The blue shark is among the most widely distributed sharks and is found in tropical and temperate areas of all oceans from about 60°N to 50°S [19
]. The species is relatively fast-growing and fecund, reaching sexual maturity in 4–7 years and producing up to 135 pups per litter (average 37 pups) after a relatively short (<12 months) gestation period compared to other shark species [20
]. Despite these characteristics, blue shark populations in the Mediterranean Sea have decreased by >95% [15
], probably due to overfishing [22
], and the species is now classified by the International Union for Nature Conservation (IUCN) as “critically endangered” in this region [23
]. This low abundance has made blue sharks very difficult to observe in the Mediterranean Sea. However, we were able to successfully observe multiple individuals in Balearic waters using our 24 h extended BRUV system. The system also allowed for the detection of residual fishing hooks inside or close to the mouth of several animals as a proxy for frequent and potentially deleterious interactions with fisheries. This result demonstrates the effectiveness of long-duration BRUVs for detecting shark species that occur at low densities, and contributes to an increased understanding of their biological status and the need to improve conservation. The fact that blue sharks were observed almost as often at night as during the day is not at all surprising, as this species has showed a clear diel behaviour, generally occupying shallower depths at night than during the day [24
]. The occurrence of only seven blue shark individuals in 55 days (1335 h) of BRUV sampling supports concerns that this species has been heavily overfished in the Mediterranean Sea, as previously occurred in the western English Channel by the 1970s [25
]. We used the preferred prey of blue sharks (squid, [26
]) as an attractant, hence it is unlikely that our methodology was unable to effectively attract the vast majority of animals reached by the olfactory stimulus. We also used cetacean flesh and oil to attract white sharks which opportunistically feed on the corpses of marine mammals [27
], and fish baits which should attract a wide range of shark species. The rarity of blue shark sightings and the absence of other sharks in BRUV videos supports the conclusion that the abundances of large epipelagic sharks remain extremely low in the Mediterranean Sea [15
Although our study was not able to provide quantitative information about elasmobranch species among the western Mediterranean in order to compare it with previous studies in the same area [28
], our long-endurance BRUV was effective in detecting the presence of elusive species, such as the blue shark or the Wreckfish, whose presence in this part of the Mediterranean was usually inferred from stomach contents analysis [29
]. Unsurprisingly, it also allowed for other more common pelagic teleost fishes and cephalopods to be detected. This device could be further modified to provide real-time observations by adding a wireless video transmitter (i.e., 1.3 GHz) to the system. This could be useful for notifying researchers of shark presence if physical samples (e.g., DNA biopsies) are needed. Such a system could also allow the development of a shark-watching industry for ecotourism purposes, such as that which exists in the Azores [30
]. It could also be a useful tool for education and outreach purposes, where live videos could be streamed to a web page that also includes BRUV ‘highlights’.
This long-endurance BRUV would improve the study of a broad spectrum of epi-pelagic marine species and would undeniably optimise the chances of observing animals in very low densities, including at night.