Development of a Quantitative Survey Method for Pelagic Fish Aggregations Around an Offshore Wind Farm Using Multibeam Sonar

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper clearly articulates the potential of offshore wind farms (OWFs) as artificial reefs and highlights the innovative application of multibeam sonar in quantifying fish aggregation. The study provides a scientific basis for the synergistic development of the fisheries and renewable energy industries, aligning with sustainable development goals.  

 

The introduction presents a weak argument for the urgency of the research. For example, it does not sufficiently explain why traditional acoustic methods (such as single-beam echosounders) are inadequate or how existing data fail to support ecological impact assessments of wind farms. It is recommended to include specific case studies or policy contexts (e.g., the current opposition to wind farm construction by fisheries in Japan).  

 

Water Column Image (WCI) Processing:  

The automated filtering process (threshold filtering, sidelobe removal, spatial filtering, etc.) effectively reduces noise, but the threshold factor (k=2) is directly cited from the literature (Nau et al., 2022) without validation of its applicability. It is recommended to conduct a sensitivity analysis (e.g., k=1.5–2.5) to verify the robustness of the threshold.  

The threshold setting for spatial filtering (1.1 times the horizontal distance) lacks biological justification and may inadvertently remove genuine fish signals. Parameters should be optimized by incorporating fish behavioral data (e.g., schooling distances).  

 

Biomass Estimation:  

 The study relies on the target strength (TS) of Japanese jack mackerel (Trachurus japonicus) for biomass estimation but does not address the impact of species mixing on TS. If other high-TS species (e.g., mackerel) are present, biomass may be overestimated. It is recommended to supplement with underwater camera data to validate the proportion of dominant species.  

 

3. Results and Discussion

- The biomass in the OWF area is 66.7 times that of the control area, but the sampling time (December) is not discussed in terms of whether it represents annual trends. It is recommended to include seasonal data to avoid conclusions based on chance.  

- Extrapolating conclusions based on single-day data requires caution. Long-term acoustic monitoring and fishery catch data should be compared to validate the accuracy of biomass estimates.  

- Additional current measurement data should be included to strengthen the explanation of aggregation mechanisms in wake zones.  

- The impact of sonar blind zones (e.g., complex seabed topography near the bottom) on the results is not discussed.  

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a multibeam sonar to quantitatively assess fish aggregations around offshore wind farm structures. The paper has following lacks

1. The survey was conducted on a single day (10 December 2014). Seasonal or multi-year data would provide more robust evidence of aggregation patterns and their consistency. The authors should acknowledge this limitation and suggest future work to address it.
2. The choice of the kk factor in the threshold filter (Equation 4) is noted as a potential source of error. The authors should discuss how variations in kk might affect results and propose methods to optimize its selection in future studies.
3. Clarify how the constant correction value was derived from the calibration process. The manuscript mentions an offset gain of +6.5 dB but does not fully explain how this translates to Const. = -55.3 dB.
4. Suggest adding a reference to the specific calibration procedure e.g., sphere calibration, standard target method for reproducibility.
5. Side-Lobe Artifact Removal (Equation 5) The equation for ASLRASLR​ assumes a linear relationship between beam angle and artifact range. However, side-lobe patterns can be nonlinear, especially at steep angles. Consider discussing potential errors introduced by this simplification.
6. Suggest validating the side-lobe removal with synthetic data or controlled experiments.
7. Figure 1: Include a scale bar for clarity.
8. Figure 6: The color scale for SvSv values could be better differentiated to improve readability.
9. Table 2: Consider merging with Table 1 or supplementing with a summary statistic for brevity.
10. Clarify why echoes <3 m deep were excluded (e.g., potential interference from surface noise).
11. Provide more detail on the spatial filter’s 1.1× threshold justification
12. Expand on the ecological significance of fish aggregations near OWF structures.
13. Address potential biases in biomass estimation due to TS assumptions for non-dominant species.
14. The key references are missing. I would suggest to cite key papers.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The majority of your comments have been fully and satisfactorily addressed, particularly the major methodological justifications. However minor revision needs to be done.

1. The authors' implementation is minimal. You should insist on a stronger, more direct acknowledgment of this limitation in the discussion and conclusion. The comparison to floating OWFs is weak.
2. The latest and key references are still mising. The new citations are outdated. Decide if you are satisfied with the new references. If not, now is the time to provide specific key citations of latest papers of 2025.
3.  Perform a quick check to confirm the English edits are sufficient.

After the minor revisions requested above, this manuscript will be significantly improved and should be considered for acceptance. The methodological contributions are clear and valuable.

Author Response

Please see the attachment.

Author Response File: Author Response.docx