PONE-D-23-05505Wastewater Plumes Can Act as Non-physical Barriers for Migrating Silver Eel.PLOS ONE

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: N/A

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

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Reviewer #1: This study offers a straightforward objective of interest – is the movement of fish modified by chemical barriers (do fish avoid wastewater plumes in the field). There is ample reason to test this and believe it to be the case, but as the authors note, it has received little attention.

This study is quite small, but well conducted. It adds interesting information and should be published. I have fairly minor comments for consideration. I think one of the figures might need attention and material could be added to the Discussion.

Specific comments

Clear abstract.

40-43. Strictly speaking, the writing is not super strong. ‘been rarely’… ‘may have a chance of…’ but this is a science paper. The message is clear enough. If the authors are after a better product, they can always have it edited.

47. I really like this chemical barrier idea. I don’t think it has been explored in this manner, at least with WW. Nice!

60. To a lot of the world, STP means scientifically treated petroleum. The other way this could go is WWTP effluent, or municipal effluent, but whatever.

91. How much of a difference could it really make? I suggest inserting a number. I bet it is not much and so meaningless in the context of sensor error etc.

113. ‘Eels’ or ‘research subjects’…

194. ‘… are reasonably possible…’ ?

Figure 5. I trust these are real, example traces? If so, please state in the caption. I am puzzled in that you state no attraction was noted, yet you have a trace that indicates at least one animal demonstrated this behavior. Is this behavior example just fiction then? Please clarify. If these are not real traces, please swap out for real ones and indicate no attraction examples existed.

Discussion. I would like to see at least a brief discussion on the variability noted in the observed responses. Is there evidence of behavioral phenotype? This might be interesting. Could something unique be said of the animals that tended to turn away, or perhaps never come back? I do appreciate the dataset is small, but even so, it is worth considering.

Reviewer #2: With much interest I have read the manuscript of Winter and colleagues about an understudied aspect of silver eel migration in Europe. Although non-physical barriers are omnipresent in the Anthropocene, their potential effect on diadromous fish are indeed hardly investigated. As such, this investigation definitely is worthwhile publishing in PlosOne.

I noticed the paper has a certain review history. I agree with the authors that both recent reviewers hardly made any usable recommendations to further improve the paper. As far as I can judge, the paper has however already significantly improved after the first review round.

For me the paper reads fluently, the analysis seem sound, the results are to the point and the discussion focussed to the research questions posed. It is a very concise paper, but according to me this is not necessarily a disadvantage.

I have some minor remarks, primarily about certain aspects that deserve a little bit more explanation.

• Lines 48-50: I doubt that no studies exist on temperature effects of plumes (effluent, cooling water) although the majority might indeed have a focus on oxygen. Maybe good to check and to adapt this statement concordantly.

• Line 101: all transmitters

• Lines 114-115: all eels used in this study were captured downstream the research location. Since all investigated eels were classified as migrating, these eels could potentially already have experienced effects of the STP-effluents during downstream migration. Why were no silver eels from upstream the STP used? Any bias by eel learning behaviour possible here?

• Line 136 a.f.: I can understand the comment by previous reviewers that the methods are a bit too concise for non-specialists to follow. I have some difficulties with the plume modelling. I think it is wise to describe in a small introductory paragraph the outline and build-up of the model. For instance, for me it is not clear to which temperature you refer to in line 156. Does it concern measured temperatures of the effluent and/or the canal water? I assume both temperatures are needed to model the plume dynamics?

• Lines 185-186: What basis do you use for this threshold? Is there literature available of European eels’ or other diadromous fish species’ capacity to sense such or other small temperature differences? Please add references if possible or explain the choice for this threshold.

• Lines 240-243: In my opinion, more explanation is needed to understand this complex figure. At least each subgraph should be mentioned and explained. I see different colours in the track locations, I assume these were related to plume interferences? But in the case of #6, these coloured dots appear far upstream from the modelled flow as shown in the subgraph. By exploring the supplementary materials I noticed a figure that explained the track colours as related to time sequence... This is confusing, please provide a clear explanation directly in the figure legend. The print quality of the figure also seems very poor. I could not read (axis)values in the subgraphs...

• Lines 286-287: I think the effects of non-physical barriers should not be underestimated. You might better point out in the discussion section that migrating eels are subject to a multitude of distracting and attracting flows in the current Anthropocene... maybe one effluent discharge point might have a minor impact but a sequence of many effluent discharge points along the migration route might cause substantial migration delays or extra energy dissipation. In this respect, also the current enrolment of aquathermic applications as renewable energy source within the EU Green deal place another and maybe comparable threat to diadromous fish. You might point this out as well in the discussion.

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Reviewer #1: No

Reviewer #2: Yes: Jeroen Van Wichelen

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Wastewater Plumes Can Act as Non-physical Barriers for Migrating Silver Eel.

PONE-D-23-05505R1

Dear Dr. van Keeken,

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Dennis M. Higgs

Academic Editor

PLOS ONE

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