Concurrent bycatch and population assessments of Burmeister's porpoises in northern Peru reveal unsustainable levels of mortality

Fisheries-related mortality has been a threat to Burmeister’s porpoises in Peru for decades. However, the magnitude of its impacts on porpoise populations has not been assessed. This study reports the first concurrent abundance and bycatch estimates to evaluate the status of Burmeister’s porpoises in northern Peru. We conducted systematic line transect surveys with 770 km of effort and concurrent towed passive acoustic monitoring as well as semi-structured interviews with gillnet fishers from Salaverry. We recorded 87 porpoise sightings during 9 survey days in February 2023 using a repurposed gillnet fishing vessel and trained observers. Using density surface modelling of the line transect sighting data Burmeister’s porpoise abundance was estimated at 1 696 individuals (CV = 0.23) in the 3 500 km2 study area. Porpoise distribution showed a spatial gradient with highest densities close to shore (<25 km) and in shallow waters (≤50 m). Visual and acoustic detections and fishers’ descriptons of bycatch areas matched closely in space. The majority (66%) of fishers interviewed (n = 32) reported porpoise bycatch in the previous year. For the area matching the abundance survey, the estimated mean bycatch rate per vessel was 6 porpoises/year which equates to an annual bycatch of 420 (95% CI: 210–700) porpoises when scaled to the entire artisanal gillnet fishing fleet of Salaverry. We then considered the potential biological removal reference limit to put into context abundance and bycatch estimates in the study area which shows that current bycatch rates by far exceed sustainable limits and highlights the urgent need for bycatch mitigation measures.