Abstract
One of the greatest challenges for environmental science is to understand how changes in global climate might affect biological systems. Marine mammals can act as sentinels of change in subantarctic environments, reflecting medium and long-term processes. Given their life histories and ecologies, baleen whales are ideal for addressing such questions. Right whales (Eubalaena sp.) were among the first globally harvested whales, brought to near extinction by 19th and 20th century commercial whaling. While protection from commercial harvesting has allowed some populations to begin recovery, others are still struggling in an increasingly urbanised world. Though some direct anthropogenic threats to right whale recovery have been identified (e.g., ship strike, entanglement in fishing gear and sub-lethal noise population), we are becoming more aware of the insidious and mounting ecosystem effects climate change can have on populations.
Unlike other Southern Ocean baleen whales for which calving areas are poorly known (e.g., blue whales, fin whales), female SRWs show high site fidelity to coastal calving areas. The Auckland Islands is one such place and is the only known contemporary calving ground of southern right whales (Eubalaena australis) in New Zealand. The population here appears to be recovering strongly after almost complete decimation due to systematic hunting, though is still at less than 12% of its pre-whaling abundance. Therefore, the nutritional state of individuals, and the reproductive output of the population, is likely to be directly limited by food availability.
Yearly abundances of reproducing females since 2008 were estimated using established photo ID techniques. I also report on results from a novel artificial intelligence matching procedure used during photo-ID processing. The reproductive portion of whales that visit Port Ross each year is increasing at an average annual rate of 12.4% (95% CL 10.9%, 13.9%). While high compared to whole-population growth rates elsewhere, this rate falls within biological plausibility and may be inflated by several non-exclusive population specific factors. Firstly, migration may be occurring from other yet undiscovered calving areas within NZ, or from other populations. Secondly, given the early stage of recovery post-whaling, the population age structure is likely skewed towards younger, more fecund whales.
I employed unmanned aerial vehicle-based photogrammetric systems to establish a new aerial photo-ID catalogue, characterize the size structure and estimate individual body conditions of SRWs at Port Ross over three consecutive calving seasons, representative of a complete reproductive cohort. Principal component analyses indicated that widths between 30–80% along total lengths represent important endogenous energy stores. Calculated body condition indices from these regions indicated that lactating females and calves were in the best condition, followed by non-breeding adults and subadults. Mother total lengths and body conditions were significantly correlated with those of their calves. A negative relationship was observed between total length and body condition in subadults, reflecting the increased allocation to somatic growth for this age class compared to others.
Using time-stamped photo-ID images collected from UAV platforms, association rates among whales present at Port Ross in 2017 and 2018 were quantified using time between images as a proxy for social closeness. While on the calving ground, associations appear relatively weak. No preferred or avoided associations were identified; demographics associated within and amongst themselves to similar degrees. From time-lag analysis, associations likely last over a single calving season.
In Chapter 6, I explored potential relationships between the number of calving females at Port Ross each year and variability in climate and prey at an inferred foraging ground. Using annual abundance estimates from Chapter 2, abundance anomalies were used as the single response variable in univariate analysis. Potential predictor variables included remotely sensed chlorophyll-a and sea surface temperature anomalies, as well as the climate indices for southern annular mode and southern oscillation. Time lags in response variables were set at 6, 12 and 18 months, representing one calving and two foraging seasons prior to annual abundance estimates. Zooplankton density from Southern Ocean Continuous Plankton Recorder (SO CPR) surveys were also included as a potential response variable, lagged at 6 and 18 months, the latter of which was identified as the best single predictor of female abundance at Port Ross. This corresponded to the amount of food available at foraging areas during the resting phase of female reproduction; more food during this period promoted increased numbers of reproducing females the following season. These results highlight the sensitivity of the SRWs of NZ to fluctuations in food availability. While considered relatively healthy and recovering strongly, climate driven fluctuations in food availability have the potential to lengthen the reproductive cycles of female right whales. If fluctuations in food become more common and severe, the current recovery of the SRWs in NZ may become compromised.