Bathymetric Structure and Recurrent Recruitment Hotspots of Juvenile Galeus melastomus Rafinesque, 1810 in the Central–Southern Tyrrhenian Sea

Recruitment and the spatial distribution of early life stages govern the resilience of exploited elasmobranch populations, yet remain poorly understood in demersal sharks. We examined the bathymetric structure and recruitment hotspot recurrence of juvenile Galeus melastomus, a frequent by-catch of deep-water bottom trawling, in the central–southern Tyrrhenian Sea (GSA 10), using MEDITS data (2017–2024). Juveniles were defined as immature individuals (maturity stages 1–2) smaller than the sex-specific length at first maturity (L50: 384.1 mm in males, 425.1 mm in females). The Recruit Density Index (RDI, juveniles/km2) was compared among depth strata, analysed with a Tweedie generalised additive mixed model accounting for station and sampling season, and mapped by Inverse Distance Weighting; hotspot persistence was tested against random expectation. Juveniles occurred in every survey year, supporting continuous recruitment. Juvenile density was highest in the 201–500 m stratum (p < 0.001), peaking near 400 m. Densities were higher in autumn–winter surveys, and the model indicated a possible decline of about 12% per year requiring confirmation. Recruitment hotspots occurred mainly in this stratum and recurred at a small number of stations more often than expected by chance. These results can inform the monitoring and management of demersal elasmobranch by-catch in the Mediterranean Sea.