Torre et al.: Abundance and diversity of fish species and blue crab in Delaware Bay 
563 
Figure 4 
Nonmetric multidimensional scaling (NMDS) or¬ 
dination plots of stations based on square-root- 
transformed densities (individuals/m 2 ) of fish 
and blue crab (Callinectes sapidus) during day 
and night in the shore zone and nearshore area 
of lower Delaware Bay during July, August, and 
September 2013. Spider plots (R, vers, 3.1.3, veg¬ 
an package, vers. 1.13-8) have been overlaid to 
show shore zone and nearshore group centroids. 
Avian predators may restrict some species to deep¬ 
er water during the day because birds are higher- 
order predators in estuarine systems and can exert 
significant top-down pressure on nekton assemblages 
(Steinmetz et al., 2003; Yeoh et al., 2017). Bay ancho¬ 
vy are known to exhibit diel vertical movement, in¬ 
habiting deeper water during day (Vouglitoisa et al., 
1987) and moving upward or into shallow water at 
night (Hagan and Able, 2008). Although we saw large 
numbers of small (<20 mm FL) bay anchovy in the 
shore zone during the day, larger individuals (mean: 
~60 mm FL) were more dense in the shore zone at 
night, possibly as a response to both increased for¬ 
aging opportunity on mysid shrimp, a staple of their 
diet (Hartman et al., 2004), and because of reduced 
avian predation. 
Gear avoidance could affect sampling efficiency of 
mobile fish species (Riha et al., 2008) and contribute 
to differences in density and species richness between 
day and night samples. However, the seining methods 
used in our study were designed to rapidly enclose the 
sampling area and minimize escape of mobile species 
(Torre and Targett, 2016, 2017). Furthermore, Rfha et 
al. (2008) reported either similar or higher sampling 
efficiency during daytime than at night with 10-50 
m seine nets. Therefore, it seems unlikely that gear 
avoidance greatly impacted the observed diel differ¬ 
ences in species densities in the shore zone. 
As described above, several species were significant¬ 
ly more abundant in the shore zone during either day 
or night, a finding that would suggest onshore-offshore 
diel migrations; however, no diel patterns were evident 
in the adjacent nearshore habitat. Lack of diel changes 
in the nearshore could be a result of sufficiently differ¬ 
ent predator-dynamics in the shore zone than in the 
more extensive and deeper nearshore. Differences in 
water depth, over a relatively small horizontal distance, 
create advantages and disadvantages for predators and 
prey in the shallow shore zone. Prey fishes moving into 
and out of the shore zone on a diel basis can take ad¬ 
vantage of a refuge from predation resulting from the 
size-specific spatial distribution patterns of piscivorous 
fish predators (Baker and Sheaves, 2007) and the as¬ 
sociated predation constraints imposed on large fishes 
in very shallow water. There are also diel movements 
of invertebrate prey, such as mysids, creating potential 
foraging opportunities for some fish species (Hulburt, 
1957; Hopkins, 1965), increased potential vulnerability 
to avian predation in shallow water (Steinmetz et al., 
2003; Yeoh et al., 2017), and the interaction of all these 
processes with differences in visibility caused by the 
diel light cycle. 
Clear diel differences in the species assemblage in 
the shore zone and distinct diel patterns in the abun¬ 
dance of several dominant species highlight that day 
sampling alone does not give a true reflection of the 
nekton assemblage in the sandy beach shore zone of 
Delaware Bay. The way we perceive habitat value and 
its functional role for fishes, including predator-prey 
interactions, are affected by a reliance on only daytime 
sampling and observations. Our results show the value 
of investigating shore-zone nekton dynamics over the 
diel cycle. Future research should include the follow¬ 
ing: sampling throughout the diel cycle and incorporat¬ 
ing tagging and movement studies (Gibson et al., 2011; 
Yeoh et al., 2017); and assessment of the influence of 
lunar, tidal, and seasonal cycles (Gibson et al., 1998) 
to more fully understand diel movement dynamics of 
nekton along sandy beach shores. 
Acknowledgments 
We thank R. Dixon, M. Davidson, R Kaestner, and A. 
Gruszkiewicz for field and laboratory assistance. We 
thank D. Miller for statistical advice and D. Taylor for 
