LAUSR

Most mobulids are listed as Near Threatened to Endangered. However, effective conservation measures are hindered by knowledge gaps in their ecology and behaviour. In particular, few studies have assessed diets and trophic ecologies that could inform methods to avoid fishing mortality. Here, a shortfall in data for the longhorned pygmy devil ray, Mobula eregoodoo was addressed by describing temporal variability in dietary preferences using stable isotope analysis. During summer and autumn in 2017, five bather-protection nets were deployed off eastern Australia (29o S, 153.5o E). From the catches of these nets, thirty-five adult M. eregoodoo had liver, muscle and stomach contents sampled to determine δ13 C and δ15 N profiles. Analyses revealed surface zooplankton and zooplanktivorous teleosts were important dietary components across short- and longer-term temporal scales. Large quantities of undigested sandy sprat, Hyperlophus vittatus in the stomachs of some specimens unequivocally support feeding on teleosts. A narrow isotopic niche and minimal isotopic overlap with reef manta rays Mobula alfredi from the same geographic region in eastern Australia implies M. eregoodoo has unique and highly specialised resource use relative to other mobulids in the area. The species is clearly vulnerable to capture during inshore migrations, presumably where they feed on shallow-water shoaling teleosts. Female M. eregoodoo likely have a low annual reproductive output so population recoveries from fishing-induced declines are likely to be slow. Measures to reduce the bycatch of M. eregoodoo in local bather-protection nets, and artisanal fisheries more broadly, should be given urgent priority. This article is protected by copyright. All rights reserved.