The cost of a meal: factors influencing prey profitability in Australian fur seals

Abstract

Knowledge of the factors shaping the foraging behaviour of species is central to understanding their ecosystem role and predicting their response to environmental variability. To maximise survival and reproduction, foraging strategies must balance the costs and benefits related to energy needed to pursue, manipulate, and consume prey with the nutritional reward obtained. While such information is vital for understanding how changes in prey assemblages may affect predators, determining these components is inherently difficult in cryptic predators. The present study used animal-borne video data loggers to investigate the costs and benefits related to different prey types for female Australian fur seals (Arctocephalus pusillus doriferus), a primarily benthic foraging species in the low productivity Bass Strait, south-eastern Australia. A total of 1,263 prey captures, resulting from 2,027 prey detections, were observed in 84.5 h of video recordings from 23 individuals. Substantial differences in prey pursuit and handling times, gross energy gain and total energy expenditure were observed between prey types. Importantly, the profitability of prey was not significantly different between prey types, with the exception of elasmobranchs. This study highlights the benefit of animal-borne video data loggers for understanding the factors that influence foraging decisions in predators. Further studies incorporating search times for different prey types would further elucidate how profitability differs with prey type.

Keywords: Animal-borne video; Benthic foraging; Crittercam; Foraging efficiency; Marine predator; Optimal foraging; Prey energetics; Profitability.

Figure 1. Representative images from animal-borne video data loggers showing captures of the various prey categories by female Australian fur seals.

From left to right, upper to lower row: schooling jack mackerel (Trachurus sp., Baitfish); an octopus (Octopus sp., Cephalopod); a dogfish (Squaliformes, Elasmobranch); a stingray (Myliobatiformes, Elasombranch); a leatherjacket (Tetraodontiformes, Solitary fish) and a gurnard (Scorpaeniformes, Solitary fish). Images are from animal-borne video cameras deployed by JPY Arnould.

Figure 2. Prey pursuit and handling times for female Australian fur seals.

Boxplots comparing the (A) pursuit time, (B) handling time, and (C) total time spent by female Australian fur seals on individual prey captures for different prey types in Bass Strait, south-eastern Australia. Ceph. = cephalopod, Elasm. = elasmobranch. Benthic prey are shown in red, demersal prey in orange, and pelagic prey in green. The median value is indicated by the thick horizontal line. Upper and lower bounds of the boxes represent the 75th and 25th percentiles, respectively, and the interquartile range (IQR) is the difference between the 25th and 75th percentiles. Whiskers represent 1.5*IQR. Any values extending beyond the whiskers are considered outliers (grey dots). Sample sizes are indicated in Table 1 (pursuit time) and Table 2 (handling and total time).

Figure 3. Diet energetics of female Australian fur seals.

Boxplots comparing (A) gross energy intake, (B) total energy, and (C) prey profitability of individual prey captures for different prey type groups captured by female Australian fur seals in Bass Strait, south-eastern Australia. Ceph. = cephalopod, Elasm. = elasmobranch. Benthic prey are shown in red, demersal prey in orange, and pelagic prey in green. The median value is indicated by the thick horizontal line. Upper and lower bounds of the boxes represent the 75th and 25th percentiles, respectively, and the interquartile range (IQR) is the difference between the 25th and 75th percentiles. Whiskers represent 1.5*IQR. Any values extending beyond the whiskers are considered outliers (grey dots). Sample sizes are indicated in Table 2.