Azorean demersal longline survey abundance estimates: Procedures and variability

Survey data from the Azores is used for fisheries stock assessments under the ICES Working Group on the Biology and Assessment of Deep-sea Fisheries Resources (WGDEEP) covering deep open ocean areas. The Azores research cruise using bottom longlines for abundance estimation is probably unique in the Northeast Atlantic Ocean. This study analyzed data from annual Azorean bottom longline surveys (ICES subdivision 10a2) from 1996 to 2018 to calculate abundance indices of commercial demersal species, to estimate the statistical significance of the annual changes in the abundance estimates, and to examine the possible factors that cause this variability. The catch rates of these surveys appear to provide accurate indices of abundance for most of the commercially important demersal species. The observed inter-annual variability of the survey abundance estimates can be explained by survey effort distribution, fishery dynamics, aggregation patterns of some species, habitat preferences, fish movements, and environmental effects. However, other factors can affect these estimates (soak time, gear saturation and competition for hooks) and we recommend that additional studies be undertaken to understand these effects. (C) 2020 Published by Elsevier B.V.