Backyard Aquaponics
A place to discuss aquaponics

Yea I was referring to profit and I know sea cage farming affects the local environment, although once the sea cage is relocated to a new area, the old location is supposed to go back to its original state after a year or two?

The other thing to think about is the question do we care about the efficiency of the bacteria?

In RAS this may be a big problem where biofilters are designed to be as small as possible but in systems that contain GBs it is probably not going to be an issue. Given that the surface area of a system incorporating GBs is likely to be at least an order of magnitude more than required (in terms of a biofilter not in terms of a solids filter) you shouldnt have a problem even if efficiency drops to 10% as long as it doesn't do so suddenly.

That's the commonly held "view"... applicable to nitrobacter....

But even the article quoted notes that nitrification in water treatment plants works effectively well outside the 7.5-7.8 "optimal" range.... all the way down to 6.0....

Which never made any sense....

Until recently.... when it was shown that it is nitrospira, not nitrobacter... that nitrifies...

Your own experience... and that of many, many others confirms that the former view is basically invalid...





No ... stocking density is a function of feed rates, filtration and oxygenation.... regardless of pH...

Unless your pH is so far beyond either limits that your bio-filtration is totally compromised...

The arguement was originally about whether or not certain pH was "optimal" for fish growth... somehow it became accepted that this happened to occur at the same pH level.. as optimal bacterial bio-filtration ...

Sadly not true.... because most people have accepted the premise of a 1:1 ratio.... which puts their filtration capacity at minimal levels....

Add a doubling of stocking density at the same time... as has become the norm for some...

And a 10% loss of filtration efficiency... could be critical...

Unfortunately not Stuart. The waste dropping from the seacage to the seafloor turns the benthic habitat anoxic, so nothing will grow there. Also this has rollover affect onto other populations ( ie plants won't grow to produce feed shelter for small fish and invertebrates etc etc)

It is my understanding that the functions of solids filter and bio-filter are quite different however in GB based systems the two functions are conducted by the one component, ie the GBs. In a fully stocked system the bacteria of the biofilter will be eating all their food (amonium/a) so if the conditions change such that their metabolism is slowed then their will be left over "food". THen the bacteria that are better adapted to the new conditions ie cooler temperature, lower pH, whatever, will have more food available to them and their frequency within the system will begin to increase. As long as the change is not too quick then the bacterial population as a whole will adapt to the changed/changing conditions. A similar change would be expected to occur amongst the bacterial populations that process the solid fraction of the waste.

I dont understand the connection though between any of my posts and the possibility of anoxic conditions due to the build up wastes beneath cage aquaculture systems.

Sorry I mention to quote thi from aquastud, Fat fingers and iPhone keypads don't combine well

Really interesting stuff, some food for thought

Far too general. Can you specify the actual density you are talking about, it might help?

For example; What are the pros and cons of running trout at 80kg/m3 verses 30kg/m3?

Is this in commercial aquaculture using RAS technology or in commercial aquaponics using gravel grow beds to provide all the filtration?

If it is the later, you may have some difficulty with oxygen demands.

If you were to look at liquid oxygen, you would be able to supply the tanks with inlet oxygen for high densities but the outlet of the fish tank will be no less than say 4 or 5mg/L no worries. The return water from the gravel beds will be require large amounts of oxygen before returning the water to the fish tank at 18mg/L to 20mg/L. I think it will run your costs per kg of fish over the top.

You could punch higher on the fish tank oxygen (not much), however you may find the grow beds stripping any available oxygen from the water quickly which may mean you are at anaerobic conditions in the grow beds. Not sure how that will benefit the plants or nitrification. You will also be topping up from a lower oxygen point on return.

You may need higher turnover rates of the tank (more pumping) to achieve the oxygen saturation required for the entire system.

This is not to say that it will not work. It may be very difficult to make it viable using gravel beds as your filters, simple because they become a very large oxygen consumer.

I work with pure oxygen ras systems (just finished building a small 20 tonne unit) and know the cost to produce per kg of fish using RAS technology or at least what costs you are aiming for. There will no doubt be a trade off on oxygen cost per kg if you supplying enough oxygen for the additional demand of the grow beds and the high density of the fish.

I think the oxygen demand of the gravel beds one of the cons in that scenario. It is because of that, all media based systems can generally only support low volumes of fish...

Nice to see you giving some concise input on the subject Earthan...

It's exactly what the "commercialisation" threads need...

Thanks John but not really what I call concise. Just a little more thought around some of the ideas floating out there wanting to be commercial in media based systems. Not once have I seen the oxygen demand per cubic meter of grow bed media loaded with fish waste mentioned.

I hear you....

But are you transposing oxygen demand figures based on waste treatment systems.... and can they be directly translated to media based aquaponics....

Personally... I think it's an area that requires some specific research....

I am not transposing anything at this point because in my opinion, media based systems belong in the backyard.

I am sure you can build a commercial media system, however that oxygen demand from the grow beds will need to be taken into account. If not you will head for a large failure or your fish densities will be very very low (in australia) and the fish feed may not be a viable source of organic nutrient.

You never know, I could be completely wrong.

Again, I hear you... and tend to agree...

And that raises a pertinent question.... given that (to date)... the profit appears to come from plant production... does the concept of low stocking densities and fish production... substantially alter the commercial viability...

Or is it merely a matter of a shift in mindset....

In Australia yes. It is very easy to step into the red in aquaculture given the current profit margins. If you are too small, the economies of scale will not work. This is to say, if you build an aquaculture system with low densities, the size and operational cost of it is multiplied significantly. If you then have low densities, you will use more electricity, more water, same labour (less fish), longer return on investment etc.

While the plant provide an income, reasonably quickly, lower densities of fish = less nutrient = less plants and so the potential reach of the plant systems is reduced by a factor of 3 to 4.