable 
habitat conditions require fewer shifts in local home 
range (Martinho et al., 2009) and allow the allocation 
of resources to the life-history processes of growth and 
reproduction instead of long-distance movements. 
