# Population self regulatory mechanism in fish



## mysiak (Jan 17, 2018)

I've stumbled upon a very interesting article which mentioned this work "Breder, C. M., Jr. & Coates, C. W. (1932): A preliminary study of population stability and sex ratio of Lebistes. Copeia, 1932(3): 147–155. "

It describes how livebearers (in this case guppies) self regulate their numbers to the "tank capacity". In their experiment the result was 9 adult guppies in 5.5 liter tanks. Even if fish had enough oxygen and food, their number did not increase.

The real life implication is interesting - most of the tanks will not overpopulate even with as prolific breeders as guppies are. Once the colony reaches the "tank capacity", fry and young fish either die prematurely or stop growing unless some of the adults are removed from the tank.

I would like to read the whole study, or at least pages dedicated to this topic, but I can't find it available anywhere. Is there anyone who could share it with me? 

Thanks!


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## mistergreen (Mar 3, 2007)

Only place I see it
https://www.jstor.org/stable/1436074?seq=1#page_scan_tab_contents


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## mistergreen (Mar 3, 2007)

Only place I see it
https://www.jstor.org/stable/1436074?seq=1#page_scan_tab_contents


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## mysiak (Jan 17, 2018)

Thanks a lot, I found it on sci hub by DOI (10.2307/1436074) from your link.

Edit: link added (I believe that scientific studies should be available for free to anyone), if it's against the forum rules, I'll remove it.


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## Michael (Jul 20, 2010)

Interesting, and fits my observations of my grossly crowded Endler and platy tanks, LOL.


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## mysiak (Jan 17, 2018)

Probably everyone heard about the usual recommendation to have 1 male to 2 females, but this is probably for the first time where it's mentioned that not only it's the usual sex ratio of fry, but any "random" population will stabilize at these numbers eventually. I always thought that it will be 1:1 if we won't interfere.

It's quite surprising that this 87 years old study isn't being mentioned among hobbyists at all. I would love to read the follow up (if there's been any) or replication study by other authors.


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## J.A. Marlow (Feb 7, 2018)

Very interesting study. Thanks for posting about it!


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## zahtar (Sep 29, 2019)

mysiak said:


> ... but this is probably for the first time where it's mentioned that not only it's the usual sex ratio of fry, but any "random" population will stabilize at these numbers eventually... I would love to read the follow up (if there's been any) or replication study by other authors.


Very interesting article and thanks for sharing! I saw on google scholar that this article has been cited several times. I didn't check those articles, but it could be a place to start.


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## dwalstad (Apr 14, 2006)

Google Scholar, yes!

Here are a couple citations to the 1932 article that look interesting. I don't yet have full papers.

'The effects of population density on fecundity and fertility in the guppy, Poecilia reticulata (Peters)'
BT Dahlgren - _Journal of Fish Biology_, July 1979, vol 15 (1) - Wiley Online Library
Experiments were performed to examine the influence of population density on the fecundity
and fertility in the guppy, Poecilia reticulata. The frequency of females with only ova or with
different combinations of reproductive units and developmental stages in the ovary was &#8230;

'Population control in guppies'
SM Rose - _American Midland Naturalist,_ 1959, vol 62, pp 475-481. - JSTOR
The production of guppies varies inversely with the concentration of adults. The number of
young eaten increases with density of the adult population. Under crowded conditions more
young guppies survive when white clouds are grown with them. Under uncrowded &#8230;


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## mysiak (Jan 17, 2018)

A bit late, but I finally got to reading the above mentioned articles, definitely worth a reading.

The effects of population density on fecundity and fertility in the guppy, Poecilia reticulata (Peters) https://sci-hub.tw/https://doi.org/10.1111/j.1095-8649.1979.tb03573.x

IMHO, the most interesting section: "The present results indicate that among females in the lowest population density and hence with reduced competition, the best reproductive strategy was to maximize the rate of natural increase (r-selection). At high population levels where a pronounced density effect and more intense competition exist, the best strategy would be to reduce the number of ova and to produce large progeny with superior competitive abilities (k-selection)."

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Population Control in Guppies https://sci-hub.tw/10.2307/2422539

" One observes in crowded and uncrowded aquaria that adults approach newly born young. This behavior seems to be a test for the young. If the young dart off on the approach of an adult they are not followed relentlessly. This is certainly true in the uncrowded, well-planted aquaria. In crowded aquaria adults, often in groups, seem to seek out the young, chase relentlessly and capture them. Possibly the initial cue for attack by the adults is a slower rate of response and motion by the young when guppy products are in high concentration when the young appear."

"The addition of unrelated species, even though this increases the need for food and oxygen, increases the survival of young guppies. It is not known how this is accomplished, but it is often found that unrelated species are mutually beneficial."


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## Michael (Jul 20, 2010)

Very interesting stuff! I will watch my Endler's and platies more closely to see if they show this behavior.


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## hoppycalif (Apr 7, 2005)

Michael said:


> Interesting, and fits my observations of my grossly crowded Endler and platy tanks, LOL.


My first aquarium was when I was 16, a 10 gallon tank, with guppies. I had about 20 guppies when I first set it up. That number topped 100 a few months later, and in a non-air conditioned house the summer heat caused all of them to die a few weeks later.


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## mistergreen (Mar 3, 2007)

hoppycalif said:


> My first aquarium was when I was 16, a 10 gallon tank, with guppies. I had about 20 guppies when I first set it up. That number topped 100 a few months later, and in a non-air conditioned house the summer heat caused all of them to die a few weeks later.


Same here. I had an under gravel filter. I suspect an ammonia spike instead of temperature though.


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## dwalstad (Apr 14, 2006)

With my guppies: 

if I keep a 1-2 breeding pairs (2-4 adults) in a 10 gal, the adults don't hurt the babies. Everything is very peaceful, even when I clean the tanks. 

In 10 gal tanks with 13-16 young adults, however, most babies don't last very long. When I'm cleaning the tanks, have removed the plants, and babies have no place to hide, I notice that the females go into a feeding frenzy and devour babies right before my eyes. They're vicious!


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