The chemical signature of retained hooks in mulloway (Argyrosomus japonicus) revealed by otolith microchemistry

A widely accepted practice to maximize the survival of mandatory or voluntary released aquatic animals that have ingested hooks is to cut the line, leaving the ingested hook in thousands of fish, sharks, turtles and marine invertebrates worldwide. Recent evidence suggests that the chemical composition (mainly metals) of ingested hooks can potentially be absorbed and retained within soft tissues of fish and can subsequently have adverse effects on their health. In this study, we tested the hypothesis that hard tissues (otoliths) should also show a chemical signal of hooking. Using the mulloway (Argyrosomus japonicus) as a case study, we have compared the microchemistry compositions (Ba, Ca, Co, Cr, Mn, and Sr) of the otoliths measured by Solution Inductively Coupled Plasma Mass Spectrometry (SO-ICPMS) among three groups: control fish (artificially-reared), treatment (deep-hooked) and wild fish. The multivariate Redundancy Analysis fitted to test the general chemical composition of the otoliths among groups revealed a significant chemical signature of hooking characterized by an increase in Sr and Co, and in a smaller effect Ca. Moreover, we found otolith microchemistry as a useful method to differentiate between artificially-reared and wild mulloway. Positive results across other taxa and different recreational and commercial fisheries should show the real magnitude of the chemical signals of hook ingestion, its utilization as a biological tag and its consequences for the health of released individuals.