Growth, Mortality, and Stock Resilience of Common Carp (Cyprinus carpio) in the Romanian Danube: Insights into Sustainable Exploitation (2019–2024)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

I have carefully reviewed your manuscript entitled “Evaluation of Stock Dynamics and Sustainable Exploitation of Common Carp in the Romanian Danube.” The topic is highly relevant, as common carp (Cyprinus carpio) represents one of the most economically important freshwater fish species in Europe, and long-term data on its population dynamics in the Danube River are indeed scarce. Your work contributes valuable baseline information that can support sustainable management and conservation of this species.

The manuscript is well organized, and the integration of growth, mortality, and exploitation parameters over multiple years provides a useful framework for stock assessment. However, several sections would benefit from greater methodological precision and a more cautious interpretation of results. In particular, many conclusions are presented too definitively given the sample size and scope of data, while key statements throughout the Discussion lack appropriate literature support. Clarification of sampling design, improved referencing, and inclusion of contextual information—such as annual catch statistics and the role of aquaculture in supporting carp production—would substantially strengthen the paper’s scientific credibility and management relevance.

Overall, I commend the effort and the importance of the dataset, but recommend a careful revision to temper speculative interpretations, improve referencing, and contextualize findings within the broader fisheries framework of the Danube Basin.

General comments:

In the Introduction, the authors should include quantitative information on the annual commercial catch of common carp from the Romanian sector of the Danube. Presenting the approximate tonnage or catch trend over recent years would provide essential context for assessing the fishery’s scale and management relevance. This would also strengthen the rationale for the study’s focus on stock assessment and sustainability.

In the Data Collection subsection, several essential details are missing. The authors state that sampling was conducted “across six seasons” between 2019 and 2024, but it is unclear which seasons or months were covered and whether sampling occurred annually or intermittently. Furthermore, the phrase “along the entire Romanian stretch of the Danube” lacks precision — the number and location of sampling sites (e.g., coordinates, river kilometers, or major sectors) should be clearly indicated. These details are crucial for evaluating spatial and temporal representativeness and reproducibility of the study.

The interpretation in the opening paragraph of the Discussion appears too definitive given the limited sample size (2646 individuals over six years). These data likely represent only a small proportion of the total population, so statements such as “significant fluctuations” or “more robust growth” should be moderated or qualified (e.g., “apparent trends” or “observed differences”). The discussion should better reflect the inherent uncertainty and sampling limitations, emphasizing that the conclusions apply to the sampled subset rather than the entire population.

Specific Comments:

Abstract

Line 14: I suggest specifying “population structure” instead of “structure” to clearly indicate that the study refers to the demographic characteristics of the Cyprinus carpio population, not to another type of structure.

 

Intoduction

 

Line 50: The Latin name of the species should not be repeated after its first mention. As the authors consistently use the common name in subsequent sentences, it would be preferable to omit “C. carpio” here for stylistic consistency.

 

Lines 50-53: A supporting reference should be added at the end of this sentence.

 

Lines 55-57: In this sentence, it would be useful to also mention fecundity and age at first maturity among the key life-history traits influencing growth, alongside natural mortality. This would provide a more complete picture of the traits shaping species’ responses to exploitation.

 

Materials&Methods

 

Lines 82-83: The sentence “Sex ratio was determined based on macroscopic gonad inspection and denoted as F (female) and M (male)” is inaccurate. Macroscopic inspection is used to determine the sex of individual fish, not the sex ratio. The subsequent sentence (“The sex ratio was calculated annually, expressing the number of females relative to the number of males...”) should follow immediately after, as it logically continues the methodological description. At present, its placement at the end of the subsection makes it appear disconnected.

 

Line 86: The use of the species name is inconsistent. The authors should either use the Latin name (Cyprinus carpio) throughout the manuscript or, preferably, mention it only once upon first appearance and use the common name (“common carp”) thereafter for consistency.

 

Line 86: the reference to Stroe et al. (2025) [Ref. 13] appears misplaced. It is not customary to cite another study as a data source in this section unless specific datasets were directly reused or integrated into the present analysis. If this is the case, the authors should clearly explain how these data were incorporated and ensure transparency in data origin. Otherwise, this reference should be moved to the Discussion section, where the comparison of results would be more appropriate.

 

Lines 126-128: The sentence on annual mean water temperature appears misplaced. Since temperature is a parameter used in Pauly’s equation for estimating natural mortality, it would be more logical to integrate this information into the description of M calculation or, alternatively, move it to the end of the paragraph. Its current position between the mortality parameters disrupts the methodological flow.

 

Lines 137-139: Sentence: “This parameter provides insight into the reproductive structure of the stock…” is more interpretative in nature and therefore belongs to the Discussion section. If the authors wish to retain it here, they should provide appropriate references to support these statements about the relevance of L₅₀ for stock sustainability and management.

 

Discussion

 

Some parts of the discussion are speculative, especially regarding the absence of adults, habitat preferences, and ecological adaptations. Please ensure that speculative statements are clearly presented as hypotheses, and provide references where possible.

 

Lines 278-282: The interpretation of sex ratio patterns appears too assertive given the dataset’s limitations. While the observed male-biased ratios in 2019 and 2021 are interesting, the explanations offered (selective removal of females or differential survival) remain speculative, as no supporting data on fishing selectivity, maturity stages, or seasonal sampling are provided. These statements should be rephrased more cautiously (e.g., “may indicate” or “could be associated with”) and discussed in the context of potential sampling bias and natural variability.

 

Lines 285-292: This paragraph provides ecologically sound reasoning regarding the influence of temperature on carp metabolism and growth, but it lacks any supporting references. Since the statements concern fundamental physiological and ecological processes, at least one or two citations (e.g., general ichthyological or thermal biology studies) should be included to substantiate these claims. Otherwise, the text reads as purely speculative and weakens the scientific credibility of the discussion.

 

Lines 295-307: The discussion of growth and mortality parameters is overly definitive relative to the data available. The observed variation in L∞ and M/K values indeed suggests potential trends, but these should be presented as possible rather than demonstrated outcomes. Statements such as “likely as a compensatory response to elevated exploitation and environmental stress” are speculative and require supporting references. The authors should moderate the tone (e.g., “may indicate” instead of “suggests”) and, where possible, cite literature linking changes in L∞, M/K, and fishing pressure. Otherwise, these interpretations risk overstating conclusions beyond what the dataset can substantiate.

 

Lines 308-319: The interpretation of growth parameter variation is speculative and lacks sufficient empirical support. The statements linking changes in K and Φ′ to environmental or density-dependent factors are presented too conclusively and require appropriate references. Moreover, the comparative references [46, 47] are only marginally relevant, as they refer to geographically and ecologically distinct systems (Ethiopia, Algeria) and therefore provide limited support for the observed trends in the Danube population. These comparisons should be either reconsidered or clearly qualified as illustrative rather than explanatory.

 

Lines 331-341: This paragraph presents biologically plausible interpretations concerning variation in L₅₀, reproductive potential, and temperature effects on fishing mortality, but it lacks supporting references. Given that these are well-documented mechanisms in fish population ecology, at least a few key sources (e.g., studies on size-selective fishing, temperature-dependent mortality, or maturity shifts under exploitation) should be cited to substantiate these claims. Otherwise, the discussion remains speculative and weakens the scientific grounding of the conclusions.

 

Lines 342-363: The discussion of the M/K ratio is overly conclusive and only partially supported by references. Interpretations regarding population vulnerability and stock destabilization should be moderated, as they go beyond the scope of the presented data. The correlation between M/K and K appears to be described as empirical, although it is largely a mathematical relationship. Moreover, references [49,50] are too general to substantiate these claims; additional literature directly addressing M/K relationships and their ecological implications should be included.

Overall, this section would benefit from a clearer distinction between observation, inference, and speculation, supported by appropriate literature.

 

Lines 366-374: The discussion linking exploitation rate (E) and length at 50% maturity (L₅₀) is presented too conclusively. The conclusions about recruitment overfishing and the need for regulatory measures are reasonable in principle but not empirically supported by the data shown. Without direct evidence of catch composition, fishing effort, or verified benchmarks for sustainable E values in this fishery, such statements should be framed as potential concerns rather than demonstrated outcomes. Additional references supporting the relationship between E, L₅₀, and recruitment dynamics would strengthen this section.

 

Lines 377-389: The concluding part of the Discussion again presents the results too definitively. Phrases such as “reveals a dynamic population trajectory” and “indicate consistently higher values” should be moderated, as they imply population-level certainty not supported by the available sample size and temporal scope. Additionally, the statement about “alternating intervals of sustainable harvest and potential overexploitation” lacks quantitative benchmarks or literature references to define what constitutes sustainability in this context. The comparison with the Caspian Sea population [48] should also be reconsidered or clearly qualified, given the ecological and geographical differences between systems.

 

Line 391: The term is incorrectly written as “Total Admissible Capture.” The correct term is Total Allowable Catch (TAC). Please correct this and ensure consistency throughout the manuscript, including figures, tables, and captions.

Lines 392-440: This subsection is informative but overly conclusive and lacks sufficient referencing to support statements about regulatory effectiveness, IUU fishing, and the spatial distribution of fishing pressure. Most assertions are general and not directly substantiated by quantitative data or citations.

In addition, it would be highly relevant to include a brief discussion of aquaculture, especially common carp production volumes in Romania. Since the section focuses on sustainability and the need to reduce fishing pressure on natural populations, referring to aquaculture as an alternative supply source would provide a more comprehensive and realistic management context. Presenting recent data or references on carp aquaculture production would help balance the discussion and highlight the potential role of fish farming in ensuring sustainable fish availability.

Lines 443-476: The final part of the Discussion provides a broad and conceptually sound overview but remains unreferenced and overly general. Statements about the paucity of research, data limitations, and governance challenges (including IUU fishing) require literature support — for example, regional management reports, FAO publications, or peer-reviewed studies addressing fisheries in the Danube basin.

While the acknowledgment of uncertainties is appreciated, the paragraph could be strengthened by integrating concrete examples or references illustrating existing knowledge gaps and management initiatives. Furthermore, mentioning aquaculture production or ongoing regional projects (e.g., EU or ICPDR frameworks) would provide a more complete sustainability perspective.

 

Conclusion

 

The Conclusion section is concise and well structured but overly definitive relative to the data scope. Statements about population stability and “generally sustainable exploitation” should be expressed more cautiously and framed as indicative trends rather than confirmed outcomes. The fact that reported catches remained below the TAC does not, by itself, substantiate sustainability.

The conclusion would be strengthened by explicitly acknowledging data limitations (e.g., sample size, temporal scope) and by situating the findings within the broader regional or European context, possibly citing comparative studies or management frameworks.

 

 

Author Response

Response to Reviewer 1

We sincerely thank Reviewer 1 for their detailed and constructive comments, which have greatly contributed to improving the quality and clarity of our manuscript. We carefully revised the text throughout to address all observations. Below we summarize the main changes implemented in response to your feedback:

 

Abstract:

Comment: Line 14: I suggest specifying “population structure” instead of “structure” to clearly indicate that the study refers to the demographic characteristics of the Cyprinus carpio population, not to another type of structure.

• Response: We specified “population structure”

Comment: In the Introduction, the authors should include quantitative information on the annual commercial catch of common carp from the Romanian sector of the Danube. Presenting the approximate tonnage or catch trend over recent years would provide essential context for assessing the fishery’s scale and management relevance. This would also strengthen the rationale for the study’s focus on stock assessment and sustainability.

• Response: Quantitative data on national and global carp production were added, providing clearer context on the scale and relevance of the fishery. The role of aquaculture in supporting sustainable carp supply was also integrated (Line 40-50).

Comment: Line 50: The Latin name of the species should not be repeated after its first mention. As the authors consistently use the common name in subsequent sentences, it would be preferable to omit “C. carpio” here for stylistic consistency.

• Response: We carefully revised the manuscript and change carpio to common carp, leaving just the first mention

Comment: Lines 50-53: A supporting reference should be added at the end of this sentence.

• Response: We added references (Lines: 62-64)

Comment: Lines 55-57: In this sentence, it would be useful to also mention fecundity and age at first maturity among the key life-history traits influencing growth, alongside natural mortality. This would provide a more complete picture of the traits shaping species’ responses to exploitation.

• Response: We mentioned fecundity and age at first maturity among the key life-history traits influencing growth, alongside natural mortality (Lines 66-68)

 

 

Materials and Methods

Comment: In the Data Collection subsection, several essential details are missing. The authors state that sampling was conducted “across six seasons” between 2019 and 2024, but it is unclear which seasons or months were covered and whether sampling occurred annually or intermittently. Furthermore, the phrase “along the entire Romanian stretch of the Danube” lacks precision — the number and location of sampling sites (e.g., coordinates, river kilometers, or major sectors) should be clearly indicated. These details are crucial for evaluating spatial and temporal representativeness and reproducibility of the study.

• Response: The sampling design was fully clarified. We specified the five sampling locations (Moldova Nouă, Calafat, Giurgiu/Oltenița, Brăila–Chiscani, and Galați), the corresponding river kilometers, and the sampling period (April–October each year). Details on sampling effort were added, including the number of hauls per site, gear types used simultaneously, and standardized duration of net deployment.

Comment: Lines 82-83: The sentence “Sex ratio was determined based on macroscopic gonad inspection and denoted as F (female) and M (male)” is inaccurate. Macroscopic inspection is used to determine the sex of individual fish, not the sex ratio. The subsequent sentence (“The sex ratio was calculated annually, expressing the number of females relative to the number of males...”) should follow immediately after, as it logically continues the methodological description. At present, its placement at the end of the subsection makes it appear disconnected.

• Response: The description of sex determination and sex ratio calculation was corrected, as suggested.

Comment: Line 86: The use of the species name is inconsistent. The authors should either use the Latin name (Cyprinus carpio) throughout the manuscript or, preferably, mention it only once upon first appearance and use the common name (“common carp”) thereafter for consistency.

• Response: Line 86: The reference to Stroe et al., 2025 was clarified, explaining that part of the 2021–2024 data set (to which the author contributed as co-author) was reprocessed and integrated with new 2019–2020 records to extend the temporal coverage of the present analysis.

Comment: Lines 126-128: The sentence on annual mean water temperature appears misplaced. Since temperature is a parameter used in Pauly’s equation for estimating natural mortality, it would be more logical to integrate this information into the description of M calculation or, alternatively, move it to the end of the paragraph. Its current position between the mortality parameters disrupts the methodological flow.

• Response: We reordered the sentences

 

Results:

Comment: Lines 137-139: Sentence: “This parameter provides insight into the reproductive structure of the stock…” is more interpretative in nature and therefore belongs to the Discussion section. If the authors wish to retain it here, they should provide appropriate references to support these statements about the relevance of L₅₀ for stock sustainability and management.

• Response: We added references to sustain our statements regarding the relevance of L₅₀ (Lines:169-172)

Discussion

Comment: Some parts of the discussion are speculative, especially regarding the absence of adults, habitat preferences, and ecological adaptations. Please ensure that speculative statements are clearly presented as hypotheses and provide references where possible.

• Response: The discussion was carefully revised to moderate speculative phrasing and ensure that all interpretative statements are clearly presented as hypotheses rather than definitive conclusions. Specifically, the sentences referring to the absence of adults, habitat preferences, and ecological adaptations were rephrased to use conditional language such as “may indicate,” “could suggest,” or “might reflect.” Where possible, supporting references were added to substantiate ecological interpretations, including works addressing habitat-related variability and population structure in cyprinid species (Lines: 447-458)

Comment: In particular, many conclusions are presented too definitively given the sample size and scope of data, while key statements throughout the Discussion lack appropriate literature support. Clarification of sampling design, improved referencing, and inclusion of contextual information—such as annual catch statistics and the role of aquaculture in supporting carp production—would substantially strengthen the paper’s scientific credibility and management relevance.

• Response: The Discussion was thoroughly revised to moderate definitive language and to frame conclusions as indicative rather than conclusive, using expressions such as “may suggest,” “could indicate,” and “might reflect.” Additional literature was incorporated to substantiate ecological and management interpretations (e.g., Jensen, 1997; Hordyk et al., 2015; Gheorghe et al., 2011; FAO, 2022; European Commission, 2023). The sampling design section (Lines 85-102) was expanded to clarify the number of sites, sampling frequency, and effort per location. Furthermore, recent data on common carp catches and aquaculture production in Romania (FAO, 2024; ANPA, 2024) were included to contextualize management relevance and sustainability.

Comment: Lines 278-282: The interpretation of sex ratio patterns appears too assertive given the dataset’s limitations. While the observed male-biased ratios in 2019 and 2021 are interesting, the explanations offered (selective removal of females or differential survival) remain speculative, as no supporting data on fishing selectivity, maturity stages, or seasonal sampling are provided. These statements should be rephrased more cautiously (e.g., “may indicate” or “could be associated with”) and discussed in the context of potential sampling bias and natural variability.

• Response: The text was revised to adopt a more cautious tone by replacing assertive expressions with conditional phrasing (“may indicate,” “could be associated with,” “might reflect”). The revised paragraph (Lines 311-314) now explicitly acknowledges the limited dataset and the potential influence of sampling bias and environmental variability on sex ratio observations.

Comment: Lines 285-292: This paragraph provides ecologically sound reasoning regarding the influence of temperature on carp metabolism and growth, but it lacks any supporting references. Since the statements concern fundamental physiological and ecological processes, at least one or two citations (e.g., general ichthyological or thermal biology studies) should be included to substantiate these claims. Otherwise, the text reads as purely speculative and weakens the scientific credibility of the discussion.

• Response: Supporting references have been added to substantiate the ecological mechanisms described. And to support statements on temperature-dependent metabolism and growth in freshwater fish species (Lines 330-331) of the revised manuscript.

Comment: Lines 331-341: This paragraph presents biologically plausible interpretations concerning variation in L₅₀, reproductive potential, and temperature effects on fishing mortality, but it lacks supporting references. Given that these are well-documented mechanisms in fish population ecology, at least a few key sources (e.g., studies on size-selective fishing, temperature-dependent mortality, or maturity shifts under exploitation) should be cited to substantiate these claims. Otherwise, the discussion remains speculative and weakens the scientific grounding of the conclusions.

• Response: Supporting references were added to strengthen this section, including studies addressing temperature-dependent mortality and size-selective fishing effects

Comment: Lines 342-363: The discussion of the M/K ratio is overly conclusive and only partially supported by references. Interpretations regarding population vulnerability and stock destabilization should be moderated, as they go beyond the scope of the presented data. The correlation between M/K and K appears to be described as empirical, although it is largely a mathematical relationship. Moreover, references [49,50] are too general to substantiate these claims; additional literature directly addressing M/K relationships and their ecological implications should be included.

• Response: The discussion of M/K ratios has been rewritten to emphasize interpretative uncertainty and methodological dependence. References were expanded to include Hordyk et al. (2015), Allen & Hightower (2011), and Jensen (1997), which directly discuss life-history relationships between growth, mortality, and demographic resilience. The empirical phrasing was corrected to reflect the mathematical basis of the M/K–K relationship (Lines: 415-419; 428-430).

Comment: Lines 366-374: The discussion linking exploitation rate (E) and length at 50% maturity (L₅₀) is presented too conclusively. The conclusions about recruitment overfishing and the need for regulatory measures are reasonable in principle but not empirically supported by the data shown. Without direct evidence of catch composition, fishing effort, or verified benchmarks for sustainable E values in this fishery, such statements should be framed as potential concerns rather than demonstrated outcomes. Additional references supporting the relationship between E, L₅₀, and recruitment dynamics would strengthen this section.

• Response: The section was revised to present these conclusions as potential concerns rather than confirmed outcomes. Conditional phrasing such as “may imply” and “could indicate” replaced definitive expressions. Furthermore, a reference to Jensen (1997) was added to highlight life-history trade-offs under elevated mortality and exploitation, providing theoretical context for the observed patterns..

Comment: Lines 295-307: The discussion of growth and mortality parameters is overly definitive relative to the data available. The observed variation in L∞ and M/K values indeed suggests potential trends, but these should be presented as possible rather than demonstrated outcomes. Statements such as “likely as a compensatory response to elevated exploitation and environmental stress” are speculative and require supporting references. The authors should moderate the tone (e.g., “may indicate” instead of “suggests”) and, where possible, cite literature linking changes in L∞, M/K, and fishing pressure. Otherwise, these interpretations risk overstating conclusions beyond what the dataset can substantiate.

• Response: Paragraphs on L∞, K, M/K, and L₅₀ variation were rewritten to reflect possible rather than demonstrated outcomes and now include comparative studies from similar ecosystems. Discussion of sex ratios, temperature influence, and recruitment was rephrased cautiously, emphasizing potential drivers and sampling limitations. The section on M/K was expanded with supporting literature to substantiate its ecological interpretation and relationship with life-history invariants.

Comment: The interpretation of growth parameter variation is speculative and lacks sufficient empirical support. The statements linking changes in K and Φ′ to environmental or density-dependent factors are presented too conclusively and require appropriate references. Moreover, the comparative references [46, 47] are only marginally relevant, as they refer to geographically and ecologically distinct systems (Ethiopia, Algeria) and therefore provide limited support for the observed trends in the Danube population. These comparisons should be either reconsidered or clearly qualified as illustrative rather than explanatory.

• Response: References to the Caspian Sea and other regions were qualified as comparative rather than directly explanatory.

Comment: Lines 377-389: The concluding part of the Discussion again presents the results too definitively. Phrases such as “reveals a dynamic population trajectory” and “indicate consistently higher values” should be moderated, as they imply population-level certainty not supported by the available sample size and temporal scope. Additionally, the statement about “alternating intervals of sustainable harvest and potential overexploitation” lacks quantitative benchmarks or literature references to define what constitutes sustainability in this context. The comparison with the Caspian Sea population [48] should also be reconsidered or clearly qualified, given the ecological and geographical differences between systems.

• Response: We rephrased the paragraph to moderate the tone and the comparison with different ecosystems (Line: 468-477)

Comment: Line 391: The term is incorrectly written as “Total Admissible Capture.” The correct term is Total Allowable Catch (TAC). Please correct this and ensure consistency throughout the manuscript, including figures, tables, and captions.

• Response: Corrected

Comment: Lines 392-440: This subsection is informative but overly conclusive and lacks sufficient reference to support statements about regulatory effectiveness, IUU fishing, and the spatial distribution of fishing pressure. Most assertions are general and not directly substantiated by quantitative data or citations.

• Response: The discussion of Total Allowable Catch (TAC) trends was rewritten to avoid definitive conclusions, emphasizing possible interpretations and data limitations. References to IUU fishing and governance issues were supported with recent FAO, EEA, and European Commission documents.

Comment: In addition, it would be highly relevant to include a brief discussion of aquaculture, especially common carp production volumes in Romania. Since the section focuses on sustainability and the need to reduce fishing pressure on natural populations, referring to aquaculture as an alternative supply source would provide a more comprehensive and realistic management context. Presenting recent data or references on carp aquaculture production would help balance the discussion and highlight the potential role of fish farming in ensuring sustainable fish availability.

• Response: A new paragraph discussing the contribution of aquaculture to carp production in Romania was added, highlighting its role in reducing fishing pressure on wild stocks. (Lines: 527-532)

Comment: Lines 443-476: The final part of the Discussion provides a broad and conceptually sound overview but remains unreferenced and overly general. Statements about the paucity of research, data limitations, and governance challenges (including IUU fishing) require literature support — for example, regional management reports, FAO publications, or peer-reviewed studies addressing fisheries in the Danube basin.

• Response: We added a few references to support statements

We hope these comprehensive revisions have addressed all your concerns and significantly improved the manuscript’s methodological transparency, scientific balance, and contextual relevance.

 

Reviewer 2 Report

Comments and Suggestions for Authors

An interesting contribution, updating important information for a valuable fishery resource. A classic methodology is applied, supported by appropriate statistical techniques and the use of current software.

The document is clearly written with a well-connected sequence of ideas and sufficient bibliographic support.

The discussion integrates and compares the results with published information and provides a general overview of the resource's health in relation to the ecosystem's environmental stability.

As a suggestion, I would mention that when using size frequency as the strategy for calculating growth values, the sample size is small, and it is necessary to specify how frequently samples were taken in a single year.

I consider the interannual comparison appropriate because it offers a longer-term perspective; however, some questions could be clarified, for example: Were the samples collected during the same climatic period each year? Was the number of sampling hauls the same each year? Were the two fishing gears described used simultaneously and with the same number of hauls in each year of the study?

Author Response

We sincerely thank the reviewer for the positive evaluation and constructive suggestions, which helped us clarify methodological details and strengthen the manuscript.

Comment: As a suggestion, I would mention that when using size frequency as the strategy for calculating growth values, the sample size is small, and it is necessary to specify how frequently samples were taken in a single year.

• Response: Regarding the sampling strategy, we have now specified in the Materials and Methods section (Lines XX–XX) that sampling was conducted annually between April and October, covering spring, summer, and autumn seasons to capture variability in fish activity and hydrological conditions. The study included five fixed sampling sites along the Romanian sector of the Danube River (Moldova Nouă, Calafat, Giurgiu/Oltenița, Brăila–Chiscani, Galați). To address the reviewer’s question, both fixed and drift gillnets were used simultaneously at all sites, with three to five hauls per sampling event, depending on local hydrological conditions. Sampling effort was therefore standardized across years to ensure comparability. These details have been integrated into the revised text for greater transparency and reproducibility (Section 2.1, Lines 85-102).

We appreciate the reviewer’s recognition of the study’s clarity, analytical approach, and contribution to understanding the status of this valuable fishery resource.

Reviewer 3 Report

Comments and Suggestions for Authors

Overall comments

The study investigates the population dynamics and sustainable exploitation of the Danube common carp, Cyprinus carpio, in the Romanian stretch of the Danube River over a six-year period (2019–2024). The authors provide a robust, quantitative assessment of the species while evaluating compliance with Total Allowable Catch regulations. They used a standardised annual sampling with fixed and drift gillnets to collect morphometric and sex data for over 2600 individuals, applying length-frequency analyses, von Bertalanffy growth modelling, mortality estimation, and statistical tests appropriate for the observed data structure.

The results align coherently with the methods and the study’s objectives. The manuscript demonstrates professional scientific reasoning, linking observed patterns to plausible ecological and anthropogenic drivers while acknowledging uncertainties and limitations, including potential biases in reported catches and the effects of IUU fishing.

In sum, the manuscript presents a thorough, evidence-based evaluation of common carp population dynamics in the Lower Danube and offers a strong foundation for adaptive, ecosystem-based fisheries management. It is coherent, professional, and largely ready for publication with minor revisions to enhance clarity and readability.

Below I list my section-specific comments for improvements.

Title

The title is a bit vague: it communicates the topic but not the scope, methods, or key insights. I suggest to rephrase it by addressing 1) what was studied, 2) where, and 3) the main outcome

Specifically:

• Evaluation: doesn’t indicate the method (modelling, field survey, statistical analysis).
• 2 Common Carp: slightly unclear… do you mean two species, two populations, or something else?
• No hint of findings: readers don’t know whether sustainable exploitation is feasible, declining, or increasing.
• Minor style: numerals should usually be written out in titles.

Abstract

Reporting growth and mortality parameters with ranges is appropriate, though it would be stronger to indicate whether ranges are confidence intervals, observed extremes, or model-derived estimates.

You might briefly mention sampling method to reinforce experimental rigor (net type, site selection, randomisation).

Introduction

You might briefly mention that quantitative indicators will be estimated using validated population models or standard fisheries assessment methods, to reinforce statistical rigor.

Methods

The statistical approach is well-detailed, but consider noting whether multiple testing

About the experimental design I would suggest you to briefly mention sampling effort per site or season to reinforce reproducibility (number of nets, sampling duration).

Results

I love that correlation results are presented descriptively without implying causation, which is scientifically appropriate.

Tests are appropriate and reported with relevant statistics, but if you want to improve it, I would suggest you to note whether correlation significance levels were tested and whether multiple testing adjustments were applied.

Discussions and conclusions

I love that you explicitly recognize the limits of inference

Comparisons to literature are appropriate, but sometimes statistical uncertainty is not emphasized.

Grammar

I’m sorry but I find that phrasing is in many instances long and dense. I strongly suggest to keep sentences short and simple to improve readability.

I noted some minor typos, please check grammar, spelling and formatting before final submission

 

 

 

 

 

Comments on the Quality of English Language

I’m sorry but I find that phrasing is in many instances long and dense. I strongly suggest to keep sentences short and simple to improve readability.

I noted some minor typos, please check grammar, spelling and formatting before final submission

Author Response

Response to Reviewer

We would like to sincerely thank the reviewer for the thoughtful and constructive evaluation of our manuscript. We appreciate the positive assessment of our methodological approach and the recognition of its scientific robustness. Below, we provide detailed responses to each specific suggestion.

Title

Comment: The title is a bit vague; please clarify scope, methods, and outcome.

• Response: The title has been revised to better reflect the scope and methods of the study:
  Revised title: “Growth, Mortality, and Stock Resilience of Common Carp (Cyprinus carpio) in the Romanian Danube: Insights into Sustainable Exploitation (2019–2024)”.
  This updated version clearly specifies the main parameters analysed, the study area, and the temporal coverage.

Abstract
Comment: Clarify whether reported parameter ranges represent confidence intervals, observed extremes, or model-derived estimates; briefly mention sampling method.

• Response: The abstract was modified to specify that all reported ranges represent model-derived estimates based on length-frequency analyses. We also added a short methodological statement indicating that sampling was performed using fixed and drift gillnets along multiple Danube sites to strengthen methodological transparency.

Introduction
Comment: Mention that quantitative indicators were estimated using validated models or standard fisheries assessment methods.

• Response: The introduction now includes a sentence explicitly stating that quantitative indicators of growth, mortality, and exploitation were estimated using validated population models and standard length-based fisheries assessment methods, including the von Bertalanffy growth model (Line: 71-74).

Methods

Comment: Briefly describe sampling effort per site or season to reinforce reproducibility.

• Response: We revised the Methods section to include full details on sampling locations, effort, and temporal coverage—five fixed sites along the Danube, three to five hauls per site, and simultaneous use of fixed and drift gillnets during April–October each year. These additions improve reproducibility and clarify spatial representativeness (Line: 85-102).

Comment: Note whether correlation significance levels were tested and whether multiple testing adjustments were applied.

• Response: We added a clarifying statement in the Statistical Analysis section specifying that correlation significance levels (p < 0.05) were tested, and due to the small dataset (n = 6), no correction for multiple testing was applied.

Discussion and Conclusions

Comment: Statistical uncertainty should be more explicitly acknowledged.

• Response: We incorporated several statements highlighting uncertainty due to sample size, temporal scope, and model assumptions, and rephrased overly definitive interpretations (e.g., replacing “suggests” with “may indicate”). This improves the balance between interpretation and caution.

Grammar and readability

Comment: Sentences are long and dense; simplify.

• Response: The manuscript was thoroughly revised to improve sentence structure, ensuring shorter, clearer phrasing and consistent scientific terminology.

We are grateful for the Reviewer’s supportive comments and constructive suggestions, which have significantly improved the clarity, structure, and interpretative balance of our manuscript.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for submitting the revised version of the manuscript and for providing detailed responses to all reviewer comments.

After carefully reviewing the revised manuscript as well as the authors’ point-by-point responses, I confirm that all my suggestions have been fully and appropriately addressed. The manuscript has been substantially improved in terms of clarity, methodological transparency, and scientific rigor.

I am pleased to recommend acceptance of the manuscript in its current form.

Kind regards,

Author Response

Dear Reviewer,

Thank you very much for your careful re-evaluation of our revised manuscript and for your positive and encouraging feedback. We greatly appreciate the time and expertise you dedicated to this review process. Your insightful comments in the previous round significantly helped us improve the clarity, methodological transparency, and scientific rigor of the paper.

We are grateful for your recommendation for acceptance and for the constructive guidance that contributed to strengthening our work.

Sincerely,
Angelica Dobre

(on behalf of all co-authors: Maria Desimira Stroe, Floricel Maricel Dima, Livia Vidu, Monica Marin, Carmen Georgeta Nicolae)

E-mail: angelica.dobre85@yahoo.com