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

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.

A long‐term shift in the summer distribution of Hector’s dolphins is correlated with an increase in cruise ship tourism

“…Thus, a marine reserve effect for this top predator is likely to be relatively small or present at longer lag times. It is clear that prey availability plays a major role in the spatial distribution of Hector's dolphins at Te P ataka o R akaihaut u (Brough et al, 2019a;Brough et al, 2020). Fine-scale species distribution modelling (Rodríguez et al, 2007), incorporating prey dynamics (Bennington, Rayment & Dawson, 2020), could be used to assess trophic health harbour-wide and further elucidate what is driving dolphin distribution at present.…”

Prey and habitat characteristics contribute to hotspots of distribution for an endangered coastal dolphin

“…Data on the relative abundance and patch characteristics of prey were collected concurrently during the dolphin-habitat surveys using hydro-acoustics (Simrad ESS7 echosounder, see Brough et al, 2019b for detailed methods). The relative abundance of Hector's dolphins showed a strong relationship with availability of potential prey represented by epipelagic schooling species (e.g., slender sprat -Sprattus antipodum, yellow-eyed mullet -Aldrichetta forsteri) and the depth of fish schools (Brough et al, 2019b). Thus, these variables were included as candidate variables in the dolphin-habitat models used to investigate the drivers of distribution.…”