Reconstructing the diet, trophic level, and migration pattern of Mysticete whales based on baleen isotopic composition

Abstract

Baleen from mysticete whales is a well-preserved proteinaceous material that can be used to identify migrations and feeding habits for species whose migration pathways are unknown. Analysis of δ13C and δ15N from bulk baleen has been used to infer migration patterns for individuals. However, this approach has fallen short of identifying migrations between regions as it is difficult to determine variations in isotopic shifts without temporal sampling of prey items. Here we apply analysis of δ15N values of amino acids to five baleen plates belonging to three species, revealing novel insights on trophic position, metabolic state, and migration between regions. Humpback and minke whales had higher reconstructed trophic levels than fin whales (3.4-3.5 versus 2.7-2.9, respectively) as expected due to different feeding specialization. Isotopic niche areas between baleen minima and maxima were well separated, indicating regional resource use for individuals during migration that aligned with isotopic gradients in Atlantic Ocean particulate organic matter. δ15N values from phenylalanine confirmed regional separation between the niche areas for two fin whales as migrations occurred and elevated glycine and threonine δ15N values revealed physiological changes due to fasting. Simultaneous resolution of trophic level and physiological changes allow for identification of regional migrations in mysticetes.

North Atlantic minke whale (Balaenoptera acutorostrata) feeding habits and migrations evaluated by stable isotope analysis of baleen

“…In line with observations for the Pacific minke whale (Mitani et al, 2006 ) and the single female North Atlantic minke whale (Riekenberg et al, 2020 ), the baleen plates for North Atlantic minke whales in the present study contain distinct δ 15 N depletions. For Lofoten/Vesterålen individuals, the timing of δ 15 N depletions occurs predominantly between May to July (Table 1 ), which can be interpreted to reflect annual shifts in the whale's foraging that coincide with the annual zooplankton productivity peak.…”

“…The average rate of baleen growth from males in the present study was 12.8 ± 0.9 cm per year, in line with the growth rate of 12.9 ± 2.1 cm reported for male Pacific minke whales (Mitani et al, 2006 ) and the growth rate of ~11.9 cm reported for a single female North Atlantic minke whale (Riekenberg et al, 2020 ). The benefit in applying stable isotopic analysis to samples taken along the length of baleen plates is that a longer temporal record is obtained, as opposed to direct observations of feeding, samples from gut contents of dead animals, or faeces, which provide point samples of feeding activity.…”

“…The estimated dates assigned to the isotope measurements (which were taken 1 cm apart) were approximately one month apart; this is in line with the monthly growth rates in the literature (e.g. ~9.9 mm for male Pacific minke whales in Mitani et al, 2006 and ~9.2 mm in a female North Atlantic minke whale Riekenberg et al, 2020 ). These estimated dates were categorised into seasons based on boreal equinox and solstice dates.…”

“…Based on δ 15 N and δ 13 C from skin samples of North Atlantic blue ( Balaenoptera musculus ), fin ( Balaenoptera physalus ) and sei ( Balaenoptera borealis ) whales, Silva et al ( 2019 ) report that winter‐feeding appears common. Riekenberg et al ( 2020 ) examined δ 15 N amino acids in minke, humpback, and fin whales and reported greater evidence that fin whales experience food stress than for either the humpback or minke whale. Mitani et al ( 2006 ) reported no detectable impact of fasting in the isotopic record in Pacific minke whales.…”

“…The field of dietary studies continues to evolve, with new techniques that provide greater accuracy in estimating trophic position and metabolic condition (e.g. amino acid nitrogen isotopic analysis, O’Connell, 2017 ; Riekenberg et al, 2020 ) and an increasing use of inter‐disciplinary approaches that provide greater insights in dietary studies (e.g. biochemical and molecular approaches, Horswill et al, 2018 ).…”