Fish eggs in the diet of White Sea cod Gadus morhua marisalbi (Gadiformes: Gadidae): species identification using DNA-based methods

Abstract

Trophic relationships reflect the peculiarities of the functioning of communities of aquatic organisms and analysis of their structure and dynamics can be used to improve fisheries management and assess the state of marine ecosystems. Populations at the margins of the species' range have particular adaptations, including feeding habits, to extreme conditions, which makes them a convenient model object for studying the possible consequences of ecosystem changes under the influence of global factors. The cod Gadus morhua marisalbi Derjugin, 1920 permanently inhabits the coastal waters of the subarctic White Sea, located at the northeastern margin of the geographic range of the Atlantic cod G. morhua Linnaeus, 1758. This coastal cod population strongly depends on the local environmental conditions and available food resources during the summer feeding period. In addition to the main food organisms (fish, polychaetes, crustaceans), the eggs of fish spawning in shallow coastal areas in summer are also important in the diet of cod at this time. According to published data, fish eggs found in significant quantities and most often in the stomachs of cod belonged to threespine stickleback and lumpfish, but this has not been confirmed by any experimental research. Besides, the possible presence of herring eggs in the diet of cod remains unproven due to the difficulties of its visual identification. In this work, we used molecular genetic methods for species identification of fish eggs found in cod stomachs. At the first stage, the species identification of DNA isolated from eggs was done using two sets of species-specific primers: the first which we designed for the mitochondrial COI gene of stickleback, cod, and herring, and the second, published for a microsatellite locus of herring. At the second stage, the species identity of DNA extracted from fish eggs was checked using DNA barcoding. The results showed that cod stomachs contained herring and stickleback eggs, and in a number of cases we found the co-occurrence of eggs of these species. Of the 29 DNA preparations studied, fourteen were successfully amplified with microsatellite primers for herring, and thirteen amplified with primers for stickleback. Interannual differences in the occurrence of herring eggs in samples were insignificant, while the frequency of occurrence of stickleback eggs in 2018 was higher than in 2017. Our analysis did not reveal significant intra-annual differences between the observed and theoretically expected number of cases of simultaneous occurrences of herring and stickleback eggs. The high frequency of occurrence of herring and stickleback eggs in cod stomachs revealed in this study indicates the importance of these components in the diet of cod and provides new knowledge on the trophic role of cod as a key species in the White Sea ecosystem. Analysis of available data on the spatial and temporal diet patterns of coastal White Sea cod shows that fish eggs can significantly contribute to the total amount of food consumed by this species during the summer.