The Role of Cold-Water Thermal Refuges for Stream Salmonids in a Changing Climate—Experiences from Atlantic Canada

Round 1

Reviewer 1 Report

This manuscript is a review about the role of thermal refuges for salmonids in the context of a progressively warmer climate. The authors go into detail about what is a thermal refuge (with examples), how they are mapped scientifically, and the fish behavior that leads them to utilize the refuges. The text finishes off by discussing management and conservation issues. For these topics, salmonid research in the Atlantic region of Canada is used for examples.

Overall, the review is a good contribution to the field, in particular the sections about behavior and management and highly important for the fisheries/nature managers cold-region countries of the world. I don’t have much serious critique, but I want to raise some discussions that may or may not lead to additions to, or modifications of, the text – I expect that the authors will consider the points that make sense and improves the manuscript, and just briefly refute the ones that don’t.

One topic that is not really covered is the temperature dependent disease severity. E.g. studies by Anti Vasemägi and colleagues suggest that brown trout can suffer substantially more from the proliferative kidney disease (PKD) at temperatures above 15 degrees. Hence, presence of disease in a population (PKD, for instance, appears to be a very common disease in brown trout!) may lead to thermoregulation behaviors. Saprolegnia may be another agent that will affect temperature choice, and there are probably many more. Maybe a mention of the issue somewhere could be motivated?

L55: Scientific names (genus and species, not “spp.”) should be in italics (here and everywhere else; MDPI will probably not fix it in copy editing, in my experience…).

L56: “Salmo spp.”, not “Salmo sp.” – sp. means one undetermined species of the genus, spp. means more than one – the Jonsson review is about two species (so technically, it is not really about “Salmo spp.” but about “Salmo salar and Salmo trutta” – other species of the genus are not covered.

L60-62: Not so sure about this statement about temperatures being expected to remain around 0 degrees (?) – in southern Scandinavia (i.e. south of the boreal zone, but still strongly seasonal) for instance, winters are getting more and more >0 degrees (I don’t have any other data than having experienced it), but there is likely analyses on it from the Scandinavian countries’ meteorological institutes. More rain and lack of ice may play a large role for water temperatures in smaller streams? With temperatures higher than historically normal in winter, I’d expect salmonids to need to start to become more active since their metabolism goes up a bit, which requires energy intake. Also, the egg development may be increased, possibly leading to mismatched hatching times. Maybe it is different in Canada, but then it should also be clear that the text in the introduction is only about this area. Of course, I may be a bit mistaken in my assessment of what is going on (I’m definitely not even close to a meteorologist) – however, I think more details about the expected changes in climate patterns for the focal geographic area of the text could be good (I.e. I would like some more specificity).

L72: Maybe better to phrase it “anadromous, potadromous and stream resident” (i.e. don’t forget the lake migrating morphs).

L77-79: I guess these cold water anomalies would be warm water anomalies in winter. Maybe there needs to be a reference to summer time here?

L109: The definition suggests that the patches are heterogenous in themselves – is this the aimed-at meaning of the definition? I’d assume that they are rather stable (i.e. the patches are thermally homogenous), but contributes to thermal heterogeneity at a larger scale within the river since they are colder (in summer) than the surrounding areas…

L127: Is metapopulation the correct term here? If so, d that mean that many different subpopulations all survive together in a single large cold water plume (then they would likely become just a “population” after a few generations of interbreeding), or that they survive in separate cold water plumes (one for each subpopulation). I haven’t read about metapopulation dynamics for many years, so I am not at all confident in knowing the exact definition – but perhaps it would be good to add a few sentences here about what exactly this statement means for river population ecology of salmonids?

Figure 1: I’d love to know where these images where from (i.e. which river, and if possible with coordinates) – that would be great to provide some context if using this figure in lectures. It could be added as text in the figure itself. Not a necessity, but I’d recommend it.

L137-161: These hyporheic patches are potentially the only cold-water refuges in small streams/brooks, where no tributaries go into the system and where springs are ? That would make them contextually highly important, I guess? E.g. highly important in small nursery streams for brown trout, or small coastal streams?

L162-179: (After writing this comment, I saw that this is mentioned in the management section, but I leave it here anyway, in case there is something in it that elicits some ideas (perhaps the winter-issue of too warm water?); no need to respond to this comment). Dams could potentially create thermal refuges in the river section just downstream of the dam, if the dam releases spill water from the lower colder section of the water column through sluices (instead of spilling surface water). Of course, in summer there is not much spilling – but it could potentially be a management option where possible (given that the hydro companies are forced to comply…). However, in winter the effects may be that the spilled water from the deeper section of the reservoir is too warm…

L217: Also, smaller streams in forested areas are probably impossible to characterize, since they are not visible?

Figure 4: Does “no thermoregulation” specifically mean that they do not seek cooler water? I’d assume that the fish, if possible, could move to warmer water if finding themselves in the lower temperature span (perhaps not so much in winter, but maybe in active periods in spring or fall (especially in spring when foraging may be of high importance to remain competitive towards conspecifics)? This scenario would also count as thermoregulation.

L333-336: Migrating adults probably want to keep their metabolic rate as low as possible to save energy for spawning and in the case of Salmo, the return to sea..? Also, cooler water would likely slow down Saprolegnia infections.

L384: Also possibly brain function – see recent papers by Fredrik Jutfelt and his colleagues (not on salmonids, but they may function in a similar way at the critical temperatures… They have a recent review in the journal Physiology.

L436. Probably better to write sex (biological and physiological characteristics of males and females), rather than gender (socially constructed characteristics of males and females).

Table 1: Scientific names need to be fixed with respect to the formatting of the name authorities.

Atlantic salmon (authority missing). Correct format:  Salmo salar L. (or Salmo salar Linnaeus, 1758)

Brown trout (erroneously within parentheses and wrong year). Correct format: Salmo trutta L. (or Salmo trutta Linnaeus, 1758)

Cutthroat trout (misspelled, a common autocorrect issue). Correct format: Oncorhynchus clarkii (Richardson, 1836)

Brook charr (missing authority). Correct format: Salvelinus fontinalis (Mitchill, 1814)

Arctic charr (wrong year). Correct format:  Salvelinus alpinus L. (or Salvelinus alpinus Linnaeus, 1758)

To explain: Parentheses around the authority are present only if the name has changed since the first description (i.e. for all Oncorhynchus and Salvelinus spp., which were historically in the Salmo genus, but not for the present Salmo spp. listed in the table). “L.” specifically means “Linnaeus, 1758” (i.e. both name AND year, referring to the first version of Systema Naturae that counts for naming of species). If you search for the species at FishBase, this database usually gets the format correct with parentheses and years. And the genus and species should be italicized (but not the authority/year), of course…

Section 5: I really like this section about behavioral thermoregulation. I think this is a great and useful contribution.

L643. It hits me now, when having read “Wolastoq|Saint John River” over and over: don’t some of the other mentioned rivers have original indigenous names as well?

Section 6: This section is also very good and very useful for both managers and scientists giving advice to managers. Perhaps some more emphasis on maintaining riparian zones for smaller streams, to hinder/reduce excessive warming, could be added, but overall it’s good in my opinion.

L840: Maybe even before-after control-impact (BACI) designs should be implemented, where possible? Not certain that it would be possible to get good matching controls though..?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

I have some comments and suggestions, particularly concerning the nomenclature of scientific names. I hope they will be useful to the authors.

Names of genera and species should be in italics. Errors in this respect have been noted at least on the lines 56, 56, 349 and 664-665.

Line 61.

Change "0 C°" to "0 °C".

Lines 92-93.

Change “Our work has mostly focused on Atlantic salmon (Salmo salar L., 1759)” to “Our work has mostly focused on Atlantic salmon Salmo salar L., 1758”. Delete the brackets. See also below my comment on lines 349-350.

Lines 349-350.

Change “brook trout (Salvelinus fontinalis Mitchill, 1814)” to “brook trout Salvelinus fontinalis (Mitchill, 1814)”. In this case, the name of the author and the year must be in brackets (since the species was described within the genus Salmo). Do not forget the italics for genus and species.

Lines 437-438. Table 1.

Change “Brown trout Salmo trutta (L., 1759)” to “Brown trout Salmo trutta L., 1758”. Brackets must be deleted since the species was described within the genus Salmo.

Change “Oncorhynchus clarkia” to “Oncorhynchus clarkii”.

Change “Salvelinus alpinus (L., 1759)” to “Salvelinus alpinus (L., 1758)”.

Lines 602-603

“steelhead trout”. Please, include genus, species, author and year.

References

Lines 946 and 1007.

References 2 and 29 correspond to a single paper. Reference 29 should be deleted, and subsequent references should be renumbered in the text and in the References list.

Be careful with genus and species names, they should be in italics and species names in lower case. Errors in this respect have been noted at least on the lines 947, 948, 960, 962, 964, 969, 993, 998, 1028, 1037, 1092, 1111, 1112, 1117, 1121, 1137, 1146, 1176, 1181, 1193, 1194, 1200, 1205, 1211, 1227, 1232, 1245, 1246, 1253 and 1259.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Dear authors,

Congratulations for your work results!

Few general suggestions:

0 Limitation of the approach should be highlighted

1 Some texts in the paper need citations.

2 Citations of some texts can be enriched to better support your paper.

3 The length of the paper induce the need of a final synthetic conclusions.

All the best

Reviewer

Author Response

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Author Response File: Author Response.pdf