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Fishery Bulletin 1 12(2-3) 
Metabolic rates of animals generally scale with body 
mass to approximately the % power (Anderson-Teixeira 
et ah, 2009; and references therein; note that in our 
study we were concerned with resource requirements 
of individual whole fish that influence residency, not 
with mass-specific metabolic rates). Therefore, larger or 
older individuals require more prey resources per unit 
of energy cost of prey acquisition (i.e. , search, capture, 
handling time, and digestion) than do smaller preda- 
tors to meet metabolic demand at a given temperature. 
Prey resources in the Navesink River include large 
numbers of small invertebrates and fishes, such as 
age-0 Atlantic Menhaden (Brevoortia tyrannus), Atlan- 
tic Silverside (Menidia menidia ), Bay Anchovy ( Anchoa 
mitchilli), mysids, and sevenspine bay shrimp ( Cran - 
gon septemspinosa ) (Scharf et ah, 2004; L. Stehlik and 
senior author, unpubl. data). 
Diet studies of small size classes (<500 mm TL) of 
predators indicate that age-0 Atlantic Menhaden (<200 
mm TL) are preferred prey that, with other small prey, 
reside in the Navesink River throughout the warmer 
months. Larger (>200 mm TL), energy rich age-l+ At- 
lantic Menhaden, consumed by the largest Bluefish 
and Striped Bass, are abundant in the river during 
late spring, but they migrate out of the tributary in 
June and July before returning again in early autumn 
(Scharf et al., 2004). Early summer egress of the larg- 
est Striped Bass and Bluefish (>500 mm TL) from the 
Navesink River coincided with the typical timing of 
egress for age-l+ Atlantic Menhaden and other large 
prey (L. Stehlik and senior author, unpubl. data). These 
large prey may be required by large fishes, particularly 
when warm temperatures increase metabolic demand. 
Metabolic demand in ectotherms is regulated by 
environmental temperatures, as well as by body size 
(Hartman and Brandt, 1995; Brown, 2004; Sousa et al., 
2010), and our GAMMs indicated that residence times 
of the 3 predators in the Navesink River were a func- 
tion of the interaction between body size and water 
temperature. For all species, threshold temperatures 
for egress and breadths of temperatures associated 
with estuarine residence decreased with increasing 
body size. The largest age-l+ Striped Bass and Blue- 
fish (>500 mm TL) released in the spring were likely to 
remain in the river only until temperatures exceeded 
23°C in the early summer. Smaller Striped Bass were 
less sensitive than large fish and remained in the 
river over a broader range of warmer temperatures. 
Smaller age-l+ Bluefish were also more likely to be 
resident at warmer temperatures ranging from 23°C 
to 26°C. Age-0 Bluefish remained in the estuary at the 
warmest temperatures recorded and were unlikely to 
leave until temperatures declined below 19°C during 
autumn. Finally, Weakfish also remained in the estu- 
ary when temperatures were warmest and were more 
likely to leave the river when temperatures declined 
below 23°C in the autumn. Smaller Weakfish, however, 
remained in the river longer and over a broader range 
of temperatures. 
The relationships between estuarine residency time, 
body size, and environmental temperature that we ob- 
served are consistent with bioenergetic studies and 
metabolic theory (Gillooly et al., 2001; Brown, 2004; 
Harris et al., 2006; Sousa et al., 2010). The species- 
and size-specific temperatures of estuarine residence 
and egress that we measured were extremely similar 
to temperatures and size-dependent, scopes for growth 
reported by Steinberg (1994) and Hartman and Brandt 
(1995). In those studies, growth potential exceeded 2% 
of body weight per day at temperatures of 12-25°C (op- 
timal 15°C) for Striped Bass, 16 -26°C (optimal 20°C) 
for Bluefish and 20-29°C (optimal 23.5°C) for Weakfish. 
Smaller individuals generally had higher optimal 
temperatures for growth because metabolic demand 
and prey requirements are generally smaller for ani- 
mals with small body sizes. For example, the thermal 
optima for age-l+ Bluefish was ~20°C, but growth po- 
tential for age-0 Bluefish reached a maximum at tem- 
peratures of ~25-27°C (Steinberg, 1994; Hartman and 
Brandt, 1995; Scharf et al., 2006). Ranges of optimal 
temperatures for various performance measures are 
also generally broader for smaller, juvenile ectotherms 
(Freitas et al., 2010), and our GAMMs indicated that 
smaller fishes were more likely than larger fish to re- 
main in the Navesink River over a broader range of 
temperatures. Because metabolic demand increases 
with temperature as well as body size, prey supply 
shortages are more likely to occur during the warmest 
summer months for large animals in small estuarine 
tributaries like the Navesink River. 
Residence time and egress of the 3 studied preda- 
tors also were related to the rate of freshwater dis- 
charge from the Swimming River into the Navesink 
River. On the basis of the 4 GAMMs that we construct- 
ed independently for the predators, we determined that 
animals were more likely to leave the small estuarine 
system when average daily freshwater discharge rates 
from the Swimming River fell below -2 m 3 s -1 than 
when discharge rates were higher. High discharge 
events ( >50 m 3 s -1 ) also appeared to affect residencies 
of Striped Bass and, perhaps, age-l+ Bluefish. Howev- 
er, in contrast with this low discharge response, high 
discharge response thresholds varied by species. The 
predators that we tagged were euryhaline and probably 
did not respond behaviorally at the scale of the whole 
estuary to the direct physiological effects of increas- 
ing or high salinities. We hypothesize that the effects 
of low discharge on residence and egress were indirect 
through hydrographic processes that control the avail- 
ability of prey resources that support the entire suite 
of predators that we tagged. 
Variability in freshwater discharge is believed to af- 
fect estuai’ine fishes primarily by changing estuarine 
hydrodynamics that control prey resource availability. 
Interactions between freshwater discharge and tides 
control gravitational circulation in estuaries and the 
advection and concentration of the essential building 
blocks of estuarine food webs. As a result, estuaries 
