We would like to acknowledge the role that conservation has played in the successful recovery of humpback whales after humpback whaling ceased. The protection level enabled through the listing of this species on the threatened species list has further contributed to the successful recovery of humpback whales in Australian waters. But without global efforts of protection of this species across the Southern Hemisphere, there may have been a different outcome. As such we applaud the progress made for this species in Australian and international waters. It is celebrated as one of the greatest success stories of conservation (Duarte et al., 2020). However, this celebration needs to be taken with caution and requires ongoing monitoring to ensure that the recovery of humpback whales continues.
Large baleen whales are facing ongoing threats and their populations may not yet be stable. There are warning examples such as the Gray whale (Eschrichtius robustus) (Stimmelmayr and Gulland, 2020). Once recovering well and removed from the threatened species list in the USA, they are now experiencing one of the greatest mortality rate recorded on the west coast of North America. It has been attributed to a food shortage likely caused by climate change (Christiansen et al., 2021).
Whaling ceased in 1978 in Australia allowing humpback whales to slowly come back from the brink of extinction. The threat of whaling has not existed for the past 43 years in Australia, but, new threats that are far more complex have emerged and they are impacting on humpback whales through underwater noise, pollution, vessel strikes, fisheries interactions, marine debris, habitat degradation, tourism pressure and climate change. In combination these threats have the potential to cause population decline of humpback whales in Australia.
Specifically, we would like to raise the following concerns in regards to the assessment of delisting of humpback whales from the threatened species list:
- We disagree that the impacts outlined in the assessment are not threatening or preventing population growth in the past years and in the future. The proposed trajectory of population growth does not take into account current and future impacts from one of the major threats: climate change. There is increasing evidence that the impacts listed and in particular climate change already have and continue to cause concerning impacts on other humpback whale populations (Ramp et al., 2015; Fleming et al., 2016; Gabriele et al., 2017; Silber et al., 2017; Cartwright et al., 2019; Derville et al., 2019; Tulloch et al., 2019; Santora et al., 2020; Kershaw et al., 2021) in particular through the decrease in prey abundance (Bengtson Nash et al., 2018; Cartwright et al., 2019; Michael et al., 2021; Prado-Cabrero and Nolan, 2021). Studies are currently underway (relevant for Criterion 1, A3, A4) investigating to what degree a population reduction may occur within the next 100 years based on current and future threats (Meynecke et al., 2020).
- In the past 5 years changes have been documented in the population that need to be reflected in the assessment, this includes shifts in calving locations and migration timing (Torre-Williams et al., 2019). Other populations have seen a possible decline in population growth in the past years (Cartwright et al., 2019; Kügler et al., 2020) and marked shifts in abundance and distribution (Becker et al., 2018; Avila et al., 2019) and increased risk of entanglements (Santora et al., 2020).
- There are indications from other data that the increase by 10-11% has already slowed down since 2010 or that sub-groups do not show the same recovery rate (Franklin, 2014). Two independent presence only survey data sets from the sub-groups in Gold Coast and Hervey Bay undertaken since 2010 indicate half the reported growth rate.
- The assessment of the delisting of humpback whale populations needs to be based on measured data over the past 10 years or 3 generations that should be publicly available. The 3-generation time window for applying the reduction criterion (A1) for a threatened category is estimated to be 1942- 2018 (Cooke, 2018). The one study used for the assessment of the population E1 of the past 10 years is based on data collected in 2007, 2010 and 2015 carried out over two month surveys (Dudgeon et al., 2018; Noad et al., 2019). The studies used to justify the delisting of humpback whales of population D are based on data collected between 1999-2008 (Hedley et al., 2011; Kent, 2012). We recommend the provision of additional references e.g. Harrison and Woinarski, 2018, updating population surveys and applying the 3 generation criterion.
- The mixing of populations in Oceania e.g. between E1 and E2 population needs to be addressed when undertaking population assessments along the east coast of Australia. In a global assessment of the IUCN Red Listing humpback whales were downgraded to least concern in 2008 apart from the Arabian Sea and Oceania population (E2, E3, F1, F2), which remained in the “Endangered” category (IUCN 2008; Cook et al 2018) until today. There is increasing evidence that the same migratory route is partially used by E1 and E2 populations along the east coast of Australia and that the number of animals sharing the same migratory path are higher than previously expected (Valsecchi et al., 2010; Garrigue et al., 2011; Clapham and Zerbini, 2015; Derville et al., 2019; Garrigue et al., 2020). This also shows that our understanding of population connectivity is not complete and assessing populations separately is misleading.
- The proposed cycle of increase and decline in population is based on the assumption that both D and E1 are stable population at present or in the future. Currently the age structure of both populations still has a strong bias towards 10-20 year age classes (Franklin et al., 2017) If the postulated “over-shoot” occurs as suggested, the population is unlikely to respond with a continues cycle of increase and decrease but with a sudden and sharp decrease taken into account the age structure and combined impacts in particular from climate change.
- The pre-whaling carrying capacity of humpback whales is unlikely going to be equal to the present and future carrying capacity considering the immense changes in predator-prey relationships caused by whaling in Antarctica since the 19th century (Roman et al., 2014; Pattyn and Morlighem, 2020) and altered environmental conditions (Ryabov et al., 2017). Carrying capacity of humpback whales should be larger than pre-whaling considering that other baleen whale species have not equally recovered and a shortage of food was unlikely since whaling ceased.
- IWC estimation for humpback whale populations prior whaling should be taken as an indication of what the total number of whales might have been. However, numbers were very likely higher and have been underestimated (Holt, 2004). Firstly, the pre-modern catches may have been larger than estimated; secondly, simulation studies showed that alternative assumptions about density-dependent limitation can result in very different past and future population trajectories (Leaper et al., 2011); thirdly, social factors need to be considered for understanding repopulation of historical breeding grounds (Clapham and Zerbini, 2015).
Notably whaling nations have argued to continue or presume whaling justified by the recovery of a species (The Guardian, 03/09/20). We are concerned that the delisting will be used by those nations to justify the continued killing of humpback whales. A delisting should be accompanied by a clear message to whaling nations that there is no “open season” for humpback whales as their populations have only partially recovered and are facing an uncertain future.
Given the projected population trajectory for E1 (Noad et al., 2019), we expect to see evidence in population changes within the next 5 years. Therefore, we urge the government to either keep humpback whales on the threatened species list for an additional 5 years or follow USA´s example and invest resources into continued population monitoring over the next 10 years and react swiftly if there are noticeable impacts on the population (National Marine Fisheries Service, 2016). In the case of delisting, we are strongly recommending a post-delisting monitoring plan with the goal to (1) detect changes in trends and associated calving rates; (2) population growth rates as well as spatial and temporal distribution and (3) monitor residual or emerging threats.