Partitioning maternal and exogenous diet contributions to otolith ⁸⁷Sr/⁸⁶Sr in kokanee salmon (Oncorhynchus nerka)

Abstract

Strontium isotopes (87Sr/86Sr) recorded in the otoliths of Pacific Salmon (Oncorhynchus spp.) are commonly used to identify natal origin. For species that migrate at or soon after emergence, the embryonic region of the otolith provides the only record of provenance. However, maternal contribution of Sr from the yolk can confound the isotopic signature of the natal site. We experimentally quantified maternal and exogenous diet contributions to otolith 87Sr/86Sr over embryonic development in Kokanee salmon (O. nerka). Eggs from two populations in isotopically distinct lakes were incubated and reared in a common water source. Timing of developmental events and proportional contribution from yolk to otolith 87Sr/86Sr differed significantly between the two populations. We suggest that the magnitude of difference in 87Sr/86Sr between yolk and water, the relative concentrations of Sr and Ca in these isotopic sources, and population-specific effects on otolith growth and composition contribute to this variation. Understanding how these factors affect otolith 87Sr/86Sr could extend the use of otolith geochemistry for determining provenance to species and populations in which natal site rearing is limited.

The incorporation of environmentally derived 87Sr/86Sr and Sr/Ca in early otolith formation of Chinook salmon

“…In contrast, Hegg et al (2018) reported the shift occurred later, approximately 225 μm, with respect to otolith elemental signatures (i.e., Mn/Ca, Ba/Ca and Sr/Ca), and thus suggested that this transition represented an ontological change within the developing fish separate from hatching and possibly related to fish emergence. Interestingly, work on a related salmonid ( Oncorhynchus nerka ) by Janak et al (2021) reported shifts in otolith 87 Sr/ 86 Sr and Sr/Ca associated with hatching and yolk sac absorption that occurred at an average otolith radius of 67 and 118 μm, respectively, which was comparable to this study.…”

“…Interestingly, work on a related salmonid (Oncorhynchus nerka) by Janak et al (2021) reported shifts in otolith 87 Sr/ 86 Sr and Sr/Ca associated with hatching and yolk sac absorption that occurred at an average otolith radius of 67 and 118 μm, respectively, which was comparable to this study.…”

“…Specifically, water 87 Sr/ 86 Sr went from 0.7067 at day 15 and 0.7069 at day 44 to 0.7117 at days 102 and 113 (Table 2). Our water 87 Sr/ 86 Sr from the Columbia River/Lake Washington treatment collected on days 102 and 113 was lower than Columbia River water reported by other studies (0.713–0.717; Janak et al, 2021, Miller et al, 2011). We suspect that our lower value was due to a residual amount of Lake Washington water that resulted in a diluted Columbia River/Lake Washington water 87 Sr/ 86 Sr. We did not report water 87 Sr/ 86 Sr for days 102 and 113 from Tucannon River because we had an insufficient amount of water for analysis.…”

“…L. Snake River was excluded from this analysis because the 87 Sr/ 86 Sr levels in the otolith and water were expected to be similar as a result of the residence of the adult females from which eggs were extracted. Although studies have indicated a 1:1 relationship between water 87 Sr/ 86 Sr and otolith 87 Sr/ 86 Sr (Kennedy et al, 2000) we did not expect otolith 87 Sr/ 86 Sr to match that of the water in which the fish resided in our study because of the dietary contribution of a marine‐based feed to otolith 87 Sr/ 86 Sr that would have a marine 87 Sr/ 86 Sr signature (Janak et al, 2021; Kennedy et al, 2000).…”

“…While Sr/Ca as a marker of maternal anadromy has been a staple of the otolith literature for some time (Kalish, 1990), the use of 87 Sr/ 86 Sr as a marker for maternal anadromy is relatively recent (Courter et al, 2013). Despite this, the mechanisms of when and how this isotopic change from maternal to natal sources of strontium occur in the developing fish have only recently been investigated (Hegg et al, 2018), and controlled experiments demonstrating these changes are lacking (Janak et al, 2021). This work provides experimental evidence of the transition from maternally to environmentally derived isotopic signatures in the developing otolith.…”