DOI: 10.5061/dryad.jwstqjq9r
title: Loss of an apex predator in the wild induces physiological changes in prey

description: Predators can impact prey via predation or risk effects, which can
 initiate trophic cascades. Given widespread population declines of apex
 predators, understanding and predicting the associated ecological
 consequences is a priority. When predation risk is relatively
 unpredictable or uncontrollable by prey, the loss of predators is
 hypothesized to release prey from stress; however, there are few tests of
 this hypothesis in the wild. A well-studied predator-prey system between
 white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus
 pusillus pusillus) in False Bay, South Africa has previously demonstrated
 elevated fecal glucocorticoid metabolite concentrations (fGCM) in seals
 exposed to high levels of predation risk from white sharks. A recent
 decline and disappearance of white sharks from the system has coincided
 with a pronounced decrease in seal fGCM concentrations. Seals have
 concurrently been rafting farther from shore and over deeper water, a
 behavior that would have previously rendered them vulnerable to attack.
 These results show rapid physiological and behavioral responses by seals
 to release from predation stress. To our knowledge, this represents the
 first demonstration in the wild of physiological changes in prey from
 predator decline, and such responses are likely to increase given the
 scale and pace of apex predator declines globally.
description: <i>Boat-based surveys
 </i> Between 2000 and 2020, white
 shark relative abundance and predatory activity during winter months at
 Seal Island were monitored daily from standardized boat-based observation
 surveys (as described in Hammerschlag et al. 2017; see electronic
 supplementary material). Water temperatures (°C) were recorded using the
 vessel’s onboard temperature sensor and the following environmental
 variables were estimated: percentage cloud cover, wind speed (kt) and
 direction, swell height (m), and water visibility (m). Additionally, the
 relative distances of rafting seal groups off the Island’s perimeter were
 estimated according to one of three distance categories: (1) seals rafting
 &lt; 5 m from the Island; (2) seals rafting &gt; 5 and &lt; 10
 m from the Island; and (3) seals rafting &gt; 10 m from the Island
 perimeter. Between 07:00 and 09:30 h, instances of
 predation by white sharks on Cape fur seals were recorded following the
 approach outlined in Fallows et al. 2016 (see electronic supplementary
 material). The duration of each observational period along with the number
 of predatory attacks by sharks on seals during this period were recorded
 to calculate white shark predation rates (i.e. number of predation events
 per hour). After 09:30 h, the vessel anchored and conducted standardized
 boat-based baited surveys of white sharks following the approach described
 in Hammerschlag et al. 2019 (see electronic supplementary material).
 Between 10:00 and 12:00 h, sharks were attracted to the boat using a large
 tuna head and a seal decoy. Individual sharks were identified based on a
 combination of unique scarring, presence/absence of claspers, and
 individual variation in pigmentation patterns. The duration of each baited
 survey was recorded, along with the number of individuals observed. The
 number of different individual sharks observed per hour during these
 baited surveys were calculated as a metric of relative white shark
 abundance. <i>Seal
 fecal sample collection and immunoassay</i> Seal fecal samples were collected from
 Seal Island during 2014 and 2015 prior to the onset of shark decline
 (results published in Hammerschlag et al. 2017) and during the decline and
 eventual disappearance of white sharks from the study site in 2016, 2017,
 and 2019. To enable scats of different individuals, ~20 g  of sample was
 collected from clearly distinct defecation piles. Each sample was placed
 in 50 ml screw-lid vials and frozen within 1-2 hours of collection, which
 occurred between sunrise and noon. Steroid hormone metabolites were extracted
 from fecal samples by drying the scat and boiling a known mass of dry
 feces in ethanol following Creel et al. 2009. Glucocorticoid metabolite
 concentrations in fecal extracts (fGCM) were analyzed as detailed in
 Hammerschlag et al. 2017 and measured using an enzyme-linked immunoassay
 with a cortisol antibody (Enzo Life Sciences ADI-900-071; see electronic
 supplementary material for details on procedural validation). We expressed
 fGCM concentrations as nanograms of cortisol immunoreactivity per gram of
 dry feces. <i>Statistical
 analysis</i> Previous analyses applied to annual
 trends in white shark relative abundance data at the study site, collected
 between 2000 and 2018, revealed a significant change point in 2015, after
 which (2016 onwards) white shark relative abundance began to precipitously
 decline (Hammerschlag et al. 2019). Therefore, we classified the period
 prior to shark decline as the years 2000 to 2015 and the post-decline
 period as years 2016 through 2020. To examine annual
 trends in white shark relative abundance and predation rates across the
 2000–2020 time-series, we calculated the mean number of white sharks
 sighted per hour and the mean number of shark predations per hour, for
 each year, following the approach of Hammerschlag et al. 2019. To compare
 changes in seal behavior in relation to white shark relative abundance and
 predatory activity, we evaluated annual trends in seal rafting distance
 from the Island by calculating the mean daily rafting category for each
 year. To compare seal stress responses to annual trends
 in white shark relative abundance and predatory activity, we calculated
 mean fGCM concentrations by sampling year and by period (pre-decline
 versus post-decline of white sharks). Previous
 laboratory studies that have subjected sea lions (<i>Eumetopias
 jubatus</i>) to an adrenocorticotropic hormone (ACTH) challenge
 found a lag of up to 4 d between ACTH injection and peak fGCM (Hunt et al.
 2004). Accordingly, here we considered that measured fGCM values reflected
 hormone values in seals based on stress experienced within the week prior.
 Indeed, Hammerschlag et al. 2017 found a very strong correlation between
 seal fGCM concentrations and mean predation rates (attacks/h) measured
 within the week prior to scat collections. Accordingly, here we used
 Spearman Correlation to test for a correlation between weekly shark attack
 rates and associated fGCM concentrations spanning the pre- and post-
 decline period. We also separately tested for correlations of seal fGCM
 levels against water temperature, wind speed, swell height, water
 visibility, and cloud cover. As in our analysis of the correlation between
 fGCM with predation rates, we used mean values of the environmental
 variables recorded during the previous week and up to the day of scat
 sampling in this analysis. Because white sharks only
 actively prey on seals at Seal Island during winter months, we restricted
 analyses to data collected from May through September, such that all data
 reflected seal behavior and physiology during the season in which seals
 historically experienced high predation risk. All statistical analyses
 were performed in SAS with P &lt; 0.05 used as a threshold for strong
 evidence of an effect.
description: The files 'Shark_Sightings_MaytoSept_summarized.csv',
 'Predations_MaytoSept_summarized.csv', and
 'Seal_Rafting_MaytoSept_summarized.csv' were all used to create
 Figure 1A, 1B, and 1C, respectively in the manuscript.
 The file 'fGCM_samples_raw_data.csv' was used to create
 Figure 2A in the manuscript, and the file
 "GW_Predations_7days_priorto_FGCM_collections_Environmental
 data.csv' was used to create Figure 2B.