Using a recreational grade echosounder to quantify the potential prey field of coastal predators.

Tom Brough,Steve Dawson,William Rayment,Judi Hewitt

doi:10.1371/journal.pone.0217013

Abstract

Quantifying the distribution of prey greatly improves models of habitat use by marine predators and can assist in determining threats to both predators and prey. Small epipelagic fishes are important prey for many predators yet their distribution is difficult to quantify due to extreme patchiness. This study explores the use of recreational grade echosounders (RGE) to quantify school characteristics of epipelagic fish and link their distribution to that of their predators at Banks Peninsula, New Zealand. The hydro-acoustic system was ground-truthed with 259 schools of epipelagic fish. During 2015 and 2016, 136 hydro-acoustic surveys were conducted with concurrent observations of Hector’s dolphin (Cephalorhynchus hectori) and little penguins (Eudyptula minor). The relative abundance of the two predator species during surveys was modelled according to the relative abundance of potential prey using generalised additive mixed models. Schools of epipelagic fish were readily detected by the RGE system and were more abundant in summer compared to winter. The models performed well, explaining 43% and 37% of the deviance in relative abundances of dolphins and penguins respectively. This is the first study to link the distribution of Hector’s dolphin to that of their epipelagic prey and confirms the utility of RGE in studies of habitat use in marine predators. Limitations associated with a lack of formal acoustic calibration and data formatting can be overcome and would make RGE valuable, inexpensive tools for investigating variability in populations of small pelagic fishes.

Highlights

The distribution of marine top predators generally reflects that of their prey [1,2,3]
Other taxa often encountered during ground-truthing events included predatory fish such as barracouta (Thyrsites atun) and kahawai (Arripis trutta) and the juvenile squat lobster (Munida gregaria)
Acoustic signals of Munida were similar to prey schools identified as fish but were typically higher intensity, shallower and had much larger dimensions

Summary

Introduction

The distribution of marine top predators generally reflects that of their prey [1,2,3] For this reason, studies investigating habitat use of predators greatly benefit from data that quantify prey [4,5,6]. Studies investigating habitat use of predators greatly benefit from data that quantify prey [4,5,6] Such data have been shown to improve the predictive power of habitat models [7,8], elucidate threats associated with prey depletion [9,10], and can contribute to marine spatial planning [11,12]. Hydro-acoustics are used to obtain data on small epipelagic fishes [14,16,17].

Methods

Results

Conclusion