Hammer Chyme: The Russian-Doll Effect Limits Reconstructing Marine Trophic Food Webs

Abstract

Increasing fishing effort, including bycatch and discard practices, are impacting marine biodiversity, particularly among slow-to-reproduce taxa such as elasmobranchs, and specifically sharks. While some fisheries involving sharks are sustainably managed, collateral mortalities continue, contributing towards >35% of species being threatened with extinction. To effectively manage shark stocks, life-history information, including resource use/feeding ecologies is pivotal, especially among those species with wide-ranging distributions and habitats. Two cosmopolitan sharks bycaught off eastern Australia are the common blacktip shark (Carcharhinus limbatus; globally classified as Near Threatened) and great hammerhead (Sphyrna mokarran; Critically Endangered). We opportunistically sampled the digestive tracts of these two species and also any whole prey; (termed the ‘Russian-doll’ approach) caught in bather-protection gillnets off northern New South Wales to investigate their regional feeding ecologies and the capacity for DNA metabarcoding to delineate trophic interactions. Sphyrna mokkaran fed predominantly on Myliobatiformes and Rajiformes, but also teleosts, while C. limbatus mostly consumed teleosts, with some inter-specific dietary overlap of prey items. Extensive cross-contamination of predator and prey digestive tracts, likely via the predator’s stomach chyme, was evident from the metabarcoding assays limiting the opportunity to delineate trophic interactions from these data. This Russian-doll effect requires further investigation in DNA metabarcoding studies focused on dietary preferences, but implies any outcomes will need to be interpreted concomitant with traditional visual approaches.