First record of Risso's dolphin Grampus griseus (Cuvier, 1812) in Icelandic waters

Abstract

In July 2022, two Risso's dolphins were reported stranded in Hrútafjörður (N65° 09,503; W21° 05,529), a fjord in northern Iceland. These events represent the first confirmed observations and strandings of Risso's dolphins in Icelandic waters. Given the uniqueness of these events, a decision was made to conduct full necropsies on these individuals. This study reports findings from viral and parasitological investigations, morphological and fitness measurements, as well as stomach and intestine content analysis for each of the Risso's dolphin specimens. The results of the necropsies do not suggest any other cause of death than lack of food and exhaustion. A large plastic fragment in one individual's stomach supports these suggestions. The presence of those specimens in the middle of the subarctic ocean illustrates ongoing changes in spatial distribution expanding northward, impacting not only Risso's dolphins but more generally marine life and biodiversity.

Keywords: North Atlantic; delphinidae; iceland; necropsy; stranding.

FIGURE 1

Risso's dolphin distribution in the North Atlantic Ocean. The red line shows the northern limit of the distribution observed in the North Atlantic (Jefferson et al., 2014). The dashed gray line shows the position of the Arctic Circle latitude (N66°50). The red star shows the location of the stranding events S2214‐1 and S2214‐2 reported in Iceland. Red dots show the location of Risso's dolphins' observations outside their normal distribution. In the right panel, the red star shows a more precise location of the stranding events in the north of Iceland and the red square shows the location of the necropsy facilities.

FIGURE 2

Pictures featuring both animals taken on the 12th of July 2022 on the stranding site (Figure 1). (a) S2214‐1 an adult female and (b) S2214‐2 a juvenile male.

FIGURE 3

Measurement and sampling positions collected during the necropsy (Lockyer & Waters, 1986). Half girth measurements were taken on position 1 (anterior to the flipper insertion), 2 (posterior to the flipper insertion), 3 (anterior to the dorsal fin insertion), 4 (posterior to the dorsal fin insertion), and 5 (midtail position). The blubber thickness was measured along the dorsal median line (from D1 to D5), along the ventral line (V1 to V5), and along the midline (M1 to M5) of the dolphin's body.

FIGURE 4

Pictures of the parasite investigation conducted during the necropsy of S2214‐2. (a) large piece of blubber with longitudinal and perpendicular incisions made every 0.5–1 cm on the whole blubber thickness: (b) discovery of cysts and (c) and (d) recovery of plerocercoid of Clistobothrium sp.

FIGURE 5

Pictures showing parts of the stomach contents of the stranded male Risso's dolphin S2214‐2: (a) and (c) a plastic bag retrieved from the stomach; (b) examples of the largest algae (mostly fucoid) retrieved from the stomach. A scale of 10 cm is indicated for (a) and (b), while a scale of 1 cm is indicated in (c).

FIGURE 6

Reconstructed Risso's Dolphin's skeleton after being preserved by the Icelandic Institute of Natural History.