Accuracy and precision in stock separation of north-east Arctic and Norwegian coastal cod by otoliths - comparing readings, image analyses and a genetic method

Abstract

The distinction between north-east Arctic cod and Norwegian coastal cod, two major groups of Atlantic cod (Gadus morhua L.), has for many years been based on different distance and shape similarities between the two first translucent growth zones in the otoliths, subjectively decided by visual inspection in a binocular. To analyse the certainty of this technique, four independent readers have classified 263 cod otoliths in total from five different geographical areas. For three of the readers, between 82% and 89% of the classification results coincided with independent results based on genetic analyses. Further, 38 cod otoliths, where the readers were certain of the classification (21 north-east Arctic cod and 17 coastal cod) were classified by several image analysis methods. A complete separation was obtained by using the ratio of the circumferences of the two zones, providing a typical ratio of approximately 2 for coastal and 1.5 for north-east Arctic cod. The otolith method for separating the two types of cod has been considered adequately accurate in assessing the two stocks of cod. However, the method is sensitive to subjective interpretation, and action needs to be taken to minimise the difference in interpretation among otolith readers.

Population connectivity among migratory and stationary cod Gadus morhua in the Northeast Atlantic—A review of 80 years of study

“…This is a remarkable omission since internal otolith morphology is used to separate the NA and NC both for management purposes, e.g. by the International Council for the Exploration of the Sea (ICES), and for scientific studies as stated by Berg et al (2005) and Stransky et al (2008).…”

A comparison of otolith microchemistry and otolith shape analysis for the study of spatial variation in a deep-sea teleost, Coryphaenoides rupestris

“…Therefore, fish and more so populations of fish with distinct spatial distributions over part or all of their lifetime can be distinguished using otolith microchemistry analysis (Elsdon and Gillanders 2004;Ashford et al 2005Ashford et al , 2006Elsdon et al 2008;Kingsford et al 2009) and otolith shape analysis (DeVries et al 2002;Turan 2004;Berg et al 2005;Stransky et al 2008a).…”

Environmental and genetic determinant of otolith shape revealed by a non-indigenous tropical fish

“…Otolith shape differences between reared and wild cod Gadus morhua (Cardinale et al 2004), flounder Paralichthys olivaceus (Katayama & Isshiki 2007) and lake trout Salvelinus namaycush (Simoneau et al 2000) also highlight how environmental factors such as temperature and feeding conditions can produce otolith shape variation in the absence of genetic differences. Consequently, shape analysis of the otolith indicates which environment a fish has experienced during its life history, and groups of fish that maintain distinct distributions for part or all of their lives can be discriminated (Turan 2000, DeVries et al 2002, Cardinale et al 2004, Berg et al 2005, Stransky & MacLellan 2005. The advantage otolith shape offers is that otoliths constitute a permanent record and no resorption occurs.…”