The relationship between body condition, fecundity, and mortality in Northern Resident killer whales

2025 July 22

Rowley A, Kay S, Mikus M, Vergara V, Visona-Kelly B, Barrett-Lennard L 2025. The relationship between body condition, fecundity, and mortality in Northern Resident killer whales. Raincoast Conservation Foundation. https://doi.org/10.70766/01.0775

Project Lead

Lance Barrett-Lennard, Project Lead

Contributors

Amy Rowley
Sharon Kay
Marie-Ana Mikus
Valeria Vergara
Brittany Visona-Kelly
Lance Barrett-Lennard

The findings presented in this report are based on preliminary analysis and have not undergone external peer review.

Funding for the work presented in this report was provided by a 2023 grant from Fisheries and Oceans Canada. The full time series of body condition data collected between 2014 and 2024 was facilitated by funders including Fisheries and Oceans Canada, the National Fish and WIldlife Foundation, Seaworld & Busch Gardens Conservation Fund, LNG Canada, the Ocean Wise Killer Whale Adoption Program, Mitacs, the Audain Foundation and the Vancouver Foundation.

Acknowledgements

We thank Holly Fearnbach and John Durban for their work developing the eye patch ratio metric of body condition and data collection in the early years of the study. We thank Gary Sutton and Dylan Smyth for data collection and data curation. Thanks also to Drew Grav-Graham and the Achiever crew. Thank you to Peter Thompson and Isaac Rankin for statistical advice. Cover image by Raincoast Conservation Foundation, SARA permit XMMS-2-2022.

Table of contents

Figures

Tables

Introduction

Since 2014, a team of scientists from the Raincoast Conservation Foundation (2022-2024), Ocean Wise Conservation Association (2014-2022), NOAA (2014-2017) and SR3 (2017) have collected aerial photographs of Northern Resident killer whales (NRKW) in the waters around northeastern Vancouver Island annually (August-September) in order to monitor body condition. In 2019, 2022, 2023 and 2024, additional field work was conducted on the Central Coast of British Columbia in June and/or July. Body condition was assessed from aerial photographs by calculating ‘eye patch ratios’ (Fearnbach et al. 2019), a measure of fatness behind the head calculated by dividing the distance between the eye patches at ¾ of their length by the distance between the anterior tips (Figure 1).

In this report, we first detail findings from field operations to assess NRKW body condition in 2023 and 2024. We then use the entire body condition time series from 2014 to 2024 to examine the relationships between body condition, survival and reproductive success in this population.

Figure 1: Measuring killer whale body condition with eye patch ratios

Killer whale eye patch ratio measurements are calculated as the length in pixels between the eye patch edges at 75% of their total length (b) divided by the length between the anterior eye patch tips (a). Photo: Ocean Wise, taken under SARA permit.

An updated and expanded 11-year time series of NRKW body condition

During the funding period from 2023-2025, we conducted four periods of field work to collect aerial photographs of Northern Resident killer whales (Figures 2 and 3). This allowed us to extend our existing time series of NRKW body condition data to 11 years, with a total of 222 whales belonging to 41 matrilines (a social unit consisting of a matriarch and her descendants) sampled at least once during the study period.

Figure 2. 2023 field trip vessel tracks

Routes taken on the (a) northern Vancouver Island (23 days) and (b) Central Coast (14 days) field trips in 2023.

Figure 3. 2024 field trip vessel tracks

Routes taken on the (a) northern Vancouver Island (14 days) and (b) Central Coast (16 days) field trips in 2024.

Table 1. Encounters with NRKW in 2023 and 2024

	Number of individual whales encountered	Number of matrilines encountered	Number of individual whales with successful aerial photographs
2023
Central Coast	42	8 (A25s, A54s, A73s, R17s, I12s, A35s, R13s, A42s + A94)	37
NE Vancouver Island	68	14 (A23s, A35s, I11s, A42s + A94, B7s, I16s, I33s, I35s, I68s, I4s, I27s, 165s, A54s, A25s)	65
2024
Central Coast	32	6 (A54s, I16s, I4s, I27s, I65s, R5s)	25
NE Vancouver Island	58	9 (A23s, A25s, A34s, A50s, A54s, I16s, I4s, I27s, I65s)	47

NRKW encountered in each sampling location and photographed for body condition analysis during the funding period from July 2023 – March 2025.

One of our objectives was to expand the geographical range of our sampling to include additional NRKW that do not frequent our typical field site off NE Vancouver Island. To this end, we spent a combined total of 30 days in 2023 and 2024 on the Central Coast, and were able to photograph 55 individuals across both years. However, we encountered only 14 individuals from five matrilines that we did not also encounter in the waters around NE Vancouver Island in the same year (Table 1). With the exception of the R17 matriline (photographed on the Central Coast in 2023), all of these whales had been encountered off NE Vancouver Island during previous years of the study. Nonetheless, sampling on the Central Coast in 2023 and 2024 did enable us to increase our sample of NRKW body condition data overall and add whales to our time series that we would not otherwise have measured in the same year (Figures 4 and 5).

Figure 4. NRKW eye patch ratio measurements, 2023

Eye patch ratio measurements taken in 2023 on the Central Coast and NE Vancouver Island.. Each point represents a different aerial photograph that was measured. Grey panels indicate which matriline whales belong to.

Figure 5. NRKW eye patch measurements, 2024

Eye patch ratio measurements taken in 2024 on the central coast and NE Vancouver Island. Each point represents a different aerial photograph that was measured. Grey panels indicate which matriline whales belong to.

There were 20 individuals for which we measured at least three different aerial photographs in both 2023 and 2024, allowing us to compare body condition between the two years (Figure 6). Of these, a majority showed a decline in fat levels. Fat loss occurs predictably at certain stages of a whale’s life history (e.g., around weaning age or while lactating) and is not always a cause for concern. The decrease observed in calves A126, A134 and I165 is somewhat typical for their ages, although, in the cases of A134 and I165, the decrease is uncommonly steep. I107’s decline in eye patch ratio is also unusual for a 20-year-old male, and may be a consequence of the death of his mother I27 between 2023 and 2024; female killer whales continue to provision their sons well into adulthood (Wright et al. 2016), so the death of a matriarch could result in reduced food intake. The relatively large declines shown by A43 (a 43-year-old female) and I51 (a 38-year-old female) could be a sign of declining health. As neither female has been observed with a new calf in recent years, this decrease in fat likely cannot be explained by parturition or lactation, and other females showed similar declines in the years before death (see Figure 9). Finally, I65’s increase in eye patch ratio, in contrast to the rest of her matriline, could be indicative of pregnancy (see Figure 10).

Figure 6: Changes in NRKW eye patch ratio from 2023 to 2024

Changes in eye patch ratio from 2023 to 2024 for NRKW individuals with three or more aerial images measured in both years. Points indicate mean eye patch ratio for each individual in each year. Panels represent NRKW matrilines.

Body condition, mortality, and reproduction in NRKW

An improved understanding of relationships between body condition and both mortality and reproduction

How does body condition influence survival in NRKW?

Mortality was rarely observed during the study period. Of 222 individuals measured at least once between 2014 and 2024, only 17 had died as of 2024. In general, focal whales that died during the study tended to be either older individuals reaching the end of their expected lifespan (8 individuals), females who recently gave birth (3 individuals) or the calves of recently deceased females (2 individuals). Collectively, this accounts for 13 of the 17 deaths of whales measured during the study. Of these 17 individuals, only eight died within a year of their body condition being assessed. This limited number of deaths complicates statistical analysis of how body condition affects survival probability in NRKW.

During the study period we observed two whales that exhibited signs of ‘peanut head’ (a concave appearance behind the head resulting from extremely low body fat) and died within the year they were photographed (Figure 7). I63, a 24-year-old female, had an eye patch ratio of 0.99, well below the expected range for adult females (Figure 8). The second whale, A37, was a 37-year-old male. Unfortunately, the research team was unable to obtain suitable images of A37 for measurement (above the water and fully exposing anterior and posterior eye patch tips). However, underwater images of A37 reveal a similar condition to I63 (Figure 7), and it is evident that A37 likely had an eye patch ratio far below the expected range for adult males (although it should be noted that, at 37, A37 would be considered old for a male NRKW).

Figure 7. Images of I63 and A37 before death

Images of I63 (left) and A37 (right) during August 2014, the year both whales were later declared deceased. Photos: Ocean Wise and NOAA taken under SARA permit.

Figure 8: Eye patch ratio of whales that died relative to predicted values for age and sex

Cubic regression splines for males (green points) and females (red points) showing the relationship between age (in years) and eye patch ratio in NRKW. Blue points indicate individuals of unknown sex. Bolded and labelled points represent the eight individuals who died within a year of being measured during the study.

While some individuals exhibited poor condition prior to death (Figure 7, Figure 8)–including I63, whose eye patch ratio before her death in 2014 was among the lowest ever measured–low body condition in late summer/early autumn generally does not appear to be associated with mortality in NRKW. However, it is important to note that the cause of death is rarely established in wild killer whales. A recent study of tropical and northeastern Pacific killer whales found that necropsied individuals died from a range of causes including infectious disease, congenital malformations, traumatic injury and malnutrition (Raverty et al., 2020). If a killer whale died from, for example, blunt-force trauma resulting from a vessel strike (a known cause of mortality in resident killer whales), body condition would be irrelevant. Such cases could therefore mask a link between body condition and mortality risk.

Another significant limitation is that NRKW were typically only measured once annually during late summer/early autumn (or twice in the case of the small number of whales sampled in both field sites in the same year). It has been shown that individual killer whale body condition can change substantially in as little as three months (Fearnbach et al., 2020). We also can often only date the death of a killer whale to within a year. Therefore, if a whale were to decline rapidly in body condition and die in less than a year, their body condition the summer prior may not be representative of the true state that preceded their death. However, we lack the temporal resolution in our dataset to investigate this more comprehensively.

Nevertheless, the very low mortality and generally good condition of NRKW assessed during the study period suggest that malnutrition is not likely to have caused many deaths in this population during this time. As a point of interest, the disproportionately high number of females who died within a few years of giving birth could potentially indicate that postpartum health issues or the costs associated with reproduction and lactation are a significant source of mortality in resident killer whales.

Figure 9. Body condition of I27 matriline over time

Variation in eye patch ratio over time within the I27 matriline. Matriarch I27 was missing during the 2024 field season and is very likely deceased.

Of all animals that died during the study period, I27 (a female who died at the age of 50 between the summer of 2023 and 2024) was measured at the greatest number of time points, allowing us to examine how her body condition changed in the years preceding her death (Figure 9). Notably, I27 experienced a substantial decline in condition between 2021 and 2022, one year prior to her presumed death, and showed no improvement in the following year. This decline was even more pronounced than I27’s decrease in eye patch ratio between 2015 and 2016 after giving birth to I153, when it is common for females to rapidly lose fat (Kay et al., in prep). While we cannot definitively rule out that I27 gave birth to a calf between 2021 and 2022 that was never documented, this would be highly unusual for a female of her age at the time (48 years). Therefore, it seems reasonable to conclude that I27’s unexplained fat loss during this period may have been an indicator of her approaching death.

How does body condition change throughout the female reproductive cycle?

Maternal condition has been shown to influence reproductive success in a variety of species. However, the inverse is also true: the energetic costs associated with reproduction and subsequent parental care can strongly affect maternal condition.

To address how female body condition varies throughout pregnancy and lactation, we evaluated the eye patch ratios of 66 reproductive-aged NRKW females (12-45 years) across reproductive stages. The results of this analysis will be presented in a forthcoming manuscript (Kay et al., in prep). For illustrative purposes, we here show the eye patch ratio of a single female measured over six consecutive years, from before pregnancy to the end of lactation (Figure 10). While the increase in eye patch ratio during pregnancy and the subsequent decline following parturition is a consistent pattern across reproductive females in our dataset, it should be noted that there is considerable individual variation, most notably in the timing of ‘recovery’ to pre-pregnancy fat levels during or after lactation (Kay et al., in prep).

Figure 10. Effect of reproductive status on the body condition of a female NRKW

Eye patch ratio of a single adult female NRKW measured in 6 consecutive years through various stages of reproduction. Points indicate mean eye patch ratio for this female in each year. Inset photo: Ocean Wise and NOAA, taken under SARA permit.

Does female body condition affect reproductive success?

To investigate how body condition influences reproductive success in female NRKW, we examined the association between eye patch ratio and calving probability in reproductive-age females (12-45 years old). Calving was treated as a binary outcome where each female was assigned “y” if they gave birth to a calf in a given year and “n” if they did not. Years in which a female could not produce a calf (i.e. if she was pregnant or nursing an existing calf) were excluded from the analysis.

The strong pattern of fat gain and loss throughout the stages of reproduction (Figure 10; Kay et al., in prep), coupled with the approximately 17-month gestation period of killer whales, makes it challenging to examine how body condition influences reproductive success. In addition, the exact timing of the birth of a calf is usually not known in NRKW, and our body condition estimates are limited to a single time point each year in the summer. Therefore, we opted to examine the effect of eye patch ratio lagged by one year (with the understanding that eye patch ratio at this time point would likely be confounded by fat gain in pregnant females) and lagged by two years (likely before the onset of pregnancy in most females) in separate models.

Across reproductive-age females in our sample, the probability of calving in any given year was approximately 0.14. Probability of calving was positively associated with EP ratio the year before the calf was first documented (Table 2, Figure 11). However, this is almost certainly due to females gaining fat during pregnancy (Figure 10). The probability of a female reproducing was not significantly associated with her EP ratio two years before a calf was documented (i.e. pre-pregnancy) (Table 2, Figure 12).

Table 2. How does eye patch ratio influence annual calving probability

Results of Bayesian logistic regression examining how eye patch ratio impacts the annual probability of a female giving birth. To control for the potentially confounding effect of age on calving probability, we included a smooth effect of age in years in the model. We also included a random effect of whale ID to account for repeated measures of individual females during the study, and a random effect of year to control for the possibility of inter-annual effects on NRKW reproduction.

Eye patch ratio lagged by one year
Fixed Effects	Slope	SE	l CI	u CI
Intercept	-16.11	9.14	-34.75	1.19
Eye patch ratio	12.36	7.68	-2.15	27.87
Age	-2.13	4.50	-11.52	6.87
Random effects	Slope	SE	l CI	u CI
Whale ID	1.59	0.74	0.31	3.23
Year	0.77	0.52	0.05	2.05
Eye patch ratio lagged by two years
Fixed effects	Slope	SE	l CI	u CI
Intercept	-1.99	8.35	-18.67	14.15
Eye patch ratio	0.13	7.08	-13.60	14.20
Age	-0.81	3.82	-9.38	6.26
Random effects	Slope	SE	l CI	u CI
Whale ID	0.90	0.62	0.04	2.33
Year	0.46	0.40	0.02	1.47

Figure 11. Effect of female body condition on calving probability in NRKW

Effects of eye patch ratio lagged by (a) one year and (b) 2 years on the probability of producing a calf among reproductive-age NRKW females. Shaded area represents 95% confidence intervals.

Overall, body condition did not appear to limit reproduction in NRKW females during the study period. One possible explanation is that female body condition remained generally high enough throughout the study to allow females to meet the costs associated with producing offspring. The steady annual increase in population size in NRKW supports this explanation. However, it is possible that body condition may begin to limit reproduction at a lower threshold of body condition than focal females exhibited during the study period.

References

Fearnbach H, Durban JW, Barrett‐Lennard LG, Ellifrit DK, Balcomb KC. 2019. Evaluating the power of photogrammetry for monitoring killer whale body condition. Marine Mammal Science. 36(1):359–364. doi:https://doi.org/10.1111/mms.12642.

Ford JKB, Ellis G. 2006. Selective foraging by fish-eating killer whales Orcinus orca in British Columbia. Marine Ecology Progress Series. 316:185–199. doi:https://doi.org/10.3354/meps316185.

Kay S, Rowley, A, Visona-Kelly, B, Barrett-Lennard, L, Thompson, P, Sutton, G, Fearnbach, H, Durban, J., Darimont, C. “Cost of Maternal Care Revealed Through Body Condition in Northern Resident Killer Whales” [manuscript in preparation]

Raverty S, St. Leger J, Noren DP, Burek Huntington K, Rotstein DS, Gulland FMD, Ford JKB, Hanson MB, Lambourn DM, Huggins J, et al. 2020. Pathology findings and correlation with body condition index in stranded killer whales (Orcinus orca) in the northeastern Pacific and Hawaii from 2004 to 2013. PLOS ONE. 15(12):e0242505. doi:https://doi.org/10.1371/journal.pone.0242505.

Wright BM, Stredulinsky EH, Ellis GM, Ford JKB. 2016. Kin-directed food sharing promotes lifetime natal philopatry of both sexes in a population of fish-eating killer whales, Orcinus orca. Animal Behaviour. 115:81–95. doi:https://doi.org/10.1016/j.anbehav.2016.02.025.

Rowley A, Kay S, Mikus M, Vergara V, Visona-Kelly B, Barrett-Lennard L. 2025. The relationship between body condition, fecundity, and mortality in Northern Resident killer whales. Raincoast Conservation Foundation. https://doi.org/10.70766/01.0775