ASSESSING THE SPAWNING STOCK BIOMASS OF ALBACORE (THUNNUS ALALUNGA) IN THE WESTERN MEDITERRANEAN SEA FROM A NON-LINEAR LARVAL INDEX (2001-2019)

Abstract

Larval abundance indices express retrocalculated abundances of larval densities at hatching time. They provide a proxy for assessing spawning stock biomass and are applied to assess population status of various species in the Gulf of Mexico and in the Balearic Sea. Recently, the methodological approach to calculate the indices was improved to accommodate for non-linear responses of environmental effects on catchability. This improved methodology is routinely applied in the Balearic Sea to assess the bluefin tuna (Thunnus thynnus) spawning stock biomass. Here we apply the same methodology to update the larval index of albacore (Thunnus alalunga) from surveys conducted from 2001 to 2019 in the Balearic Sea, the most relevant spawning ground of this species in the Western Mediterranean. Albacore larval abundances show a decreasing trend and significant lower abundances from 2013 onwards, despite a slight recovery between 2016 and 2017. This larval index, standardized for gears, sampling coverage, salinity, date and sea surface temperature, provides information on the dynamics of the western Mediterranean stock of albacore, which is considered a data poor stock.