Juveniles are preferably used because they allow the observation of pollutant's interference on tissues development during the growth.

Moreover, using immatures, it is possible to discover long term effects of the pollutant on the body (like chronic effects).

I agree with you dear Riccardo. I add few more reasons.

The juvenile fishes are very much perfect for showing the effects of pollutants at very low level even as their vital index is low.

In addition to this, taking the big fish as a model organism means more intoxication of the environment than taking the small fishes for one simple reason: concentration factor.

Thanks for asking the question dear Milad.

Dear Milad,

This question has been treated here:

https://www.researchgate.net/post/Why_do_fish_toxicity_researchers_use_juvenile_immature_fish_to_measure_toxic_compounds?_tpcectx=qa_overview_following&_trid=54d257efd039b1f90d8b4569_

1) Small sized fish require very small amounts of toxic substances to effect them,as compared to bigger ones which require higher amounts. 

2) They are easy to observe & handle them during the studies. If large sized are used, it is possible that they may jump out of the container & may die otherwise.

3) using smaller sized ones would require smaller tanks for experiments,as compared to larger sized ones,which require larger tanks,& all may not be of the same size & weight. The normal protocol is to use a maximum of 2 gms size @ of 1/L.

4) More of time,energy,money need to be spent on rearing to bigger sized ones.

5) It would be easier to collect small ones from nature,as compared to larger ones, for those laboratories which doesn't have the facility of a fish hatchery.

Small fishes are suitable for toxicity study in  Laboratory conditions becoz of easy to set up  with minimum requirements and and maintenance. Moreover it helps to study toxicity for long term (chronic  study).

Hi Milad

Juvenile fish are often used for such studies, because, In addition to above-mentioned issues,:

It is easy to rearing them in laboratory conditions rather than the mature fish, especially commercial fishes. However, for the study of toxicant effects on reproduction and on progeny, mature fish of small model fishes, such as goldfish, zebrafish and medaka, are usually used because of their short generation and reproductive period.

Hope to be useful.

An additional information: immature fish are sometimes also used because of regulations in animal experimentation. Until yolk sac resorption, fish are considered as invertebrates, you can thus use  them in toxicity studies without any habilitation. After fish starts to fed (opening of the mouth), they are considered as vertebrates and you need an habilitation to realize experiments on such animals.

There's a lot of thruth in the answers from collegues above. But there's also a very simple yet important reason. Juvenile fish are easy to age in contrast to adults of which it is very hard to get an exact age and to determine life history. This can severely add variability to experiments.

Dear Milad,

Aquatic toxicology is actually moving more toward using early life stages of fish (i.e. embryo and eleutheroembryo). There are many benefits, including 1) quicker acclimation times of fish, 2) less oxygen demands, 3) no feeding required (as these live stages are feeding on their yolk sacs, 4) less volumes of water needed etc. I have posted a few papers (one from my research) that touch on the benefits for using ELS fish. Many governments promote using these life stages for toxicology research (e.g. EU). Cheers, Konstantine

There are many reasons for use juvenile:

1) It’s a critical life-stage for the survival and growth of a population

2) which  are more sensitive to toxic pollutant.

3) juvenile are more small and more practical for laboratory experiment with many individuals

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There are a number of good answers above as well as the reference to a question asked earlier. 

The sensitivity of the organism will change with each life stage.  Yolk-sac fry do not require feeding and expend less energy during this phase.  Once the yolk is exhausted the fry become more mobile in order to hunt for and consume food.  They enter a different energy phase and often exhibit greater sensitivity to toxicants.  Growth becomes exponential as energy (food) is converted to biomass.  Internal organs will also develop further as the organism grows. 

Pay attention to the life stage that is being exposed and differentiate among each regarding response and life stage.  

Smaller organisms facilitate handling because of lower space requirements, less water volume and total toxicant mass required (as above).  Smaller organisms also facilitate great replication of controls and exposure treatments which improves confidence in the results.  Smaller organisms can be more easily examined histologically for organ effects (gills, liver, others? ).  Growth (a very sensitive response) can be measured by image processing and is a sensitive parameter because it is such complex process.   

Do take care to feed small organisms frequently because they are growing so quickly ; another good reason to track growth throughout - if the controls are not growing the entire test system may be in a starvation condition at which point the effects of two stressors are being measured and the estimate of toxicant exposure will be overstated.