High-resolution fishing activity data of gillnetters reveal unexpected fishing pattern on western Baltic cod during spawning closure

The understanding of small-scale fishing activities in shallow coastal waters is scarce because fishery-dependent data are often not collected at the required spatio-temporal resolution. During a two-month fishing closure of cod in 2018 in the western Baltic Sea, German fishers self-reported their at-sea activities in waters shallower than 20 m at a high spatio-temporal resolution using a smartphone application. Linear mixed modelling was used as an explorative approach to analyse 365 trips from 23 cod gillnet fishers. The aim was to identify a set of explanatory variables that would best describe the pattern of the cod landings per unit effort. In addition, it was expected that fishers would on average set the nets as deep as possible to still reach parts of the cod spawning aggregations. In the end, it was not possible to identify a clear single best model and the high-resolution fishing activity data showed that fishers stayed in waters shallower than expected. Together with available biological data from regular monitoring, the results suggest that fishers targeted the dominant 2016-cohort of cod in 2018, i.e. the age-2 cod, in shallow coastal waters. This indicates that the tactic of small-scale fishers was altered due to changes in resource availability. The opportunity to analyse highly resolved spatio-temporal fishery-dependent data of a European small-scale fishery highlights the value of such data to improve the understanding on how fishing activities are shaped by the distribution of a target species.