Spatial and temporal patterns of capelin (Mallotus villosus) spawning sites in the Barents Sea

Identifying spawning sites of fish often involves extensive egg and larval sampling surveys over potential spawning sites, or by backward-tracking advected larvae to their source. Due to the vastness of the Barents Sea capelin spawning areas, back-tracking methods have limited application. Egg and larval surveys that provide information about spawning sites have also been discontinued in recent years. This paper aims at using alter-native data sources to egg and larval distribution information, to infer potential spawning regions of the Barents Sea capelin during the period 1994-2020. We use the K-Means clustering technique to cluster historical spawning sites into spawning regions, and the Self Organizing Map (SOM) algorithm to define observed data clusters, which we assign to specific regions. The observation data consists of survey data sets from capelin pre-spawning and post-spawning periods during winter and spring respectively, as well as data from the Norwegian Directorate of Fisheries Electronic Reporting System database (ERS). Our method was efficient in reproducing capelin spawning regions and approximate time windows for commencement of spawning. The results showed that spawning occurred mainly over the eastern part of historical spawning areas during the whole period. A westward extension of the preferred spawning areas occurred in several years regardless of the rising Barents Sea water temperatures, especially during the second half of the period. The ERS data can be used to identify the arrival times and migration fronts of pre-spawning capelin along the coast.