The cost of reproduction is a key parameter determining a species' life history strategy. Despite exhibiting some of the fastest offspring growth rates among mammals, the cost of reproduction in baleen whales is largely unknown since standard field metabolic techniques cannot be applied. We quantified the cost of reproduction for southern right whales Eubalaena australis over a 3 mo breeding season. We did this by determining the relationship between calf growth rate and maternal rate of loss in energy reserves, using repeated measurements of body volume obtained from un manned aerial vehicle photogrammetry. We recorded 1118 body volume estimates from 40 female and calf pairs over 40 to 89 d. Calves grew at a rate of 3.2 cm d −1 (SD = 0.45) in body length and 0.081 m 3 d −1 (SD = 0.011) in body volume, while females decreased in volume at a rate of 0.126 m 3 d −1 (SD = 0.036). The average volume conversion efficiency from female to calf was 68% (SD = 16.91). Calf growth rate was positively related to the rate of loss in maternal body volume, suggesting that maternal volume loss is proportional to the energy investment into her calf. Maternal in vestment was determined by her body size and condition, with longer and more rotund females investing more volume into their calves compared to shorter and leaner females. Lactating females lost on average 25% of their initial body volume over the 3 mo breeding season. This study demonstrates the considerable energetic cost that females face during the lactation period, and highlights the importance of sufficient maternal energy reserves for reproduction in this capital breeding species.

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.

Animal body size and growth patterns play important roles in shaping the life history of species. Baleen whales include the largest animals on the planet, with somatic growth costs expected to be substantial. We used unmanned aerial vehicle photogrammetry and long-term individual sighting histories from photo identification (1991-2019) to estimate the cost of somatic growth for southern right whales (SRWs) Eubalaena australis. A Richards length-at-age growth model was developed, based on 161 calves, 20 yearlings, 1 juvenile and 23 adults, ranging in age from newborn to 27 yr. Predicted lengths were 4.7 m at birth, 12.5 m at minimum age of first parturition (6 yr) and an asymptotic length of 14.3 m. A volume-at-age curve was estimated from the body volume versus length relationship, and converted to a mass-at-age curve, using data on body tissue composition of North Pacific right whales E. japonica (n = 13). The energetic cost of growth was estimated using published estimates of tissue lipid and protein concentrations. The cost of growth for SRWs (in MJ d-1) was 2112 at birth, 544 at 4 mo, 314 at 1 yr (~weaning age), 108 at 5 yr (minimum age of sexual maturity), 51.5 at 10 yr and 5.2 at 30 yr. The cumulative cost to age 30 was 764.3 GJ, but varied widely (458-995 GJ) depending on the tissue energy content. Our estimates represent a healthy SRW population, and provide a baseline to investigate individual and population level impacts of anthropogenic disturbance (including climate change).

Seasonal trends in the distribution and relative abundance of southern right whales (SRWs) Eubalaena australis, were assessed in Australia's largest calving aggregation ground at the Head of the Great Australian Bight, in the Commonwealth Marine Reserve, South Australia. Annual cliff‐based surveys were undertaken between June and October from 1992 to 2016. SRWs were primarily distributed in a 15 km by 2 km area within the 10 m depth contour (with 95% of whale sightings made within a 10 km2 area). The distribution of SRWs at Head of Bight varied within an individual season but was consistent among the years. The composition of SRW sightings was 70% female–calf pairs and 30% unaccompanied whales. Peak abundance occurred between mid‐July and end‐August for female–calf pairs and unaccompanied whales (juveniles or adults not accompanied by a calf), earlier than previously reported. A mean of 16% (range 8–28%, SD = 6.5, 95% CI = 0.15) of calving females were present at the site in mid‐June and a mean of 37% (range 13–61%, SD = 15.8, 95% CI = 0.37) remained at the site at the end of September. Based on nearest‐neighbour distances of 150 m, the area occupied by 95% of SRWs at Head of Bight could reach carrying capacity at 68 female and calf pairs. Results suggest that the primary aggregation area at Head of Bight may have reached saturation capacity and that habitat expansion can be expected as the population increases. This study provides information on SRW seasonal trends in distribution and abundance, timing of arrival and departure from the site and peak abundance periods relevant to application to conservation and marine park management. As management requirements increase with a growing population, there is a need to complete an Australia‐wide assessment of SRW connectivity and habitat expansion.

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