Karl W. Flessa*
David L. Dettman- Department of Geosciences, University of Arizona, Tucson, AZ, United States
by Rodríguez-Pérez M-Y, Sánchez-Velasco L, Rosas-Hernández M-P, Hernández-Camacho CJ, Cervantes FA, Gallo-Reynoso JP, Arreguín-Sánchez F and Godínez VM (2024) Front. Conserv. Sci. 5:1490262. doi: 10.3389/fcosc.2024.1490262
Introduction
Rodrı́guez-Pérez et al. (2024)’s discussion of our analyses of oxygen isotopes in mollusk shells and fish otoliths (Rodriguez et al., 2001; Dettman et al., 2004; Rowell et al., 2005, Rowell et al., 2008) is both misleading and wrong.
Discussion
Rodríguez-Pérez et al. (2024) state, “This [isotopic] enrichment in clams and fish has been related to a population decline in species such as Mulinia coloradoensis, Cynoscion othonopterus, and Totoaba macdonaldi and has been attributed to high temperatures due to environmental change caused by the blocking of the flow of the Colorado River toward the UGC [Upper Gulf of California] (Rodriguez et al., 2001; Dettman et al., 2004; Rowell et al., 2005, 2008).”
Although we (Kowalewski et al., 2000) documented a decline in the population of the bivalve mollusk Mulinia californiensis (now Mulinia modesta), we did not document changes in the population sizes of the sciaenid fish Cynoscion othonopterus and Totoaba macdonaldi.
Rodríguez-Pérez et al. (2024) are wrong to state that we attributed the change in δ18O values to higher temperatures. We showed that the change resulted from a decrease in Colorado River influx and the consequent increase in salinity in the delta region of the UGC.
δ18O values in skeletal carbonates (both mollusk shell and fish otoliths) are sensitive to both the temperature and δ18O of the surrounding water. Warmer water will reduce δ18O values, as will a mix of Colorado River and Upper Gulf of California (UGC) marine water. Figure 2 in Dettman et al. (2004) documented the relationship between δ18O of UGC water and salinity and compared δ18O values in skeletal carbonates from before the cessation of Colorado River flow to δ18O values in skeletal carbonates after the cessation of Colorado River flow to the UGC. As Dettman et al. (2004) further noted, the effect of the δ18O of the water is far greater than the effect of temperature on the δ18O of the skeletal carbonates. We corrected for the temperature effect on skeletal carbonate by comparing δ18O values for the same season of the year.
Dettman et al. (2004) concluded: “Almost all our fossil samples are more negative in δ18O than the no-flow bivalve record. This offset to more negative values is due to growth in less saline water in the delta region.” A very similar conclusion on fish otoliths was reached in Rowell et al. (2005) and Rowell et al. (2008).
At no point did we suggest that the cessation of the Colorado River flow led to an increase (or decrease) of the temperature of the UGC. Our stable isotope data demonstrated that salinity changes are responsible for the large difference in UGC conditions before and after the limitation/cessation of Colorado River flow.
Rodríguez-Pérez et al. (2024) do not present any evidence that Colorado River water is colder than water of the UGC. In fact, the river’s temperature near its mouth on August 4, 2024 was 32.2°C (Waterdata.usgs.gov, 2025), greater than the 31.3°C average August maximum of UGC water at San Felipe, Mexico (Seatemperature.org, 2025). A cessation of flow would most likely cause a summer decrease in UGC water temperature, not an increase.
Conclusion
We agree with Rodríguez-Pérez et al. (2024) on the importance of understanding the biological consequences of the cessation of Colorado River flow to Upper Gulf of California habitats. Stable isotope studies of animal remains are valuable for understanding environmental conditions encountered by animals in changing habitats. We hope that our comment adds to this understanding and improves the scientific rigor of research on this topic.
Author contributions
KF: Writing – original draft, Writing – review & editing. DD: Writing – review & editing.
Funding
The author(s) declare that no financial support was received for the research and/or publication of this article.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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The author(s) declare that no Generative AI was used in the creation of this manuscript.
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References
Dettman D. L., Flessa K. W., Roopnarine P. D., Schöne B. R., Goodwin D. H. (2004). The use of oxygen isotope variation in shells of estuarine mollusks as a quantitative record of seasonal and annual Colorado River discharge. Geochim. Cosmochim. Acta 68, 1253–1263. doi: 10.1016/j.gca.2003.09.008
Kowalewski M., Avila Serrano G. E., Flessa K. W., Goodfriend G. A. (2000). Dead delta’s former productivity: Two trillion shells at the mouth of the Colorado river. Geology 28, 1059–1062. doi: 10.1130/0091-7613(2000)28%3C1059:DDFPTT%3E2.0.CO;2
Rodriguez C. A., Flessa K. W., Dettman D. L. (2001). Effects of upstream diversion of Colorado River water on the estuarine bilvalve mollusc Mulinia coloradoensis. Conserv. Biol. 15, 249–258. doi: 10.1111/j.1523-1739.2001.99463.x
Rodríguez-Pérez M. Y., Sánchez-Velasco L., Rosas-Hernández M. P., Hernández-Camacho. C. J., Cervantes F. A., Gallo-Reynoso J. P., et al. (2024). Stable isotopes of carbon (δ13C) and oxygen (δ18O) from vaquita (Phocoena sinus) bones as indicators of habitat use in the Upper Gulf of California. Front. Conserv. Sci. 5. doi: 10.3389/fcosc.2024.1490262
Rowell K., Flessa K. W., Dettman D. L., Román M. (2005). The importance of Colorado river flow to nursery habitats of the gulf corvina (Cynoscion othonopterus). Can. J. Fish. Aquat. Sci. 62, 2874–2885. doi: 10.1139/f05-193
Rowell K., Flessa K. W., Dettman D. L., Román M. J., Gerber L. R., Findley L. T. (2008). Diverting the Colorado River leads to a dramatic life history shift in an endangered marine fish. Biol. Conserv. 141, 1138–1148. doi: 10.1016/j.biocon.2008.02.013
Seatemperature.org (2025). San Felipe average August sea temperature. Available online at: https://www.seatemperature.org/central-america/Mexico/san-felipe-august.htm (Accessed January 15, 2025).
Waterdata.usgs.gov (2025). Colorado River below Cooper Wasteway, Near Yuma, AZ. Available online at: https://waterdata.usgs.gov/monitoring-location/09522005/dataTypeId=continuous-00010-0&period=P365D&showMedian=true (Accessed January 15, 2025).
Keywords: vaquita, Phocoena sinus, stable isotopes, Gulf of California, Colorado River
Citation: Flessa KW and Dettman DL (2025) Commentary: Stable isotopes of carbon (δ13C) and oxygen (δ18O) from vaquita (Phocoena sinus) bones as indicators of habitat use in the Upper Gulf of California. Front. Conserv. Sci. 6:1564652. doi: 10.3389/fcosc.2025.1564652
Received: 21 January 2025; Accepted: 07 April 2025;
Published: 28 April 2025.
Edited by:
Biao Yang, China West Normal University, ChinaReviewed by:
Roberto Velazquez Ochoa, National Autonomous University of Mexico, MexicoMario A. Pardo, Center for Scientific Research and Higher Education in Ensenada (CICESE), Mexico
Copyright © 2025 Flessa and Dettman. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Karl W. Flessa, a2ZsZXNzYUBhcml6b25hLmVkdQ==