4.2 Food gain and Energy balance is similar
The two feeding strategies (i.e. head-dipping in shallow waters and
pursuit diving in deep waters) resulted in significantly different catch
rates, and apparently the duck was more efficacious in shallow waters
with a catch rates of 0.28 fish/minute (in comparison, the average catch
rate was 0.13 fish/minute in deep waters. Fig. 5A). This is in
consistency with studies on other piscivorous waterbirds. For example,
Shags (Phalacrocorax aristotelis ) has higher prey–capture
performance in shallower (Wanless et al., 1993). The difference in
foraging catch rates could be linked to prey abundance and
vulnerability, which is strongly affected by water depth (Lantz, Gawlik,
& Cook, 2011; Monaghan, Walton, Wanless, Uttley, & Bljrns, 1994;
Schekkerman & Beintema, 2007). As the key components of habitat
complexity for stream fish, water depth has an important effect on the
distributions and size of fish (Lonzarich & Quinn, 1995; Rose, 2000)
and macroinvertebrates as well (Lantz et al., 2011). Small and juvenile
fish intended to inhabit shallow waters to avoid predatory fish (Blaber,
1980; Sheaves, 2006). In addition, it would be easier to catch a prey in
shallow waters.
Although more fish were caught in shallow waters, the total gains in
terms of biomass at different habitats were similar due to the larger
size of fish in deep waters. That’s to say, the higher prey quality
offset the lower prey quantity in deep waters (Brodmann, Reyer,
Bollmann, Schläpfer, & Rauter, 1997), resulting in slightly higher
energy gains (6.99±0.94, and 6.08±1.08 KJ/minute in deep waters and
shallow waters, respectively. Fig. 6B). However, the energy gains in
shallow waters may be underestimated as macroinvertebrates were excluded
in the study due to technical difficulty, and predation of
macroinvertebrates mainly occurs in shallow habitats.
Different foraging patterns imply highly different energy costs (Godfrey
& Bryant, 2000). In deep waters, the bird spent significantly more time
in diving, which has high basal metabolic rate (Table 1 and (Leeuw,
1996)). The frequent pursuit diving resulted in significantly higher
energy cost in deeper water (Fig. 6C). However, as the forging energy
consumption of M. squamatus was only a fraction of energy intake
(less than 10%), it had little influence on the foraging energetics:
the net energy gain was slightly higher in deep waters but the
difference was not statistically significant (Fig. 6C).