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Referee comments: Referee 1

Posted by PLOS_ONE_Group on 27 Mar 2008 at 18:06 GMT

Referee 1's review:

The manuscript by Caut et al. (Isotope analysis reveals two feeding ares for Atlantic leatherback turtles) describes the isotopic values in egg yolk and blood fractions of leatherback turtles nesting at the largest rookery in the world. The results showing two distinct isotopic profiles for the 2- vs. 3-year remigrant turtles provide an encouraging account of how stable isotope analyses link with information on animal movements derived from satellite telemetry. Further, the confirmation that egg yolk isotopic values are consistent with signatures from blood is a very important finding as it provides a basis for future studies that aim to use egg component isotopic values to interpret foraging area characteristics. I am quite pleased with the inferences made regarding the isotopic differences in the 2- vs. 3-year remigrants as well as the findings that egg yolk isotopic values link with blood values.

However, I have two major concerns with the manuscript and several minor issues. First, although the study clearly shows two distinct isotopic groupings, we don't know for sure if these are indeed reflective of 2 feeding areas as is suggested in the title and throughout. An alternate scenario is that these signals are relective of multiple feeding areas or 'regions' within the Atlantic, each of which falls into one of the two isotopic groupings. Although telemetry (Hays et al. 2004, Ferraroli et al. 2004, Eckert 2006) is suggestive of two general regions, there are still too few satellite tracks to unequivocally state that there are only two distinct foraging areas. Better to restate the title to something along the lines of "Isotope analysis reveals foraging area dichotomy for Atlantic leatherback turtles". Second and perhaps most importantly, the authors significantly overstate the significance of their findings with respect to the lack of d15N variation over the nesting season. The paper does well in reporting the isotopic values of jellyfish adjacent to the nesting beach - which do not correspond well with the observed d15N values of leatherback plasma, a fact interpreted as evidence that foraging does not occur over the course of the nesting season. However, as has been shown in at least two papers for turtles (Seminoff et al. 2006, 2007), isotopic turnover of blood plasma in turtles is too slow to expect that foraging during the nesting season would manifest as a change in d15N values. Several studies showing the similarity in leatherback and Cheloniid turtle metabolism (e.g., Wallace et al. 2005) indicate that the results from Seminoff et al's studies are informative for leatherbacks as well. As such, it is plausible that foraging has occurred, but simply not yet manifested as a change in isotopic signal. Further, the presence of foraging during the nesting season is weakly supported by the fact that a change in d15N that comes with nutritional stress associated with lack of foraging during the nesting season is also not observed. While lack of foraging is clearly equivocal based on the above reasons, the argument is further limited by the fact that there are no convincing data presented on d15N flux within individuals over the course of the nesting season. Overall, I do not believe this is a fatal flaw to the paper; however, this aspect of the manuscript must be removed before the paper is acceptable for publication. Another concern I have is not with the data presented, but rather with redundancy throughout the paper. Perhaps this is a result of the requested format of Plos One, but I still believe that the text could be tightened substantially to reduce some of the repetition.

In addition to these two aspects, there are a number of minor issues throughout the paper that are commented on below. If the above concerns in addition to these minor aspects are addressed then I believe that this manuscript is worthy of publication.

MINOR COMMENTS

Page 2

L22: Mortality in fisheries bycatch is ONE of the primary causes of population declines. Egg harvest and killing of nesting females have also been implicated in this decline. For example, see Sarti et al. 2007 and Kaplan 2005 (Reference list at bottom of review).

L31: this statement is not so. You have used stable isotopes to reveal the stable isotopic characteristics of foraging grounds for leatherbacks. Unless this is explicitly linked with isotopic values from the component habitats, the foraging grounds have not been determined. Further, foraging activity per se has not been revealed. Please reword sentence accordingly.

Page 3:

L50: True, leatherbacks are listed as 'Critically Endangered' on the IUCN Red List, however to invoke this listing for a paper on Atlantic leatherbacks is misleading as Leatherbacks in the Atlantic Ocean are increasing or stable at virtually all of the nesting sites in this ocean basin. (For references see 2007 Special focus issue on leatherback turtles in Chelonian Conservation and Biology).

L61: Use of the Davenport 1998 citation to suggest leatherbacks eat 50% of their body mass each day is misleading and taken out of context from the Davenport paper. In fact, the 50% figure comes from the need for heating foods when consumed in temperate and cold waters. At tropical or subtropical foraging areas this need to consume 50% is probably not the case. OK to use the Davenport citation, but put in appropriate context.

L62-66: Statement of "Diving activity.....foraging opportunities" would be more appropriate at the end of the paragraph.

Page 4:

L69-70: The use of 'Pacific strategy" for describing relatively short migrations would be appropriate in describing leatherback post-nesting migrations for the eastern Pacific rookeries in Costa Rica and Mexico. However, it is incorrect if applying to entire Pacific ocean basin, as it has now been shown that western Pacific nesting populations are similar to Atlantic migrations in that they traverse the entire ocean basin (see Benson et al. 2007). Please change appropriately.

L73: Change from "on this aspect of turtle biology" to "on the feeding ecology of leatherback turtles"

L91: add "and varying proximity of foraging areas to nesting beaches"

Page 5:

L97: change to "time-integrated information on foods assimilated"

L106: the change in d13C with water temperature is more due to changes over geologic rather than ecological time scales and is not entirely appropriate for this context. Please delete.

L108: the statement that this is the 'first isotopic analysis of the critically endangered leatherback turtle' is entirely untrue. See Wallace et al. 2006.


L113: when talking about offshore/inshore continuum in d13C, please refer to this as determining origins of carbon sources rather than 'trophic scale', since the latter is not correct.

L116: important to point out that variations in plasma values would only be expected if isotopic signatures of foods adjacent to nesting beaches are different than those at the distant foraging areas. Also, as it is important the temporal range for inferences is greater than turnover (see above).

Page 6

L118-119: use of egg analysis as a less invasive technique is a very key point and should be highlighted. It's a very important finding for this paper!!

L125-126: in addition to number of clutches, it would be informative to mention the total duration of these intervals.

Page 7

L150-151: yes, values may not have changed, but as this is a key component of the paper it is appropriate to show the data in summarized format either as a table or figure.

L152: were the jellyfish data derived from whole-body analysis? If so, please state this.

Page 8

L175: insert 'during internesting intervals' after 'females feed'

L177: citation [12] does not present video data, so please remove from the citation string.

L186-186: the logic is flawed for the statement that Seminoff et al's data is not informative for turnover rates over shorter time frames. In fact, regardless of the time duration of other studies, that for Seminoff illustrated that turnover in Chelonia mydas plasma is >181 days. It is entirely informative for other studies - even those of shorter duration, and the data from the Seminoff study underscores the fact that for the present study (one nesting season), turnover is too slow to expect full integration of isotopic values of foods consumed during the nesting season (see above).

L188: assumption on turnover is incorrect.

L202: by 'discussed', do you mean 'debated'? If so, please state.

L205-207: statement starting with "This suggests..." is not clear. How exactly is this linked to leatherback turtles? It is important to give further information on the effects of nutritional stress and the timeframe over which this has happened in other species. Also, the expected isotopic results of this possibility is the the results that would be expected if 'no feeding occurs during the nesting season'. This disparity in findings is worthy of additional discussion.

L214: That stable isotope studies have 'just begun' with marine turtle is not entirely accurate. Godley et al. (1998) first addressed isotopes in marine turtles nearly 10 years ago. Since then, there have been at least 8 papers that have addressed stable isotope questions in marine turtles. Perhaps state that 'relative to other taxa, there is a paucity of stable isotope studies in marine turtles'

Page 11

L245-246: again, not accurate to make conclusion on lack of feeding during nesting season.

Page 12

L270: Line starting with "this suggests...." Is unclear. Perhaps restate to 'this indicates that stable isotope analysis of egg components is a viable method for assessing foraging ecology questions in marine turtles"

L272: Not true that blood sampling and lavage are 'highly stressful'. Instead state that SI analysis is a less invasive procedure compared to lavage. Since your SI study does indeed sample blood, this is more of a comparison of blood/tissue sampling to lavage sampling.

L277: change 'non-invasive' to 'less invasive'

L287: this is a good spot to talk about future study strategies for this nesting population. For example, it would be very interesting to compare isotopic values in tissues from the same individual between seasons.

L288: as stated above, fisheries bycatch is among the primary causes for population declines, but not the only. Important to mention egg harvest and harvest of nesting females.

Page 13:

L 292: statement that this study highlights the need to deliniate foraging areas is not accurate. Instead, this study responds to an aforementioned need to delineate foraging areas. See US Recovery Plan for leatherbacks, IUCN Red List assessments, and other such management plans that call for such studies.

L 316: important to cite Wallace and George 2007 for blood sampling from the venous sine in rear flippers of leatherbacks.

Page 14

L323: give specifics of blood centrifugation. How long, and what is the rpm of the centrifuge. Also, how was blood stored?

Page 18

L418: please provide authors for citation #16

L423: please provide journal for citation #17

Page 24

Figure 1A

This figure is inadequate in its present form. Please provide a map inset showing where French Guiana is within a wider geographic context. The shading over the atlantic waters is distracting, and the offset of the Yalimapo study site is not very clear. Perhaps eliminate the shading in the Atlantic and make the map as three panels, one of wider geographic area, the second of FG, and the third of Yalimapo. Also, please make the kilometer marker more prominent.

Figure 1B

This figure is unnecessary and does not contribute substantially to the paper since it is not possible to see where blood is being drawn from. Further reducing the need for this figure is the citation of Wallace and George (2007) which clearly shows the area at which blood is drawn from in the hind flipper. Consider removing this figure.

Page 25

Figure 2.

This figure is central to your study and should absolutely be included. However, it would be more informative if A and B are combined onto the same figure. This can easily be done with the use of additional shapes to discern the different tissues. Also, I question the need for the leather back icons on the two migratory radiations from the study site. As is they are distracting and oculd be easily replaced with a bit more explanation in the figure caption.


Page 26

This figure is not necessary as it adds little to the paper, since the regression equations and significance tests are already given in the text. Consider changing this to tabular format to provide the specific regression equations and statistical findings.

Literature Cited:

Benson et al. 2007. Post-Nesting Migrations of Leatherback Turtles (Dermochelys coriacea) from Jamursba-Medi, Bird's Head Peninsula, Indonesia. Chelonian Conservation and Biology 6:150-154

Eckert 2006. High use areas for atlantic leatherback seaturtles identified using satellite telemetered dive and location information. Mairne Biology 1491257-12678.

Ferraroli et al. 2004. Where leatherback turtles meet fisheries. Nature 429, 521-522

Godley et al. 1998. The trophic status of marine turtles as determined by stable isotope analysis. Marine
Ecology Progress Series 166:277-284.

Hays et al. 2004. Pan-Atlantic leatherback turtle movements. Nature 429, 522 .

Sarti et al. 2007. Conservation and Biology of the Leatherback Turtle in the Mexican Pacific. Chelonian Conservation and Biology 6:70-78.

Seminoff et al. 2006. Stable isotope discrimination (δ13C and δ15N) between soft tissues of green sea turtles Chelonia mydas and their diet. Marine Ecology Progress Series 308: 271-278.

Seminoff et al. 2007. Stable carbon and nitrogen isotope discrimination and turnover in Pond Sliders Trachemys scripta: insights for trophic study of freshwater turtles. Copeia 2007(3):534-542.

Wallace et al. 2005. Bioenergetics and diving activity of internesting leatherback turtles Dermochelys coriacea at Parque Nacional Marino las Baulas, Costa Rica. Journal of Experimental Biology; 2005, v. 208, no. 20, p. 3873-3884

Wallace and George 2007 Alternative Techniques for Obtaining Blood Samples from Leatherback Turtles. Chelonian Conservation and biology 6:147-150

Wallace et al. 2006. Leatherback turtles as oceanographic indicators: Stable isotope analyses reveal a trophic dichotomy between ocean basins. Marine Biology 149:953-960.

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N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication the manuscript has been revised in light of these comments and to address other editorial requirements.