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).