The Population Development of the Invasive Round Goby Neogobius melanostomus in Latvian Waters of the Baltic Sea

Round 1

Reviewer 1 Report

Dear Authors, 

I found this research well structured, clear and significative to the field. The study of the spatio-temporal variation of the invasive round goby was performed using data from three different independent surveys, and I think it is good to assess an evaluate the status of populations of this invasive species. Also, methods are good and results adequate. However, you mentioned that this invasive species has several predators, such as cod, pikeperch, European perch and other ones. On the basis of this, I think that this part must be better considered and developed in the discussion section, because also the role of predators could be of some relevance. Consider and quote at this regard the manuscript of Tiralongo et al., 2021 (Invasive Species Control: Predation on the Alien Crab Percnon gibbesi (H. Milne Edwards, 1853) (Malacostraca: Percnidae) by the Rock Goby, Gobius paganellus Linnaeus, 1758 (Actinopterygii: Gobiidae) published in Journal of Marine Science and Engineering (https://doi.org/10.3390/jmse9040393).

Dear Editors, 

the English style requires a moderate revision.

Author Response

Response to Reviewer 1 Comments

Point 1: Round Goby has several predators, such as cod, pikeperch, European perch, and other native species. On the basis of this, the reviewer thinks that this part must be better considered and developed in the discussion section because also the role of predators could be of some relevance. The reviewer suggests quoting the manuscript of Tiralongo et al., 2021

Response 1: Yes, Thank You for Your suggestion, this is a good point to highlight in the discussion part also the reference to the Rock Goby predation on the alien crab study gives relevance to the absorption of a new species in the local food web. We incorporated the suggested changes in the revised draft.

Point 2: The English style requires a moderate revision

Response 2: As not one of the referees has suggested that the manuscript should undergo

extensive English revisions according to the MDPI’s Special Issue Editor, we decided not to use external English language editors’ services and did our best to improve the language of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

This is a very good article, recommended for acceptance and publication

1. Line 229-230, please follow the journal formula format.

2. Line 326-327, regarding “whereas a similar comparison for 2017 was about 200 times higher”, it is recommended to check whether the result is correct. This rapid growth should have suitable growth conditions. After 2018, it seems that there are also unfavorable conditions that lead to rapid decline.

3. Line 422, regarding high fishing pressure [2], can it be checked by the amount of fishing effort invested? At this time, does the price increase of this fish species lead to an increase in demand?

4. Line 432, on a strong decline, whether more climatic factors or abnormal bottom hydrology can be explored.

Author Response

Response to Reviewer 2 Comments

Point 1: Line 229-230, please follow the journal formula format

Response 1: Yes, Thank You for Your correction, further on we follow the MDPI’s Microsoft Word template and punctuate equations as regular text in “MDPI_3.9_equasion” style

Point 2: Line 326-327, regarding a similar comparison for 2017 was about 200 times higher, it is recommended to check whether the result is correct. This rapid growth should have suitable growth conditions. After 2018, it seems that there are also unfavorable conditions that lead to a rapid decline.

Response 2: Thank You for the comment, we agree that the rapid growth of the population values in this time period is eye-catching but we double-checked the results and they are correct. We tried to incorporate other potential factors affecting population dynamics in the discussion.

Point 3: Line 422, regarding high fishing pressure, can it be checked by the amount of fishing effort invested? At this time, does the price increase of this fish species lead to an increase in demand?

Response 3: Yes, this is an important issue to assess, and as we mentioned in the methods “2.5. Commercial fisheries data”- the current state of the official commercial records data does not allow for the development of mathematically reliable calculations. However, we think that this is a very complex analysis to be made and is the basis for a different study towards developing a Stock assessment for the Round Goby in The Baltic Sea (The authors are working on it right now and a proper paper is in progress).

The Round Goby opened a completely new sector in the Latvian coastal fishery so there are no doubts about the increase in demand as the industry developed and the price rose from “0” to 100%. As we mentioned in the conclusions the effective acquisition of the new resource could be an important factor in the population drop after 2017.

Point 4: Line 432, on a strong decline, whether more climatic factors or abnormal bottom hydrology can be explored

Response 4: Yes, Thank You for mentioning this issue, we added a small paragraph on this topic in the discussion.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this manuscript, Authors attempted to characterize the spatial-temporal population development of the round goby in Latvian coastal waters using data from three different surveys, as well as data from commercial fisheries landings. Obtained results suggest a rapid exponential increase in population numbers of the round goby peaking in 2018, followed by a sharp decline, which as suggested by the authors, was largely caused by intensive commercial fishing. Authors indicate that this study could contribute to the general knowledge base on the mentioned species and how fisheries can aid in limiting the development of invasive fish populations. Authors also provide recommendations for better future monitoring program designs on the round goby.

Unfortunately, most of the manuscript is focused on methodology for calibrating different fishing gear, and creating a population change model. The whole issue of round goby invasion seems to be of secondary importance.

Because of that I strongly suggest authors to rethink what is the main topic of this work. Method for intercalibration of different fishing gear they describe is quite interesting, so I would advise authors to modify the manuscript title and abstract to be more focused on that part of the research.

Specific remarks

Title

Title should be changed to be more in line with what’s the manuscript about

Abstract

A above, especially that key contribution fits quite well with actual scope of the manuscript

Introduction

This paragraph needs a rework. For manuscript concentrating on RG I would expect more information and literature on this specie invasion history in Baltic sea. On the other hand, in manuscript focused on methods some additional background information is also needed.

Material and Methods

Figure 1 maps a) and b) seem unnecessary, either use only one of those or combine them in some way.

Line 143 “Given the common courtesy…” not sure what authors are trying to convey here.

Line 178 missing space.

Line 188 missing bracket.

Line 229 and further in the manuscript, formulas should be written with equation tool and numbered.

Line 246 equation

Line 260 equation

Line 265 missing space

Line 273 please consider “does not allow for the development of…”

Results

Line 306 missing space before unit.

Discussion

Line 380 I would like to see some more citations on this method, as well as how is it applicable to fishes.

Kind regards

Reviewer

Author Response

Response to Reviewer 3 Comments

Point 1: Unfortunately, most of the manuscript is focused on methodology for calibrating different fishing gear, and creating a population change model. The whole issue of round goby invasion seems to be of secondary importance

Response 1: Thank You for Your comment on this, the main idea of the paper remains to describe the Round Goby population so we tried our best to improve the manuscript and emphasize the ichthyological aspects of the study by supplementing references on the invasion history and factors that could affect the population development. See the introduction and discussion parts.

Point 2: The reviewer suggests that authors should rethink what is the main topic of this work. The method for inter-calibration of different fishing gear is described as quite interesting, so it is advised to modify the manuscript title and abstract to be more focused on that part of the research.

Response 2: The methods used in this study are well-established and described as well as used in different studies, but the novelty of the research should reflect the Round Goby population development. We decided not to shape the manuscript in a methodological way and to focus on improving the manuscript to the overdue description of the Latvian case of the Round Goby invasion.

Point 3: The title should be changed to be more in line with what’s the manuscript about

Response 3: Given the arguments presented above, we decided not to change the Title in a methodological way

Point 4: The abstract, especially that key contribution fits quite well with the actual scope of the manuscript

Response 4: Thank You, We have done our best

Point 5: The introduction needs a rework. For a manuscript concentrating on RG, I would expect more information and literature on this species’ invasion history in the Baltic Sea. On the other hand, in the manuscript focused on methods some additional background information is also needed.

Response 5: Thank You for the suggestion, as you can see in the previous responses we chose to concentrate on the Round Goby and added information and literature on the species invasion history

Point 6: Figure 1

maps a) and b) seem unnecessary, either use only one of those or combine them in some way.

Response 6: Thank You for this advice, see line 147

Point 7: Line 143 “Given the common courtesy…” not sure what the authors are trying to convey here.

Response 7: Thanks, we agree with You and changed the sentence, see line 215

Point 8: Line 178 is missing space, Line 188 missing a bracket, and Line 229 and further in the manuscript, formulas should be written with an equation tool and numbered for lines 246 and 260

Response 8: Thank You, Our apologies for somehow skipping those inaccuracies in the writing process, now we follow the MDPI’s Microsoft Word template and punctuate equations as regular text in “MDPI_3.9_equasion” style

Point 9: Line 273 please consider “does not allow for the development of…”

Response 9: Thanks, We Appreciate Your suggestion, see line 380

Point 10: Line 380 I would like to see some more citations on this method, as well as how is it applicable to fishes

Response 10: We did our best to improve the description of the method (see line…) but unfortunately we are not aware of any peer-reviewed publications or other study reports on the method being used in fisheries, nevertheless we consider this well-established statistical modeling method suiting properly on any kind of reliable wildlife monitoring data

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report