Saturday, June 18, 2016

Fathead Minnow - The Perfect Backyard Pond Fish

Backyard ponds are wonderful additions to wildlife gardens!  They are magnets for birds, who use them to bathe, drink, and cool down in the summer heat.  They provide habitat for amphibians, dragonflies, and other aquatic animals.  Not to mention the beauty and serenity they bring to a backyard getaway!

However, backyard ponds have a nasty problem - mosquitoes!  Even in ponds with fast-moving water, mosquito larvae will find small pockets of still water in which to thrive. Many pond-owners turn to chemical additives and Bti to control mosquitoes, which can also affect some beneficial insects, including craneflies, whose larvae feed on mosquitoes. Others turn to goldfish, which are effective mosquito predators when small and young, but inevitably grow large enough that they begin to feed on important native species, including dragonfly larvae, amphibian eggs, tadpoles, and smaller fish!

Goldfish are popular pond fish, but unfortunately prey on native wildlife, such as frogs, dragonflies, and other fish. 
Another fish species commonly used to control mosquitoes is the mosquitofish (Gambusia), so named to market it towards mosquito control.  However, most mosquitofish available in pond supply stores or pet shops are western mosquitofish (Gambusia affinis), which are not native to Maryland and have a detrimental effect on native fishes when they are accidentally transferred to natural streams and ponds.  Further, while they eat mosquito larvae, their preferred prey are other beneficial aquatic insects and other fishes' fry!

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The western mosquitofish unfortunately prefers other insects and fish fry over mosquitoes.
Image public domain via Wikimedia Commons
So what is a conscientious pond-owner to do? Many mosquito control options can actually harm native wildlife in some form or another! Fortunately, Maryland has a perfect, native solution - the fathead minnow (Pimephales promelas)!

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The star of this post, the fathead minnow!
Image public domain via Wikimedia Commons
Fathead minnows are native to most of the central United States, with the Appalachian Mountain range creating a natural barrier to their eastward spread.  They naturally occur in Maryland at the extreme western edge of the panhandle, before the start of the Chesapeake Bay drainage system. However, they are a popular bait fish, and have been introduced into freshwater areas across the state by anglers dumping their unused live bait.  NEVER release live bait into a native ecosystem, as introduced species can become invasive or carry disease to native fish populations.  Fortunately, fathead minnows set up breeding populations east of the mountains with little to no adverse effect on the native wildlife.  While every effort should be made NOT to introduce bait or pets to new habitats, the fathead minnow seems to be a relatively innocuous species, and accidental introductions have been fairly harmless, unlike those of goldfish and western mosquitofish.  I'd wager that if the mountains weren't in the way, the fathead minnow would have been a natural member of most of Maryland's freshwater ecosystems.

So we know that fathead minnows are a gentler, more wildlife-savvy option than most other pondfish, but their appeal doesn't stop there!  Many of their natural attributes make them well suited to backyard ponds.  Their popularity as bait fish and as feeder fish comes from the fact that they are readily consumed by a large number of predatory fish.  This means that their preferred habitats are pretty much anywhere the large predators are NOT. Because of this, they are not particularly picky about habitat conditions, and are tolerant of a wide range of water conditions that might be unfavorable by predators.  This includes shallow pools, turbid water, water with high acidity or pollutants, low oxygen, and quick and frequent changes in water quality. Anglers sometimes call fathead minnows "tuffies" due to their ability to "tough out" the experience of being kept in an unregulated bait bucket for the entire day.

While you should always strive to make a good, healthy home for your fish, the fatheads' tolerance for poor water conditions also makes them tolerant to human mistakes when regulating an artificial pool.  Algal blooms, drops in oxygen, pump and filtration problems, and other surprise pond problems are less likely to adversely affect fathead minnows than they are many more sensitive fish.

A fathead minnow hunts for mosquito larvae along the edge of a pond. 
Because they are often found in murky water, fathead minnows have many wonderful adaptations to help them thrive in poor visibility. Like most members of the carp/minnow family (Cyprinidae), fathead minnows have excellent hearing. A series of bones connects their swimbladder to their inner ear, amplifying sounds and making them exceptionally sensitive to vibrations in the water.

They also use a variety of chemical cues to communicate, such as releasing pheromones when breeding or alarmed.  Using their chemical scents, they are even able to discern individuals they have met before from strangers!  During breeding, males develop sensitive tubercles on their heads that help them sense females, rival males, and eggs, as well as select and maintain breeding sites.  The large heads of the males that accommodate these tubercles give rise to the species' common name of "fathead" minnow.

Like many true minnows (Leuciscinae), fathead minnows tend to be small (about 3in at the largest) and silvery in color.  Females and juveniles have silver sides, a pale belly, and a brownish back, with a dark line running the length of their sides. Mature males have large black heads and a dark body with one or two pale bands. Fathead minnows have short lifespans, living only about 1-3 years. Because of this, they are eager breeders, since many may only survive for one breeding season.

Fathead minnows begin to breed when water temperatures are above 60F (15C) and continue to reproduce throughout the warm months (typically May-September). Males who are ready to breed find and maintain nesting sites in small "caves" under horizontal surfaces, such as flat rocks, logs, and water lily leaves.  Pond-owners can create these nesting sites with landscaping rocks, cored cinder blocks and broken pottery.  Many locally-run garden centers will give away broken pots for free!  The Loading Dock in Baltimore City often has free ceramic tiles available on their lot, as well.

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A male fathead minnow, showing its large head and tubercles on its snout.
Image public domain via Wikimedia Commons
Once a male has set up his site, he vigorously defends it against all intruders - even females that do not seem ready to breed, as they may eat other females' eggs. Females "shop" for males and seem to prefer large males who already have eggs. Once a female has selected a male and is ready to lay, the pair remains in almost constant contact inside their little cave. When the female releases her eggs, they float upwards and stick to the roof of the nest site with a natural adhesive.  The female then leaves, but the male stays and dotes on the eggs, cleaning them, wafting them with fresh, oxygenated water, and defending them from predators.

Several females may lay in the male's nest, and as long as there are eggs to care for, the male will not leave them to eat.  He primarily uses his energy reserves to sustain himself during his parental duties, but if he comes close to starving, he will eat some of the eggs. The eggs typically hatch within a week, but because he may continue accepting females during this time, the male may be rearing broods non-stop during the breeding season.  Many males die towards the end of the season due to the extreme energy depletion involved with caring for the eggs.

Upon hatching, the minnow fry are extremely tiny, only about 1.5mm (0.06in) in length.  They spend about 1-2 days living off of the nutrients in their yolk sacs, then become active and eager little feeders!  Fathead minnows eat minute food - algae, bites of vegetation, and tiny invertebrates, like mosquito larva.  They rarely, if ever, feed on fish fry, and amphibian eggs and tadpoles are simply too large for them.  My fatheads are well-fed on the natural bounty of my pond, though I net a few out every week to judge body condition.  If your fathead minnows are thin or are not able to find enough food in your pond alone, they can be fed bloodworms, brine shrimp, small goldfish pellets, and even betta pellets.

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The "rosy red" or xanthic color phase of the fathead minnow, male in foreground and female in back
Image public domain via Wikimedia Commons
One downside to typical fathead minnows is that they can be difficult to see and enjoy, not only for their camouflaging coloring, but also because of their habit of lurking under rocks at the bottom of the pond when they are not hunting.  However, a selectively-bred color mutation helps correct this aesthetic problem.

These fish, called "rosy red minnows" have a simple mutation that makes them lack melanin.  They are essentially albinos, but because fish produce more pigments than simply melanin, they are not pure white.  Instead, lovely yellow and orange (xanthic) pigments are revealed, making the minnows bear a striking resemblance to small goldfish.  Because there is no dark pigment to mask the blood vessels beneath their skin, they are also rosy-pink in some areas of their body, such as around their gills and along their lateral line, hence the name "rosy red."

"Rosy red" is also the common name given to both dark and xanthic fathead minnows in pet shops, as it is sounds more attractive than "fathead."  The mutation is also known as "golden" or "pink" in other species.  Some fathead minnows are even "pied," exhibiting spots of normal dark color on an otherwise xanthic fish. If you like the look of goldfish, but not their size or voracious appetites, rosy red minnows are for you!

A rosy red fathead minnow does a convincing impression of a small goldfish. 
Fathead minnows are readily available from bait shops and pet stores, where they are sold as feeder fish for less than a quarter a piece. Since they are normally sold to be fed to larger animals, they often do not receive particularly attentive care and are frequently underfed or ill.  NEVER release fish from bait shops or pet stores into natural ponds or streams, as they can introduce illness to the native populations.  If you are adding newly purchased fish to an artificial pond that already contains fish, it is prudent to keep the new fish in a separate quarantine tank for 14-30 days while they receive preventative treatments.  Work with your vet to determine what treatments may be reasonable for your fish before adding them to your pond.

To learn more about fathead minnows, check out Robyn's Rosy Red and Fathead Minnow Page on fishpondinfo.com. It is an excellent site with a wealth of information on their care and rearing.

RESOURCES:

Montana Field Guide, "Fathead Minnow - Pimephales promelas"
http://fieldguide.mt.gov/speciesDetail.aspx?elcode=afcjb32020

Animal Diversity Web, University of Michigan
http://animaldiversity.org/

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