Eurasian Otters’ Urban Pond Use Patterns in Southern Spain: A Case Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The research seeks to highlight the impact of otters on small bodies of water in urban areas compared to natural zones, The subject is important from an ecological point of view but has many aspects that need to be improved and some shortcomings which, in view of the experimental design, can hardly be solved but which can be described in discussions as a limitation to work.  The bibliography can be improved on a number of specific topics. The names in vulgar and in Latin of the species must be harmonized remembering that the name in Latin of the species must always be in italics and mentioned only in the first quotation of the species. I suggest to introduce photos of the ponds, in order to allow a visual comparison of the 5 sites, which are often described by single location but should always be remembered also for the typology (urban vs natural). Monitoring is carried out in different months and years across different areas, this should be discussed. The results are influenced by the different sizes of the ponds and this is a key element to discuss. In the discussions also in relation to the lack of results on some tables should emphasize the limits of research, in terms of numbers , poor harmonization of monitoring systems and their results, also in relation to the use of complex models such as GLMM which is de facto applied to a limited number of study areas. I recommend that the site codes (A, B, C, Co 1 Co2) be introduced after each monitoring site. To simplify the reading I suggest to introduce a table, with described for each monitoring area the number of cameras, the monitoring effort applied both for the presence of otter and study of the presence of fish species and other species , This is a relevant aspect because it informs the reader of the effort in terms of finding markings (spraints), depredations, photo-trap periods, also in relation to the study period.  The part of the discussions is quite complete, but I repeat the results must also be commented on because of the large number and the possible effects of a lack of time harmonization and sampling effort , for the different areas; some parts of the discussion seem contradictory and it would be better to harmonize them (otters benefit from high food availability and/or easily accessible zones ?. English needs to be revisited. I add some specific notes: I suggest introducing images of the different areas of study.

Row 16 eurasian otter

Row 47 add citations for mammals  it’is not enough a single reference to birds

Row 58, the name in Latin is used only once in the text, in the case of the first quotation and is put in italics , ow 61 add more quotes

Row 68-73 the period should be better harmonized with the previous and following one ,

Row 93 and 96 complete the measures of fish size: it seems incomplete

Row 106 specify the name and code of ZSC

Figure 1 it is necessary to introduce the scale especially in the last image

Author Response

Answer to reviewers

 

We would like to thank reviewers their time spent on our work. All suggestions are welcome, and we appreciate them. After this revision the paper has improved considerably.

 

Reviewer 1

The research seeks to highlight the impact of otters on small bodies of water in urban areas compared to natural zones, The subject is important from an ecological point of view but has many aspects that need to be improved and some shortcomings which, in view of the experimental design, can hardly be solved but which can be described in discussions as a limitation to work. 

The bibliography can be improved on a number of specific topics.

> We have introduced new paragraphs with basic bibliography in the introduction as well as new references across the paper.

The names in vulgar and in Latin of the species must be harmonized remembering that the name in Latin of the species must always be in italics and mentioned only in the first quotation of the species.

> Latin species name have been harmonized.

I suggest to introduce photos of the ponds, in order to allow a visual comparison of the 5 sites, which are often described by single location but should always be remembered also for the typology (urban vs natural).

> We have not photos of all the ponds but have included some of them.

Monitoring is carried out in different months and years across different areas, this should be discussed.

The results are influenced by the different sizes of the ponds and this is a key element to discuss. In the discussions also in relation to the lack of results on some tables should emphasize the limits of research, in terms of numbers, poor harmonization of monitoring systems and their results, also in relation to the use of complex models such as GLMM which is de facto applied to a limited number of study areas.

> GLMM already consider the years as random factors, and they do not have significant effects on the results. Survey months comprised in all cases the period spring-summer (April-May to early October) except for Campanillas and Nueva Andalucía ponds that were spring and summer, respectively. In any case, all periods are similar to otters. We are not sampling during autumn or winter. All the sampling was carried out during the dry Mediterranean season.

We have also tested possible effects of pond size in the models without significant results. Further, there is not significant differences between urban and natural pond sizes (Anova test; F_(1,4) = 2,705; p = 0,175). In addition, the smallest ponds (Campanillas and Nueva Andalucía) do not contribute to GLMM analyses.

Nevertheless, we have included a comment about the limitation of the research in the discussion section.

I recommend that the site codes (A, B, C, Co 1 Co2) be introduced after each monitoring site.

> Done. We have harmonized the use of these site codes instead the names of the ponds in all the paper.

To simplify the reading I suggest to introduce a table, with described for each monitoring area the number of cameras, the monitoring effort applied both for the presence of otter and study of the presence of fish species and other species , This is a relevant aspect because it informs the reader of the effort in terms of finding markings (spraints), depredations, photo-trap periods, also in relation to the study period. 

> Added.

The part of the discussions is quite complete, but I repeat the results must also be commented on because of the large number and the possible effects of a lack of time harmonization and sampling effort , for the different areas; some parts of the discussion seem contradictory and it would be better to harmonize them (otters benefit from high food availability and/or easily accessible zones ?.

> Discussion has been revised/harmonized and a limitation of the study section is now part of ir discussing sampling effort and other effects.

English needs to be revisited. I add some specific notes: I suggest introducing images of the different areas of study.

> Photos of areas of study are now introduced, as noted before. English is reviewed.

Row 16 eurasian otter

> Modified.

Row 47 add citations for mammals  it’is not enough a single reference to birds

> Added.

Row 58, the name in Latin is used only once in the text, in the case of the first quotation and is put in italics

> Modified.

Row 61 add more quotes

> Description of species ecology improved in introduction. There are now more and detailed references.

Row 68-73 the period should be better harmonized with the previous and following one

> I don't understand what period needs to be harmonized. This paragraph only provides context for the reader regarding the evolution of the species in the study area. It is not the study period. However, we have introduced a clarification about our study period.

Row 93 and 96 complete the measures of fish size: it seems incomplete

> Modified. The symbol ± was missing.

Row 106 specify the name and code of ZSC

> Added.

Figure 1 it is necessary to introduce the scale especially in the last image

> This figure is composed of three different scaled areas (country, province, and municipalities). It is difficult to add the three scales and we think that will not significantly improve the understanding of the figure. The idea of the figure is just to put the reader in a geographic context noting that the study area is as far south as possible on the Iberian Peninsula (Andalucía region, Málaga province).

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is interesting and brings a new approach to otter research. I think it has potential for publication in a scientific journal.

I have a few suggestions:

Line 90, 94, 100 – latin name in italic

Line 96 - unclear measure

Methodology

I am not sure how well the data collection methodology is described. Otter abundance estimation and monitoring methodology - it might be good to add a reference if available.

Results

3.1 - is it possible that apart from predation, there is also intraspecific or interspecific predation of certain fish species, as well as natural mortality? If possible, then the otter predation data is incorrect.

Regarding the carcasses, is there a possibility that not all carcasses have been recorded?

3.2 - same as before, is there a possibility that the goldfish were predated by some other predatory species of birds or mammals (if it was the only species of fish in the lake)

 

In the entire study, i.e. the results presented, the authors did not estimate the number of otters. The predation caused by the otter is not well separated from the predation of other predators. I think the monitoring methodology should be described better, I think it was not carried out systematically.

In the discussion, the authors touched on one thing that would be a significant scientific contribution. The density of fish in a pond at which otters stop attacking!! I think it would be desirable to address this issue a little better (if there are observations for such an analysis).

It is not new that otters are coming to artificial ponds. It would also be desirable to indicate the distance or to make spatial analyses of how far away the natural habitats of otters are from artificially constructed ponds.

 

Author Response

Answer to reviewers

 

We would like to thank reviewers their time spent on our work. All suggestions are welcome, and we appreciate them. After this revision the paper has improved considerably.

 

Reviewer 2

The manuscript is interesting and brings a new approach to otter research. I think it has potential for publication in a scientific journal.

I have a few suggestions:

Line 90, 94, 100 – latin name in italic

> Modified.

Line 96 - unclear measure

> Modified. The symbol ± was missing.

Methodology

I am not sure how well the data collection methodology is described. Otter abundance estimation and monitoring methodology - it might be good to add a reference if available.

> Added.

 

Results

3.1 - is it possible that apart from predation, there is also intraspecific or interspecific predation of certain fish species, as well as natural mortality? If possible, then the otter predation data is incorrect.

> In urban pools, there was only one species of fish, and they were very uniform in size. This rules out interspecific predation and greatly reduces intraspecific predation. In natural pools, the possibility was somewhat higher, but mortality was not estimated there. In any case, we add these issues to the limitation of the study section in discussion.

Regarding the carcasses, is there a possibility that not all carcasses have been recorded?

> There's always the possibility that some carcasses might go undetected, even though the search was conducted early in the morning by both the staff and our team. However, if this were to happen, it would only be an underestimation of the predation rate, which would be faster and more intense than we assumed. In this case, our conclusion (massive predation or depletion in urban ponds) would be more consistent.

3.2 - same as before, is there a possibility that the goldfish were predated by some other predatory species of birds or mammals (if it was the only species of fish in the lake)

> It's likely that a heron or kingfisher caught some goldfishes, as occurred in the large lagoon of Torre Leoneras. However, predation by species other than otters was negligible there. Furthermore, the fact that the fished disappeared quickly (within almost thirty days…) points to otters (the same pattern as in the other ponds). In addition, the otter was observed in this pool. Anyway, this consideration is also added to the limitations of the study.

In the entire study, i.e. the results presented, the authors did not estimate the number of otters. The predation caused by the otter is not well separated from the predation of other predators. I think the monitoring methodology should be described better, I think it was not carried out systematically.

> It is not possible to us estimate the number of otters causing these predation patterns. Camera trapping methods did not allow to identify individually otters and at that moment reliable methods as REM were not used. We recognize that this is precisely one weakness of the study, but we have no doubt that most of the predation was due to otters. In fact, the patterns between ponds (and years) are not significantly different, and the daily presence of otters is confirmed by the various sampling methods used and direct observation. Sampling was systematic: tracking of spraints and prey daily and by two teams (the staff and our team). Nevertheles, we have improved monitoring description in the method section.

In the discussion, the authors touched on one thing that would be a significant scientific contribution.!! I think it would be desirable to address this issue a little better (if there are observations for such an analysis).

> Many thanks for this observation. We add some comments on this issue in discussion section.

It is not new that otters are coming to artificial ponds. It would also be desirable to indicate the distance or to make spatial analyses of how far away the natural habitats of otters are from artificially constructed ponds.

> The distance data to natural habitats are shown in Table 1 (between 55 to 190 meters away). All urban ponds are very close to natural habitats, except for urban pond B, which is slightly further away. This variable is not significant (tested in preliminary analyses) and do not allow for further spatial analyses. We modify table 1 to make clearer that distance to nearest water stream is the same than natural habitat.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents an important and timely study on the use and predation patterns of Eurasian otters in urban versus natural ponds in southern Spain. Understanding how protected carnivores like otters interact with new anthropogenic habitats is of broad interest in the fields of urban ecology, conservation, and human-wildlife conflict. The study offers valuable observations and robust datasets but also has several significant areas that require attention before it can be considered for publication. Detailed comments are as follows:

1) The logical flow is generally clear, but the Results and Discussion are lengthy, sometimes repetitive, and key points are buried in detailed site descriptions. Use clearer section/subsection headings for each study site in Methods and Results. Summarize main findings up front within the Results section to aid reader navigation. Briefly refer to detailed observations (e.g., carcass descriptions) in main text, but move exhaustive detail to supplementary data or appendices if appropriate.

2) The statistical analysis is adequate but sometimes lacks clarity, specifically regarding sample sizes for camera trap data and the precise meaning of "weekly otter visits" estimates. Clearly describe how visits were differentiated between individuals (any way to distinguish multiple otters?). For sites without camera trap data or missing values ("not estimated"), clarify how this affects the interpretation. Where predation could not be attributed solely to otters, clearly separate those findings and discuss limitations. Address the potential biases introduced by relying on reports from property staff, especially in the absence of photographic evidence.

3) The main hypothesis predicted little difference between urban and natural sites, but the most interesting result is the non-random, rapid depletion of prey in urban patches. This pivotal point deserves greater emphasis and interpretation. Highlight the implications of this finding for urban prey base sustainability and potential otter behavioral plasticity. Discuss more deeply how otter exploitation of urban ponds may affect both urban fauna and otter population dynamics at the landscape scale.

4) The manuscript registers human conflict only after much narrative detail. The significance for management is strong but would benefit from tighter integration. Raise the implications for conflict and mitigation earlier in both the Introduction and Discussion. Present the human dimension as a key parallel research finding, not just an add-on.

5) The writing is generally clear but can be verbose and indirect, with some awkward constructions and minor typographical errors. Condense sentences for clarity and directness. Carefully proofread for typographical errors, inconsistencies (e.g., spacing, units), and use of scientific names. Consider minor language editing for readability.

6) The figures and tables are generally well-presented, but legends sometimes lack essential context. Some data (e.g., carcass images) are described in text but could be better used as supplementary figures or appendices. Ensure all figures/tables are fully self-contained, with axes and units clearly labeled. Consider moving lengthy photographic descriptions to supplementary material.

Author Response

Answer to reviewers

 

We would like to thank reviewers their time spent on our work. All suggestions are welcome, and we appreciate them. After this revision the paper has improved considerably.

 

Reviewer 3

The manuscript presents an important and timely study on the use and predation patterns of Eurasian otters in urban versus natural ponds in southern Spain. Understanding how protected carnivores like otters interact with new anthropogenic habitats is of broad interest in the fields of urban ecology, conservation, and human-wildlife conflict. The study offers valuable observations and robust datasets but also has several significant areas that require attention before it can be considered for publication. Detailed comments are as follows:

1) The logical flow is generally clear, but the Results and Discussion are lengthy, sometimes repetitive, and key points are buried in detailed site descriptions. Use clearer section/subsection headings for each study site in Methods and Results. Summarize main findings up front within the Results section to aid reader navigation. Briefly refer to detailed observations (e.g., carcass descriptions) in main text, but move exhaustive detail to supplementary data or appendices if appropriate.

> Results and discussion have been improved, key points highlighted, and subsections added (results already had).

2) The statistical analysis is adequate but sometimes lacks clarity, specifically regarding sample sizes for camera trap data and the precise meaning of "weekly otter visits" estimates. Clearly describe how visits were differentiated between individuals (any way to distinguish multiple otters?). For sites without camera trap data or missing values ("not estimated"), clarify how this affects the interpretation. Where predation could not be attributed solely to otters, clearly separate those findings and discuss limitations. Address the potential biases introduced by relying on reports from property staff, especially in the absence of photographic evidence.

> A new table has been added to improve clarity regarding sampling effort data, sample sizes. The limitations of the study have also been added to the discussion.

> Otters weekly visits is just a relative abundance index, a way to measure the presence of otters in the ponds in a consistent number, since otters did not appear every day. Obviously, this index does not allow for differentiation between different individuals, just as camera trapping did not. As explained to a previous reviewer, we do not use methods to estimate population abundance without individual recognition (such as REM). Therefore, we cannot know whether it was always the same otter or different individuals. That's why we always refer to otters in the plural. And in any case, this consideration does not affect the fact that prey populations were depleted. We have discussed the possibility that in some cases predation was carried out by females with cubs. Anyway, this argument adds to the study's limitations.

> The ponds without camera traps were not included in the analysis, so they do not affect the results or interpretation. In any case, the limitations of the work already mention that the sample size is small and further research on this topic is needed.

3) The main hypothesis predicted little difference between urban and natural sites, but the most interesting result is the non-random, rapid depletion of prey in urban patches. This pivotal point deserves greater emphasis and interpretation. Highlight the implications of this finding for urban prey base sustainability and potential otter behavioral plasticity. Discuss more deeply how otter exploitation of urban ponds may affect both urban fauna and otter population dynamics at the landscape scale.

> We have reinforced this result in the discussion, emphasizing its relevance and significance. We have also improved interpretation in the discussion regarding otter populations.

4) The manuscript registers human conflict only after much narrative detail. The significance for management is strong but would benefit from tighter integration. Raise the implications for conflict and mitigation earlier in both the Introduction and Discussion. Present the human dimension as a key parallel research finding, not just an add-on.

> We have added some comments on implications of wildife-human conflict in introduction and improve discussion section about this topic highlining the relevance of findings.

5) The writing is generally clear but can be verbose and indirect, with some awkward constructions and minor typographical errors. Condense sentences for clarity and directness. Carefully proofread for typographical errors, inconsistencies (e.g., spacing, units), and use of scientific names. Consider minor language editing for readability.

> Typographical errors have been corrected and a second professional proofreading by a native English carried out trying to improve readability.

6) The figures and tables are generally well-presented, but legends sometimes lack essential context. Some data (e.g., carcass images) are described in text but could be better used as supplementary figures or appendices. Ensure all figures/tables are fully self-contained, with axes and units clearly labeled. Consider moving lengthy photographic descriptions to supplementary material.

> Figure and table legends have been revised to improve units and labels. Instead of moving photographic description to supplementary material we have modified them to make clearer and shorter.

 

 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

All suggestion are accepted.

Author Response

There are no reply becouse there no questions.

Reviewer 3 Report

Comments and Suggestions for Authors

This study documents Eurasian otter visits and predation at three urban ponds and two control (natural) plunge pools in Málaga province (2017–2019). The authors combine spraint counts, camera-trap visit rates, prey counts and Kaplan–Meier survival curves to compare use patterns in urban versus natural ponds, test randomness of visit sequences (Wald–Wolfowitz), and describe human–wildlife conflict and mitigation attempts. Main findings: (1) weekly spraint abundance did not differ by pond type; (2) otter visit frequency was lower in monitored urban ponds than in controls; (3) visit timing to urban ponds was non-random/clustered and associated with concentrated prey depletion; (4) urban ponds can create conflicts with owners/managers. 

Weaknesses that block acceptance at present: limited replication and uneven effort, incomplete statistical reporting, and some attribution/seasonality uncertainties. If the authors satisfactorily address the major points above, I recommend acceptance pending minor further edits.

1) Only three urban ponds and two control plunge pools (single replicates per category are low). The GLMM includes site and year as random effects but with only few levels this provides weak inference about “site type” generality. The authors acknowledge small sample size, but the discussion and conclusions still at times over-generalize (e.g., “otters hunted extensively in urban settings” as if broadly representative). Tone down general statements about urban otter behaviour or present results strictly as case studies. If possible, perform sensitivity analyses: (a) run models with sites removed one-by-one to show robustness; (b) report effect sizes with 95% CIs (not only Wald χ2 and P-values). Add explicit caveat paragraph in Discussion about inference limits from low site number and how this affects generality.

 

2) Camera trapping and daily monitoring occurred at some ponds (Urban pond A small pond: 3 cameras; natural pond Co2: 3 cameras) but not others. Spraints were removed daily in some sites and weekly in controls—differences in survey frequency and methods can bias spraint counts and visit estimates. This heterogeneity is not fully accounted for in analyses. 

Quantify and present detection effort per site clearly (you partly did in Table 2 — good). Then: (a) include sampling effort as an offset or covariate in GLMMs where appropriate (e.g., camera trapping days as exposure for visit counts); (b) assess whether spraint count rate correlates with sampling frequency; (c) explicitly state limitations due to uneven effort and how they might bias findings.

 

3) Model diagnostics and effect sizes are not shown. GLMM families chosen are reasonable, but no model tables with estimates, standard errors, confidence intervals, or goodness-of-fit / overdispersion checks are reported. For Poisson GLMM, overdispersion is plausible and not addressed. The runs test is reported but no effect sizes or plots of visit sequences are shown. Kaplan–Meier curves are shown but sample sizes/events and censoring details are sparse. Provide (either in main text or Supplementary): full model output tables (estimates, SEs, 95% CIs, p-values), overdispersion checks for Poisson models (if overdispersed, consider negative binomial), residual diagnostics, and number of events/censored observations for K–M. Add plots: model-predicted means (with CIs) by site type; raster/sequence plot of visit dates per pond to illustrate clustering; survival curves with number at risk table.

 

4)  In some ponds (large pond), multiple predators (kingfishers, herons) were observed. The study uses spraints and carcasses to infer otter predation, but evidence is indirect for some removals. This weakens conclusions about otters being sole drivers of prey depletion at all sites. 

Clarify which prey mortality events are confidently attributed to otters (camera-confirmed) versus inferred (scales/spraints). Re-run analyses of survival/predation excluding events/sites where otter attribution is uncertain, or present those results side-by-side. Be explicit in Discussion about the uncertainty and how it affects interpretations.

 

5) Monitoring periods differ across sites and years (e.g., urban pond A monitored 2017–2018, others only short periods). Since otter activity and prey availability vary seasonally, this can confound comparisons.

 Include sampling period (season/month) as a fixed effect or covariate where possible, or at least show summary statistics by month/season and discuss potential seasonal bias. Refrain from statements implying equivalence across seasons if not supported.

 

6)  The paper documents escalation to illegal or ethically questionable measures (snares). The manuscript should make clearer whether the authors intervened, whether they reported illegal actions, and whether the work had ethics approvals or permits for monitoring, spraint removal, and camera trapping. v2 states compliance with laws but lacks permit numbers or institutional approvals. 
Add a brief ethics/permits statement with permit numbers and approval bodies. Clarify authors’ role regarding the mitigation actions (did they advise? record only?). If snares or lethal measures were illegal, mention whether incidents were reported to authorities (if known), while being careful legally.

Author Response

Plase, see the attachment.

Author Response File: Author Response.pdf