Marine Mammals Thematic Assessment

The impacts of climate change on marine mammals are not fully understood but will be multifarious

Key messages:

• Changes in environmental conditions driven by climate change and ocean acidification are likely to impact marine mammals by influencing the availability of suitable habitats and the timing and distribution of primary production, increasing the prevalence of and susceptibility to disease, and shifting the phenological timing of key life history events. 
• Increasing evidence indicates that geographic range shifts in marine mammals are largely driven by changes in sea surface temperature, since species track changing environmental conditions as well as the abundance and distribution of prey. Climate change impacts are expected to be more pronounced in Arctic Waters, where species are more restricted in their ability to adapt and move further north. 
• Observed environmental changes as a result of climate change, such as storm frequency, suspended sediment and sea ice coverage, can impact the acoustic properties of seawater, influencing social communication, predation risk and both inter- and intraspecific competition across marine mammal species. 
• As sea-ice cover declines in the Arctic, the increased commercial interest in the region (for example, increased shipping activity) afforded by the longer ice-free season exposes polar marine species to higher risk of both collision and incidental by-catch as well as noise disturbance. Similarly, changes in the distribution of prey may shift the location of fishing activity, thereby affecting the impact of underwater noise across different parts of the environment.  
• As marine renewable energy developments increase in an to attempt to minimise future climate change effects, the risk of marine mammal exposure to underwater noise throughout all stages of the development also rises. Impulsive and continuous noise can result in disturbance and injury to marine mammals, inhibiting foraging and feeding behaviours. 
• Arctic environmental changes can alter exchange processes between atmospheric, terrestrial and aquatic reservoirs of persistent chemicals like polychlorinated biphenyls (PCBs) and mercury, and the rates of their bioaccumulation in marine mammals. Most of the available data derive from studies conducted in Arctic waters. They show how the polluting impact of climate change on marine mammals’ varies greatly across the Arctic region. For example, significantly decreasing trends of mercury were found in ringed seals from the Canadian Arctic (-2,4 to –8% / year), whereas no trend was observed in Greenland ringed seals. Shifts in food web structure and prey abundance due to climate-driven environmental changes seem to be most important drivers of such changes. 

The impacts of climate change on marine mammals are driven by a multitude of factors including sea temperature, sea ice availability, salinity, prey availability and distribution, exposure to disease and toxins, among many others. Observed geographical range shifts in some cetacean species across the North-East Atlantic are driven, at least in part, by increased sea temperatures and associated range shifts in both primary production and prey species (zooplankton, fish, cephalopods or crustaceans) (Williamson et al., 2021) as well as by physical thermal tolerances and habitat availability (Moore et al., 2019; Gulland et al., 2022; Orgeret et al., 2021). Corroborating this, northward shifts in commercial fish species have been well documented in recent years; several of these species overlap with the diet of marine mammals across seas in OSPAR Regions II-IV (Evans and Waggitt, 2020; Williamson et al., 2021; Coombs et al., 2019; Strand et al., 2020; Evans, 2020; Ashlock et al., 2021; Sanderson and Alexander, 2020; Tracy et al., 2019; Byers, 2021; Montero-Serra et al., 2015; Edwards et al., 2016). The impacts are predicted to be most pronounced in Arctic Waters, where marine mammal species are physically restricted in their responses to change (e.g., ice-obligate species and those reaching, or already at, their thermal tolerances; Biuw et al., 2022; Kovacs et al., 2020; Vacquie-Garcia et al., 2017; Bestley et al., 2020; Orgeret et al., 2021; Albouy et al., 2020).  

Increased storm frequency and sea level rises leave those seal species which breed or haul out along low-lying coastal areas particularly vulnerable to storm surges (Evans and Bjorge, 2013; Zicos et al., 2018). Approximately 35% of the global population of grey seals breed in the United Kingdom (SCOS, 2020). Severe storms in 2017 led to particularly high levels of grey seal pup mortality in the United Kingdom, with 75% of grey seal pups around all major breeding sites in Wales reported lost (SCOS, 2018). Similarly, in 2021 at least 225 pups were found washed up along the Scottish east coast after a storm event. Increased intensity and frequency of storms may also impact the foraging behaviours of marine mammals (Smith et al., 2013; Fandel et al., 2020). Encounters of coastal bottlenose dolphin populations in both the US Middle Atlantic Bight and the Mississippi Sound decreased significantly during storms, with individuals thought to be travelling further to forage (Smith et al., 2013; Fandel et al., 2020). Conversely, the number of foraging encounters was significantly higher after the storm than before (Smith et al., 2013; Fandel et al., 2020). These changes in foraging have been linked to changes in the distribution and behaviour of prey species and can have impacts on population dynamics and key life history events such as reproduction (Smith et al., 2013; Fandel et al., 2020).  

Increasing sea temperatures and changing prey distributions are predicted to impact the phenology and recruitment success of marine mammal species. The extent of harmful algal blooms is predicted to increase along with the temperatures and stratification in the water column. When combined with the geographic range shifts that introduce individuals to novel pathogens and thermal stress, lowering their immune system functioning, this may increase both the prevalence and susceptibility of disease in marine mammals (Cohen et al., 2018; Mann et al., 2013; Sanderson & Alexander, 2020; Fernandez et al., 2022). Salinity changes and thermal stratification of the water column impact the seasonal timing of plankton production at the base of the food web. Changes to plankton blooms influence the recruitment success of marine mammal prey species, which time their life events to these cycles. In turn, the reproductive activities of marine mammals are similarly affected by the trophic mismatch (Holt et al., 2010; Edwards et al., 2020; Sharples et al., 2020; Sharples et al., 2013; Frost et al., 2016). Observed increases in instances of skipped breeding attempts as well as phenological shifts in the start of the breeding season of grey seals have been associated with increasing sea surface temperature and changing prey distributions. Sub-optimal environmental and body condition has been seen to increase the incidence of skipped breeding attempts and it is hypothesised that skipping a season may be a mechanism by which individuals prioritise survival and increase the likelihood of future reproductive success (Smout et al., 2019; Bull et al., 2021; Caillat et al., 2019). 

Shorter sea-ice seasons and reduced ice extent in Arctic Waters have substantially increased access and opportunity for marine traffic, whose predicted increase will alter the soundscape and increase the risk of ship strikes on slow moving species such as bowhead whales (Stephenson et al., 2013; Hauser et al., 2018). Increased human activities, either from climate change or government policy ambitions to mitigate and reduce carbon emissions, expose marine mammals to added pressures. The construction and operation of marine renewable developments, such as offshore windfarms, introduce increased levels of underwater noise into the marine environment. Both impulsive and continuous noise have the potential to injure or disturb species, mask communication, and restrict access to important breeding or feeding grounds by both cetaceans and pinnipeds. 

Changes in all these factors have major consequences for the sources and fate of both naturally occurring and human-made chemicals entering the ocean. Sea-ice decrease, temperature and precipitation increase, current shifts and changes in the structure and biodiversity of local food webs are modifying the bioaccumulation rates of persistent chemicals (e.g., mercury or PCBs) by marine mammals. A shift in mercury levels in Greenland Sea ringed seals has been related to higher terrestrial/freshwater spring run-off (Pinzone, 2021), whereas an increase in Persistent Organic Pollutants (POPs) levels in North Atlantic whales (e.g., killer whales) was in part linked to a shift from a fish-dominated to a seal-dominated diet in new Arctic habitats (Remili et al., 2021). At the same time, changes in the proportion of Arctic versus sub-Arctic seal species in Hudson Bay polar bears' diet has been associated with an increase in adipose concentrations of total poly-brominated diphenyl ethers (PBDEs) between 1990 and 2007 (Borgå et al., 2022). Climate change impact on marine mammals’ chemical pollution is therefore strongly variable in time and space, underlining the necessity for more species-specific case-studies. Moreover, the largest number of observations derive from Arctic Waters, as here the cause-effect relationships between environmental changes and pollution are easier to study. A greater effort of data acquisition is needed from the Greater North Sea (Region II) to the Wider Atlantic (Region V) in order to forecast future trends at the entire OSPAR scale. 

To improve confidence in OSPAR’s assessment of the impacts of climate change and ocean acidification on marine mammal species and the prediction of future impacts, numerous strands of evidence are required. Long-term monitoring of the abundance and distribution of cetacean species across the North-East Atlantic is already being undertaken using regionally coordinated aerial and ship surveys in a six-yearly cycle. However, the availability of fine-scale data in between these surveys limits the power of the assessments of seasonal trends. Robust information on abundance and distribution is even more limited for those offshore and deep-diving species which are particularly challenging to monitor using traditional methods. Research into the impact of cumulative human-induced and climate change pressures in the marine environment, as well as the responses of marine mammal species to these pressures, is necessary in order to better understand how marine mammal species will adapt to environmental changes and human activities in the marine environment.