The Former Pelagic Longline Fishery of a Large-Scale Marine Protected Area

Abstract

The establishment of large-scale marine protected areas (LSMPAs) has emerged as one of the defining trends in ocean conservation over recent decades. To assess the potential benefits of such designations, it is necessary to understand the nature of the threats that have been excluded. Here, we summarise over 25 years of historical catch and effort data for a pelagic longline fishery that formerly operated within the recently designated LSMPA surrounding Ascension Island (UK), using data compiled from logbooks and observer programmes. Licenced fishing by foreign vessels (primarily flagged to Taiwan and Japan) operated intermittently in the Ascension Island exclusive economic zone (EEZ) between 1988 and 2016, with catch peaking at over 5000 t year⁻¹ in the early 1990s. Bigeye tuna (Thunnus obesus) was the dominant species targeted (76% of total catch weight) whilst oceanic sharks (e.g. blue shark Prionace glauca) and other predatory pelagic finfish (e.g. longnose lancetfish Alepisaurus ferox) appear to have presented a sizable bycatch risk, accounting for 37% of total individuals caught in local observer data. The fishery displayed strong seasonality, with two thirds of activity occurring between December and March and was consistently concentrated in the northwest of the EEZ. This distribution closely aligns with recent satellite-derived vessel tracking data which suggests that a regional longline fishing hotspot remains in the high seas area adjacent to the northwest of the Ascension Island MPA. Our results suggest that predatory pelagic fish and sharks will be the most direct beneficiaries of the Ascension Island MPA, although the high mobility of these species may lessen any conservation impacts, given intense the fishing effort in adjacent high seas areas. While illegal fishing remains a potential threat, the spatiotemporal predictability of the historic fishery may be useful in identifying areas of elevated risk for targeted enforcement in this large, remote MPA.