Fish Growth Inhibition

Photo: A 55-gallon vat with 144 adult male cichlids 5+” long, showing that our system allows crowding without growth inhibition. See below for the possible reasons.

Recently, a hobbyist messaged me:

“I’ve heard that fish release a hormone that will slow the growth of other similar fish. Do you agree or would you not be able to tell even though you raise so many close together without water changes because the total water amount in your system is so great?”

This was my brief response:

“There are reports in aquaculture of fish excreting substances that inhibit growth of other fish. I’ve not seen that happen in our systems. While we have lots of water, we also have lots of fish biomass. In addition to the fish in our vats, we have many feral fish living and breeding in the sumps and floor gutters. I suspect all the biologic activity from the plants and microorganisms in our systems have some impact. This subject requires more time and space than I have here, so you’ve nudged me into doing a blog about it. When I publish the blog, I’ll provide a link to it in a reply to you. Maybe a week?”

As I thought about his question, I realized the answer is holistic and, therefore, complicated. A full answer requires a description and analysis of our entire system and procedures. So, here goes! This will be stream-of-consciousness so bear (Or is it “bare?’ I always have to stop and think about that.) with me.

First, let’s consider our system, which it turns out is at present really two systems (although this winter I’m planning on connecting the two to make one). We have three greenhouses, but Greenhouse 2 and Greenhouse 3 share a common sump, making them one system. Greenhouse 1 has its own sump and is a separate system. For those willing to spend the time, Cory McElroy of Aquarium Co-op made an hour plus YouTube video about our systems (see https://www.youtube.com/watch?v=nE3zTisZ_c0). Now, I must admit that I’ve only watched the first 10 minutes of the video; I’m in it way too much for my tastes. That said, I’ve gotten comments and questions from around the world about the video. I suppose this winter when days are short and I can’t work outside or in the greenhouses as much, I’ll watch it all the way through. On a similar topic, I was just reminded that Goliad Farms™ has a YouTube channel (whatever that is). I suppose I should do something with it or find a victim to do it for me.

Okay, enough of that, back to the physical descriptions of our systems.

First, I’ll describe Greenhouse 1 and Greenhouse 2, which are each 3,000 square feet (~280 m²). They both have four-foot (~1.2 m) sidewalls of metal poles and wooden rails with two layers of aquaculture liner forming the walls. Their structures above that are pipe arches and purlins for support of a double layer of greenhouse clear plastic film. During the summer (about nine months of the year here) there is a layer of greenhouse shade cloth over the plastic film. For additional cooling, on hot days large fans force air into the greenhouses to force out hot air through vents at the top of the greenhouse ends. During the winter air is pumped between the layers of film to provide an insulating layer of air for heat retention. Propane powered heaters provide supplemental heat that is usually only required during long periods of low temperatures and cloudiness, usually around three to four nights a year. The rest of the time, the greenhouses gain sufficient solar heating during the day to keep water temperatures above 70°F (~21°C). The floors of the greenhouses are lined with pond liner and filled with water; we call these floor gutters. The water is about 6 inches (~15 cm) deep in the floor gutters. Both Greenhouse 1 and Greenhouse 2 have sumps. Each sump is located at the far (south) end away from our work area and door and is four feet (~1.2 m) deep. Above the sumps, on metal platforms (actually metal car ramps since they are cheap and readily available), are six above ground water pumps for each greenhouse. Each of the pumps moves about six to seven thousand gallons (2,300 to 2,700 L) of water an hour. They pump into 2-inch (~5 cm) PVC piping that delivers water from the sumps to each vat (more about the number of vats later). Each vat receives at least 50 gallons of water hourly. With that much water delivery, we found there is no need for aeration, so we have removed our air blowers and associated piping. Each vat is equipped with 3/4″ (~2 cm) overflows to return the water to the floor gutters.

Greenhouse 3 does not have a sump, but water in its floor gutters return by gravity to the Greenhouse 2 sump via four 2” (~5 cm) water lines (this will change this winter as we rebuild Greenhouse 3 and add 4” (~10 cm) return lines).

One other thing I should mention is our walkways. Between the rows of vats, we place 8” by 16” (20 cm by 40 cm) cinderblocks on their sides positioned so that the holes allow waterflow. This allows us humans to avoid wet feet as we work in the greenhouses. We once used wooden walkways supposedly water-proofed with marine paint. Did that work? NO! They rotted and fell apart. Concrete doesn’t do that and the cinder blocks are fungible since they aren’t cemented in.

Okay, let me describe the water flow through the systems. As said earlier, there are above ground water pumps above the sumps. These pumps have 2-inch (~5 cm) PVC intake lines that suck water from the sumps. The intake lines are in cylindrical filter cages made of aquaculture netting to prevent fish and debris from entering the pumps. By the way, the fish in the sumps and floor gutters are escapees and their descendants; we call them “feral fish.” The water then flows through 2-inch (~5 cm) PVC lines running above each row of vats. In Greenhouse 1 there are two water valves for each vat. Greenhouse 2 has only one. (Another winter-time project is replacing its water lines to provide two valves for each vat. This has been done for three rows of vats so far, with five rows to go.) Water flows into a vat, forcing surplus water to flow out through strainers that prevent fish from overflowing as well. The water then runs down the side of the vats into the floor gutter. By gravity, water flows through the floor gutter back to the sump where it is picked up again by the pumps. I just described a recirculating system. Did you notice there was no description of filtration or any form of water treatment (think ozone, UV, activation carbon)? Why? Because we use none, instead we rely upon plants, which I’ll talk about later.

Now let’s look at the vats. Currently, in the greenhouses we have about 700 vats. Some of these are out of service as we reconfigure Greenhouse 2 and Greenhouse 3. Some of the vats are outside and are used to grow plants, scuds and other livefoods, and some native and cold hardy fish. Greenhouse 1, used mainly for cichlid production, currently has 240 55-gallon vats, which were originally food-grade plastic 55-gallon barrels, and 28 300-gallon Rubbermaid livestock water troughs. Greenhouse 2 has 160 55-gallon vats, 23 110-gallon water troughs, and 30 300-gallon vats. Greenhouse 3 has an assortment of 45-gallon water troughs and 110-gallon vats. The rest of the vats are outside and include a number of 300-gallon vats, some 400-500-gallon and two 1,200-gallon fiberglass tanks, and many 45-gallon and 55-gallon vats.

So, we’ve covered the greenhouse structures and contents. Let’s discuss filtration (remember no filters!) and biological activities that might get to the hobbyist’s question about growth inhibiting hormones (you thought I’d forgotten about that, hadn’t you?).

Before we go further, let’s discuss the idea fish may excrete hormones or other substances that inhibit growth. There are reports of some food fishes excreting hormones that inhibit growth in other fish. If that happens, either the fish we raise don’t do it or something we are doing prevents it. Let’s assume it could happen. One obvious way to prevent it then would be via dilution, i.e., water changes. Can that be a factor in our system? Not likely. We don’t do water changes. We replace water lost due to our operations and water lost to evaporation, especially in the summer when fans are running. Water lost to operations could amount to maybe 200 gallons a day. Since our two systems contain over 75,000 gallons of water, this wouldn’t be much dilution. Evaporation, on the other hand, can be as high as 700 gallons a day for both systems when fans are running, so about one percent per day. But, since only pure water evaporates, there would be no loss of organic molecules such as hormones. So, dilution isn’t the answer.

Let’s digress here (I know! There has been more digression than content in this blog so far!) and discuss our water. We’ll start with our well water. We pump water from a well that takes water from the second water bearing sand. The first sand starts at a depth of 18 feet (~5.5 m), but we don’t use it. Although when I was a child (this farm has been in the family since 1870), we drank water pumped from that sand via a windmill. The water bearing sand we pump from starts at 75 feet (~23 m) deep and extends down at least another couple of hundred feet (~60 m) from there. Our well water is rock hard, has arsenic slightly over recommended drinking levels, comes up at 73°F (~23°C), and, prior to the gassing out of CO₂, has a pH of 7.0. Rock hard? It has 285 ppm calcium carbonate. If you let a faucet drip, you get a stalactite and a stalagmite. pH? Within minutes of coming up the pH goes up to about 8.3 as the carbon dioxide escapes. How does this water behave once it’s in our systems? Well, the temperature varies a lot with time of the year and time of the day. During the winter we endeavor to keep the morning low water temperature above 70°F (~21°C). Due to solar gain, evening temperatures are usually 5°F (~3°C) higher. During the summer, we try keep afternoon temperatures from reaching above 88°F (~31°C). Summertime morning temperatures are usually 5°F (~3°C) lower. pH year round varies from about 8.0 in the morning to 8.5 in the evening. Why? Because we have tons of plants and during the day, they convert CO₂ to food via photosynthesis, thereby producing oxygen and increasing the pH. At night they respire releasing, releasing CO₂ and lowering the pH.

One question I get when I give talks to fish clubs around the country (all cancelled this year due to the pandemic) is why doesn’t our water hardness increase as evaporation concentrates the calcium carbonate? Remember, only pure water evaporates. I also wondered about this, but our well water and system water both test at about 285 ppm calcium carbonate. What gives? Okay, remember the stalactites and stalagmites? We get sheets of calcium carbonate on the sides of the vats and anywhere that water evaporates. As much calcium carbonate precipitates out of the system water as comes in from the well.

So, we’ve determined that dilution doesn’t account of the lack of hormone induced growth inhibition in our fish. Our fish grow large and do so rapidly. Our African cichlids grow an inch a month during the nine summer months in south Texas. Could it be that our fish are serendipitously unaffected by growth inhibiting hormones? I guess so. But, even if they are affected, I think other factors would prevent that. Are we finally getting to my answer? We’ll see.

Let’s consider our plant filtration. We rely exclusively on plant filtration. How does this work? Plants remove nitrogen excreted in the form of ammonia in urine by the fish. Ammonia and its metabolites nitrate and nitrite are toxic to fish. Ammonia and nitrate being the most toxic. Ammonia is metabolized to nitrate by a group of bacteria called Nitrosomas. The nitrite is then metabolized to nitrate by Nitrobacter bacteria. Hobbyists use this cycle in aquaria to neutralize ammonia and nitrite. They then remove the excess, but not as toxic, nitrate via water changes. Plants, however, crave nitrogen as ammonia, nitrite, or nitrate. We’ve found a group of plants that are exceptionally good at this. These include submersed plants such as hornwort (Ceratophyllum demersum) and guppy grass (Najas guadalupensis) and emersed plants such as hoja santa (Piper auritum), Bacopa, red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), and house ivy (Pothos sp.). Our ammonia and nitrite levels, despite my very heavy hand at feeding (Susie, wife and tyrannical business manager, complains vociferously about our cost of food), stay at zero. The plants suck out nitrogen to grow their stems, leaves, and roots; all of which happens rapidly since we have ideal growing conditions including direct sunlight, warm temperatures, high humidity, and plenty of nitrogen from the fish. During the summer months when plants are growing very rapidly, wWe throw away 25-50 pounds (~10-20Kg) of hornwort daily to prevent it from crowding out the fish in the vats.

I imagine that in addition to removing nitrogen, our plants might also remove other organic molecules and could be, therefore, removing any growth inhibitors as well.

For more about plant filtration, here are a couple of blogs:

https://goliadfarms.com/plant-filtration/

https://goliadfarms.com/monster-plant-dieffenbachia-species/

In additional to plant filration, we have huge surface areas due to the floor gutters, side of vats, sides of the sumps, and the cinderblocks. These surfaces provide ample real estate for Nitrosomas and Nictrobacter. Also, while mulm (or “gunk” as we call it) doesn’t accumulate in the floor gutters and sumps (I guess from the actions of our feral fish), it does accumulate in the vats that don’t have huge fish populations. This gunk is loaded with Paramecium and other microscopic critters that might also metabolize organic molecules as well. By the way, we don’t have to culture Paramecium. We simply use a 10-inch brine shrimp net to scoop up gunk from the bottom of a vat, place it is a five-gallon bucket, fill the bucket with water, and wait a few hours. The Paramecium form a milky cloud in the water and we simply, when we need them, decant them and the water off feed tiny fry.

Okay, so my answer is:

1. Our fish don’t excrete growth inhibiting hormones, or
2. Our fish are immune to growth inhibiting hormones, or
3. Our system removes growth inhibiting hormones.

This seems like a good research project for someone.

Good fishkeeping!