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I think that you did a really good job in improving the manuscript and addressing the issues raised by the reviewers.
Now it's ready for publication.
Congratulations!
[# PeerJ Staff Note - this decision was reviewed and approved by James Reimer, a PeerJ Section Editor covering this Section #]
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Dear authors,
As you can see, two reviewers commented on your manuscript. Overall, they suggested that a moderate amount of revisions is still needed before the acceptance. Most of the comments are related to the clarification of some important points of the manuscript (including the data used and the statistical analyses) and to language editing aimed at improving the readability of the paper. Please carefully take into account both the general and the specific comments, including the ones on the annotated pdf.
[# PeerJ Staff Note: Please ensure that all review and editorial comments are addressed in a response letter and any edits or clarifications mentioned in the letter are also inserted into the revised manuscript where appropriate. #]
The MS has cited Ioth & Takahashi (2019) for several times. First, the document was an "update report" and did not provide complete information so is difficult for readers to get whole picture. One of the examples was that it is difficult to find information of the statement of line 426. Suggest the MS citing additional documents. Second, the document provided several series of indices. The MS needs to identify which series they were referring to. Third, after statement of line 425, the MS cite this document and finally says " As such, similar recent trends have been seen in both the Japanese and Korean longline fisheries." However, the document showed the opposite result in its Fig. 4 that the nominal CPUEs were higher than standardized indices for recent years. This confusion needs to be clarified.
Some editorial issues:
(1) line 367: hard to read month information from Fig. 12D. (2) Fig. 2: missing color scaler. Better to explain the meaning of dark black color. (3) the MS does not have area 7 (line 257); (4) no explanation of Figs. S6 and S7; (5) order of figures (sudden jump from Fig. 4 to Fig. 9 in line 256; from Fig. 6C to 12D in line 286); (6) Fig. 12 is one of the important results but is so vague to read and understand.
Some explanations are needed: (1) line 204: how to get the degrees of freedom of 5, 3, and 4? (2) line 362: how did the MS get the "catchability about 3%"? (3) Table 1: missing information to support the statement of line 320-321 " Dropping each variable from the lognormal constant models and the delta lognormal models (Table 1) indicated that all explanatory variables were statistically significant"; (4) no unit for CPUE.
1. The data exploration sub-section of the section Result provides observations in detail on HBF, nominal catch rates, catch species, etc., by statistical areas, based on available logbook data. It was informative but hard to catch the fleet dynamics of Korean longline fleet in the region which can help to understand the spatiotemporal change of target strategies of the fleet. The 65 vessels (Fig. S3) should be able to be grouped into several sub-fleets. For example, one of which might be composed of SBT professional vessels that fishing for SBT in area 9 in March to July/August (clusters 1 and 3, Fig. 9) and moving to area 8 for the period of July to December (clusters 2 and 3). Another possibility was that SBT might be just a seasonal target species to the fleet; so that there might be several sub-fleets and one of which was composed of professional ALB/YFT vessels and seasonally fishing for SBT in area 9 or area 8 in different season. It seems there was a new sub-fleet emerged from 2010 onwards fishing specifically in southwest part of area 9 (no vessels or interest fishing in area 8) (Fig. 9) when the SBT stock was recovering. With this information, the MS can then discuss gear features and fishing strategies of the sub-fleets. It is also suggested to provide a graph with time series of catch composition by area. I believe this can show a more interesting story about the seasonal fleet dynamics of Korean longline fishery in the region and provide good background to the current subsections of data exploration and clustering.
2. Vessel effect is one of the most important variables in the standardization result (line 322). Providing information on abovementioned vessel dynamics can help to understand the importance of this variable and can demonstrate if there were non-SBT vessels left the fishing ground (line 421). Also, from Fig. S3, there were many vessels fishing in the region for only one year in early period. It would be interesting to see the features of these vessels and the effect of excluding these vessels in the analysis. The clustering effect in this study seemed not so important; so, if the vessels of fishing non-SBT could be identified easily, estimating abundance indices from the professional vessels (or core vessels) might be more convenient and straightforward.
3. Areas 8 and 9 roughly overlapped with albacore fishing ground, and albacore composition would be high even the target was SBT. So, it needs more supporting information to say shifting of (or "a trend towards") targeting albacore, especially in the period when SBT abundance was very low. The MS needs to consider the effect of declining abundance of SBT to the 'catch composition' before making statement of shifting target.
4. Decline of fish stock may cause range contraction, and the fishing effort may concentrate to the relatively abundant area (or the "hot-spots"). It is interesting to see the another observation here that "As the stock has increased, fishing effort has tended to concentrate". The MS is suggested to provide more explanation on this argument. In addition, it might be arguable that when effort concentrates to an area, the nominal CPUE might be higher than usual, or might be higher than standardized index, which was not seen in Fig. 11. Line 425 explains "the standardized area 9 indices were higher than the nominal indices in recent years due to increased effort in areas with (on average) lower catch rates". The MS is better to provide rationale for the argument that, when SBT stock was recovering, the fishers "tended to increase effort in areas with lower catch rates". It is also suggested to check if the "concentration" was a result of a decline of fishing vessels.
The manuscripts reads as if there are two different authors, one for the introduction, methods and results and the other for the discussion. This manuscript would benefit from a closer edit of the language and sentence structure particularly in the introduction, I think this would improve the manuscript cohesiveness and coherence of the reader.
The introduction also needs further citations, certain areas lack citations and I noted this on the PDF.
The research question is an interesting and an important one. However, it is not clear from the methods how the data selection was used to determine shifts in targeting. Hooks between floats as well as lights sticks, time of day are generally used to determine gear shifts related to target shifts but this was not explicitly addressed. Additionally, only 9-12 HBF were retained so I am unsure how a target shift can be determined from such a constrained number of HBF. What is the range that is generally observed? The target change related data selection section needs more information overall.
These findings are interesting and the provided materials support them. The discussion and conclusion are well stated but would benefit from some elaboration. In particular, are the indices split in the two areas (8 and 9) when entered into the stock assessment? Is this what is done with the Japanese LL? If they are not split into statistical areas, why was statistical area not included as a covariate and one index produced?
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