Population comparison of right whale body condition reveals poor state of the North Atlantic right whale

Abstract

The North Atlantic right whale Eubalaena glacialis (NARW), currently numbering <410 individuals, is on a trajectory to extinction. Although direct mortality from ship strikes and fishing gear entanglements remain the major threats to the population, reproductive failure, resulting from poor body condition and sublethal chronic entanglement stress, is believed to play a crucial role in the population decline. Using photogrammetry from unmanned aerial vehicles, we conducted the largest population assessment of right whale body condition to date, to determine if the condition of NARWs was poorer than 3 seemingly healthy (i.e. growing) populations of southern right whales E. australis (SRWs) in Argentina, Australia and New Zealand. We found that NARW juveniles, adults and lactating females all had lower body condition scores compared to the SRW populations. While some of the difference could be the result of genetic isolation and adaptations to local environmental conditions, the magnitude suggests that NARWs are in poor condition, which could be suppressing their growth, survival, age of sexual maturation and calving rates. NARW calves were found to be in good condition. Their body length, however, was strongly determined by the body condition of their mothers, suggesting that the poor condition of lactating NARW females may cause a reduction in calf growth rates. This could potentially lead to a reduction in calf survival or an increase in female calving intervals. Hence, the poor body condition of individuals within the NARW population is of major concern for its future viability.

Have whales returned to a historical hotspot of industrial whaling? The pattern of southern right whale Eubalaena australis recovery at South Georgia

“…These values are similar to our mean body condition results (1.15 ± 1.49) Overall, North Atlantic right whale health parameter scores were higher (reflecting poorer condition) during calving years and for presumed dead whales (Pettis et al 2004), underscoring the fact that visual health assessment methods are useful for measuring individual fitness when individuals are assessed over multiple seasons. A similar pattern has been seen using overhead images (Christiansen et al 2020), with juvenile, adult and lactating female SRWs across 3 calving grounds found to be in better body condition than North Atlantic right whales at equivalent stages.…”

Decreasing body lengths in North Atlantic right whales

“…4,5 Decreasing body size may therefore be associated with smaller calves and lower calf survivorship, or potentially delayed first calving and lower reproductive success in females. NARW exhibit generally poor body condition compared to other populations of right whales, 17,18 which could contribute to synergistic negative effects where females in poor condition produce smaller calves that ultimately reach smaller maximum sizes, further contributing to reduced calf growth and declining calf condition. In addition, our results suggest that sub-lethal entanglements constrain overall body size in NARW, which may in turn make them less resilient to future entanglements by reducing their absolute energetic reserves and increasing the probability of a lethal entanglement.…”

Developing protocols for in‐water morphometric measurements of cetaceans using stereo‐videogrammetry

“…If the issues with identifying reference points in this area can be resolved (e.g., software program modifications), the low variation achieved for replicate TBL measurements (1%–2%) suggests that small 1–2 cm changes in width would be detectable and thus useful for assessing body condition of dwarf minke whales and other cetaceans. Alternatively, further processing of suitable frames through an additional program that automatically calculates width at select intervals along the body (e.g., Christiansen et al, 2020; Torres and Bierlich, 2020) may provide a solution to the issue of manually identifying reference points inherent to the image processing software used herein.…”

“…Determining growth rates and size at sexual maturity can provide insights into population and social structures (Shane, Wells, & Würsig, 1986), reproduction, and other life history characteristics (Blueweiss et al, 1978). Morphometry can also be used to assess body condition/fitness and provide insights into sublethal (i.e., energetic) threatening processes such as declining food availability (Bradford et al, 2012; Christiansen, Dujon, Sprogis, Arnould, & Bejder, 2016; Christiansen et al, 2020; Lockyer, 1986; Miller, Best, Perryman, Baumgartner, & Moore, 2012). Concerns for cetacean populations are increasing due to a number of threats, including anthropogenic disturbance (Evans, 1996; National Research Council, 2005; Pirotta, 2018) and climate change, in particular the effects of changes in food availability for baleen whales in high latitude feeding areas (Fabry, McClintock, Mathis, & Grebmeier, 2017; Tulloch et al, 2019).…”