Predation of archival tagged Dolly Varden, Salvelinus malma, reveals predator avoidance behaviour and tracks feeding events by presumed beluga whale, Delphinapterus leucas, in the Beaufort Sea

Background We report compelling evidence suggesting a predation event of a pop-up satellite archival tagged anadromous Dolly Varden (Salvelinus malma) by a marine mammal during summer in the Beaufort Sea based on abrupt changes in temperature and vertical movements. This observation provides insight on predator avoidance behaviour by Dolly Varden and the predator’s feeding frequency while the tag was ingested. Based on published distribution and ecology information, we presumed the predator was a beluga whale (Delphinapterus leucas). Supplemental satellite telemetry data from previously tagged Dolly Varden and beluga whales were used to determine the extent of spatial and vertical overlap between species in the area where predation occurred. Results Prior to the predation event, depths and temperatures occupied by the tagged Dolly Varden averaged 1.1 m and 3.1 °C, respectively. On July 7, 2020, depths remained shallow apart from a sudden dive to 12.5 m (16:45 UTC) followed by a precipitous increase in temperature from 4.4 to 27.1 °C (16:52 UTC) suggesting predation by an endotherm. Subsequent readings indicated the endotherm had a resting stomach temperature of 36.1 °C. Including the predation event, eight separate feeding events were inferred during the 20-h period the tag was ingested (before presumed regurgitation) based on subsequent declines in stomach temperatures (mean decline to 31.1 °C) that took an average of 24.1 min to return to resting temperature. The predator occupied mainly shallow depths (mean = 2.3 m), overlapping with tagged belugas that spent 76.9% of their time occupying waters ≤ 2.5 m when frequenting the area occupied by tagged Dolly Varden in the Canadian Beaufort Sea in July. Back-calculation based on tag drift and mean displacement by tagged belugas indicated the predation likely occurred west of the Mackenzie Delta. Conclusion Our findings provide new information on both anti-predator behaviour by, and marine predators of, Dolly Varden in the Beaufort Sea. We provide the first estimate of feeding frequency and stomach temperature recovery in a presumed wild beluga, and evidence for shallow foraging behaviour by belugas. Elucidating the likely predator and exploring the extent of overlap between Dolly Varden and beluga whales contributes towards knowledge on the trophic interactions in the Beaufort Sea.

Cell composition of blood for Salvelinus malma (Salmonidae) from the Raduga River (Kamchatka)

Hematopoiesis is a mechanism of multifunctional adaptation in vertebrates, including fish. Adaptive capabilities of populations are studied for a species of genus Salvelinus with high ecological plasticity. Char Salvelinus malma is represented in Kamchatka by the anadromous, river, brook, and several lake forms. Peripheral blood cells are investigated for this species juveniles and adults from the Raduga River in the lower Kamchatka River basin. The fish sampled in August 2013 were divided into two groups: sexually mature individuals (10 specimens) and juveniles (10 specimens). The body length (AC) of mature chars (mean ± standard deviation, range in brackets) was 338.90 ± 12.64 (304.0–380.0) mm, the body weight — 437.40 ± 57.74 (235.0–735.0) g, their sex ratio was close to 1 : 1. The length of juvenile chars was 150.50 ± 9.14 (104.0–195.0) mm, the weight — 33.90 ± 6.19 (10.1–68.8) g. Blood was sampled from the caudal vessel of alive fish immediately after catching, the blood smears were dried and fixed with May-Grunwald dye and then stained with azure-eosin. Composition of leukocytes and erythrocytes was determined on the stained blood smears. The blood cells were classified according to Ivanova (1983) and Zhiteneva et al. (1989). Statistical significance of the blood cells composition differences between fish groups was assessed using the Mann-Whitney U-test, after checking for normal distribution of characteristics (Kolmogorov-Smirnov test). The erythroid row of S. malma was represented by proliferating (erythroblasts, basophilic and polychromatophilic normoblasts), maturing (oxyphilic normoblasts) and mature erythrocytes, the leukocyte row — by blast cells, agranulocytes and granulocytes. The juveniles were characterized by intense proliferative processes (great number of immature erythrocytes, large and medium lymphocytes, presence of blast cells) and appearance of poikilocytes; the sexually mature chars were distinguished by greater number of granulocytes, mainly due to stab and segmented neutrophils (p < 0.01). Parasitological analysis showed different infestation of the juvenile and adult chars by pathogenic helminths (Anisakis simplex, Eubothrium salvelini, Neoechinorhynchus salmonis) that influenced on their blood. Broad adaptive capabilities of S. malma hematopoiesis provided by wide morphological and functional diversity of blood cells are concluded that maintains homeostasis of the species. Blood indices for char from the Raduga River can be used to form reference intervals for monitoring physiological state of this species.

Freshwater early life growth influences partial migration in populations of Dolly Varden (Salvelinus malma malma)

Populations of northern Dolly Varden (Salvelinus malma malma) exhibit partial seaward migration, yet little is known about this phenomenon in Dolly Varden populations. Our study analyzed data from three different Dolly Varden populations in the western Canadian Arctic in order to determine if: (1) differences in size-at-first seaward migration exist between fish that migrate at early and late ages among populations inhabiting different river systems, and (2) annual growth influences anadromous or resident life history choice. Otolith strontium analysis and back-calculation were used to determine age- and size-at-first seaward migration, respectively. Differences in age- and size-at-first seaward migration were determined across river system and migration age. Back-calculated fish lengths were compared using a mixed effect model to determine how early growth influences migratory tactics (early or late aged smolt, or resident). Our results indicate that fish exhibiting faster early growth migrated in earlier years and at smaller sizes than slower growing fish, however size- and age-at first seaward migration varied by river system. Faster growing Dolly Varden tended to become either residents or early smolts, while slower growth was associated with smolting later in life. This is contrary to life history theory where the fastest growing fish in a population should mature as a resident. Our results indicate factors other than growth may be influencing life history ‘decisions’ in Dolly Varden. Future work on growth efficiencies and metabolic rates is needed to assess how they affect migratory behaviours.