This manuscript describes the movement of a male humpback whale from a breeding ground in the eastern Pacific Ocean to another in the western Indian Ocean and add to the growing body of evidence that as populations of humpback whales grow, more interchange occurs. This record is unique because it suggests movement across an ocean basin (that is through the Atlantic Ocean), which has not been documented before. The manuscript is well written, and this unique record deserves publication, pending minor revision. I have added a few suggestions to improve the text (see below) for consideration by the authors and I wish them good luck in getting the paper published.

L. 37 and 38. Perhaps rephrase here a bit as humpback whale migration may no longer be the longest migration of all mammals (see for example Mate et al., 2015). Perhaps indicating they represent one of the longest migrations among mammals would be more accurate (as stated in the abstract on L. 16).

L. 46: I am not sure the migration routes and movement patterns of humpback whales are “fairly unknown”. In the last 15 years, a number of studies on photo-identification, genotyping and telemetry have elucidated the migratory routes and movements in both feeding and breeding for most stocks, especially in the Southern Hemisphere. Compared to other baleen whales, the movement patterns of humpback whales are fairly well known.

L. 72: Should the reference here (Ransome et al., 2023) be indicated by a number like all other references?

L. 75: Same for Acevedo et al. here?

L. 90: It is unclear in Fig. 1 which semi-circle represents sub-stock C1 and which one represents sub-stock C2. Given they are mentioned in the text extensively, this should be clarified in the figure.

L. 104: Replace “Columbia” by “Colombia”.

L. 117: I think the evidence is that the South Shetland area is a primary (not an occasional) feeding ground for stock G (see for example Stevick et al., 2004; Modest et al., 2021).

L. 120: It is difficult to estimate age of humpback whales based on field observations. There is no rationale provided to estimate this whale was at least 5 years old (other than the statement that the estimation is based on “behavior and size”, which are not necessarily good predictors of age). Perhaps indicate that this was a large individual or an adult whale would suffice? Or expand on what features related to behavior and size that led to the conclusion this whale was at least 5 years old.

L. 138: Replace “Columbia” with “Colombia”.

L. 139-150: Without additional evidence, the assumption of “multiple latitudinal movements” of this whale between Colombia and the Western Antarctic Peninsula, along with the various estimated distances and potential routes (as shown in Fig. 3) are highly speculative. For example, given information presented in the manuscript and references there-in, it is plausible that this whale moved from stock G to stock A, then to stock B (and used feeding grounds associated with these stocks) before moving to Zanzibar. The text should be modified to make it clear that the areas visited between the last sighting of Colombia in 2017 and the first sighting off Zanzibar in 2022 are unknown and that multiple possibilities exist. The bottom line is that a whale from Stock G moved to Stock C within a period of 5 years across feeding grounds associated with multiple breeding stocks.

L. 171-173: Estimated age is provided here. See comment above related to L. 120. Please provide a more detailed rationale to consider this whale at least 5 years old. This does not seem to matter as long as it is known this was an adult whale.

L. 177: I like the discussion on the possible drivers of the unusual movement of the individual described in the manuscript. Climate change/environmental shifts have been proposed as a reason for western South Atlantic humpback whales to shift their foraging habitats, which may expose this whale to animals from other breeding grounds and possibly promote greater interchange (see for example Bedriñana-Romano et al., 2022) and that should be acknowledged. Population increases (which have been documented for almost all Southern Hemisphere breeding grounds and relevant references should be added here) are also an appropriate explanation. However, I am puzzled by the hypothesis related to whaling and the “lack of resources and mates”. The two populations considered here (stocks G and C) and those in between Colombia and Zanzibar (stocks A and B) have shown strong signs of increase in the last couple of decades so there is no evidence that there is a “lack of mates”. This section of the text needs to be deleted (or better explained in case I missed the point).

L: 205-213: The last two paragraphs of the discussion can be integrated as they share some common elements (e.g., need for long-term collaborative projects to understand humpback whale dispersal and their effects on population genetics and recovery.

Fig.1: I don’t think Fig. 1 is needed. The authors can point out to the relevant sources (e.g., the IWC reports or references already in the text). The important information for the manuscript is already in Fig. 3.

Fig. 2: That is a good figure, which clearly shows the ids of the different sightings.

Fig. 3: I suggest removing the dotted lines as they imply some sort of movement direction and route, which is of course unknown. If Fig. 1 is deleted, information related to stocks A and B could be added to Fig. 3 to make this a more complete figure for reference in the text.

References:
Bedriñana-Romano, L., Zerbini, A.N., Andriolo, A., Danilewicz, D. and Sucunza, F. 2022. Individual and joint estimation of humpback whale migratory patterns and their environmental drivers in the Southwest Atlantic Ocean. Scientific Reports 12(1): 7487.

Mate, B.R., Ilyashenko, V.Y., Bradford, A.L., Vertyankin, V.V., Tsidulko, G.A., Rozhnov, V.V. and Irvine, L.M. 2015. Critically endangered western gray whales migrate to the eastern North Pacific. Biology Letters 11: 20150071. 4pp.

Modest, M., Irvine, L., Andrews-Goff, V., Gough, W., Johnston, D., Nowacek, D., Pallin, L., Read, A., Moore, R.T. and Friedlaender, A. 2021. First description of migratory behavior of humpback whales from an Antarctic feeding ground to a tropical calving ground. Animal Biotelemetry 9(1): 42.

Stevick, P.T., Aguayo, A., Allen, J., Avila, I.C., Capella, J., Castro, C., Chater, K., Dalla Rosa, L., Engel, M.H., Félix, F., Flórez-González, L., Freitas, A., Haase, B., Llano, M., Lodi, L., Munoz, E., Olavarría, C., Secchi, E., Scheidat, M. and Siciliano, S. 2004. Migrations of individually identified humpback whales between the Antarctic peninsula and South America. Journal of Cetacean Research and Management 6(2): 109-13.