• biologia della pesca
• otoliti
• pagellus erythrinus
• shape analysis

The stock represents the fundamental unit in fisheries science: it is defined as a population, or portion of a population, characterized by similar biological and ecological parameters, distributed in a specific geographical area and subject to fishing exploitation. Stock identification and knowledge on stock spatial distribution is pivotal to understand fish population dynamics, perform robust stock assessment, and support fisheries management. Otolith shape analysis has proven to be an efficient tool for stock identification, because of its low cost and easy preparation procedures as well as to provide high accuracy in results. Otoliths are calcareous structures located in the head of fishes and they are considered as a reliable biological and environmental archive of fish life history. They grow throughout the entire life of the fish, and they are metabolically inert: for this reasons otolith shape remains unaffected by short-term changes in fish condition. As a result, otoliths represent a powerful tool for fish stock identification. Otolith’s shape is species-specific, and the morphology can often vary geographically within a species. Environmental factors, behaviour (food or spatial-temporal differences), somatic growth rate, variations in diet habits and genetics could modify otolith's growth rate and, therefore, otolith shape.
Common Pandora, Pagellus erythrinus (family Sparidae), is a demersal species and is widespread throughout the Mediterranean. Along the Italian coasts, this species is caught in large quantities by bottom trawling and trammel nets.
The aim of this study was to evaluate the stock structure of Common Pandora in the Ligurian and Northern-Central Tyrrhenian Sea (GSA 9). To this end, the area was separated into two sub-area (A: Ligurian Sea – B: Northern-Central Tyrrhenian). In addition, common Pandora otoliths were gathered from other three areas of Mediterranean Sea (GSA 10 - Southern Tyrrhenian Sea, GSA 18 - Southern Adriatic Sea, and GSA 19 - Ionian Sea); different statistical approaches (univariate and multivariate analyses) were used to test differences in otolith shape among the four areas (GSA9, 10, 18 and 19).
520 individuals were collected during the MEDITS survey (MEDiterranean International Trawl Survey) carried out in November 2020. For each individual, total length and total weight were recorded, and sex and maturity stage were determined by means of the macroscopic investigation of the gonads. For each specimen, both right and left otolith were extracted and subsequently photographed.
In addition, population dynamics features, namely sex ratio, length-at-first maturity, and length-weight relationship, were analysed. For each otolith, morphometric parameters (feret length, feret width, perimeter, area, weight) and Shape Indices (Roundness, Circularity, Rectangularity, Aspect Ratio, Ellipticity and Form Factor) were recorded.
Otolith shape analysis was performed using statistical analysis (PCA and DA) on shape indices; the Elliptical Fourier Analysis (EFA) and Wavelet Transform were used to investigate the degree of similarity in the otoliths and detect the reciprocal variability through Canonical Analyses of Principal Coordinates (CAP).
Sex ratio did not differ significantly from the expected value (1:1) (χ2=1,875; χ20.05 <3,84), while sex ratio by length showed a predominance of females in the smallest size classes, while males are predominant in the largest size classes.
Statistically significant differences (p<0.05) in length-at-first maturity (L50) were found between females (15,1 cm TL) and males (16,9 cm TL)
Length-weight relationship showed a negative allometric growth (b < 3; p < 0,05); b values did not differ significantly from males and females (t = 0,03; p > 0,05). Regarding length-weight relationship of otoliths, a negative allometric growth was observed (t=8,00; p<0,05) and no difference between males and females (t=0,142; p>0,05), and right and left otoliths (t=0,03; p>0,05) was detected.
The analysis of otolith morphology did not show any difference in shape within the samples from GSA 9 (area A – B), while significant differences in shape indices, Fourier shape descriptors and Wavelet coefficients were observed among the specimens from GSA 9, 10, 18, 19. This result could be related to genetic and local environmental factors.