The tub gurnard, Chelidonichthys lucerna, is considered the most important commercial fish species of the family Triglidae captured by the Portuguese traditional fisheries. However, the data available about its population structure, movement patterns and habitat connectivity is, at present, scarce. In this study, otolith chemistry and shape analyses of 90 individuals captured, between March and June 2016, in the three main fishing grounds of the Portuguese mainland coast (Matosinhos, Aveiro and Peniche) were investigated. Otolith morphological (shape indices and elliptic Fourier descriptors) and elemental (Ba:Ca, Fe:Ca, Mg:Ca, Mn:Ca, Sr:Ca and Zn:Ca) signatures of individuals with similar length range were assessed through uni and multivariate statistical tests. The overall combination of otolith elemental chemistry and morphology techniques revealed the highest re-classification success (74%) of samples to their original location and the existence of significant differences among sampling regions. However, linear discrimination function analyses and canonical analyses of principal coordinates did not fully discriminate C. lucerna individuals from the three sampling regions of Portugal. Moreover, the hereby data revealed a partial overlap among individuals from the different sampling regions. The obtained shape and chemical signatures suggest that C. lucerna is apparently a unique, although not necessarily homogenous, single population-unit in Portugal mainland, and that these fish populations should not be managed separately for fisheries purposes.

Tub gurnard, Chelidonichthys lucerna (Linnaeus, 1758), is a Mediterranean-Atlantic benthic species usually captured as by-catch by Portuguese traditional fisheries and considered the most important commercial fish species of the family Triglidae. However, to date, little is known about its habitat residency and whether the species can be considered a marine estuarine-dependent fish. Otolith microchemistry has proven effective in providing information about fish movement patterns throughout different water salinities. In this study, core-to-edge transects of Sr:Ca in the fish saccular otoliths of 35 juveniles of C. lucerna captured in March 2007 by a scientific survey along the Portuguese coast were used to assess the species movement between brackish and marine waters. Data suggest that most individuals (97%) have an estuarine-dependent profile, with 63% showing a clear presence in marine waters during the early life history periods. Evidence of an estuarine residence throughout the fish life cycle was found in only 3% of individuals. Ba:Ca profiles did not reflect an inverse relationship with Sr:Ca salinity results but corroborated findings from other authors about the influence of upwelling processes and freshwater runoffs on Ba incorporation into the otoliths of coastal fish. Furthermore, the data also showed that C. lucerna can occupy and migrate among habitats with diverse salinity degrees, showing high environmental plasticity and adaptation.

The study of geometric morphometrics among stocks has proven to be a valuable tool in delineating fish spatial distributions and discriminating distinct population units. Variations in fish body morphology can be linked to genetic factors or to phenotypic adaptability in response to environmental variables. The tub gurnard (Chelidonichthys lucerna) is a demersal species that usually lives in the bottom of the continental shelf, being widely distributed along the northeast Atlantic, Mediterranean and Black seas. Worldwide interest in the species has increased since 2006, when ICES recognized its potential for commercial exploitation. However, despite its broad geographic occurrence, to date, research on C. lucerna population structure at large spatial scales is still lacking. In this paper, body geometric morphometrics, using a landmark-based truss network, was applied in order to discriminate C. lucerna populations caught in three different fishery grounds areas along the northeast Atlantic: Conwy Bay (United Kingdom), Biscay Bay (Spain) and Matosinhos (Portugal). The results obtained in this study revealed a high overall relocation success (95%) of samples to their original locations, thus demonstrating the existence of significant regional differences and indicating that we are dealing with different fish population units. Moreover, the data revealed a partial overlap between individuals from Spain and United Kingdom, suggesting that in geographically distant areas these populations may inhabit similar environments. However, to corroborate these findings, future works using a holistic approach with alternative and complimentary stock assessment tools (e.g., genetic and phenotypic natural tags) are highly recommended.