Maturity assignment based on histology‐validated macroscopic criteria: Tackling the stock decline of the Mediterranean swordfish (Xiphias gladius)

Abstract

Effective stock assessment relies on the evaluation of the reproductive potential of a population, but this is often achieved using fast, inexpensive, and inaccurate macroscopic indicators of gonadal maturity. Indeed, this approach suffers from high error rates as a result of misclassification and operator errors. Although histology is considered the gold standard to assign reproductive stages, it represents a time‐consuming and expensive method. Therefore, cost‐effective approaches are currently needed to assess the species‐specific reproductive potential and define the correct size at first maturity (L50). In the present study a histology‐calibrated method based on the gonadal index (GI) was developed to assign maturity stages to the overfished Mediterranean swordfish (Xiphias gladius), supported by histological classification of gonadal development, an estimation of L50, and an analysis of the reproductive output of this species. The performance of this new method had an overall agreement of 90% with histological data, and the L50 calculated by the GI‐based method compared with histology analysis was not statistically different (p > 0.05). Moreover, in order to apply this method without bias, a thorough validation of the relationship between length and gonad weight was carried out and the suitability of the GI to compare individuals of different size and maturity stages was confirmed. The present findings provide a comprehensive picture of the patterns characterizing Mediterranean swordfish reproduction and a promising method to assign maturity stages based on a macroscopic indicator. This method has the potential to be extensively applied during routine fish sampling activities in the field because of its simple requirements.

Mediterranean swordfish (Xiphias gladiusLinnaeus, 1758) population structure revealed by microsatellite DNA: genetic diversity masked by population mixing in shared areas

“…In the North Atlantic, reproduction takes place in the western subtropical area, with year-round spawning and seasonal peaks ( Arocha, 2007 ). In the Mediterranean Sea, spawning takes place between June and August ( Palko, Beardsley & Richards, 1981 ; Arocha, 2007 ; Marisaldi et al, 2020 ) and it is restricted to three main spawning grounds. The first is located in the western Mediterranean, from the Strait of Gibraltar up to the Balearic Islands ( Arocha, 2007 ; Neilson et al, 2013 ); the second one extends from the Strait of Messina to the Gulf of Taranto in the Ionian Sea ( Cavallaro, Potoschi & Cefali, 1991 ; Arocha, 2007 ).…”

Reproductive Biology of Male European Hake (Merluccius merluccius) in Central Mediterranean Sea: An Overview from Macroscopic to Molecular Investigation

“…Histological validation is strongly recommended to ensure a proper calibration between visual and microscopic staging [ 73 , 74 , 75 ]. In this study, it is proved that European hake males’ maturity staging necessarily requires histological analysis and that the macroscopic classification causes many evaluation errors in all stages.…”

Loss of Mitochondrial Genetic Diversity in Overexploited Mediterranean Swordfish (Xiphias gladius, 1759) Population

“…Specifically, mtDNA is maternally inherited therefore, the analysis of mtDNA variability reflects only the history of the Mediterranean swordfish female portion. Swordfish females reach larger sizes than males, outnumbering males in larger size classes (LJFL > 140 cm) [5,39,42]. Therefore, an increase in the average size may be related to an increase in the number of mature females exploited.…”

“…The growth parameters are different, and the sexual maturity is reached at younger ages than in the Atlantic [4]. In the Mediterranean, the estimated size at which 50% (L 50 ) of the female population is mature occurs from 131.5 cm [5] and 140 cm [6], corresponding to 3-4-year-old fish [2]. Males reach sexual maturity at smaller sizes and mature specimens have been found at about 90 cm (Lower jaw fork length, LJFL) [7].…”