Backyard Aquaponics
A place to discuss aquaponics

Eating algae??? Eating each others poop?

i was looking at some fcr data a while back, and couldn't figure out how the fish could gain so much weight in comparison to the food it ate.. all the extra weight is simply from water - dry food, gets wet and absorbs moisture, thus getting more "food"

Trout are carnivorous, at least they're not primarily herbivorous; they aren't eating the algae in sufficient quantities to be identified as eating it. The measured FCR is not less than 1.0 (but there is a margin of error) so they're not putting on more weight than the quantity of feed added, but the averages so far are leaning towards a FCR much closer to 1.0 (again, with a margin of error) than the nominal, often cited FCR value of 1.2

Even if the fish eat each others' waste, transferring weight around between individuals doesn't affect the measurement of the totals.


During winter we get very few bugs around and the FT is 2/3 covered with shade-cloth which would be impervious to all but the smallest midges (only around here in summer). I don't think extra-curricular feeds are the answer here, otherwise we'd be looking at swarm proportions of insects.


The mass of the dry-feed going into the system is "known" and the water that feed is absorbing is from inside the system, so any "extra" weight from water absorption is irrelevant? If you started with a known weight of dry food, then wet it with extra water, then added it to the system you might have to account for some of the extra mass added, but not if the food is weighed and added whilst dry? (I'll have to think more about mass, densities, composition etc.)

FCR is a measure of the efficiency of an animal's digestive processes. FCR also changes during the lifecycle (for most animals) and can also change with environmental factors. Trout and many species of fish are actually quite efficient feeders with FCR in the order of 1.1-1.2, whereas some other animals such as chickens (pullets) have an FCR of about 2-3, and adult humans have an FCR of about 40-50 (depending on which "literature" you choose to follow; and allegedly varies wildly (very high standard deviation) depending on the individuals). With many fish having such low FCRs, any errors with input measurement will have a more significant impact on the results than for other animals with much higher FCRs. As I am only looking at a "small window" of the fish' life, the changing FCR with age and environment could affecting the numbers producing these "strange" results?

The most likely explanation for unaccounted weight stems from errors in measurement, which means the next time I buy a bag of feed, I am going to weigh it on the same scales used to weigh the fish (and assume the accuracy of the scales doesn't change! I'll use a standard weight for calibration purposes.)

As I said before, I'm not complaining that my fish are putting on almost as much as weight as I am feeding them, but it's in my nature to try to figure out why. Just as much as I believe in the KISS principle, I also believe in Occam's (or Ockham's depending on where you come from) Razor (or a variation of it), "The simplest explanation is usually the best." The answer might not be found with this current batch of fish, it might take two or three more seasons, but it's a good distraction to ponder over.



Scott

I've pulled a couple more of good sized trout from the system. Seems the monster I had the other week was oversized compared to the rest ( must have been a "glandular thing" ) Today's was a nice fish, 496g cleaned.

I'm going to buy a proper smoker soon as the wok on the BBQ doesn't get it just the way I would like. Any recommendations?

Bought a "deluxe" smoker, didn't come with instructions, so I just fumbled along. The old wok didn't smoke enough, the new one produces a fish so smokey it might have been a cigar! Next time I guess I will use only a single burner and less time than the "standard" 20 minutes.

It seems a post went astray in the recent forum machinations? Anyway...

It seems winter for me is officially over as all of the remaining winter veges have been harvested; Summer is yet to officially start as I still have some trout running. I have been taking some nice fish from my system, although I have now run out of food, so they are purging for the next couple of days (whether they like it or not!) We're having friends over on Saturday, so that'll be the last day for the fish; the Governor (SWMBO) has signed their warrants of execution.

Not much to brag about with these recent extractions; just healthy, tasty fish.

Looks very braggable to me , top job

800 + grams deserves a spot in the 2012 trout thread

Looks very braggable to me too.

That is a huge fish

In the recent extractions, the fish have been of good size and weight, 503-826g, with the exception of one which was barely 15cm and only 174g (the fish went into the tank ~25g on 27 March 2012) This fish is smaller than the first casualty of the 2012 season on 19 Jun which was 180g. Of course this was the runt of the tank, but to have such a small fish in amongst all those bigger fish is rather baffling? I have had two "extremes" in the fish this season, one over 1kg and one exceptionally small for their age when compared to their peers.

It turns out some of our friends don't like fish, so for recent soirées I didn't have to extract as many fish as planned.

Total feed provided to fish, not more than 8kg. Total weight of fish killed (so far, approximated where required): 9460g. I'm going to guess my three remaining fish are on the smaller side, 3 fish at maybe 500g is 1500g, totalling nearly 11kg in total fish weight, but of course, the fish harvested earlier didn't get the opportunity to eat the feed added after their demise, but I am reckoning the FCR still must be less than parity? We don't get that many bugs around here, so something seems awry; maybe I have a neighbour feeding my fish when I'm not looking?

[edit -- added chart]



It's a little too early to close the books on this season, with the three survivors still hanging in there but so far expenditure has been -->
water: not counting as I use the bore. I did have to top up the other day for the first time since last summer. Guestimate 2000L total.
fish purchase cost: $48.00 (for 21 of Gavin's trout, I think the price is close as I didn't actually write this down at the time!)
power: $37.50
feed: $88.00
which, so far, is over $8.20 per fish, or even more if we only count the harvested fish! If you had to count water, labour etc then the cost per fish is higher again. I haven't been as efficient as I could have been, but it does demonstrate it must be very difficult to run a backyard system as a profit-making commercial enterprise (note: I didn't say impossible)

You haven't accounted for the increased water content in the growth of the fish. The human body is about 70% water, and I assume that the fish are about the same. If the fish food was 100% converted to fish flesh, you would expect that additional mass to be about 30% of the total increased weight.

Consider the equivalent with humans. If your weight increased by 10kg, only 3kg would be from the food you consumed, 70% would be water content for the increased tissue.

The traditional method for computation of FCR is the weight of food which must be consumed to result in one unit of biomass increase, i.e. how many kilograms of feed have to be consumed for a fish to gain one kilogram in weight.

Depending on which research you read, published FCRs for rainbow trout are generally in a range of 1.2 to just under 2.0 (from memory) and none of those papers have subtracted the probable weight of water in that biomass. Some of these studies used dry feeds to obtain these values, other studies have compared wet and dry feeds (some even compared with live feed too), where on average the wet feed has a caloric mass of about one third that of similar dry feed, which results in FCRs of about 2.5 to over 5.0, or about three times the FCR of the dry fed fish. It's hard to compare all of the studies, because metabolic rates are dependent on a wide variety of variables such as temperature, stock density, pH etc. Still, the values I get in my backyard experiments vary wildly from those of these published papers. If you discount a 70% mass of water, the biomass gain over time in my system is about 2800g for 8000g of feed, so an FCR' of 2.8 for dry feed is just as disperse from those published studies.

Trout are all done now...

My system has no fish, and has no fish tank for the mean time too, I shall jury rig something to keep the plants going for a while until normality is restored. The last trout to be extracted were pretty good size and weight, 593, 680 and 567g.

Total harvest for 2012 trout season: 11.3kg from 21 fish added on 27 Mar (average including early deaths is 530g, average of the harvested fish 590g). Of course we had heaps of veges! Total expenditure etc didn't change much from the stats posted earlier, so no point in repeating them.

Well done!!

Yep, nice work and great fish

Two jumpers from the 25 trout added yesterday managed to find their way through a narrow gap between the edge of the FT and the cover. 23/25 after one day is not the start I was hoping for.