Spatiotemporal changes in the Atlantic cod (Gadus morhua) stock at Flemish Cap (1993–2019) and their relationships with demersal communities

Changes in Atlantic cod (Gadus morhua) abundance at Flemish Cap, likely due to exploitation and perhaps also to changing environmental conditions, have been well documented since 1980s. While the ecological implications of cod fluctuations have been explored in relation to dominant and commercially important species including redfish (Sebastes spp.), northern shrimp (Pandalus borealis) and Greenland halibut (Reinhardtius hippoglossoides), the broader ecological impacts, e.g. on less abundant species, remain less well explored. This study aimed to analyse spatiotemporal variation in the distribution and abundance of cod, and identify associated changes in distribution and abundance of other species with various trophic relationships to cod. This analysis used a delta Generalized Additive Model (GAM) approach, incorporating binomial and quasi-Poisson GAMs fitted to EU bottom trawl survey data from 1993 to 2019. Trophic species and guilds were defined based on the sizes and feeding habits of each species, as established in previous studies. Atlantic cod is considered to comprise of two trophic species: cod under 46 cm and larger cod. Model predictions were used to construct distribution maps and estimate distribution range and annual total abundance. Bottom temperature was a more important predictor in abundance (quasi-Poisson) models than in presence (binomial) models. The observed decline in cod abundance was associated with contraction in the distribution range. Significant negative correlations were identified between cod trophic species and all but one of the other trophic species in the same trophic guilds, for both distribution range and abundance. Species in other trophic guilds that rely on northern shrimp as prey also exhibited negative correlations with cod. The abundances of the main prey of cod, namely juvenile redfish and northern shrimp, showed negative correlations with cod abundance but no relationship was seen for distribution range. The abundance of large Acadian redfish (S. fasciatus) and large beaked redfish (S. mentella), which are major prey species of cod, was positively correlated with that of large cod, suggesting that the abundance of these prey species depends more on external variables, such as intense exploitation, than on their predator-prey relationships. These findings highlight the importance, for fishery management, of considering both the direct effects of fishing mortality and the indirect effects via trophic relationships.