The Influence of Body Size on Behavioral Thermal Preference in Atlantic Cod (Gadus morhua): Larger Fish Favor Colder Waters
, John Fleng Steffensen 2 and Morten Bo Søndergaard Svendsen 2,3,4Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
Comments
The title feels like a tautology, the question is what causes larger fish to stay in colder waters.
This hypothesis would only work if global warming causes larger fish to select cooler waters than done in previous climates. This is not the case as findings are in line with earlier ones.
l. 46 “a definitive consensus remains elusive” is presented as if this is evidence for the reasoning presented in this paper in search for alternatives?
The introduction should consider the lifestage dependent changes in the width of thermal ranges in fish as compiled by Dahlke et al, 2020 (Science 369, 65–70, 2020) in hundreds of fish species. Thermal windows start narrow in embryos, widen in juveniles and adults, and narrow again with increasing body sizes until spawning stages. In cod and others this pattern is reflected mostly in shifting upper thermal limits, with a clear metabolic background that overrides gill surface area as a primary driving factor. A synthetic view has been proposed earlier which is not really applied here. Authors should discuss to what extent thermal preference follows such trends rather than being a driver of body size shifts. In fact the present data can be interpreted differently and in harmony with OCLTT findings and even seems to provide evidence for such interpretation. In that sense the presentation and discussion would benefit from a clarification and possibly change in rationale. At least, alternative ways of interpretation should be presented and a balanced discussion presented.
L. 128 T pejus has been defined otherwise, why cause confusion by developing a second definition rather than using a different term.
l. 189 etc. This reasoning is fine but feels somewhat misplaced when biogeography is understood as a consequence of equilibrium body temperatures. The role of shorter term transition periods associated with different time dependencies of changes in body temperatures would be interesting but should be identified as such.
ll. 306 to 325 This seemingly controversial discussion is not needed and blurs the need for a synthetic effort to understand cause and effect and the complexities involved in a systemic to molecular hierarchy of thermal tolerance. Authors should acknowledge that earlier findings are in line with what they have observed.
l. 335 to 336: larger fish do have lower thermal limits, see unifying patterns available as in Dahlke et al. 2020.
Author Response
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Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsGeneral comments
In general, I found the manuscript original and well written. However, I detected some potentially serious flaws linked to the experimental design or at least to its description. Specifically, at line 178, the 25-frame running median of the distance covered per 25 frames (that is, in one second) is defined as the maximum swimming speed of a fish (Umax). However, as far I can understand, this is actually the “instantaneous” speed of the fish under investigation, not the maximum speed. Instead, I guess the maximum speed should be the largest speed value calculated over the 15 min observation period following each one of the heating phases, i.e. the maximum over a total of 60x15=900 instantaneous speed values. If it is so, the procedure for the calculation of Tpref and Tpejus appear to be reasonable, but this should be clearly described in the text accordingly. In addition, in the same description of the calculation for Tpref and Tpejus there is some confusion between ambient temperature (Ta) ad body core temperature (Tb). Is Tb the temperature on the x axis of Figure 2, as I guess? If yes, this should be indicated in the text and in the caption of the graph.
Accordingly, most of the suggested amendments/fixes listed below in my “Specific remarks” are about the about the quality of presentation, provided that my interpretation of the experimental design is correct.
Specific remarks
Page 5, line 172 (Figure 1): black arrows should be filled with black, and grey arrows with grey, in order to be clearly distinguished from the "white" arrows. Does “T” stands for "Tank"? If so, it can be removed from the graph.
Page 5, line 178: provided that my interpretation of the description of the experimental design is correct, “maximum swimming speed” should the replaced by “instantaneous swimming speed”. At the end of the sentence, a new sentence could be inserted for defining Umax as the largest instantaneous speed value calculated over the 15 min observation period following each one of the heating phases.
Page 6, lines 204-206: replace “dVmax/dTb” with “dUmax/dTb”.
Page 6, line 211 (Figure 2 caption). Replace “upper panel” with “Upper panel”. Why the units for Umax are indicated as “ Body lengths s-1”? I guess it should be “cm s-1”. In the same sentence, specify “core temperature of a 45 g cod”. After “Lower panel: the calculated”, insert “rate of variation of Umax”. Delete “is” between “temperature” and “shown”.
Page 9, line 300. Delete “emphasizing the ecological importance of size-dependent temperature preference.”
Author Response
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Reviewer 3 Report
Comments and Suggestions for AuthorsIn this manuscript, the authors investigated the behavioral response of Atlantic cod (Gadus morhua) to increasing water temperatures. The experiments were carried out on 29 individuals with body weight ranging from 4 to 2625 g. The temperature was increased gradually at established points, with 15-minute observation periods. The authors results were consistent with literature data and showed that size-dependent temperature preference correlates well with both measures of physiological performance and in situ temperature preference in Atlantic cod.
The authors place great emphasis on linking their research to climate change, especially the concept of fish "shrinking" due to rising water temperatures as a physiological response, even going so far as to propose an alternative hypothesis based on a behavioral response. Climate change is, of course, a hot topic, but the results obtained in this study do not justify such lofty conclusions. This issue aside, the research conducted by the authors divinely can serve as an interesting introduction to more advanced research on the behavioral responses of fish.
Specific remarks
Introduction
Line 104 not sure why there is a dash here
Line 102 it is generally recommended to use notation with space between number and degree symbol, also this form seams to be used in the rest of manuscript, please verify an unify the notation
Methods
In general I would appreciate if authors provide more detailed information on where and when the fish were catch? In what conditions were they kept on bord? I am not familiar with Danish Experimental Animal Inspectorate guidelines, so at least a citation would be helpful.
Line 136 degree symbol seams not to be correct, please verify
Line 144 missing numbered reference
Line 182 again missing reference number
Line 191` missing space, incorrect degree symbol
Line 192 as above
Line 195 as above
Line 196 as above
Results
Line 244-248 something is wrong with paragraph formatting, please verify
Discussion
Line 265, 266, 269, 270 missing subscript for Tpref and Tpejus
Conclusions
I understand the need to engage with high stake topics such as climate change, however it is important to understand limitation of own data. Not all studies have to provide solutions to global problems.
Kind regards
Reviewer
Author Response
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Round 2
Reviewer 1 Report
Comments and Suggestions for Authors
The revision is minimal and the key line of thought of the paper has not been changed in proposing thermal preference as a key mechanism driving change. This reviewer thinks otherwise and that in line with evidence reported in the literature e.g. in amphibians that lowered thermal preference in the heat results from oxygen limitations (hypoxia causing behavioral hypothermia). So physiology is the reason for thermal preference, not the other way round. Such mechanism-based reasoning is not presently visible in the text, in fact a solid mechanism-based framework is missing. The downward shift of Tpref is a progressive process with increasing body size. In fact as performance maxima and limits are correlated, simply due to the thermal performance curve, this interpretation would be in line with what the authors found and discuss, namely that thermal preference follows thermal optima. In their list of replies to my comments the later ones are unclear, are there errors involved in the writing?
Overall, without opening the text for alternative explanations (one of which in the eye of this reviewer are the more convincing ones anyway) the interpretation seems to follow one line of beliefs more than evidence and could be misleading for the reader. Authors should be aware that their findings may be interpreted differently in future literature.
Figure 2: How does the upper depiction relate to the temperature dependent performance curve (e.g. according to Brett)? How do growth curves as another performance indicator match these observations?
l. 310: There is too much emphasis on gill surface area in the new section. There are other allometric tradeoffs in tissue biochemistry to be considered.
l. 358: this hand-waiving reasoning shows the lack of reference to existing hypotheses.
Author Response
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Reviewer 2 Report
Comments and Suggestions for AuthorsAll my comments have been duly addressed. Congratulations to the authors for their work.
In my opinion, now the manuscript can be accepted in its present form. Just add an asterisk after the name of the last author, if he is going to be tagged as an additional corresponding author.
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