Directional Bilateral Asymmetry in Fish Otolith: A Potential Tool to Evaluate Stock Boundaries?

Kélig Mahé,Oussama Hamed,Alba Jurado-Ruzafa,Joanne Smith,Patrícia Gonçalves,Rachid Amara,Angelique Jadaud,Romain Elleboode,Andrea Massaro,Marine Randon,Aikaterini Anastasopoulou,Alaia Morell,Hélène De Pontual,Zouhir Ramdane,Djamila Ider,Kirsteen Mackenzie,Karen Bekaert,Bruno Ernande,Chryssi Mytilineou

doi:10.3390/sym13060987

Abstract

The otolith, found in both inner ears of bony fish, has mainly been used to estimate fish age. Another application that has been developing significantly in recent years, however, is the use of otolith shape as a tool for stock identification. Often, studies have directly used the shape asymmetry between the right and left otoliths. We tested the magnitude of directional asymmetry between the sagittal otoliths (left vs. right) of 2991 individuals according to their catch locations, and we selected species to evaluate whether directional asymmetry may itself be a tool to evaluate stock boundaries. Elliptical Fourier descriptors were used to describe the otolith shape. We used a flatfish, the common sole (Solea solea, n = 2431), from the eastern English Channel and the southern North Sea as well as a roundfish, the bogue (Boops boops, n = 560), from the Mediterranean Sea. Both species showed significant levels of directional asymmetry between the testing locations. The bogue otoliths showed significant asymmetry for only 5 out of 11 locations, with substantial separation between two large areas: the Algerian coast and the western part of the Italian coast. The sole otoliths showed significant asymmetry in the shape analysis (3.84–6.57%), suggesting a substantial separation between two large areas: the English and French parts of the English Channel and the southern North Sea. Consequently, directional bilateral asymmetry in otolith shape is a potential new method for stock identification.

Highlights

Otoliths are calcified structures, located in the inner ear cavity of all teleost fish, that aid in hearing [1,2,3]
After principal components analysis (PCA) on the elliptic Fourier descriptors (EFDs), only the first six principal components (PCs) for both species were kept for the shape matrix, according to the broken-stick model
The partial redundancy analysis (pRDA) performed on data across all sampling sites detected a significant directional asymmetry between left and right otolith shape (p = 0.018 for bogue and p = 0.005 for the common sole)

Summary

Introduction

Otoliths are calcified structures, located in the inner ear cavity (left and right) of all teleost fish, that aid in hearing [1,2,3]. Otoliths have been primarily used as a tool for age determination in many fish species, thanks to the ability to track growth periodicity, from daily to annual growth increments. Otolith shape remains unaffected by short-term changes in fish condition [5] or environmental variation [1]. Otolith shape has been used as a tool to identify species, reconstruct the composition of predator diet (fishes, seabirds, seals, etc.), and discriminate between fish stocks. Elliptical Fourier analysis (EFA) remains the most widely used and robust method to describe otolith shape. 91 papers on the identification of marine fish populations or stock structure using otolith shape were published between 1993 and 2017

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