Thinking of Fish Population Discrimination: Population Average Phenotype vs. Population Phenotypes

Joana Vasconcelos,Antoni Lombarte,Víctor M Tuset,Rodrigo Riera,José Luis Otero-Ferrer,Alba Jurado-Ruzafa

doi:10.3389/fmars.2021.740296

Abstract

The genetic polymorphism and phenotypic variation are key in ecology and evolution. The morphological variability of the contour of fish otoliths has been extensively used for the delimitation of stocks. These studies are conventionally based on average phenotype using elliptic Fourier analysis and lineal discriminant analysis as classifier. Considering new analytical options, such as the wavelet transform and non-parametric algorithms, we here analyzed the otolith shape ofTrachurus picturatus(blue jack mackerel) from mainland Portugal, Madeira, and the Canaries. We explore the phenotypic variation throughout a latitudinal gradient, establish a hypothesis to explain this variability based on the reaction norms, and determine how the use of average phenotype and/or morphotypes influences in the delimitation of stocks. Four morphotypes were identified in all regions, with an increase of phenotypes in warmer waters. The findings demonstrated that stocks were clearly separated with classification rates over 90%. The use of morphotypes, revealed seasonal variations in their frequencies and per region. The presence of shared phenotypes in different proportions among fishing grounds may open new management approaches in migratory species. These results show the importance of the phenotypic diversity in fisheries management.

Highlights

Life cycles of small and medium-size pelagic fish (SMPF) show high dispersion and mobility in marine environments
Our findings revealed the existence of four morphotypes (M1, M2, M3, and M4) whose frequencies varied according to a latitudinal gradient: M1 mostly occupied mainland Portugal and Madeira, M2 Madeira and the Canary Islands, and M3 and M4 the Canary Islands
The presence/absence of antirostrum is genetically codified (Vignon and Morat, 2010), many Trachurus spp. show similar variations in the antirostrum silhouette (Lombarte et al, 2006), and it cannot be an indicator of a genetic polymorphism

Summary

Introduction

Life cycles of small and medium-size pelagic fish (SMPF) show high dispersion and mobility in marine environments. The capacity to perform larger migrations is positively linked to fish body-length (Brochier et al, 2018), slower growth rate, larger size-at-maturity, and changes in morphological traits (e.g., body shape and swimming performance) (Tamario et al, 2019) This adaptive phenotypic variation may occur at different spatial scales; and the production of local phenotypes non-randomly distributed is expected (Lind et al, 2015). Studies have demonstrated that the source of variation can stem from local differences at loci (genetic polymorphism) underlying ecological adaptation (Catanese et al, 2017), coined as ecotypes (Turesson, 1929; Turrill, 1946) In this case, different genotypes display contrasting phenotypic responses to environmental change or crossing norms of reaction (Pigliucci et al, 2001; West-Eberhard, 2003). Complementary to genetic approaches, several tools and body characters or elements have been extensively used to explore phenotypic variation such as, morphometry, pigmentation, otoliths, among others

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Conclusion