First documentation of putative mating behavior in blue sharks (Prionace glauca) reveals a potential reproductive area in the Northeast Atlantic

Abstract

Reproductive behavior in sharks remains poorly understood, with direct observations of mating reported in only a few species. The blue shark (Prionace glauca) is a widely distributed, placental viviparous species, yet direct evidence of mating behavior remains undocumented. Here, we describe the first visual documentation of a putative mating attempt involving blue sharks in the Bay of Biscay, off the Basque coast, observed during a shark ecotourism dive in July 2023. An adult male and an immature female exhibited a sequence of behaviors consistent with shark courtship, including parallel swimming, following, a courtship bite, and an inversion of both individuals. Additionally, we documented females with mating scars across 4 consecutive years, the majority of which were considered immature. These observations align with prior reports suggesting mating attempts between adult males and immature females. The combination of direct behavioral observation and repeated evidence of mating scars highlights the potential reproductive significance of the region and underscores the need for further research on the demographics, habitat use, and reproductive ecology of blue sharks in the Northeast Atlantic.