The Rapid Population Collapse of a Key Marine Predator in the Northern Antarctic Peninsula Endangers Genetic Diversity and Resilience to Climate Change

Abstract

Antarctic fur seals (AFS) are an ecologically important predator and a focal indicator species for ecosystem-based Antarctic fisheries management. This species suffered intensive anthropogenic exploitation until the early 1900s, but recolonized most of its former distribution, including the southern-most colony at Cape Shirreff, South Shetland Islands (SSI). The IUCN describes a single, global AFS population of least concern; however, extensive genetic analyses clearly identify four distinct breeding stocks, including one in the SSI. To update the population status of SSI AFS, we analyzed 20 years of field-based data including population counts, body size and condition, natality, recruitment, foraging behaviors, return rates, and pup mortality at the largest SSI colony. Our findings show a precipitous decline in AFS abundance (86% decrease since 2007), likely driven by leopard seal predation (increasing since 2001, p << 0.001) and potentially worsening summer foraging conditions. We estimated that leopard seals consumed an average of 69.3% (range: 50.3–80.9%) of all AFS pups born each year since 2010. AFS foraging-trip durations, an index of their foraging habitat quality, were consistent with decreasing krill and fish availability. Significant improvement in the age-specific over-winter body condition of AFS indicates that observed population declines are driven by processes local to the northern Antarctic Peninsula. The loss of SSI AFS would substantially reduce the genetic diversity of the species, and decrease its resilience to climate change. There is an urgent need to reevaluate the conservation status of Antarctic fur seals, particularly for the rapidly declining SSI population.

Untangling unexpected terrestrial conservation challenges arising from the historical human exploitation of marine mammals in the Atlantic sector of the Southern Ocean

“…Recent detailed genetic studies support the two regional populations being distinct (Paijmans et al 2021). Although the South Shetland Islands population appeared to have stabilised at a level of about one eighth of its pre-exploitation level by the early twenty-first century (Hucke- Gaete et al 2004;Hoffman et al 2018), it has subsequently shown a further substantial decrease in numbers suggested to be a consequence of a sustained increase in top-down predation of pups by leopard seals (Krause et al 2022). However, a study of seal hair abundance in a core of accumulated terrestrial sediment on a coastal raised beach on King George Island (Sun et al 2004) inferred large levels of variation in the size of the local fur seal population over the last 1500 years.…”

“…Large populations of fur seals on the sub-and other peri-Antarctic islands, but particularly on South Georgia and the South Shetland Islands (Fig. 4), were the first to suffer uncontrolled overexploitation, almost being driven to extinction (Bonner 1968;Forcada and Staniland 2009;Paijmans et al 2021;Krause et al 2022). The sequence of rampant overexploitation was followed by the great whales, with the development of, initially, shore-based whaling stations on several peri-Antarctic islands in the early twentieth century, and later of the pelagic whaling industry (e.g.…”

“…Boyd 1993;Forcada and Staniland 2009;Foley and Lynch 2020). The much smaller South Shetland Islands population, the species' southern-most breeding population, most likely recovered from a very small remnant after near extirpation, possibly reinforced by some migration from South Georgia (Reid et al 2006;Krause et al 2022). Recent detailed genetic studies support the two regional populations being distinct (Paijmans et al 2021).…”

“…With increasing evidence that at least some Antarctic whale populations are now recovering (Zerbini et al 2019;McCormack et al 2021), it is not clear whether the fur seal population 'rebound' will remain at its current level, or whether there will be some form of rebalancing with the increased number of whales. However, the consistent and large recent decreases in fur seal pup numbers and, by implication, adult populations on the South Shetland Islands (Krause et al 2022) and South Georgia (Forcada and Hoffman 2014) over the last two decades, while having a number of possible contributing causes, may suggest this is happening already. Further important unknowns are the distribution and future trajectory of human exploitation of the krill resource, the primary food source exploited by fur seals and often described as one of the greatest remaining largely unexploited sources of protein on the planet (McBride et al 2014;Trathan et al 2021).…”

“…The International Union on Nature Conservation defined the Antarctic fur seal as a species of least concern and not threatened in any part of its distribution in 2014 (Hofmeyr 2016). However, recent research has questioned this decision, on the basis that genetic studies have confirmed the existence of at least four genetically distinct sub-populations (South Shetland Islands, South Georgia, Bouvetøya, eastern sub-Antarctic islands) (Paijmans et al 2021;Krause et al 2022). As noted earlier, both the South Georgian and South Shetland Islands populations have consistently declined over the last two decades, and the decline in the latter has recently been described as potentially catastrophic, risking loss of an important component of the species' genetic diversity (Krause and Hinke 2021;Krause et al 2022).…”