Marine non-native species in the Southern Ocean and Antarctica

For this article, we focus on free-living non-native marine species (NNMS) in general, which occur outside their natural range due to human activities, rather than invasive species, a small subset of NNMS with harmful impacts to biodiversity and ecosystem functions (see definitions by Hughes et al., 2023, CEP 2019). In contrast to the Arctic, while NNMS have been detected outside their typical range in Antarctica and the Southern Ocean, no confirmed spreading NNMS populations or negative impacts have been reported yet (McCarthy et al., 2019).

Currently, the six NNMS that were likely introduced by humans (i.e., via biofouling on ships or in ballast water) are Ulva instestinalis (grass kelp), Hyas araneus (great spider crab), Bugula neritina (brown bryozoan), Ciona intestinalis (vase tunicate), Ectopleura crocea (pinkmouth hydroid), and Mytilus sp. (Chilean blue mussel) (McCarthy et al., 2019, Cárdenas et al., 2020). Furthermore, 55 different taxa have been found on the hulls of Antarctic-going ships, 15 of which have distributions in or records from the Arctic or sub-Antarctic and therefore may be tolerant of Antarctic environmental conditions (see a complete list in McCarthy et al., 2019). Ten of the 55 taxa are considered invasive in parts of their distribution range (McCarthy et al., 2019). However, these taxa are yet to establish self-sustaining populations in Antarctic waters.

In contrast to terrestrial non-native species (see summary by Hughes et al., 2023), the current NNMS research focuses on understanding the main pathways to Antarctica, as there is little or no scope for removal once the NNMS population becomes established in the Antarctic environment. The risk of NNMS introduction to Antarctica is determined by the biodiversity in gateway ports, i.e., the abundance and type of organisms, as well as their physiology and ability to colonise higher latitude locations (McCarthy et al., 2019, 2022). Additionally, the ship’s route to Antarctica and port residence time in Antarctica and its port of origin have a major impact on the abundance and type of transported organisms (Davidson et al., 2009; Sylvester and MacIsaac, 2010; Sylvester et al., 2011; McCarthy et al., 2022).

The most common pathway of non-native marine species transportation into the Southern Ocean and to the Antarctic near-shore environment is likely to be biofouling on ships (Lewis et al., 2005), especially in sheltered areas of the ship, such as sea chests, moon pools, outlet ports, and internal seawater systems (Coutts and Dodgshun, 2007; Frey et al., 2014). Ballast water is considered a lower threat than biofouling, because vessels rarely release ballast water in the region (also see Section 5) and rather take up ballast water after resupplying Antarctic research stations (Lewis et al., 2003, Hughes and Ashton, 2016). Although moving through sea ice might scrape off encrusting organisms on the ship’s hull (Lewis et al., 2004; Lee and Chown, 2009; Hughes and Ashton, 2016), ice scour will not remove NNMS from recessed areas on the hull and the survival of NNMS on voyages to Antarctica remains unclear (Lee and Chown, 2007; McCarthy et al., 2019). Moreover, sea ice does not affect all vessels equally. For example, ships that visit the sub-Antarctic islands or Antarctic Peninsula in summer may choose to avoid ice or travel later in the season when sea ice cover is reduced (Lewis et al., 2004; Argentina, 2015; Hughes and Ashton, 2016; McCarthy et al., 2019). Indeed, the vast majority of voyages within the Antarctic take place within the summer months (Bayley et al., 2024; McCarthy et al., 2022).

The most commonly travelled route to Antarctica is from South America or South Atlantic ports to the Antarctic Peninsula, often via South Georgia and the South Orkney Islands (Figure 1; Lynch et al., 2010; Bender et al., 2016, McCarthy et al., 2022). Consequently, 19 of the top 20 most vulnerable regions for the introduction of marine invasive species in Antarctica, based on shipping activity, were identified along the Antarctic Peninsula southwest of Anvers Island, the South Shetlands Islands and the South Orkney Islands (McCarthy et al., 2022). Most recordings of NNMS in Antarctica have been received for this region (Hughes et al., 2015; McGeoch et al., 2015, Cárdenas et al., 2020). In contrast, due to lower shipping activity and, hence, propagule pressure, the East Antarctic coast in the Indian Sector of the Southern Ocean is currently less vulnerable (Holland et al., 2021, McCarthy et al., 2022).

The Antarctic Circumpolar Current (ACC) reduces the passive oceanographic transport of new species to Antarctica due to its general west-to-east flow (Clarke et al., 2005; Fraser et al., 2018). In addition, the harsh environmental conditions in Antarctica and the surrounding Southern Ocean south of the Polar Front, including freezing temperatures, physical disturbance from icebergs, and seasonal variation in light availability and water chemistry represent barriers to the survival and establishment of potential NNMS (Aronson et al., 2015; Byrne et al., 2016). Moreover, the available habitat for new species is limited by ice coverage in the shallow-subtidal environment (Peck, 2018). The deep continental shelf of Antarctica, caused by the weight of the ice cap, further restricts the habitat availability for potential ship-borne colonisers (Peck, 2018; McCarthy et al., 2019). Typically, non-Antarctic organisms do not have the same physiological adaptations to life in Antarctic waters (e.g., production of antifreeze proteins, Johnston et al., 1998; Peck, 2018). Their life histories are not adapted to the disturbance regimes, e.g., caused by ice scour, which affects 30-95% of the seabed community each year (Barnes and Conlan, 2007; Barnes et al., 2014).

While physical and physiological barriers have previously prevented the establishment of NNMS populations in Antarctica, this situation might change in future with (i) the increase in ship traffic, which increases the chance of NNMS introductions and (ii) climate change which increases the chance of NNMS survival and establishment.

Since the 1960s, voyages around Antarctica have increased from 75-100 to more than 500 voyages during the 2017-18 season (Headland, 2009; CCAMLR, 2017, 2018; COMNAP, 2018; IAATO, 2018a, 2018b), mainly due to the ongoing expansion of the tourism industry (IAATO, 2023a). Tourist visitor numbers to Antarctica are currently at an all-time high. In the 2022/23 season, the members of the International Association of Antarctica Tour Operators (IAATO) undertook 540 voyages to Antarctica (IAATO, 2023a), of which 494 went to the Antarctic Peninsula alone (IAATO, 2023b). In addition to tourism operations, more than 100 voyages during the 2017/18 season were for research and resupply of the c. 75 stations operated by c. 31 countries in Antarctica and the sub-Antarctic islands (COMNAP, 2023; McCarthy et al., 2019). Building new or redeveloping old bases requires additional logistic support and further increases shipping activity (McCarthy et al., 2019). The risks of NNMS introduction by the fishing industry depend on target species, fisheries location, and gear types (McCarthy et al., 2019). The highest threat from the fishing sector will likely be IUU (illegal, unreported, and unregulated) fishing vessels that do not adhere to internationally agreed hull and ballast water biosecurity measures (McCarthy et al., 2019). Between 2014 and 2024, IUU fishing activity was reported on 20 occasions to CCAMLR (CCAMLR, 2024).

In addition to increasing shipping activity, climate change-driven declines in sea ice coverage and increasing water temperatures could create favourable conditions for NNMS while being less hospitable to native species (McCarthy et al., 2019). In particular, the Antarctic Peninsula region has undergone substantial changes in temperature and seasonal ice dynamics (Henley et al., 2019), and some native invertebrate species in the region show limited capacity to adapt to the increasing water temperatures (Meredith and King, 2005; Ashton et al., 2017). Meanwhile, there are already ice-free habitats on the Antarctic Peninsula that could serve as stepping stones for NMMS, if species were introduced there by increased shipping activity. For example, Port Foster (Deception Island) is mostly ice-free due to volcanic activity, previously reported NNMS, and is also one of the most popular tourist landing locations (Sturz et al., 2003; Smith, 2005; Chown et al., 2012; Berrocoso et al., 2018; McCarthy et al., 2019; Avila et al., 2020; McCarthy et al., 2022). Beyond temperature increases and decreased sea ice cover, how other climate change effects will impact native Antarctic species and communities and influence rates of NNMS establishment is currently uncertain (McCarthy et al., 2019). Nonetheless, climate change is expected to remove some of the physiological barriers currently preventing NNMS from establishing within the Antarctic region (Holland et al., 2021; López-Farrán et al., 2021; Navarro et al., 2024). Factors such as an increase in light availability, ocean acidification, and increased disturbance due to ice shelf calving need to be considered in any future research. Enhanced quarantine measures and monitoring are needed to detect potential NNMS in the early stages of settlement, as removal methods of established NNMS populations in the highly interconnected marine environment are largely unavailable.

The management of NNMS introductions to Antarctica is regulated by regional (Antarctic Treaty System) and global (International Maritime Organisation (IMO)) agreements.  Currently, 56 nations have acceded to the Antarctic Treaty and 42 nations have signed the Protocol on Environmental Protection to the Antarctic Treaty. In contrast, 175 member nations (as of April 2024) are required to comply with the regulations of the IMO, which also has jurisdiction over the waters around Antarctica.

In 2006, the ATCM adopted the “Practical Guidelines for Ballast Water Exchange in Antarctic Waters” which requests that ships undertake ballast water exchange before entering Antarctic waters at the Polar Front (ATCM, 2006). In large part, the Guidelines have been superseded by the IMO Ballast Water Management Convention, which requires ships to treat and manage their ballast water to reduce the risk of transferring potentially invasive species. By 8 September 2024, the IMO will require all ships to limit the number of viable organisms released when ballast water is exchanged through an installation of a ballast water management system (Clarke et al., 2023, IMO, 2024).

Compared to ballast water treatment, the introduction of NNMS via biofouling has received less attention in the Southern Ocean. The IMO Polar Code, which entered into force in 2016 requests measures to be taken to prevent the introduction of NNMS including through biofouling (IMO, 2016). However, this is only mandatory for certain ships under the SOLAS and MARPOL Conventions (IMO, 2016). Of relevance globally, the non-binding “IMO Guidelines for the control and management of ships’ biofouling to minimize the transfer of invasive aquatic species” were adopted in July 2023 and provide advice for managing biofouling, for example, inspection, cleaning, and use of antifouling systems (Biofouling Guidelines, IMO 2023). While this is a step toward more consistent management of biofouling globally, it does not consider measures specific to the polar regions (Hughes and Ashton, 2016). A future focus on enhancing quarantine measures and monitoring would likely help to reduce the rate of NNMS introductions to the Antarctic ecosystem (Holland et al., 2021) and the likelihood of their establishment under climate change conditions. Furthermore, some countries with Antarctic gateway ports have national biofouling regulations that are compulsory, which may reduce the risk of introducing NNMS to Antarctica via vessels that pass through those particular ports.