Telemetry‐validated nitrogen stable isotope clocks identify ocean‐to‐estuarine habitat shifts in mobile organisms

Abstract

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Isotopic Tracers Suggest Limited Trans-Oceanic Movements and Regional Residency in North Pacific Blue Sharks (Prionace glauca)

“…This differs from more typical isotopic applications to ecology, which generally use δ 15 N to estimate trophic dynamics and δ 13 C to trace energy source and foraging location(s), due to lower trophic fractionation of δ 13 C than of δ 15 N (Post, 2002). However, more recent studies have demonstrated that in some systems and for certain predators, baseline δ 15 N values across ecoregions can result in predators acquiring regional δ 15 N signatures that outweigh trophic effects (Graham et al, 2010;Hobson et al, 2010;Madigan et al, 2017;Shipley et al, 2021). Isoscapes will be basin-and ecosystemdependent.…”

“…While SI-inferred movements are limited by isotopic turnover rates of analyzed tissue, prior movements can be characterized due to mobile predators integrating prey isotopic signatures during movements through isotopically distinct regions (Graham et al, 2010;MacKenzie et al, 2012;Carlisle et al, 2015;Madigan, 2015;Trueman and Glew, 2019). As such, isotope-and tag-inferred migration patterns are effective complementary techniques (Carlisle et al, 2012(Carlisle et al, , 2015Madigan et al, 2015Madigan et al, , 2018Shipley et al, 2021). Here, comparing isotopic estimates to past conventional tagging suggests that blue sharks make both eastward and westward trans-Pacific migrations, though the number of sharks that make these migrations appears to be low (<5%).…”

“…Comparatively, WPO waters are nutrient-poor, which promotes dominance of nitrogen-fixing picophytoplankton at the base of the food web and lower (i.e., 15 N depleted) δ 15 N composition of regional prey pools (Takai et al, 2007;Fujinami et al, 2018;Ohshimo et al, 2019). Knowledge of regional isotopic baseline variation can be combined with measured predator stable isotope ratios and tissue-specific incorporation rates to identify individuals that have recently migrated from one system into another (Madigan et al, 2021;Shipley et al, 2021). Tissues of recent migrants will reflect an isotopic mix of prey baselines from the prior and current regions, provided that sampling has occurred prior to the consumer reaching isotopic steady-state with prey from their current region (Heady and Moore, 2013;Moore et al, 2016;Madigan et al, 2021).…”

“…This technique has been applied to characterize trans-Pacific migrations of Pacific bluefin tuna (Thunnus orientalis), using machine learning algorithms to define migrants vs. residents, and thus predicting the extent of mixing between WPO and EPO populations (Madigan et al, 2014). More recently, SIA approaches have been used to determine movement transitions across systems spanning marine, brackish, and freshwater habitats (Moore et al, 2016;Shipley et al, 2021), and have proven to be a robust and insightful approach for clarifying aspects of animal migration.…”

Inferences to estimate consumer’s diet using stable isotopes: Insights from a dynamic mixing model

“…Most animals move across certain areas when foraging and/or over the course of their growth. To date, isotopic studies that considered λ explicitly in their inferences have investigated individuals migrating between isotopically distinct habitats [ 33 , 34 ] and individuals exploiting seasonally available resources [ 35 ]. The combination of ontogenetic shifts, suboptimal physiological states of consumers, changes in availability and/or quality of prey resources [ 21 ], and habitat use impact δ c , highlighting λ as a key parameter for the integration time of the incorporated δ s , to track animal movements and niche shifting [ 30 , 36 , 37 ].…”