Digestibility of Local Feed Ingredients in Tilapia Oreochromis niloticus Juveniles, Determined on Faeces Collected by Siphoning or Stripping

Round 1

Reviewer 1 Report

Three Tanzanian together with three Swedish researchers have carried out an experiment with Nile tilapia where they tested digestibility of different feed ingredients easily (?) available in Tanzania. In principle, such experiments could be useful in promoting tilapia farming also on small farms by providing information of the quality of feedstuffs. This work also compared two different methods for analyzing digestibility and there was no apparent difference in this respect between the methods how to collect the faeces. Despite interesting results of the digestibility of eight tested ingredients, the reader would really need know what is the outcome when using these ingredients, i.e. how well the fish grow using these different diets. Even if the authors report differences in digestibility it would be essential to know how these differences are reflected in growth, growth heterogeneity, body composition and mortality. Without reporting such data the authors cannot make any conclusion and certainly not recommendations about their use, as they now have done.

I do not know the situation in Tanzania, but are marine and FW shrimp so abundant and cheap there that it would make sense to replace FM with these? Would there actually be any savings, as claimed in the ms. The authors make conclusion about replacing FM with these different ingredients, but only about 1/3 of the FM was replaced, while what they write implies that all FM could be replaced (in Implications).

In discussion the authors try to advocate the use of plant proteins rather than FM in fish feeds, apparently to avoid competition of resources with humans and other animals. This raises some questions: why FM should rather be fed to other animals but not fishes? Secondly, also many plant ingredients could be directly used as human food, so this raises a question why it is better to feed plant ingredients rather than FM to fishes. The authors mention that they are cheaper, but isn’t there competition for these ingredients as well? Just a point to consider. In the present work e.g. duckweed is such an ingredient which I assume is not used as human food anywhere (not sure though) and therefore the use of such ingredients should be promoted in fish feeds.

The ms was rather complicated to read due to high number of abbreviations. I suggest making the paper easier to read and follow by not using abbreviations at all in most places, including tables. To streamline the text, instead of using crude protein, crude fiber and crude fat, just use protein, fiber and fat (including tables) without abbreviating them. Also, for example CF could easily be misunderstood as crude fat but in this instance it referred to fiber. It was confusing that the abbreviation for crude fat was EE. Pay also attention when the verb should be in singular or plural form. The authors have failed to follow the journal rules for citing articles. Also the reference list is incorrectly formatted.

Minor comments

Introduction:

L38: “often combined with”, just thinking if “often” is needed here, delete it?

M&M:

There was no explanation how energy was measured. Please give information how the fish were handled: with or without anaesthesia. You should also inform the reader if there was a permission from the local animal welfare committee (if it exists) to carry out this experiment.  

Be consistent when using a number (8) or when writing a number (eight).

L61-62: according to my calculation 2*27 is not 56.

L65. what do you mean by “according to Cho & Slinger”, please clarify.

Please clarify the feeding and faeces collection part. It was very hard to follow and understand what happened and when.

L88. I do not understand what has been mixed. Did you pool samples and mixed them? Or what does this mean?

L102-3: I do not quite understand what you have done

L141: Results (plural)

In discussion, try to decrease the number of sentences starting with “however” and “moreover”. In some places their use is overwhelming.

I attach the marked and commented ms. Before submitting the next version, it would be important that all authors read and accept the ms, which is one of the general rules for having an authorship in an article. It is difficult to believe that this has been the case with this first draft.

Comments for author File: Comments.pdf

Author Response

Reviewer 1

“Despite interesting results of the digestibility of eight tested ingredients, the reader would really need know what is the outcome when using these ingredients, i.e. how well the fish grow using these different diets. Even if the authors report differences in digestibility it would be essential to know how these differences are reflected in growth, growth heterogeneity, body composition and mortality. Without reporting such data the authors cannot make any conclusion and certainly not recommendations about their use, as they now have done.”

Response: We agree that growth performance studies are needed to for a complete evaluation of the feed ingredients. Our study provide an essential piece of information to allow formulation nutritionally balanced diets. We have made conclusions regarding the potential use of the ingredients based on their digestibility but we have not given made any recommendations with respect to diets composition or diet formulations.

 

“The authors make conclusion about replacing FM with these different ingredients, but only about 1/3 of the FM was replaced, while what they write implies that all FM could be replaced (in Implications).”

Response: It is correct that only 30% of the reference diet was replaced with the test ingredient, but the digestibility of the test ingredient was calculated (by difference) to assess the digestibility of the test ingredient alone (see Table 5). These data are the basis for our conclusions about replacing fish meal with the respective test ingredients.

 

“The ms was rather complicated to read due to high number of abbreviations. I suggest making the paper easier to read and follow by not using abbreviations at all in most places, including tables. To streamline the text, instead of using crude protein, crude fiber and crude fat, just use protein, fiber and fat (including tables) without abbreviating them.”

Response: We have introduced abbreviations to shorten the length of the manuscript and to facilitate the presentation of our data. We regret that the reviewer find the ms complicated to read due to the abbreviations. However, we have decided to keep the abbreviations in the manuscript as we think that they all are clearly defined both in the text and tables.

 

“The authors have failed to follow the journal rules for citing articles. Also the reference list is incorrectly formatted.”

Response: We apologise for these mistakes. The citing of articles has now been corrected to follow the journal rules. The formatting of the reference list has now been corrected and the whole manuscript is now following the journals template.

 

Minor comments

Introduction:

L38: “often combined with”, just thinking if “often” is needed here, delete it?

Response: “often” has been deleted.

 

M&M:

There was no explanation how energy was measured.

Response: Information added under M&M.

 

Please give information how the fish were handled: with or without anaesthesia.

Response: Fish were anaesthetized with cloves oil. Information added.

 

You should also inform the reader if there was a permission from the local animal welfare committee (if it exists) to carry out this experiment.  

Response: Information regarding ethical considerations with animal experiments (The present study was carried out in accordance with the law on the protection of animals against cruelty (Act no. 12/1974. of the United Republic of Tanzania) has been added under M&M.

 

Be consistent when using a number (8) or when writing a number (eight).

Response: Noted.

 

L61-62: according to my calculation 2*27 is not 56.

Response: We agree. This has been corrected, should be 2*28.

 

L65. what do you mean by “according to Cho & Slinger”, please clarify.

Response: The following sentence has been added to clarify: The experimental diets were formulated as suggested by Cho & Slinger.

 

Please clarify the feeding and faeces collection part. It was very hard to follow and understand what happened and when.

Response: This section has been rewritten to better describe the feeding and the collection.

 

L88. I do not understand what has been mixed. Did you pool samples and mixed them? Or what does this mean?

Response: The text has been revised to clarify the pooling and mixing of faeces from the fish tanks.

 

L102-3: I do not quite understand what you have done

Response: The text has been revised to clarify.

 

L141: Results (plural)

Response: Thank you. This has been corrected.

 

In discussion, try to decrease the number of sentences starting with “however” and “moreover”. In some places their use is overwhelming.

Response: The text has been revised and the number of ”however” and ”moreover” used has been markedly reduced.

Author Response File: Author Response.pdf

Reviewer 2 Report

The study is well carried out and I think the authors can improve on the quality of the manuscript in a few ways. The objective of the study is described as determining the digestibility of several locally available potential feed ingredients through stripping and siphoning. I think the authors do themselves a disservice by not focusing more on the comparison between methods. The authors have a nice study where they have used both methods in sequence with the same fish, so the comparison of methods is very strong. The fact they found no significant differences between the methods is a major point that can benefit many other researchers, but is actually relatively rapidly glossed over in the text. I would also suggest changing the concept from "siphoning and stripping" to "siphoning or stripping" as both methods were not used at the same time to collect samples and data. 

   The introduction needs to be greatly expanded. A lot of research has been done on digestibility and alternative ingredients in tilapia and the authors need to go into more detail about studies that have looked at similar ingredients or used similar approaches. The authors also elude to the economic advantages to switching from fishmeal to more cost effective ingredients, however they offer no current pricing or cost analysis of the ingredients tested to determine how much money may be saved by switching to alternative ingredients. The concept of using more locally sourced ingredients and by-product ingredients from other industries is not novel, and there is a great deal in the literature the authors should reference and rely upon to set the stage for this study.

The major point that needs to be addressed by the authors from an experimental standpoint is the overall very low digestibility numbers across the board, including the fishmeal reference diet. these diets typically have protein digestibility in the high 80's to 90's but here they are all lower. Does this have to do with the % in the reference diet? the specific type of fishmeal? a specific strain of tilapia tested. The low digestibility for almost all ingredients seems odd in comparison to other studies and needs to be justified or explained.

Author Response

Reviewer 2

“The authors have a nice study where they have used both methods in sequence with the same fish, so the comparison of methods is very strong. The fact they found no significant differences between the methods is a major point that can benefit many other researchers, but is actually relatively rapidly glossed over in the text. I would also suggest changing the concept from "siphoning and stripping" to "siphoning or stripping" as both methods were not used at the same time to collect samples and data. “

Response: Thank you! We have changed the concept as suggested. The discussion has been revised and changes have made to better high-light the importance of our findings.

 

“The introduction needs to be greatly expanded. A lot of research has been done on digestibility and alternative ingredients in tilapia and the authors need to go into more detail about studies that have looked at similar ingredients or used similar approaches.”

Response: We agree that there is a large number of studies performed on the digestibility and alternative ingredients in tilapia. We found it difficult to do justice to all and therefore decided to select some key references that we consider representative for the work done. Our focus was on the situation in Tanzania, where there is very limited data available.

 

“The authors also elude to the economic advantages to switching from fishmeal to more cost effective ingredients, however they offer no current pricing or cost analysis of the ingredients tested to determine how much money may be saved by switching to alternative ingredients.”

Response: The statement about the economic benefits of replacing fishmeal with alternative feed ingredients has been deleted from the conclusions.

 

“The concept of using more locally sourced ingredients and by-product ingredients from other industries is not novel, and there is a great deal in the literature the authors should reference and rely upon to set the stage for this study.”

Response: We agree that the concept of using locally sourced ingredients and by-product ingredients from other industries is not novel. We have tried to do justice to earlier published papers but have limited the discussion to studies that could be of interest to the situation in Tanzania.

 

“The major point that needs to be addressed by the authors from an experimental standpoint is the overall very low digestibility numbers across the board, including the fishmeal reference diet. these diets typically have protein digestibility in the high 80's to 90's but here they are all lower. Does this have to do with the % in the reference diet? the specific type of fishmeal? a specific strain of tilapia tested. The low digestibility for almost all ingredients seems odd in comparison to other studies and needs to be justified or explained.”

Response: Except for duckweed (AD of CP, 46%), all test ingredients showed high AD of CP (> 77%). These data are well in line with other published data in tilapia fed feed ingredients of varying origin.

The low AD of CP in duckweed was un-expected but could be due to lower CP content and higher CF content than in other studies, which will have an impact on the digestibility. The chemical composition of duckweed has been reported to vary widely depending on plant age, environmental temperature and nutrient content of the aqueous environment. The plant age of the duckweed used in the present study was unknown.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

There are clear improvements in the current version of the ms. However, I was disappointed to see that the authors were reluctant to follow my suggestions regarding not using too many abbreviations and including the growth data. Even though the ms is about the digestibility of ingredients, including growth data would have made the ms much stronger but of course it is also possible the growth was not measured. I hope that the final and rather strong conclusion “there is great potential for using several of these feed ingredients as a replacement for fishmeal in diets for tilapia” is true also in the real farming conditions, not only based on AD values obtained in the present experiment. I would like to remind that the researchers have the responsibility over their results and in the lack of a growth experiment, I feel that this statement is promising far too much. Can a Tanzanian fish farmer after reading this article count on the AD values and expect that the fish will also grow properly with these ingredients by replacing FM? This statement does not define how much of the FM could be replaced, but the reader easily interprets that all FM can be replaced. In addition, does the “great potential” refer also to the reduced price by replacing FM?

Minor comments

It was frustrating to see, that the minor comments written directly to the pdf had not been taken into account in several places. Recheck and correct if still needed.

Species’ scientific names should be given when mentioned for the first time and all scientific names should be in italics

L65: incomplete sentence

L70: spell out DM

Tables 1 and 2 (and paragraph 2.1) belong to M&M

In Table 4 the authors have decided NOT to use abbreviations. This looks much better and same format should be adopted in the other tables. Also in Table 2 Cassava flour and sunflower oil, for some unknown reason, have not been abbreviated, and I suggest to do the same for the other items.

Table 5: remove “ing” from the abbreviation (i.e. AD, not ADing). Some values in this table exceed 100% which should not be the case. This should be discussed: is it a methodological problem or is there a mistake?

Paragraph L172-181 reads nicely as the authors have not used any other abbreviation than AD. Please follow this style also elsewhere. In that paragraph you should anyway define what “high” and “lower” mean.

L 209: clove oil

Section 4.3. Nothing is told about feeding the fish.

L213-216: scientific names missing in many places

L223-225. Something must be wrong here regarding the faeces collection.

L229-230, and 235. Fish feeding does not belong here.

L238: change “Recording” to “Water quality measurements”. In this paragraph you must also give the actual values what you have got. It is not sufficient just to say what has been measured.

L264: give details of the calorimeter used.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have improved the quality of the presentation and writing. The only suggestion now would be to clarify the focus on the situation with tilapia in Tanzania and how this study will affect the research and industry there. this would be critical without an exhaustive review of global tilapia research in the introduction. Maintaining focus on the Tanzanian industry is fine, as long as that is clear and any/all work done in Tanzania is being reviewed and referenced.

Author Response

Please see the attachment

Author Response File: Author Response.pdf