Large marine mammals can serve as an indicator of the overall state of the environment due to their apex position in marine food webs and their functions as sentinels of change (Moore, 2008; Williams et al., 2013). Changes in environmental conditions can result in decreased prey availability at the base of the food web, which impacts predators further up the trophic chain (Acevedo-Whitehouse and Duffus, 2009; Benoit-Bird and McManus, 2012; Soledade Lemos et al., 2020; Rasmussen et al., 2021). In the case of whales, reduced food availability results in visible reductions in fat stores (Lockyer, 1986; Konishi, 2006; Christiansen et al., 2013; Braithwaite et al., 2015). Whales with a compromised body condition may be unable to take on the challenges of their extreme life history, such as their long migrations and periods of fasting, which can lead to reduced survivorship and reproductive fitness (Greene et al., 2003; Lockyer, 2007; Bradford et al., 2012; Soledade Lemos et al., 2020). This can have adverse impacts on the population or species as a whole, which is compounded in recovering populations (Acevedo-Whitehouse and Duffus, 2009).

These impacts are magnified in pregnant and lactating females who rely on bountiful foraging seasons in order to sustain themselves and their calf during the different phases of pregnancy (Lockyer, 1984; Miller et al., 2012; Christiansen et al., 2013; Christiansen et al., 2014). Body condition affects the fertility of many mammalian species, including whales, where a minimum threshold of body fat is needed for ovulation, and therefore, pregnancy (Young, 1976; Frisch, 1984; Lockyer, 1987; Miller et al., 2011). Poor foraging can also influence future pregnancies through a process known as the carry-over effect- where a reduced body condition of a reproductive female can decrease her ability to carry a pregnancy to term in future years, ultimately impacting population growth rates (Soledade Lemos et al., 2020).

Blue whales (Balaenoptera musculus) are the largest animal on Earth and thus have the highest prey demands (Savoca et al., 2021). These large animals feed almost exclusively on euphausiids (krill) and consume up to 22 tons of prey per day, often employing energetically costly maneuvers to obtain their prey (Acevedo-Gutiérrez et al., 2002; Croll et al., 2005; Calambokidis et al., 2008; Goldbogen et al., 2011; Goldbogen et al., 2015; Barlow et al., 2020; Savoca et al., 2021). Blue whales were hunted extensively during the whaling era, facing reductions in populations of 90% or more (Jefferson et al., 2015; Thomas et al., 2015). After they became a protected species by the International Whaling Commission in 1966 blue whale populations started to increase, but they are still considered endangered. The eastern North Pacific population of blue whales is currently estimated to be at 1,898 (lower and upper 20th percentile values of 1,767 to 2,038) individuals (Calambokidis and Barlow, 2020) and is considered to be at or near pre-whaling estimates (Monnahan et al., 2015).

Due to their narrowly focused prey regime, blue whale health is closely linked with environmental factors that determine prey abundance and concentration (Croll et al., 2005; Calambokidis et al., 2007; Silber et al., 2017). The eastern North Pacific blue whales forage partially in the highly productive California Current System which spans from British Columbia, Canada (~50°N) to Baja California, Mexico (~15-25°N) (Huyer, 1983; Di Lorenzo et al., 2008; McClatchie et al., 2008; Checkley and Barth, 2009; Palacios et al., 2019). Productivity in the California Current System is driven by patterns of variability on various spatial and temporal scales from local and seasonal upwelling to ocean basin level interannual/decadal Pacific Decadal Oscillation. From 2014 to 2016 the California Current System also experienced a large marine heat wave with sea surface temperature anomalies exceeding 3°C, which was the warmest three- year stretch on record (Schwing et al., 2006; McClatchie et al., 2008; Checkley and Barth, 2009; Di Lorenzo and Mantua, 2016; Gentemann et al., 2017; Thompson et al., 2018).

Body condition is commonly used to assess the health of individuals in both terrestrial and aquatic ecosystems (Lockyer et al., 1985; Lockyer, 1986; Batzli and Esseks, 1992; Pettis et al., 2004; Konishi, 2006; Acevedo-Whitehouse and Duffus, 2009; Bradford et al., 2012; Williams et al., 2013; Braithwaite et al., 2015; Soledade Lemos et al., 2020; Akmajian et al., 2021; Rasmussen et al., 2021). Body condition studies are important to gauge the health of recovering populations of animals and the environment as a whole (Williams et al., 2013). However, measuring large whale body condition at sea is logistically challenging due to their size, sighting frequency, and remoteness (Pettis et al., 2004; Konishi, 2006). Previous studies have used blubber thickness and whaling measurements of girth in carcasses to estimate health (Lockyer et al., 1985; Lockyer, 1986; Konishi, 2006; Williams et al., 2013; Braithwaite et al., 2015). But these studies have limitations as few countries have allowed the lethal removal of whales since the 1980s when international whaling was outlawed. Other studies have used ultrasound to measure the blubber thickness on live right whales (Miller et al., 2011), but such methodology would be challenging for faster, more streamlined whales.

Alternatively, the use of photographic data has been developed as a low cost, easy to implement, and non-invasive way to monitor whale health. Qualitative visual body condition assessments of target areas make this methodology valuable for long-term studies (Pettis et al., 2004; Bradford et al., 2012; Akmajian et al., 2021). Photographic studies show temporal trends in the body condition of baleen whales over years for species such as North Atlantic right whales (Eubalaena glacialis) (Pettis et al., 2004), and gray whales (Eschrichtius robustus) (Bradford et al., 2012; Soledade Lemos et al., 2020; Akmajian et al., 2021).

The aims of this study were (1) to determine if photographs, especially those used for individual identification (ID) of blue whales can be used to assess body condition, (2) to determine if body condition varied by year or reproductive class, and (3) to investigate the relationships between blue whale body condition and environmental indices representative of upwelling strength and productivity in the California Current System. This research provided a non-invasive model for monitoring blue whale body condition in the future and determining how blue whales may respond to a quickly changing ocean.

This study shows that visual health assessments based on foundational studies (Pettis et al., 2004; Bradford et al., 2012) can be applied to blue whales to track body condition over time. The lateral flanks varied in the amount of subcutaneous fat in that area which made differentiation between condition scores possible. This region had the added benefit of being the target for photo identification images, making it possible to use historical data in our analysis. Our study could be used as a basis to investigate other streamlined whales such as the minke (Balaenoptera acutorostrata), sei (Balaenoptera borealis), and fin (Balaenoptera physalus) whales.

The reproductive class of blue whales had a highly significant impact on body condition. Calves had the best body condition scores in the population and females who were seen with dependent calves had the worst scores, though this study did not account for variation among other demographic groups. Other body condition studies have found that cows who are nursing their calves are in the worst condition of observed whales (Pettis et al., 2004; Bradford et al., 2012; Soledade Lemos et al., 2020). The high reproductive costs of nursing a mysticete whale calf has been well documented, as cows must consume enough during their feeding season to sustain themselves and their quickly growing calf while in calving grounds, although all North Pacific blue whales forage year-round (Busquets-Vass et al., 2021). Blue whale calves are nursed over a period of 7-8 months during which their size more than doubles (Mackintosh and Wheeler, 1929; Jefferson et al., 2015) and this period of lactation is the costliest part of reproduction, requiring 3-5 times more energy than gestation (Miller et al., 2012).

Like Bradford et al., 2012, we saw small variations in the body condition of calves across years with 76% being scored as having good body condition scores (score 0) despite a variation of conditions being observed in the other groups. This could be due to the link between health and reproductive success seen in many species (Lockyer, 1984; Lockyer, 1986; Greene et al., 2003; Christiansen et al., 2013; Williams et al., 2013; Soledade Lemos et al., 2020). Females in poor body condition may be unable to become pregnant (Young, 1976; Frisch, 1984; Lockyer, 1987; Miller et al., 2011), and those that do then must minimize the energy expended on their fetus to maximize their own chance of survival (Christiansen et al., 2014). This means poor foraging conditions can lead to drops in calving rates- something that is especially concerning for an endangered species (Greene et al., 2003). A year of bad foraging can also influence more than just the pregnancies for that year, as a reduced body condition of a reproductive female can decrease their ability to carry a pregnancy to term in future years (carry-over effect; Christiansen et al., 2014; Soledade Lemos et al., 2020). If foraging conditions are poor for consecutive years, the long term recovery of endangered populations, especially if they are dealing with the effects of human activities, can be in jeopardy.

Blue whale body condition varied by year, but overall the proportion of whales in poor body condition remained generally low, except for two years (2015 and 2017) when more than half of the whales were in poor health. While this study focused on the blue whales feeding on the US west coast, the eastern North Pacific population feed year-round and has a much broader feeding range that extends from the Eastern Tropical Pacific to Alaska (Mate et al., 1999; Stafford et al., 1999; Calambokidis et al., 2009; Monnahan et al., 2014). After their numbers were greatly reduced from whaling, blue whales were rarely seen in Alaska; but starting in the late 1990s an increase of whales in more northern waters coincided with a decrease of blue whales off California (Calambokidis et al., 2009). It has been hypothesized that when foraging around California is poor due to decreased krill availability or increased competition, some blue whales may continue elsewhere to look for better foraging opportunities. It is likely that the pattern in blue whale body condition we observed results from a combination of some whales shifting foraging grounds and others being influenced by the feeding conditions off of California (Calambokidis et al., 2009; Busquets-Vass et al., 2021).

Variation in energy reserves (body fat) has been linked to prey availability, where oftentimes environmental conditions are used as a proxy (Braithwaite et al., 2015; Soledade Lemos et al., 2020). We looked at several environmental indices that we believed would drive prey availability and therefore body condition. PDO and LUSI were the environmental drivers selected in our final model. While our final model included data from environmental indices taken in the same year as the sightings, other models that performed well had one- or two-year lags for the environmental indices, which may reflect that blue whale body condition is a function of both conditions in the current year, as well as previous years.

PDO varied significantly by year over our study period with high values in 2014-2016. Only four years of our study period had positive PDO values (2014-2017), which followed the same pattern of poor body condition. PDO reflects sea surface temperature in the Pacific Ocean basin with positive PDO values signifying warmer ocean temperatures (Mantua and Hare, 2002; Brinton and Townsend, 2003; Di Lorenzo and Mantua, 2016). We also looked at LUSI since upwelling occurs on a smaller temporal scale than PDO, seasonally leading to high levels of primary and secondary production (Huyer, 1983; Fiedler et al., 1998; Croll et al., 2005; Checkley and Barth, 2009). The seasonal upwelling coincides with the summer feeding period for blue whales (Croll et al., 2005; Barlow et al., 2020) and therefore its length could drive overall foraging success. The worst body conditions occurred in years with the shortest upwelling seasons. Our results suggest that warm sea surface temperatures driven by positive PDO values and short upwelling seasons are detrimental to blue whale health by reducing food resources for the whales feeding off the US west coast. PDO was linked to changes in blue whale distribution and their use of areas off the US west coast versus further north into the Gulf of Alaska for present day as well as for historical whaling data showing that PDO may predict broad shifts of blue whale distribution in the eastern North Pacific (Calambokidis et al., 2009).

While not all environmental indices are highly correlated, they do play off one another and have a cumulative impact on the California Current System and the organisms that live there. Future studies may be required to quantitatively link specific indices and environmental events to whale body condition. It would be interesting to perform a similar study on other whales that forage in the same region, like humpback whales (Megaptera novaeangliae). Unlike blue whales, humpbacks forage only seasonally so the environmental factors may have a larger impact on their body condition.

We provided a basis for evaluating large whale health using historic records that can provide context for present and future conditions. This methodology can also be used to monitor populations over the long term which will only become more important in the context of a changing climate. Marine heat waves are expected to become more frequent and more intense in the future (Oliver et al., 2018) due to anthropogenically driven climate change (Frölicher et al., 2018; Oliver et al., 2018). This study indicates that with an adequate scoring method, photographs collected during boat based surveys can be used to effectively evaluate whale health in response to a changing ocean.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The animal study was reviewed and approved by Cascadia Research IACUC.

The authors confirm contribution to the paper as follows: RW, JC, and KF conceived the study. JC and KF collected original data. RW scored the data and created the model. RW was the lead author on the draft and revisions. JC and KF gave assistance in analysis and editing the manuscript. All authors contributed to the article and approved the submitted version.

Support for long term photo ID effort was provided through various contracts and grants over the years from the National Oceanic and Atmospheric Administration (NOAA) as well as support for this project through a NOAA Section 6 grant through the Washington Department of Fish and Wildlife. Support for initial assessment of blue whale body condition provided through a grant from the Office of Naval Research with funding from the Bureau of Ocean Energy Management through the National Oceanographic Partnership Program.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.