Pimephales promelas
Black-head minnow
(Also: Fathead minnow)

Fathead minnows are native to the Nearctic region. The northern limits of their geographic ranges extends from Quebec to Alberta and Northwest Territories, Canada. Their southern limites of their geographic range extends as far southward as Alabama, Texas, and New Mexico. Fathead minnows are most abundant in the Prairie Pothole Region of the northern Great Plains. Bait-bucket introductions have also occurred in the Mobile Bay drainage, Colorado River drainage, Alabama, Arizona, New Mexico, and Mexico. They are generally absent in mountainous regions as well as on the Atlantic Slope of Delaware River. They have also been introduced in Europe (Belgium, France, Germany, and the United Kingdom), Puerto Rico, and Iran. (Luna, et al., 2010; Page and Burr, 1991; Zimmer, et al., 2002)

In addition to small rivers and ponds,fFathead minnows are commonly found in muddy pools of headwaters and creeks. They also appear to tolerate habitat conditions that exclude many freshwater fishes such as high turbidity and temperature, variable pH and salinity, and low oxygen. Residing in such habitats may be important in decreasing risk of predation, as many predatory fish are intolerant of such conditions. (Nelson and Paetz, 1992; Page and Burr, 1991)

Fathead minnows are characterized by deep, compressed bodies, typically five to eight centimeters in length, and a short head that is dorsally flattened with a blunt snout, round lateral eyes, and terminal, upturned mouth. With the exception of a dark blotch at the rostral end of the dorsal fin, their fins are generally clear. Fathead minnows are dark-olive colored with a dusky, dorsal and lateral stripe, and yellow to white underbelly. They have an incomplete lateral line, 8 dorsal rays, 7 anal rays, 14 to 17 pectoral rays, 7 to 8 pelvic rays, pharyngeal teeth, and smaller scales along the nape. Males and females vary in size, banding, and secondary sexual characteristics, however, males and females are virtually indistinguishable until reproductive maturity. Adult males range from 3 to 5 g, while adult females are slightly smaller, ranging from 2 to 3 g. Additionally, males have dark heads with 2 white to gold vertical bars posterior to the head and dorsal fin. Males also have a fleshy, dorsal pad and 16 nuptial tubercles on the lower jaw. Although females do not undergo such obvious changes in morphology, they do develop a fleshy ovipositor approximately a month prior to spawning. (Ankley, et al., 2000; Page and Burr, 1991; Ross, 2001; Wisenden, et al., 2009)

After fertilization, fathead minnow eggs are approximately 1.4 mm to 1.6 mm in diameter. Following several well-defined embryonic stages, eggs hatch within 4 to 5 days at 25°C. Upon hatching, fathead minnow larvae absorb the yolk sac within 1 to 2 days, afterwhich larvae become active feeders preying upon live food. These protolarvae range in length from 4.0 mm to 5.2 mm and can be characterized by an incomplete mouth, dark eyes, rudimentary pectoral fins, melanophores, which are widely distributed on the yolk sac and concentrated in regions posterior to the vent. Contrastingly, mesolarvae and metalarvae possess high concentrations of melanophores on the ventral surface of gill covers. The number of myomeres between protolarvae and mesolarvae, and metalarvae differ only slightly, with the metalarvae possessing a slightly more in the predorsal region and a few less in the postanal region. All larval stages have rounded rather than flattened eyes, similar to 'bluntnose minnows Pimephales notatus'. (Ankley and Villeneuve, 2006; Ankley, et al., 2000; Ross, 2001)

Reproductive maturity in fathead minnows is identified by a number of morphological changes in both males in females. For example, males develop a dorsal pad, tubercles on their lower jaw, and banding changes. Just prior to maturation, females develop urogenital papillae. Fathead minnows reach sexual maturity within 4 to 5 months after hatching in optimal habitat conditions (e.g., water temperature of 25°C and photoperiod of 16 hours of light). Once mature and under appropriate seasonal conditions, minnows can spawn continually for a period of several months. (Ankley and Villeneuve, 2006; Ankley, et al., 2000; Ross, 2001)

Fathead minnows are polygynandrous and spawn between the months of May and September, producing anywhere from 1000 to 10000 offspring per season. During breeding season, reproductively mature males are responsible for the selection and preparation of nest sites on the underside of horizontal objects (e.g., rocks, wood, and vegetation). Nest sites typically occur in shallow water, typically on a sandy substrate. Preparing nest sites requires males to utilize their mouth and tubercles to form a depression in the substrate. Once the nest is complete, males become highly territorial. Agonistic behavior is directed toward other male conspecifics, heterospecific intruders, and initially, mature females. In addition, males perform a variety of courting behaviors to attract females to their site. If a mature female is persistent, and not interested in consuming eggs that may already be present in the male's nest, the male grants her access to the nest site. (Andrews and Flickinger, 1974; Ankley, et al., 2000; Ross, 2001; Wisenden, et al., 2009)

Spawning behavior in fathead minnows involves close lateral contact, body vibrations, and swimming back and forth in the nest area. Once adequate stimulation is obtained, males make contact with the urogenital region of the female, causing the release of eggs along with the simultaneous release of milt prior to their sudden separation. This occurs sporadically until the male aggressively drives away the female. All of the buoyant, adhesive, fertilized eggs are then deposited in a single layer on the ceiling of the nest site and thus, the male is left to care for the eggs on his own. (Andrews and Flickinger, 1974; Ankley, et al., 2000; Ross, 2001; Wisenden, et al., 2009)

Being oviparous and a fractional spawner (i.e., females spawn multiple times per breeding season), female fathead minnows may deposit 400 eggs per spawn, normally taking approximately 2 hours to perform each spawning sequence. Females may spawn between 16 and 26 times between May and September. Eggs are normally deposited at night, and each fertilized egg takes 4 to 5 days to hatch at 25°C, though it can take up to 13 days at cooler temperatures (15°C). Both sexes reach reproductive maturity between 4 and 5 months after fertilization. Fathead minnows grow rapidly, and despite high postspawning mortality, multiple generations may be alive at the same time. (Andrews and Flickinger, 1974; Ankley and Villeneuve, 2006; Ankley, et al., 2000; Herwig and Zimmer, 2007; Miller and Robinson, 1934; Ross, 2001; Wisenden, et al., 2009)

Male fathead minnows invest a great deal of time and energy into caring for fertilized eggs. To sustain energetic demands, males rely on somatic energy reserves. If these reserves are insufficient, male fathead minnows tend to consume a number of their eggs in order to meet energetic demands. In addition to creating the nest site, males fan nest eggs to maintain sufficient oxygenation, utilize their dorsal pad to clean eggs, and defend against predators until hatching occurs. Fathead minnows spawn numerous times a season, thus, male energy expenditure during this time is significant. In addition, females prefer to deposit eggs in the nest of males that already possess eggs, while alloparental care augments the likelihood of new fertilization. (Ankley, et al., 2000; Ross, 2001; Wisenden, et al., 2009)

On average, fathead minnows live two to three years in the wild and may be limited by high levels of postspawning mortality. Fathead minnows can live for up to 4 years in captivity. (Ross, 2001; Werner, 2004; Wisenden, et al., 2009)

Fathead minnows are strictly aquatic and stay within schools of familiar individuals, which allows them to act as a more cooperative, cohesive unit in comparison to schools with unfamiliar individuals. Chemical cues allow minnows to discriminate between familiar and unfamiliar individuals and may also play a significant role in courtship behaviors and the degree to which reproduction is successful. For instance, the dorsal pads and tubercles of reproductively mature males act as a female attractant. Males also release pheromones to attract females to nest sites during breeding season. Males are highly territorial of nest sites. Evidence suggests that fathead minnows may select habitats that reduce risk of predation. By constructing nests in areas that are low in dissolved oxygen, fathead minnows appear to be seeking out habitats with decreased abundances of potential predators. (Sloman, et al., 2006)

There is no information available regarding the average home range size of fathead minnows.

Fathead minnows utilize chemical cues to differentiate between familiar (i.e., shoalmates) and unfamiliar individuals. Olfactory signals vary in relation to diet, social status, parasite load, and predation risk. Chemical signaling can occur involuntarily due to the release of an alarm signal as a result of mechanical cellular damage, or voluntarily as in the release of sexual pheromones during courtship. During breeding season, males also perform both stationary and dynamic courtship displays in order to attract females to nest sites. (Chivers, et al., 1996; Ross, 2001; Sloman, et al., 2006)

Fathead minnows are opportunistic omnivores that can be characterized as benthic filter feeders, sifting through mud and silt in order to find food. Freshwater sediments often contain a large abundance of algae and protozoans, which represent a significant proportion of the fathead minnow diet. Secondary prey items include diatoms, filamentous algae, small crustaceans, and insect larvae. In addition, brook sticklebacks represent one of few species also common within the Prairie Pothole Region that also prey heavily on zooplankton, placing a high probability of interspecific competition between brook sticklebacks and fathead minnows. Differences in size-selection and feeding strategies, however, allow fathead minnows to consume a broader variety of zooplankton prey. Flexibility in feeding may explain why greater densities of fathead minnows exist in this region as compared to brook sticklebacks. (Ankley and Villeneuve, 2006; Herwig and Zimmer, 2007; Laurich, et al., 2003; Miller and Robinson, 1934; Ross, 2001)

Fathead minnows are commonly preyed upon by piscivorous fish such as northern pike, yellow perch, largemouth bass and walleye. An important antipredator tactic is their ability to warn conspecifics of potential threats via pheromones. From distinctive epidermal club cells, fathead minnows release an alarm substance in response to mechanical cell damage other species of the nearby threat. Such cues also have the potential to attract additional predators that could disrupt the predation event. If others can disrupt the predation event and facilitate escape, warning signals may benefit other species as well as wounded prey. Interestingly, mature males lose this capability during the breeding months. (Chivers, et al., 1996)

Secluded basins and harmful conditions in the Prairie Pothole Region result in a simple fish community, with fathead minnows and brook sticklebacks being the most common species in this region. Fathead minnows are often the dominant species and experience explosive population growth with the absence of piscivorus fish and their high reproductive rate. This allows fathead minnows to reach biomass estimates ranging from 144 to 482 kg/ha during the breeding season. With both high population numbers and biomass, it is not a surprise that fathead minnows possess significant influence on the aquatic macroinvertebrate populations, one of their primary food sources. Populations of zooplankton, aquatic insects, and ostracods are greatly affected by fathead minnow predation, with peak daily consumption ranging from 10.1 to 62.6 kg/ha. Thus, fathead minnows indirectly affect species dependent on aquatic invertebrates as food, such as ducks and their young, larval salamanders, and a number of passerine birds. Higher turbidity and phytoplankton biomass also result in wetlands with the presence of fathead minnows. However, the structure of these systems is strongly dependent on the presence and abundance of fathead minnows as a low- to middle-level consumer, whether it be due to direct or indirect effects. In addition, because they are tolerant of a broad range of environmental conditions, fathead minnows are common among in a large variety of aquatic habitats throughout its geographic range. Fathead minnows represent an important food source for piscivorous fishes, as they often the only species present in human-made retention ponds due to their habitat adaptability. (Stewart and Watkinson, 2004; Zimmer, et al., 2002)

Fathead minnows are significant contributors to global toxicology and behavioral ecology research due to their relatively short lifespan and high reproductive rate. Likewise, it has become the most widely utilized North American model for ecotoxicology since the mid-twentieth century. They also play a significant role in fisheries as prey for important commercial and recreational fish species. Albino fathead minnows, often referred to as "rosy-red minnows," are commonly used as bait for recreational fishing, and are also used as a feeder fish in bass-bream ponds. Albino fathead minnows are commonly found in aquaria, with both sexes possessing red-orange body and fins. (Ankley and Villeneuve, 2006; Page and Burr, 1991; Ross, 2001)

Although widely distributed across North America, introductions of fathead minnows as a bait species in Europe have resulted in devastating effects on the wildlife in northern Europe. Its introduction in Europe resulted in the co-introduction of enteric redmouth disease, an organism that negatively affects wild and cultured trout and eels. (Ankley, et al., 2000; "NatureServe Explorer: An online encyclopedia of life", 2010)

Fathead minnows are among the most prevalent fish species in eastern North America. The species is considered secure and has stable populations. The widespread nature of this species may be attributed to its ability to adapt to a variety of aquatic habitats and conditions as well as its high reproductive rate. ("NatureServe Explorer: An online encyclopedia of life", 2010; Page and Burr, 1991)