Feeding ecology of harbor seals (Phoca vitulina) in the outer Oslofjord quantified by DNA metabarcoding of scat soft-matrix and morphological analysis of hard prey remains

Abstract

The feeding ecology of harbor seals in the outer Oslofjord was investigated during late summer and autumn of 2019 using DNA metabarcoding and morphological hard-parts identification. To evaluate potential competition between seals and fisheries, the annual consumption of fish by harbor seals was estimated and compared to the fisheries landings in the outer Oslofjord. Fish species from 16 and 18 families were identified among the feces samples included in the DNA metabarcoding (n = 44) and hard-parts analysis (n = 77), respectively. In total, fish from 25 families were identified as prey. In addition to bony fishes, the DNA metabarcoding revealed that birds and skates may also be components of harbor seal diet in the region. The hard-parts analysis indicated cephalopods were prey as well. The results from molecular and morphological analyses were similar in regard to important prey species, but finer taxonomic resolution of important prey groups was achieved using DNA metabarcoding compared to the more traditional morphological analysis. Additionally, individual scats tended to contain a greater diversity of prey when analyzed by the DNA metabarcoding. In both methods of analysis, gadoid fishes comprised the largest part of the diet. In the hard-parts analysis, the most important prey in terms of relative diet contribution were fish in the unresolved cod/pollack/saithe group (13.0%), followed by Trisopterus spp. (Norway pout/poor cod/bib; 11.2%), Atlantic cod (8.7%), and haddock (8.5%). Similarly, through DNA metabarcoding the most important prey were found to be Atlantic cod (25.5%), haddock, (14.4%) and Trisopterus spp. (12.6%). Thus, the combined approach gives us reason to believe that much of the prey identified as cod/pollack/saithe in the hard-parts analysis may in fact be Atlantic cod. Pleuronectid flatfishes were also common prey in both methods of analysis. The total annual prey consumption by ca. 620 harbor seals in the outer Oslofjord (Færder and Hvaler sub-areas) was estimated as 1009 tons. Our data indicates increased predation on 0-group cod in a year when juvenile cod were exceptionally plentiful. Though prey size estimates showed that seals predate primarily on small fish below minimum allowed landing size for most commercial species, we speculate that harbor seals may expose preferred prey, such as coastal cod, to a “predator pit” phenomenon, and thus question whether seal predation is constraining the recovery of coastal cod in the outer Oslofjord and adjacent areas. Further investigation of diet across multiple seasons in subsequent years is needed to understand how harbor seals regulate prey populations.