Boreogadus saida (Lepechin, 1774)

2.1 | B. saida View in CoL sample collection

B. saida View in CoL were sampled on board R.V. Polarstern (Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, 2017) using a Surface and Under-Ice Trawl (SUIT) during PS 80 in the Eurasian Basin of the CAO in late summer/autumn ( August 2 – October 7, 2012), north of Svalbard during PS 92 in spring ( May – June 2015), and north of Svalbard during PS106/ 2 in spring/early summer ( June – July 2017). The SUIT sampled the upper 2 m of the water column in open water and under-sea ice. It consisted of a steel frame with a 2 2 m opening and two 15-m-long nets attached. One net was a 7-mm half-mesh shrimp net, and the other was a zooplankton net with a 300 μm (PS80, PS92) or 150 μm (PS106/2) mesh. Due to an asymmetric bridle, the net was forced to tow off at an angle allowing it to sample the surface water directly underneath sea ice, outside the ship's wake ( Van Franeker et al., 2009; Flores et al., 2012). Using the SUIT, 53 individual B. saida View in CoL were collected for diet analyses at 11 stations during PS80 ( Kohlbach et al., 2017), 12 individuals at 6 stations during PS92, and 5 individuals at 3 stations during PS106/2 (Figure 1).

B. saida View in CoL were also collected during PS122 (MOSAiC expedition) on board R.V. Polarstern, which took place from September 20, 2019, to October 12, 2020. The expedition was subdivided into five legs. Four legs took place starting from the initial deployment in late September 2019 north of the Laptev Sea, then drifting south within the Transpolar Drift, and ending in Fram Strait in July 2020. The ice station was relocated for the fifth leg to near the North Pole in August 2020, where it remained until the end of the expedition in late September 2020. In total, 16 B. saida View in CoL suitable for diet studies were collected from underneath the ice during Legs 1, 4, and 5 (Figure 1). One fish was collected in winter during Leg 1 ( December 7, 2019) with a remotely operated vehicle (ROV) operating at 10 m depth on December 7, 2019. Most specimens ( n = 9) were collected in a single sampling event on July 25, 2020, during Leg 4 ( June 4 – August 12, 2020), using Castellani's stick-and-scoop method. This method consisted of using a long thin stick to gently disturb the fish in a crack from the bottom of the crack so that they moved toward the surface, where they were scooped out of the water by hand. The method proved to be very successful when B. saida View in CoL were residing in narrow cracks in the ice where no net can be used. Six more individuals were caught in the period between end of July and mid-September 2020. During Leg 5 ( August 12 – October 12, 2020), B. saida View in CoL were caught with hand-held sieves or a ring net from holes in the ice and sampled unintentionally by the ship's seawater intake.