The demographic decline of a sea lion population followed multi-decadal sea surface warming

Abstract

the population growth of top predators depends largely on environmental conditions suitable for aggregating sufficient and high-quality prey. We reconstructed numerically the size of a resident population of California sea lions in the Gulf of California during 1978-2019 and its relation with multi-decadal sea surface temperature anomalies. This is the first multi-decadal examination of the sea surface temperature of the Gulf of California and of one of its major predators. A three-decade sustained warming explained the population's trend accounting for 92% of the variance, including a 65% decline between 1991 and 2019. Long-term warming conditions started in the late 80s, followed by the population's decline from 43,834 animals (range 34,080-58,274) in 1991 to only 15,291 (range 11,861-20,316) in 2019. The models suggested a century-scale optimum sea surface habitat occurring in mildly temperate waters, from 0.18 to 0.39 °C above the 100-year mean. The mechanistic links of this relation are still untested, but apparent diversification of pelagic fish catches suggests a reduction of high quality prey. We propose this population should be considered vulnerable to any disturbance that could add to the negative effects of the current warm sea surface conditions in the Gulf of California. The physical structure of oceanic habitats often determines, through a multi-step process, the success of animal populations 1. Sea surface temperature and its variability is widely known to affect oceanic top predators such as pinnipeds, through bottom-up mechanisms 2-4. Abrupt or sustained changes of sea surface temperature affect the abundance and diversity of plankton communities 5 , pelagic fishes 6 , and ultimately marine mammals 7,8. The latter typically respond with alterations in foraging habits 9,10 , key physiological processes 11,12 , reproductive success, or survival 4. Although many marine mammal populations can withstand and recover from short-term sea surface warming conditions (e.g. El Niño) 13 , sustained positive environmental trends at a multi-decadal scale typically cause important shifts in the base of marine ecosystems 14 , and arguably the diversity of potential prey. These shifts could lead to the decline of some marine mammal populations as the conditions move away from the optimum habitat to which the species have adapted 15. In the Gulf of California (hereafter "the gulf "), there is a resident population of California sea lions (Zalophus californianus) 16 genetically isolated from the other populations of the Northeast Pacific Ocean 17. Although a ~ 20% reduction of this population during the 1990s has been proposed based on partial counts spanning 1997-2004 18,19 , its total size and temporal trend, as well as the historical environmental context, remain unknown. A recent review mentioned that the population decreased 44% between 1979 and 2016, based on unpublished data 20. Negative interaction with fisheries, and a temporal decline of the Pacific sardine (Sardinops sagax) in the early 1990s, as a resul...

Declines and recovery in endangered Galapagos pinnipeds during the El Niño event

“…The oceanographic conditions recorded in the Galapagos Archipelago show that the effects of the El Niño event on the population dynamics of GSL and GFS tend to remain for at least the next two years, and after this time there is a notable recovery. Both species are top predators in the region 24 , 51 , therefore, long-term warming would produce a progressive habitat change affecting multiple generations 70 . California sea lions ( Zalophus californianus ) and Guadalupe fur seals ( Arctocephalus townsendi ) also showed a decline in the pups’ production during the El Niño event 2014–2015, which, in both otariids, was associated with nutritional stress in females and the potential impact on reproductive success 15 .…”

Effect of environmental variables on the number of births at California sea lion (Zalophus californianus) rookeries throughout the Gulf of California, Mexico

“…Considering the climatology of the entire GoC for the period from 1920 to 2018, Adame et al (2020) argue that positive SST anomalies began to be registered around 1990, about the same time that the California sea lion population in the GoC began to decrease. They propose that the increase in the SST has resulted in a diversification of pelagic fish catches by fisheries, which suggests a reduction in highquality prey for California sea lions.…”

“…In the central region, the growth rate has remained relatively low (1%) and the decline has been continuous since 1978, reducing the population to a level that may represent a threat to the species' persistence. Not all reproductive colonies were included in our analysis; however, previous studies examining all colonies have reported the same trend (Szteren, Aurioles-Gamboa & Gerber, 2006;Adame et al, 2020).…”

Multi-locus evaluation of gastrointestinal bacterial communities from Zalophus californianus pups in the Gulf of California, México

“…The California sea lion ( Zalophus californianus ) is the predominant pinniped species in in the Gulf of California (GoC). However, a recent survey revealed a notable population decline over nearly three decades from approximately 43,834 (1991) to 15,291 (2019) sea lions ( Adame et al, 2020 ) distributed among thirteen islands of GoC ( Szteren, Aurioles & Gerber, 2006 ; Masper et al, 2019 ). Due to this rapid population decline, the California sea lion is now listed as Endangered on the Red List of the International Union for Conservation of Nature (IUCN) and by the Official Mexican Standard (NOM-059-SEMARNAT).…”