Quantifying thermal cues that initiate mass emigrations in juvenile white sharks

Abstract

While the function of migration varies among species, environmental temperature is known to be one of the most important abiotic variables that drive animal migration; however, quantifying the thresholds and timing of the cues that influence a mass emigration is difficult, often due to lack of monitoring resolution, particularly for large, highly mobile species. We used acoustic telemetry tracking and high-resolution water temperature data over a relatively large spatial scale (5.5 km2) to identify and quantify a thermal threshold for mass emigration of juvenile white sharks. Sixteen tagged sharks were observed to initiate a search for warmer water within 10–12 hours of an upwelling event where water temperatures dropped below 14 °C. Eleven sharks traveled ~ 35 km away where they experienced similar cold temperatures before returning to the aggregation site within 24 hours. Five days following the upwelling event, most sharks emigrated from the site for the season. Quantifying movement patterns across different spatial and temporal scales is necessary to understand cues and thresholds influencing animal migration, which may be greatly affected by climate anomalies and climate change, resulting in potential impacts on the dynamics of local prey species, management, and conservation policy and practice.

Diverse habitats shape the movement ecology of a top marine predator, the white shark Carcharodon carcharias

“…Additionally, juvenile white sharks also have a different foraging ecology and lower thermal inertia (Anderson et al, 2022; Harasti et al, 2017; Tamburin et al, 2020). The influence of these physiological factors is thought to limit juvenile white sharks to warmer coastal areas in the Northeast Pacific (Anderson et al, 2021; Lowe et al, 2012; Spurgeon et al, 2022), which means they are only present at Aptos from late spring to early fall. Such constraints are likely drivers of the differences between movement behaviors of sharks tagged at Aptos and all other areas.…”

Evidence of increasing juvenile white sharks' (Carcharodon carcharias) habitat use at the Northern Channel Islands

“…There was high variation in the number of sharks detected each year at CINMS, with the highest number of individual sharks detected in CINMS in 2018 and 2020/21 (Figure 1b). It is also possible that these episodic interannual patterns of detections are attributed to new habitat exploration of the CINMS, initiated by rapidly changing environmental conditions along the mainland nursery areas (Spurgeon et al, 2022). Unfortunately, the highly variable number of detections across years made comparing with environmental conditions with detection rates at CINMS infeasible across the whole timeseries.…”

“…This occurred shortly after a major coastal upwelling event cooled nearshore waters across the southern California Bight, particularly at Santa Barbara County beaches, where a large aggregation of JWS had formed. Between November 8 and 14, 2020, sea surface temperatures along Santa Barbara County dropped below 14°C, the lower thermal preference threshold for JWS (Weng et al, 2007; White et al, 2019), which likely caused a number of sharks to search for more suitable habitats (Spurgeon et al, 2022). This turnover in coastal water temperatures occurred concurrently with offshore waters in the CINMS having a higher (1–2°C) than average temperature, associated with a large marine heatwave that formed in 2019 and persisted through early 2021 (Weber et al, 2021).…”

“…However, the majority ( n = 10, 77%) of tagged sharks that came to CINMS in that winter period returned to coastal nursery locations in early 2021. After March 2021, only three tagged sharks were detected on the coastal nursery aggregation array, likely because spring upwelling rapidly cooled local nearshore water temperatures driving JWS to seek warmer habitats (Spurgeon et al, 2022). Thus, the CINMS may have effectively served as short‐term thermal refugia during this environmental anomaly, temporarily providing more suitable habitat within preferred temperature bounds.…”

Emerging human–shark conflicts in the New York Bight: A call for expansive science and management

“…Inspiration can be taken from several coastwide monitoring techniques applied to juvenile white sharks (JWS) in coastal California, which are greatly aiding in understanding the likelihood of NHSIs. Here, techniques including drone surveys (Rex et al, 2023), omnidirectional and traditional passive acoustic telemetry (Anderson, Burns et al 2021; Anderson, Clevenstine et al 2021; Spurgeon et al, 2022), environmental DNA (Lafferty et al, 2018), and potential future applications of automated underwater vehicles (AUVs, Lowe et al, 2018), have provided key information that can be used to inform public safety. For example, systematic drone surveys have identified specific user groups exhibiting the highest spatiotemporal overlap with JWS, including standup paddle borders and surfers (Rex et al, 2023).…”