Fish samples

Seven fish species were selected for this study: herring Clupea harengus Linnaeus, 1758, sprat Sprattus sprattus (Linnaeus), whiting Merlangius merlangus Linnaeus, pout Trisopterus luscus (Linnaeus), dab Limanda limanda (Linnaeus), flounder Platichthys flesus (Linnaeus), and plaice Pleuronectes platessa Linnaeus. All species were sampled in 2019 while whiting and pout only were collected in 2020. A total of 1,163 and 423 specimens were collected by bottom trawling during the International Bottom Trawl Survey in the English Channel and North Sea in January 2019 and 2020, respectively [39, 40]. The initial protocol included the collection of 40 individuals in each geographical area. The sampling area was divided into five sub-areas defined based on environmental characteristics (temperature, salinity) (Fig. 1). Individuals of all species were sampled in all areas, except for pout in the east North Sea in both years (Table 1). For 2019 sampling, all fish were frozen at −20 °C rapidly on board. For 2020 sampling, individuals were eviscerated and kept fresh at 1 °C for a maximum of 11 days until parasite inspection at the laboratory.

Sampling locations in the English Channel and North Sea with indications of number of sampled fish at each station. Size of the circle is proportional to the total number of fish (n fish) of the five species sampled at each station. Stations names with a code beginning with an X and with full circle were sampled in 2019 and by a Y and empty circle in 2020.

Geographical origin and biometric data of fish species, including weight and length ± SD (min–max).