Mobulid ray by-catch in longline fisheries in the south-western Atlantic Ocean

This paper presents the first by-catch assessment focussed on mobulid rays in pelagic longline fisheries based on on-board scientific observer data (1998–2013) over the south-western Atlantic. Mobulid diversity, as well as their catch distribution and disposition, were investigated. Mobulid by-catch frequency was low, but by-catch probability increased at lower bottom depths (i.e. continental shelf and slope), higher fishing effort and during the warm period (summer–autumn). Photographic and video records allowed the identification of two devil ray species, the spinetail devil ray (Mobula japanica) and the bentfin devil ray (Mobula thurstoni), but the number of species interacting with longliners in the region could also be greater. Southernmost captures of both species (36°S) exceed previous reported records by almost 12°, therefore greatly expanding their known distribution ranges towards southern waters in the south-western Atlantic. Mobulids were never retained on board, and most of them were released alive. However, hooks were often not removed from their bodies and on several occasions fishermen caused injuries while trying to cut the branch line, so some uncertainty remains regarding their post-capture mortality. More specific studies that assess these particular aspects are needed to better understand the potential effect of longline fisheries on mobulid populations.