During the primary feeding season, marine mammals often accumulate fat reserves, primarily in the form of blubber. Despite the ecological significance of feeding, uncertainty remains surrounding the timing of energy store accumulation in Hudson Bay beluga whales (Delphinapterus leucas (Pallas, 1776)). Blubber samples were collected from whales hunted by Inuit along the whale’s migration route, from 2015 to 2021 (excl. 2018). Sampling occurred in spring and fall, assumed to represent feeding in winter and summer, respectively. We analyzed blubber for lipid content and adipocyte size, two related indices of lipid dynamics, across three blubber sections (outer, middle, and inner). We found interannual variability in the season with the highest fat content, with some years showing higher lipid content in spring than fall. While adipocyte size did not differ seasonally, minima were observed in 2017 and 2019. Fat stores differed across blubber sections, with the highest lipid content and largest adipocytes in the middle section. The observed seasonal variation indicates there is no consistent season in which Hudson Bay beluga whales predominantly accumulate fat. Consequently, building energy stores in the form of blubber may not be the primary driving force behind the beluga whale’s migration between the wintering and summering areas.

The dataset includes data on 53 Hudson Bay beluga whales harvested near Sanikiluaq, Nunaut. Primary analysis was on lipid content and adipocyte size, used to evaluate the primary feeding season of these whales. Subsamples of blubber were collected from the outer, middle, and inner sections of the blubber. The data includes when the whale was harvested (year, month, season), genetically determined sex, and lipid content measured during fatty acid analysis. Histological analysis was conducted to obtain adipocyte size (average area). Age of the individuals was determined using teeth. Note: The blubber was seldom present down to the muscle. Blubber length represents the length of the blubber available and not necessarily the blubber depth of the whale. 

Teeth were aged by Matson's Laboratory. 

Sex was determined genetically at the Freshwater Institute (Fisheries and Oceans Canada) with DNA extracted from skin tissue (Qiagen DNeasy Kit) and analyzed following PCR procedures described in Rosel (2003).

Lipids were extracted by placing ~0.5g of blubber in 7 mL of chloroform for a minimum of 24 hours. After, 3.5 ml of methanol with 0.01% butylated hydroxytoluene (BHT) was added and vortexed. The blubber was compressed to extract the lipids, and the remaining tissue was removed. Next, 2.6 mL of NaCl was added to the tube and vortexed. The lower phase containing the chloroform and lipids was extracted and placed in anhydrous sodium sulfate to remove excess water, which was subsequently rinsed with chloroform twice. Chemicals were obtained from Fisher Scientific Canada, Greenfield Global, and Avantor (previously VWR). The remaining solvent was evaporated using a nitrogen stream evaporator (N-EVAP) with silver beads (Organomation ©, USA) at 25-30°C. The final lipid mass was recorded and compared to the initial blubber sample weight to calculate the percent lipid content of the initial wet sample.

Histological Analysis: Samples were no more than 0.4 x 1.5 x 2 cm in size, with the longest measurement parallel to the skin. ). The samples were placed in Falcon tubes with 50 ml of 10% neutral buffered formalin and fixed at room temperature on a shaker for a minimum of 48 hours. The samples were subsequently transferred to 50 ml of 70% ethanol for shipping to The Centre for Phenogenomics (Toronto, ON), where histological analysis was conducted. The samples were processed, embedded, cut, stained, scanned, and analyzed by the lab. Samples were embedded using the maximum surface area, cut to their standard 4 µm, stained using an H&E stain (Harris hematoxylin and eosin Y; Table S2), and scanned with a Hamamatsu NanoZoomer at 20x magnification. Images were analyzed using an algorithm typically used to measure vacuoles, but was instead used to measure the area of the fat cells.