September 07, 2017

A brief intro into optimizing your fish population in the botanical-style, blackwater aquarium...

Part one of a periodic series on "functionally modeling" our aquariums on natural ecological niches...

One of the things I love about creating a dynamic blackwater, botanical-style aquarium is how you can really "optimize" it on many levels- aesthetically, functionally, and biologically. And the most interesting part of the "biological" side is...how you populate it with fishes, right?

Yeah, for many of us...although I confess that I do have a certain fascination with adding worms and crustaceans and such (as do some of YOU, I know!), which we've talked about before and no doubt, will again.

And the neat thing about the leaf-litter-dominated habitats, like the seasonally-flooded igapos, which we love so much is that surprisingly, these habitats host a variety of fishes living within and upon the allochthonous input from the forest. Within submerged litter banks, a diverse group of fishes forming surprisingly dense local populations has been found by researchers. And the population of these areas is often sequential, depending upon the appearance of available food sources.

How can this information be utilized in our aquarium efforts?

Well, it all starts with food. Rather, the type of food and how it occurs and where it's found...

According to researchers, there is no significant "in situ" primary production of food in these litter beds, and the food web depends on allochthonous inputs, such as "fruits, blossoms, leaves and wood from the surrounding forest" (Fittkau et al.,1975; Sioli, 1975; Junk, 1983). At the base of the igapo food chain are the decomposing fungi, which work to decompose the leaves and botanical materials. These are fed upon by chironomids and other small organisms, which colonize the litter. Chironomids (yeah, like, Blood Worms!) are an important food for about many species of fish which inhabit the litter.

Because leaf litter banks in flooded forests in regions like The Amazon are known to have finite life spans (ultimately being changed and eventually washed away by currents of around 20 years or so), the animals within the leaf litter must be adaptable and capable of colonizing the habitat quickly.

It's thought by researchers that, during periods of high water, the resultant inundation produces new habitats, which are colonized rapidly by the fishes and their prey organisms. In fact, a lot of young fishes are found in the igapo vegetation when the waters rise, which suggests that, even though fish densities may be lower at that time, the fishes are adaptable and reproduce as new habitats become available.

What fishes are you likely to find in these newly inundated areas? Well, research suggests a fair amount of "the usual suspects", like Elachocharax or Characidium species ("Darter Tetras"), which are prime residents of these beds. And of course, other characins as well, drawn to the abundant food sources (the aforementioned Chironomids and such).

Quite simply, fishes "follow the food", and will populate and reproduce wherever the food is most easily accessible and abundant.

If we were really trying to recreate a naturally-occurring igapo-type system, it would make sense to initially stock the aquarium with small characins, like the Darters, Pyrrhulina, and Pencilfishes which seem "first to market" in these areas.

Later, other opportunistic fishes, such as Rivulus, will take advantage of the abundant insect life near the surface of these inundated areas of the forest, further diversifying the fish population.

And ultimately, you'll see the Apistogramma and other small cichlids move into the neighborhood, exploiting more available food as the leaf litter begins to break down and support more and more insect larvae and such.

And of course, as water levels remain higher, a wider variety of fishes, ranging from Amblydorus catfishes to Kinfefishes, start moving into the neighborhood.

And ultimately, you'll even find true piscivores, like the Amazon Leaf Fish (Monocirrhus), taking advantage of the abundance of small fishes which form the bulk of their diets.

Monocirrhus polyacanthus (image by Daiju Azuma, used under CC BY-SA 2.5)

The complexity of the fish population in these seasonally-inundated habitats evolves along with the food sources, forming an ever-changing habitat with a surprisingly dynamic diversity.

By studying the natural "habitation sequence" and the reasons why the population changes, we can create interesting and very dynamic stocking sequences for our aquariums that seek to replicate this amazing habitat! Cory Hopkins' amazing tank, which we've featured a lot lately, bears a striking resemblance to the recently-inundated igapo forest floor, and is a great example of an aquarium which can serve as an aesthetic and functional representation of this niche.

Managing the population of fishes along the lines discussed above in an aquarium such as Cory's would be a fascinating first step in better understanding the benefits of the concept of "fish stocking optimization!"

So much more to discover, define, and discuss on this subject!

Stay engaged. Stay excited. Stay observant. Stay creative.

And Stay Wet.

Scott Fellman

Tannin Aquatics