Zebrafish Girella zebra (Richardson 1846): Biological Characteristics of an Unexploited Fish Population

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General comments

 

This paper focuses on the biological characteristics of the unexploited zebrafish (Girella zebra), trying to investigate key life-history traits of this non-targeted fish species found off the southern coast of Western Australia, including its growth, longevity, mortality, and reproductive patterns. Key findings highlight that the Girella zebra is long-lived (up to 45 years) and exhibits a "square" form of growth, where growth is rapid initially but minimal later in life. This study is significant because it provides baseline biological data for a fish population that has not been impacted by commercial fishing, offering a crucial "virgin state" reference for other related species, the Kyphosidae.

 

The objective of the manuscript, in general, is interesting from a fisheries management perspective. However, the manuscript requires some improvements to meet publication standards. The manuscript should become acceptable for publication pending suitable minor revision considering the comments appended below.

 

More specific comments:

                                                                                                                                               

Introduction:

 

Line 47-49: "Although omnivorous during their juvenile stages, the diet of girellids and kyphosids once maturity is attained, primarily consists of algae [10,11].". Please briefly mention the trophic link this shift creates. The ecological significance of this family is strongly tied to their herbivory.

 

Line 50-52: "Kyphosids comprise a significant component of temperate reef fish communities, contributing > 80% of the fish biomass in some cases [12-15], and are thus likely to be ecologically important.". This is a very strong statement. Try to specify which temperate reef fish communities and which species are responsible for the >80% biomass to anchor the claim.

 

Line 60-61: "Very few studies on the biological characteristics of teleosts can claim to have been undertaken on unfished species or populations [exception: 20].". Please strengthen this claim.

 

Material and methods:

 

The Materials and Methods section is highly detailed and generally robust.

 

Results:

 

Line 201-203: "Therefore, the first opaque zone is laid down in the otoliths of G. zebra during the first winter (June-July) of life and becomes delineated from the edge of the otolith during the first full spring (September-November) of life when fish are ∼12 months old.". Explicitly state the evidence for the age at the formation of the first opaque zone. The text describes the sizes and months of capture for the smallest fish but does not directly link this to the formation timing (June-July).

 

Line 207-209: "The mean monthly MIs for otoliths with 2-4, 5-9 and ≥ 10 opaque zones all declined from high levels (≥ 0.41) in those months between June and November to minimas ≤ 0.25 between January and March (Figure 2).". Please clarify the relationship between the peak/minimum MI and the timing of zone formation. The opaque zone is interpreted as forming annually when MI is at its minimum.

 

Line 224-236: "The von Bertalanffy growth curves (vBGCs) for female and male G. zebra were significantly different (P<0.01)... As these differences are very small... the differences were assumed to be of little or no biological significance... Thus, the TLs at age for female and male G. zebra were combined...". The authors should report the separate vBGC parameter estimates in the results, even if they choose to use the combined curve for comparative purposes. Please present the specific test statistic and df from the likelihood-ratio test, and then both the combined (Table 1) and the sex-specific vBGC parameters.

 

Line 252-255: "The Alverson and Carney [33] method produce the lowest estimates of M of 0.07 and 0.08 year-1 for females and males, respectively, which were identical or very similar) to those estimates of total mortality (Z) obtained from catch curve analysis (i.e. 0.07 and 0.09 year^-1, respectively.". Provide the r² or p-value for the catch curve analysis regression used to determine Z.

 

Line 262-267: "The mean monthly gonadosomatic index (GSI) for female G. zebra ≥ the estimated TL50 increased from 0.9 in July to a maximum of 3.7 in October, ...". Please state the total sample size (n) used for calculating the GSI, as this is a subsample (fish ge TL50) of the original 757 fish. While Figure 5 provides sample sizes per month, stating the total here enhances context.

 

Line 302-312: "The likelihood ratio test demonstrated that the estimated TL50s for female and male G. zebra were significantly different (P < 0.001). The TL50s for females and males were 290 and 269 mm, respectively and the estimated TL95s were 346 and 322 mm, respectively (Table 2).....". Report the specific test statistic value and df for both the TL50 and A50 likelihood-ratio tests. While the p-value is given, the test statistic is essential for the statistical finding.

 

Discussion:

 

Line 335-338: " The formation of the first growth zone in the otoliths of G. zebra at the end of the first full spring of life when fish are ∼1 year old parallels that for another kyphosid S. aequippinis in the same region [9].". The authors should briefly discuss the potential environmental cue responsible for the delineation of the opaque zone in spring/early summer (September–November).

 

Line 340-342: "This maximum age is nearly identical to that of G. elevata [22] (i.e. 45 years), which inhabits rocky reefs and is taken in small quantities by recreational anglers and spear fishers [38].". To strengthen the "unfished" argument, the authors should quantify what "small quantities" means for G. elevata if possible (e.g., specific catch data or stock status reference). This helps justify the comparison of G. elevata's 45 years to G. zebra's 44 years as a natural benchmark, rather than one still slightly influenced by fishing pressure.

 

Line 361-364: "A "square” form of growth adopted by some species is proposed to lead to the accumulation of multiple similarly sized adult cohorts, enabling the retention of individuals within stable, low-turnover populations that persist in nutrient-sparse environments [23].". The statement that "square growth" is proposed to lead to the accumulation of similarly sized cohorts is somewhat vague. Please clarify the biological implication.

 

Line 408-410: "The similar size of the gonads of female and male G. zebra strongly suggests that this species spawns in schools or large groups, which is consistent with their observed schooling nature [6].". This is a key conclusion about reproductive behavior. Please mention the lack of direct observation of this behavior, which is a limiting factor in the conclusion.

Author Response

This paper focuses on the biological characteristics of the unexploited zebrafish (Girella zebra), trying to investigate key life-history traits of this non-targeted fish species found off the southern coast of Western Australia, including its growth, longevity, mortality, and reproductive patterns. Key findings highlight that the Girella zebra is long-lived (up to 45 years) and exhibits a "square" form of growth, where growth is rapid initially but minimal later in life. This study is significant because it provides baseline biological data for a fish population that has not been impacted by commercial fishing, offering a crucial "virgin state" reference for other related species, the Kyphosidae.

 

The objective of the manuscript, in general, is interesting from a fisheries management perspective. However, the manuscript requires some improvements to meet publication standards. The manuscript should become acceptable for publication pending suitable minor revision considering the comments appended below.

Thank you to the reviewer for taking the time to undertake the review, it is greatly appreciated.

More specific comments:

                                                                                                                                               

Introduction:

 

Line 47-49: "Although omnivorous during their juvenile stages, the diet of girellids and kyphosids once maturity is attained, primarily consists of algae [10,11].". Please briefly mention the trophic link this shift creates. The ecological significance of this family is strongly tied to their herbivory.

 The following text has been added at lines 47-51: “With a shift toward a diet dominated by algae, and thus a role as primary consumers, herbivorous fish serve as a key pathway for energy between primary producers (algae) and higher trophic levels.”

Line 50-52: "Kyphosids comprise a significant component of temperate reef fish communities, contributing > 80% of the fish biomass in some cases [12-15], and are thus likely to be ecologically important.". This is a very strong statement. Try to specify which temperate reef fish communities and which species are responsible for the >80% biomass to anchor the claim.

Sentence has been updated: “Members of the Kyphosidae, such as Kyphosus bigibbus, K. sydneyanus and K. gladius, and Girellidae, such as Girella tricuspidata and G. elevata, comprise a significant component of shallow temperate reef fish communities in Australia, contributing > 80% of the fish biomass in some cases …..” (lines 52-55)

 

Line 60-61: "Very few studies on the biological characteristics of teleosts can claim to have been undertaken on unfished species or populations [exception: 20].". Please strengthen this claim.

 Sentence has been modified and an additional reference provided: “Very few studies on the biological characteristics of teleosts have been undertaken on unfished or very lightly fished species or populations” (line 63-64).

Material and methods:

 

The Materials and Methods section is highly detailed and generally robust.

 

Results:

 

Line 201-203: "Therefore, the first opaque zone is laid down in the otoliths of G. zebra during the first winter (June-July) of life and becomes delineated from the edge of the otolith during the first full spring (September-November) of life when fish are ∼12 months old.". Explicitly state the evidence for the age at the formation of the first opaque zone. The text describes the sizes and months of capture for the smallest fish but does not directly link this to the formation timing (June-July).

The following text, already in the manuscript, states when the age when the first opaque zone is formed: “Therefore, the first opaque zone is laid down in the otoliths of G. zebra during the first winter (June-July) of life and becomes delineated from the edge of the otolith during the first full spring (September-November) of life when fish are ∼12 months old.” (lines 205-207)

 Line 207-209: "The mean monthly MIs for otoliths with 2-4, 5-9 and ≥ 10 opaque zones all declined from high levels (≥ 0.41) in those months between June and November to minimas ≤ 0.25 between January and March (Figure 2).". Please clarify the relationship between the peak/minimum MI and the timing of zone formation. The opaque zone is interpreted as forming annually when MI is at its minimum.

 Sentence has been modified to the following: The progressive rise in the mean monthly MI values followed by a single, sharp decline in those values after the outermost opaque zone has become fully formed  indicates that a single opaque zone is formed in the otoliths each year and thus the number of those zones can thus be used for ageing individuals of G. zebra. (lines 213-217).

Line 224-236: "The von Bertalanffy growth curves (vBGCs) for female and male G. zebra were significantly different (P<0.01)... As these differences are very small... the differences were assumed to be of little or no biological significance... Thus, the TLs at age for female and male G. zebra were combined...". The authors should report the separate vBGC parameter estimates in the results, even if they choose to use the combined curve for comparative purposes. Please present the specific test statistic and df from the likelihood-ratio test, and then both the combined (Table 1) and the sex-specific vBGC parameters.

 The test statistic and DF are now presented along with the p-value (line 232). Table 1 now includes parameters for vBGCs fitted to female and male length-at-age datasets

Line 252-255: "The Alverson and Carney [33] method produce the lowest estimates of M of 0.07 and 0.08 year-1 for females and males, respectively, which were identical or very similar) to those estimates of total mortality (Z) obtained from catch curve analysis (i.e. 0.07 and 0.09 year^-1, respectively.". Provide the r² or p-value for the catch curve analysis regression used to determine Z.

P value added on line 263.

 

Line 262-267: "The mean monthly gonadosomatic index (GSI) for female G. zebra ≥ the estimated TL50 increased from 0.9 in July to a maximum of 3.7 in October, ...". Please state the total sample size (n) used for calculating the GSI, as this is a subsample (fish ge TL50) of the original 757 fish. While Figure 5 provides sample sizes per month, stating the total here enhances context.

As the Reviewer has pointed out, the sample sizes used in calculating the mean monthly GSIs are shown in Figure 5. Providing the reader with a total sample size values adds nothing if, for example, 80% of fish were from a single month. I have chosen to not make this change.

 

Line 302-312: "The likelihood ratio test demonstrated that the estimated TL50s for female and male G. zebra were significantly different (P < 0.001). The TL50s for females and males were 290 and 269 mm, respectively and the estimated TL95s were 346 and 322 mm, respectively (Table 2).....". Report the specific test statistic value and df for both the TL50 and A50 likelihood-ratio tests. While the p-value is given, the test statistic is essential for the statistical finding.

 The test statistic and DF are now presented along with the p-value (lines 311-312).

Discussion:

 

Line 335-338: " The formation of the first growth zone in the otoliths of G. zebra at the end of the first full spring of life when fish are ∼1 year old parallels that for another kyphosid S. aequippinis in the same region [9].". The authors should briefly discuss the potential environmental cue responsible for the delineation of the opaque zone in spring/early summer (September–November).

 New sentence included. “The completion of the opaque growth zone at that time of year is consistent with increasing in water temperature during spring (see Figure 1 in Coulson et al. [9]) and thus the commencement of the new translucent zone.” (lines 346-348)

Line 340-342: "This maximum age is nearly identical to that of G. elevata [22] (i.e. 45 years), which inhabits rocky reefs and is taken in small quantities by recreational anglers and spear fishers [38].". To strengthen the "unfished" argument, the authors should quantify what "small quantities" means for G. elevata if possible (e.g., specific catch data or stock status reference). This helps justify the comparison of G. elevata's 45 years to G. zebra's 44 years as a natural benchmark, rather than one still slightly influenced by fishing pressure.

The contemporary status of the stock of Girella elevata in New South Wales is not monitored and there appears to be little recent information from recreational fishing catches. Historical information regarding the catch of G. elevata indicates that this species makes constitutes ~9% of the catch at spearfishing competitions. This information is now added and an additional reference provided (352-353): “For example, G. elevata has historically constituted as little as 9% of catches at spearfishing competitions [].”

Line 361-364: "A "square” form of growth adopted by some species is proposed to lead to the accumulation of multiple similarly sized adult cohorts, enabling the retention of individuals within stable, low-turnover populations that persist in nutrient-sparse environments [23].". The statement that "square growth" is proposed to lead to the accumulation of similarly sized cohorts is somewhat vague. Please clarify the biological implication.

I feel that this is already answered above (lines 367-369): “This form occurs when a large portion of growth is undertaken in the first 5-8 years of life, after which little growth occurs throughout their extended adult lives.”

Line 408-410: "The similar size of the gonads of female and male G. zebra strongly suggests that this species spawns in schools or large groups, which is consistent with their observed schooling nature [6].". This is a key conclusion about reproductive behavior. Please mention the lack of direct observation of this behavior, which is a limiting factor in the conclusion.

In the sentence that the reviewer has highlighted I state that this “strongly suggests” spawning in schools or large groups. The use of the word “suggest” indicates that the data presented does not provide conclusive evidence of this type of spawning behaviour but suggests G. zebra exhibits in this type of spawning behaviour. I have removed the word “strongly” (lines 421).

Reviewer 2 Report

Comments and Suggestions for Authors

 

General comments

This study is the first to report the biological baseline data of the unimpacted Girella zebra population, filling the gap in the ecological research of the species in the Girella family. Girella zebra is a common fish species in temperate and subtropical coral reef communities. However, due to its low edible value, it is usually not caught and has become an ideal subject for studying the biological characteristics of fish that have not been affected by fishing. However, Girella zebra can inhibit the excessive growth of large algae and maintain the balance between corals and algae, having important ecological functions. Through in-depth analysis of the spotfin fish, this study revealed its biological characteristics under the condition of no fishing impact, providing important data support for future fishery management and ecological research. The research topic of this manuscript is novel, the design is rigorous, and the data is reliable. The conclusion is of great value for the protection of temperate reef fish. I suggest it be revised and published.

 

Special comments

1. The abstract lacks the description of the estimation methods for age and natural mortality. I suggest adding a more appropriate introduction to the methods in the abstract.

2. Fish were sampled using spearfishing. Does the specification of the spear have any impact on the sampling and the research results? For example, the ratio of males to females and the mortality rate estimation.

3. The sampling was concentrated in the shallow waters of Albany and Bremer Bay, representing the complete distribution depth of this species. In Australia, what is the maximum distribution depth of this species? Does it exceed 20 meters?

4. The area where the samples were collected is very concentrated. Why is that? So, is there any issue of insufficient representativeness of the samples? (Lines 75-76)

5. Why do we need to simultaneously analyze the relationship between the total weight and the total length as well as the fork length? (Formula 1 and Formula 2)

6. The Hamels formula valuation is too high (0.22 for female fish vs. Z = 0.07 for the fishing curve). This needs explanation: It is suggested to emphasize that "the mortality model based on the developed population may overestimate the M value of the undeveloped population. It is recommended to preferentially adopt the Alverson-Carney method."

7. The peak value of male fish GSI occurred earlier than that of female fish (August vs. October), suggesting a gender-asynchronous reproductive strategy. This finding can be further explored in the discussion.

8. Figure 3: Combined frequency distribution chart of body length/age for both males and females. Are there any significant differences between the two sexes?

Author Response

General comments

This study is the first to report the biological baseline data of the unimpacted Girella zebra population, filling the gap in the ecological research of the species in the Girella family. Girella zebra is a common fish species in temperate and subtropical coral reef communities. However, due to its low edible value, it is usually not caught and has become an ideal subject for studying the biological characteristics of fish that have not been affected by fishing. However, Girella zebra can inhibit the excessive growth of large algae and maintain the balance between corals and algae, having important ecological functions. Through in-depth analysis of the spotfin fish, this study revealed its biological characteristics under the condition of no fishing impact, providing important data support for future fishery management and ecological research. The research topic of this manuscript is novel, the design is rigorous, and the data is reliable. The conclusion is of great value for the protection of temperate reef fish. I suggest it be revised and published.

My sincere thanks to the reviewer for taking the time to review this manuscript and for their positive comments.

Special comments

1. The abstract lacks the description of the estimation methods for age and natural mortality. I suggest adding a more appropriate introduction to the methods in the abstract.

The abstract is limited to 200 words, in which I need to convey what was done, why it is important, key results and their implications. I don’t feel that this is the place to be describing the methods, particularly for routine aspects of fisheries science such as age and mortality estimation.

2. Fish were sampled using spearfishing. Does the specification of the spear have any impact on the sampling and the research results? For example, the ratio of males to females and the mortality rate estimation.

Any type of sampling method will have some form of bias, i.e. selectivity of gillnets, selectivity of different hook sizes. In undertaking this study, I was conscious of collecting the full size range of fish and thus hopefully providing results of the true population characteristics of this species, including length and age structure of females and males and mortality rates.

3. The sampling was concentrated in the shallow waters of Albany and Bremer Bay, representing the complete distribution depth of this species. In Australia, what is the maximum distribution depth of this species? Does it exceed 20 meters?

Key information regarding fish species in in Australian waters provided by the Fishes of Australia website indicates that the maximum depth for G. zebra is 20m. While it is likely that individuals are found in deeper waters, 20 m appears to be the typical maximum depth of this species.

4. The area where the samples were collected is very concentrated. Why is that? So, is there any issue of insufficient representativeness of the samples? (Lines 75-76)

Sampling area was restricted by funding and time restraints. In addition, the availability of samples from the sampled region enabled monthly samples to be collected easily. As this species is not fished throughout its southern Australian distribution the samples collected from Albany and Bremer and likely to be representative of other regions in southern Western Australia.

5. Why do we need to simultaneously analyze the relationship between the total weight and the total length as well as the fork length? (Formula 1 and Formula 2)

This was done to condense the text as the relationships are essentially the same, just different length measurements.

6. The Hamels formula valuation is too high (0.22 for female fish vs. Z = 0.07 for the fishing curve). This needs explanation: It is suggested to emphasize that "the mortality model based on the developed population may overestimate the M value of the undeveloped population. It is recommended to preferentially adopt the Alverson-Carney method."

Sentence in discussion has been changed to the following: “In contrast, other methods such as Heonig [30], Then et al. [31] and Hamel and Cope [32], based on data for heavily fished stocks, may overestimate the value of M for unfished, or lightly fish populations. It is thus recommended that the Alverson-Carney [33] equations provides a better estimate of M for pristine or baseline scenarios” (lines 388-392). I have also replaced the Cope reference with the more recent Hamel and Cope (2022) reference in which it clearly states their equation.

7. The peak value of male fish GSI occurred earlier than that of female fish (August vs. October), suggesting a gender-asynchronous reproductive strategy. This finding can be further explored in the discussion.

Without collecting sufficient samples over multiple years, it is not possible determine if the differences in the peak GSI values are true or just an artifact of the number of samples collected, size of fish collected or some other variable. I don’t believe that this detail is worthy of further exploration due to these uncertainties.

8. Figure 3: Combined frequency distribution chart of body length/age for both males and females. Are there any significant differences between the two sexes?

The results of Kolmogorov-Smirnov test on the length and age distribution have now been added (lines 228-230). A line in the methods section has also been added to indicate that these tests were undertaken (lines 94-95).