Nutrient Assimilation and Retention in Baleen Whales Feeding on Capelin and Krill

Baleen whales are essential marine nutrient cyclers that move nutrients across depth and latitude. Through consuming high amounts of prey, whales release nutrients through excreta that fuel surface primary productivity. Understanding nutrient assimilation and retention is critical for quantifying whale contributions to nutrient cycling, yet these parameters remain unknown for whales. We measured assimilation and retention of 15 trace and six macro elements in minke whales (Balaenoptera acutorostrata) and fin whales (Balaenoptera physalus) feeding on capelin (Mallotus villosus) and krill (Meganctyphanes norvegica & Thysanoessa inermis). Nitrogen (N), phosphorus (P), iron (Fe), and zinc (Zn) were the most abundant nutrients in capelin and krill, and these same nutrients predominated in whale excreta. Dry matter assimilation in whales ranged from 83% to 92%. Minke whales showed high N assimilation (94.11 ± 2.81% for capelin, 92.81 ± 3.94% for krill), and retention (69.06 ± 17.73% for capelin, 62.23 ± 23.24% for krill). Elements such as Cadmium (Cd) and cobalt (Co) showed negative assimilation in two of three whale-prey groups, suggesting active excretion to avoid toxic accumulation. This study provides the first assessment of nutrient assimilation and retention in whales, highlighting the physiological basis for their role in marine nutrient movement.