Backyard Aquaponics
A place to discuss aquaponics

A guy that I know who did fisheries studies suggested to me that fish growth tends to be fast to a point and then slows down significantly as they get bigger. For example, a fish may double its size every few weeks for the first few months, but will slow and may only grow 10% a month after a point (I am just using these figures to explain where I am coming from).

Does anybody know anything about this in terms of fresh water fish, particularly the ones we are using. In aquaculture the idea is to identify the point at which it is most economical to harvest having regard to future expected growth rates.

I would be interested in knowing how long it might be before my 22cm silver perch are say 35cm and ready to eat. It might be bloody ages (though I will probably never see the day as I reckon they will die).

I was doing another fin count today and noticed that there were HUGE differences in the sizes of my barra... Some were just barely larger than when I got them 4 weeks ago, while others (about 4) were almost 3 times the size they were 4 weeks ago... SO I am not sure if it would work that easily, unless you take the average size from those inbetween and calculate it on them.

Highest growth rates would be a good indication.

Yes growth rates can vary a lot between fish. Generally there are two different harvest points VB, one is at plate size 350-500g, the other is fillet size about 750g .

When you have a large system with a lot of fish it works quite well that they have such different growth rates. It allows you to be pulling a few of the biggest ones out along the way, before then harvesting whats left once they have reached size.

I would suggest going for plate size, it's less time to wait...

My point may have been missed a little - I am not very good at explaimung myself, particularly in this type of medium .

What I am saying is that eventhough a fingerling may get to 200 or 300 grams relativelly quickly, the next 100, 200 or 300 grams gain may take a lot longer to achieve.

I note that in the book Joel talks about it taking 12 - 18 months to grow out a silver perch, yet often now we are starting to refer to periods closer to 6 months. Is this because we are going to eat our fish small, we are expecting optimal levels of growth due to some other factors, or because we are trying to kid ourselves (or more likely our partners) .

It can depend on so many different thing VB, water temperature, water quality, feed rated, % of protein in the feed thats being used, and the size of the fingerlings when bought. Yes, fish do slow down their growth as they mature, and unless your happy waiting for them to get really big, it's much more efficient to eat them younger at plate size, and depending on the conditions that your raising your fish in this can be achieved fairly quickly... I definately managed to grow my barra from 45g fingerlings to an average of around 400g in 6 months....

VB,
I am interested to know something along those lines as well. Some of my Jades have just doubled in size in the last two weeks, whereas some of the smaller ones do not appear to be growing at all.
I guess it does not matter much in the overall scheme of things.
Murray

Do you think that aquafarms or the DPI&F would know what these rates of growth would be? and how long it would perhaps take to get to plate size and to fillet size?

AM, I take comfort in Joel's experience in these matters, I know full well as soon as one of my Jades are plate size, the poor little fellow will be offered up to the great tummy god, then, may the next one achieve the same size as rapidly as possible.
Mojo

LOL
Best not let the barra I have know that growing at different rates is a good thing as it " spreads the food out" ... THat tummy god can be a demanding god!

There's lots of information about fish growth rates around, and the economies of when to harvest, can't remember any specific sources ATM...

i wonder if years of harvesting the largest fish will somehow spark an awareness in the fish that eating is bad only joking guys. LOL

I wonder if it wouldn't be like natural selection. Eat the biggun's and the littlies will get a better go at the food....

so we are doing the little ones a faviuor then
Aren't we kind!

Actually, from watching my indoor tank i think that monya's assumption is correct. the big silver and jade perch eat first, and chase away the smaller ones, then when they have had their fill the next size eats, and so on...........