Authors - Andy J. Danylchuk, William M. Tonn, and Cynthia A. Paszkowski; University of Alberta, Department of Biological Sciences
Publication Date - October 2011
|Common Name:||Fathead Minnow|
|Other Common Names:||Northern fathead minnow, blackhead minnow, tuffy minnow, fathead, blue-headed chub, crappie minnow, rosy-red minnow (for red color morph only)|
|Scientific name:||Pimephales promelas|
|Wisconsin Synonyms:||Pimephales melanocephalus (Hoy 1883)|
Pimephales promelas promelas (Greene 1935; Hubbs and Lagler 1964)
|Etymology:||Pimephales - Greek, meaning fat head|
promelas - Greek, pro, meaning in front, and melas, meaning black
The fathead minnow is a member of the minnow family (Cyprinidae). It was originally described by Rafinesque (1820) from Ohio. The species is morphologically variable across its range, and until at least the 1960s three subspecies were commonly recognized, Pimephales promelas promelas, P. p. confertus, and P. p. harveyensis (Hubbs and Lagler 1964; Vandermeer 1966; Scott and Crossman 1973). The northern fathead minnow, Pimephales promelas promelas, was thought to be the subspecies present in Wisconsin (Greene 1935; Hubbs and Lagler 1964). However, Vandermeer (1966) compared meristic and morphometric characters throughout the presumed native range of the fathead minnow and concluded that the subspecies designations were unworkable and unwarranted. He found that the traits used to distinguish among subspecies varied substantially and non-concordantly within and among the ranges of the subspecies. Furthermore, because variation in each character was complex, identifying subspecies would make the taxonomy of the fathead minnow unnecessarily cumbersome and would oversimplify the true nature of the morphological variation within the species.
The evolutionary relationships and systematic position of the fathead minnow within the genus Pimephales are uncertain. Based on an analysis of morphological characters, Mayden (1987) proposed that the fathead minnow was most closely related to the bluntnose minnow. Alternatively, based on chromosome attributes, Li and Gold (1991) hypothesized that the fathead minnow was most closely related to the slim minnow (Pimephales tenellus), a species found in the south-central U.S. Using analyses of mitochondrial DNA, Schmidt et al. (1994) reported some support for a close relationship between the fathead minnow and the bluntnose minnow but little for a close relationship between the fathead minnow and the slim minnow. Conversely, using a different dataset of mitochondrial DNA, Bielawski et al. (2002) argued that the fathead minnow was most closely related to the slim minnow and less so to the bluntnose minnow.
The fathead minnow has a body that is slightly laterally compressed and a head that is slightly flattened dorsally.
The snout is blunt, especially in males. The mouth varies from terminal and strongly oblique to superior and almost vertical and extends back to below the anterior nostril.
The pharyngeal tooth formula is 0,4-4,0, and the teeth are slender with elongate cutting surfaces (Becker 1983). The origin of the dorsal fin is located directly above to slightly in advance of the origin of the pelvic fin. The dorsal fin has eight rays, and the first ray is noticeably shorter than the others.
The anal fin has seven rays, the pectoral fin usually has 15 (range 14-18), and the pelvic fin has eight. There are 41-54 scales in the lateral series; the lateral line itself is short and incomplete (Scott and Crossman 1973; Becker 1983; Nelson and Paetz 1991).
The fathead minnow exhibits strong sexual dimorphism.
During the breeding season, males develop a broader head, a dorsal pad of spongy tissue on the top of the head and nape
, and tubercles on the snout and lower jaw
(Wynne-Edwards 1932; Markus 1934). Females become rotund when bearing large quantities of eggs and develop a relatively pronounced urogenital papilla (ovipositor) at least one month prior to spawning (Flickinger 1969).
Adult fathead minnow are dark brown to dark olive dorsally, have slightly silvery sides, and a silvery-white ventral surface. The peritoneum is uniformly black and can often be seen through the belly.
Outside the breeding season, both males and females have a narrow dusky lateral stripe extending from the caudal peduncle to the head; this stripe becomes faint to absent in males during the breeding season. There is no dark outline on scales (Cross 1967); however, the anterior scale pockets above the lateral stripe may be edged in pigment whereas scale pockets below the lateral line are only slightly pigmented (Becker 1983). Hatchery-reared fathead minnow occasionally exhibit substantial variation in coloration, with a very small fraction being red, yellow, or white (Robison and Buchanan 1984). Selective breeding of reddish individuals has led to development of the bright "rosy red" form, widely sold for fishing bait and as an aquarium pet. During the reproductive season, males develop a dark to black head, a dark body with tan bars on the sides (sometimes called "bumblebee stripes"), and tan tubercles (Wynne-Edwards 1932; Markus 1934). Fins of males can become dark and an area of dark pigment forms on the anterior rays of the dorsal fin.
The fathead minnow is generally similar in appearance to many other species of minnows and can be difficult to identify. The fathead minnow and other members of the genus Pimephales along with the creek chub can be distinguished from all other Wisconsin minnows by the relative size and number of the pre-dorsal scales.
In Pimephales and creek chub, these scales are noticeably smaller and more numerous and crowded together than the scales on the sides. In other minnows, the pre-dorsal scales are similar in size, number, and crowding to the scales on the sides. The creek chub differs from Pimephales in having a relatively large mouth that extends to the middle of the eye (vs. no further than the anterior edge of the eye in Pimephales) and 8 anal fin rays (vs. 7). Within Pimephales, the fathead minnow differs from the bluntnose minnow and bullhead minnow in having a terminal and strongly oblique to superior and almost vertical mouth vs a terminal and slightly oblique to subterminal mouth in the other two species. For more details on the appearance and identification of the fathead minnow, see the website http://wiscfish.org.
The fathead minnow is a small species. The maximum adult sizes reported for fathead minnow populations range from 75 to 102 mm TL (McCarraher and Thomas 1968; Scott and Crossman 1973; Robison and Buchanan 1984; Nelson and Paetz 1991; Danylchuk 2003). Mean adult size varies from 50 to 60 mm TL among populations (Carlson 1967; Scott and Crossman 1973; Becker 1983; Sublette et al. 1990). It is commonly observed that males grow more rapidly and to a larger size than females (Andrews and Flickinger 1974; Becker 1983).
Growth to maturity is relatively fast in the fathead minnow, especially in warm, food-rich waters characteristic of southern portions of its range (Scott and Crossman 1973; Becker 1983). For instance, Markus (1934) found that fathead minnow under artificial conditions reached an adult size of 50 mm TL or more within two months of hatch. For natural populations in Wisconsin, Becker (1983) reported that young fathead minnow could reach 58 mm TL in 120 days, and in Elk Creek (Buffalo County), young of year collected 27 September 1975 were 26-48 mm TL. Although there has not been direct examination of growth rates in more northerly natural populations, fathead minnow reared in experimental ponds in Alberta, Canada, only grew to an average of 36 mm TL (range 18-42 mm TL) by the end of their first growing season (Grant and Tonn 2002).
The fathead minnow is a relatively short-lived species. Most reports state that fathead minnow rarely exceed age 3 (e.g., Brown 1971), but only a few studies have directly aged this species. In the Des Moines River, Iowa, nearly all fathead minnow were ages 1 or 2 based on scale ageing, with only one specimen (65 mm TL) classified as age 3 (Carlson 1967). In Wisconsin, fathead minnow collected from Sawyer Bay (Door Country) on 5 June and aged with scales were 45-60 mm TL at age 1 and 52-70 mm TL at age 2 (Becker 1983). Andrews (1971) noted a maximum average age of 2.5 years and a relatively small size at age for a population at higher elevations in Colorado, with the latter likely coinciding with a shorter growing season. Using otoliths for ageing, Danylchuk and Tonn (2006)found that age structure varied among four fathead minnow populations in north-central Alberta, Canada, with maximum ages ranging between 2 and 5 years. In these populations, size at age and growth rates were negatively associated with longevity.
The fathead minnow is one of the most widely distributed fishes in North America, occurring naturally between the Rocky Mountains and the Appalachians in the Mississippi, Hudson Bay, and Great Lakes basins and from Louisiana, Texas, and Chihuahua, Mexico, in the south to the Alberta-Northwest Territories border and northern Ontario in the north (Brown 1971; Scott and Crossman 1973; Lee and Shute 1980; Nelson and Paetz 1991).
It has been widely introduced outside of its native range, mostly accidently or intentionally via its widespread use as bait for angling, including many areas west of the Continental Divide and east of the Appalachians as well is in Belgium and France (Bell 1956; Sublette et al. 1990; Schade and Bonar 2005; Kottelat and Freyhof 2007). This species is one of the most common minnows in several upper mid-western states, Ontario, and Canadian Prairie Provinces (Bailey and Allum 1962; Brown 1971; Nelson and Paetz 1991) but normally is rare or absent in areas of higher elevation or in coastal drainages (Jenkins and Burkhead 1994). Globally and nationally, the fathead minnow is considered "secure" and is either "secure" or "apparently secure" in the states and provinces in which it is clearly native (NatureServe 2007).
Because of frequent introductions, however, it has been difficult to define the fathead minnow's original distribution in some areas (Etnier and Starnes 1993). For instance, Andrews (1971) reported an altitudinal range extension for the fathead minnow in Colorado, with a self-sustaining population in a mountain lake at 3,034 m elevation, although this population was likely introduced since fathead minnow is not found in any of the major drainages in the region.
Throughout its current distribution, the fathead minnow can be found in a variety of water bodies, including slow-moving shallow streams with emergent vegetation (Falke et al. 2010), semi-isolated pools along the shoreline and turbid backwaters of larger rivers (Starrett 1950; Carlson 1967), prairie pothole lakes and wetlands (Burnham and Peterka 1975; Zimmer et al. 2002), small productive and shallow lakes of the Canadian Boreal Plains (Robinson and Tonn 1989; Price et al. 1991; Tonn et al. 2003), small shallow stained lakes of the Great Lakes region (Scott and Crossman 1973), and many small reservoirs and dugouts throughout its natural and current range. However, fathead minnow are usually scarce or absent in larger and deeper clear-water lakes and the main channel of larger streams and rivers where predatory fishes are present (Tonn and Magnuson 1982; Rahel 1984; Matthews 1985; Lyons et al. 1988; Potthoff et al. 2008).
In Wisconsin, the fathead minnow is widely distributed, occurring in all three major drainage basins (Mississippi River, Lake Superior, and Lake Michigan) (Becker 1983).
Its conservation status within the state is "apparently secure" (NatureServe 2007). The species resides in a wide variety of Wisconsin water bodies, from boggy lakes, ponds, and streams in the north to small ponds and low-gradient streams and ditches in the south (Becker 1983).
The fathead minnow is a characteristic member of fish assemblages that inhabit low-gradient, warmwater small-stream environments in areas of Wisconsin with little topographic relief (Lyons 1989; Lyons 1996). Newall and Magnuson (1999) found that although fathead minnow was also mainly encountered in such stream environments in the St. Croix River drainage of northwestern Wisconsin, the species was not particularly common anywhere there. Although the fathead minnow has clear habitat preferences and is only able to persist in certain types of streams and lakes, because of its extensive use as bait, single individuals of the species may be encountered in nearly any body of water in the state.
The reproductive biology of the fathead minnow has been well studied. Individuals can mature rapidly, even within their first year (Markus 1934); however, maturity is more frequently reported at age 1 or age 2 (Carlson 1967; Becker 1983; Danylchuk 2003). Larger males nest earlier and are more likely to be aggressive than smaller males (Divino 2005). Spawning commences in mid-April to early June (Markus 1934; McCarraher and Thomas 1968; Brown 1971; Cooper 1983; Robison and Buchanan 1984; Danylchuk and Tonn 2006; Falke et al. 2010), with exact timing controlled by day length, temperature, and water level (Andrews and Flickinger 1974; Falke et al. 2010). Spawning begins when water temperature reaches approximately 15°C but can be inhibited if water temperatures exceed 30°C. In southern Wisconsin ponds, Thomsen and Hasler (1944) found that spawning commenced in late May and lasted until the middle of August. In the Madison area, spawning tends to peak in early July (Becker 1983). Reproductive activity can continue to early September (Andrews and Flickinger 1974).
The timing of reproductive activity in individual male fathead minnow is plastic and can be modulated by population structure, through its effects on social status. Specifically, small males will advance their reproductive condition, hold nests, and spawn earlier in the reproductive season when large males are absent or removed from a population (Danylchuk and Tonn 2001). Even where large males are present, their numbers decline naturally over the spawning season as they suffer high mortality due to the energetic costs and increased vulnerability to predation associated with nest defense and courtship; their replacement by smaller, later-maturing males can extend the spawning season by several weeks. This in turn can result in offspring from the same year differing in age by many weeks, and as a result, the earliest hatched offspring will have a correspondingly longer first growing season. This can affect, in turn, the size attained by young of year at the end of the season, their survival over the winter, and ultimately their age and abundance at first reproduction (Divino and Tonn 2007).
The spawning behavior of the fathead minnow is well documented. Males begin to develop secondary sexual characteristics approximately 30 days prior to spawning (Markus 1934), concurrent with the final stages of sperm development (Smith 1978) and early stages of nest defense (McMillan and Smith 1974). Prior to spawning, males search for, hold, and aggressively defend nesting sites that are usually associated with the underside of relatively flat substrates, such as lily pads, woody debris, or rocks (Wynne-Edwards 1932; McMillan and Smith 1974; Ming and Noakes 1984). Fathead minnow also successfully spawn on introduced substrates such as wooden boards, metal signs and cans, and ceramic tiles (Benoit and Carlson 1977), but fathead minnow use of such substrates can be dependent on their size, surface texture, and location within a water body (Ming and Noakes 1984; DeWitt 1993; but see Divino 2005). The number, availability, and quality of spawning substrates can determine the amount of spawning, modify spawning behaviors, and influence egg hatching success (Bessert et al. 2007; Divino and Tonn 2008; Wisenden et al. 2009).
Male fathead minnow are very active during nest defense and courtship of females. When defending nests, males hover below the nest site, touching, circling, and rubbing the surface with the spongy pad on the top of their heads (McMillan and Smith 1974). Males actively court females through a series of advances and retreats back to their nest. Once a female selects a mate, she will deposit buoyant sticky eggs on the underside of the substrate using her ovipositor (Flickinger 1969; McMillan and Smith 1974); egg deposition usually occurs at night (Andrews and Flickinger 1974).
Males continue to defend their nest after spawning is finished. Once eggs are present in the nest, males become extremely alert, circling the egg mass and boldly charging, butting, and biting potential egg predators (McMillan and Smith 1974), including other small-bodied fishes, crayfish, and some other macroinvertebrates. Cannibalism by other fathead minnow can be a major source of egg mortality, especially at high population densities when the availability of other foods may be limited (Vandenbos et al. 2006). In addition to courtship, males will often display aggression towards females, because non-breeding females may try to eat eggs and because some apparent "females" may actually be "sneaker" males mimicking females and trying to "sneak" fertilizations during spawning bouts between males and true females (Jenkins and Burkhead 1994). Recent genetic analyses suggest that this sneaking behavior is sometimes successful (Bessert et al. 2007).
Other predators can also influence fathead minnow reproductive activities. When fish predators large enough to eat the fathead minnow males are present, the males adjust their nest guarding activities to be less conspicuous and to take longer to return to their nests following disturbance (Jones and Paszkowski 1997). This response potentially increases the susceptibility of eggs to predation by smaller fish, crayfish, and other macroinvertebrates. In this way, the presence of large predators may indirectly suppress fathead minnow reproductive success. When crayfish are present, fathead minnow eggs hatch sooner, producing hatchlings of smaller length than if crayfish had been absent and hatching had taken longer (Kusch and Chivers 2004). By hatching sooner, the larval fathead minnows escape the threat of crayfish predation, but their smaller size may make them more vulnerable to predators of larval fish.
Males continue to guard and care for eggs until they hatch (Andrews and Flickinger 1974). Care includes agitating the water around the eggs to prevent fouling, displacing sediments, and removing waste material and fungus-infected eggs (McMillan and Smith 1974). Nests where paternal care was observed during monitoring in experimental ponds were larger, lasted longer, and more likely to produce hatchlings than nests where care-giving was not observed (Divino 2005). A single male may guard the eggs of several females that have deposited them in his nest at different times; Markus (1934) observed from 36 to 12,000 eggs per nest. Females are known to be fractional spawners (Andrews and Flickinger 1974; Gale and Buynak 1982; Danylchuk, unpublished data) and have been observed to lay from 80 to 370 eggs during single spawning bouts (Thomsen and Hasler 1944). Individual females almost certainly lay their eggs in the nests of multiple males (Bessert et al. 2007). Fecundity of age 1 females is reported at 1,000-10,000 eggs (Andrews and Flickinger 1974).
Guarding the eggs of several females may prolong the length of time a male remains on territory and could affect a male's ability to successfully defend his nest and care for his eggs. Indeed, an index of aggressive care was also associated with improved nest performance (Divino 2005). Successful males have been shown to maintain a stable body weight throughout the nesting period by continually replacing depleted energy stores with water, which, in turn, may allow them to deceive male intruders and avoid eviction from the nest (Unger 1983). Initially females spawn randomly with available males but then prefer to spawn with males that are already guarding eggs (Unger and Sargent 1988), likely because egg survival increases with increasing nest size (Sargent 1988; Divino 2005). As such, newly reproductive males will prefer to evict a parental male guarding a nest with eggs rather than initiate a new nest at an adjacent and physically identical location (Unger and Sargent 1988).
The larval development of the fathead minnow is described by Buynak and Mohr (1979) and Fuiman et al. (1983) Egg diameter is between 1.1 and 1.3 mm, and the time to hatch is from 4.5 to 7 days (Scott and Crossman 1973; Grant and Tonn 2002; Danylchuk and Tonn, unpublished data). Newly hatched larvae range in size from 4.75 to 5.2 mm TL (Markus 1934; Buynak and Mohr 1979; Grant and Tonn 2002), with transformation to early and late postlarval stages occurring at 5.6 mm and 13.6 mm, respectively (Buynak and Mohr 1979). Burnham and Peterka (1975) found that high salinities in prairie pothole lakes may reduce egg and larval survival.
The ubiquity of the fathead minnow is largely due to this species' ability to tolerate a wide range of environmental conditions (Scott and Crossman 1973). Considered a pioneer species, the fathead minnow is often cited as the first species to invade intermittent drainage channels after flooding, and one of the last species to disappear from small, muddy, isolated pools that remain in stream channels during drought (Cross 1967; Sublette et al. 1990).
With the exception of high acidity (pH < 6.0; Rahel and Magnuson 1983), the fathead minnow is able to survive a diversity of extreme water quality conditions. Fathead minnow are often found in shallow Great Plains ponds and wetlands, where they may tolerate highly alkaline conditions (2,000 mg/l CaCO3 equivalent) and salinities up to one third of full seawater (>10,000 mg/l; McCarraher and Thomas 1968; Scott and Crossman 1973; Burnham and Peterka 1975). They are also found in drying pools in New Mexico, where they tolerate high water temperatures and low dissolved oxygen levels (Sublette et al. 1990). Thermally, fathead minnow are considered a warmwater species in Wisconsin, with preferred water temperatures of 20.9-29.0°C and a maximum tolerance of 33.2-40.2°C (Lyons et al. 2009). Generally, adult fathead minnow can tolerate dissolved oxygen levels as low as 1 mg/l for extended periods, although reproduction is reduced at 2 mg/l and larval growth at 4 mg/l (Brungs 1971; Gee et al. 1978; Igram and Wares 1979; Wares and Igram 1979; Robb and Abrahams 2003).
Several studies have focused on the fathead minnow's ability to survive low oxygen levels during winter months in the northern part of its range in so-called "winterkill" lakes (e.g., Magnuson et al. 1989; Danylchuk and Tonn 2003). The fathead minnow possesses a number of traits that allows it to survive severe winter hypoxia (Gee et al. 1978; Klinger et al. 1982; Magnuson et al. 1985). The small size of the fathead minnow reduces the absolute amount of oxygen needed to support metabolic processes (Klinger et al. 1982). During progressive hypoxia, fathead minnow remain active and move upward in the water column, potentially allowing them to locate areas of higher dissolved oxygen, such as trapped air bubbles or oxygenated inlet and outlet streams (Gee et al. 1978; Klinger et al. 1982; Magnuson et al. 1985). As oxygen levels decline, fathead minnow increase the frequency of opercular movements, likely in an attempt to increase the volume of water and hence the total amount of dissolved oxygen flowing across gills (Gee et al. 1978; Klinger et al. 1982; Robb and Abrahams 2003). The fathead minnow is also physostomous with a direct connection between the swim bladder and the esophagus; however, there is no direct evidence that the swim bladder can be used as an accessory respiratory organ as is the case with some other physostomous species (Klinger et al. 1982). Likewise, although a few other minnow species, such as goldfish and crucian carp (Carassius carassius), have evolved the capacity to switch to anaerobic respiration when exposed to low oxygen levels (Marchand 1987; Holopainen et al. 1997), there is no evidence that the fathead minnow has developed such a novel metabolic pathway to contend with winter hypoxia (Shoubridge and Hochachka 1980; Klinger et al. 1982).
In Alberta, Ontario, and Wisconsin, a dichotomy exists among fish assemblages of small lakes. Small-bodied fishes, including fathead minnow, reside in relatively isolated water bodies prone to winter hypoxia, whereas large-bodied predatory fishes, such as northern pike reside in lakes that are either inherently less susceptible to this disturbance (Harvey 1981; Robinson and Tonn 1989) or where stream connections allow for seasonal migrations or post-winter recolonizations (Tonn and Magnuson 1982). Although fathead minnow can and do survive in water bodies less prone to winter oxygen depletion, larger piscivorous fish species generally restrict the abundance and even presence of fathead minnow in those lakes (Robinson and Tonn 1989; Jones and Paszkowski 1997; Duffy 1998). Thus, where not regularly introduced as forage (see Importance and Management below), the fathead minnow is most commonly found in water bodies devoid of predatory game fish (Brown 1971; Tonn et al. 2003).
Despite their apparent high susceptibility to large piscivorous fishes, fathead minnow, along with a number of fishes, have an adaptation to avoid predation that has attracted substantial scientific study. When the skin of a fathead minnow is broken, a specific chemical, termed an alarm substance or alarm pheromone, is released into the water. Fathead minnow can smell this chemical in the water at very low concentrations, and when they smell it they may engage in a variety of behaviors designed to reduce their vulnerability to a predator including moving to shelter, greatly reducing activity to become less conspicuous, swimming rapidly and randomly to avoid and confuse the predator, and forming tightly organized schools to reduce predator effectiveness (Lawrence and Smith 1989; Mathis and Smith 1993; Chivers and Smith 1994; Wisenden et al. 2003; Pollock et al. 2006; Carreau-Green et al. 2008; Wisenden et al. 2009). Thus, when a predator injures or consumes a fathead minnow, the event typically releases the alarm substance and triggers behavioral changes in other fathead minnow that should make them less likely to be eaten. The behavioral responses of fathead minnow to the alarm substance occur in larvae at young as 28-37 days post-hatch, indicating an instinctive component (Carreau-Green et al. 2008), but also tend to increase in intensity with size, age, and experience, indicating a learned component as well (Ferrari and Chivers 2006; Pollock et al. 2006). Other fish species also have alarm substances, and fathead minnow can learn to recognize and react to those substances (Pollock et al. 2003; Wisenden and Barbour 2005) as well as to the smell of the predator itself (Brown et al. 2001; Fincel et al. 2010). In the absence of additional predation events, the alarm substance disperses and breaks down within a few hours and normal fathead minnow behaviors resume (Wisenden et al 2009). However, if predation occurs frequently and alarm substance is constantly present, then the degree of the fathead minnow behavioral response may become reduced (Ferrari and Chivers 2006).
In waters that lack piscivorous predators, the fathead minnow can become an important component of and may even dominate the small-bodied fish assemblages of ponds and small lakes, especially in more species-poor northern areas (Robinson and Tonn 1989; Abrahams 1994; Danylchuk and Tonn 2003). In such ponds and lakes, the fathead minnow may outcompete other species, such as the brook stickleback (Abrahams 1996). Starrett (1950), however, suggested that the fathead minnow does not compete well with other minnows in stream environments, potentially related to the species' difficulty in feeding in turbulent waters (Landry et al. 1995).
Life history traits of the fathead minnow can be influenced by environmental factors such as water quality, predation, and competition. In laboratory settings, high concentrations of lead caused reduced nesting behaviors in male fathead minnow and reduced egg deposition by females (Weber 1993; Alados and Weber 1999). In experimental ponds, nutrient enrichment increased the number of eggs laid by fathead minnow and enhanced survival of age-0 fish, contributing to a high number of young of year at the end of the growing season relative to non-enriched treatments (Grant and Tonn 2002). Although in unenriched ponds growth of age-0 fathead minnow is typically reduced at high densities of fry (Vandenbos et al. 2006), young-of-year fish were larger in nutrient enriched treatments despite the higher densities (Grant and Tonn 2002). Larger young of year, in turn, had increased overwinter survival. Differences in predation pressure also influences the body size of fathead minnow, with larger individuals occurring in water bodies with fewer piscivores (Duffy 1998).
Life history traits of fathead minnow can also be related to the incidence of natural disturbance (Danylchuk 2003). In the boreal region of western Canada, fathead minnow in lakes more prone to frequent and/or severe winterkills were shorter lived, grew faster, allocated a greater proportion of their body mass to gonad development, and tended to mature earlier compared to fathead minnow in lakes with less winterkill (Danylchuk and Tonn 2006). In addition, spawning activity tended to begin earlier in the year in lakes with more frequent or severe winterkills. These trends are consistent with predictions for organisms in variable, unpredictable environments and, given that fathead minnow are tolerant to a wide range of environmental conditions, suggest that variation in life history traits among populations is likely a product of both natural selection and plasticity in the expression of traits (Danylchuk 2003).
Parasite load in the fathead minnow can also affect life history traits, including growth and survival (Lemly and Esch 1984). Parasites of the fathead minnow include Protozoa, Trematoda, Cestoda, Nematoda, and Crustacea (Scott and Crossman 1973). The infestation of fathead minnow by parasites can be high even when other associated fish species are only lightly infested (Scott and Crossman 1973). In fathead minnow collected from four pristine lakes in north-central Alberta, the trematode Ornithodiplostomum ptychocheilus was the most common and abundant of 14 parasites infecting fathead minnow but was absent in the other fish species that occurred in the lakes (finescale dace and brook stickleback) (Sandland, 1999). McCarraher and Thomas (1968) noted heavy infestation of cestodes in alkaline lakes in Nebraska, with over 80% of fathead minnow infested with Ligula intestinalis. For some trematode parasites, intensity of infestation in fathead minnow can be partially attributed to variation in host size, as well as to factors that influence the transmission of earlier life stages of the parasite such as water depth, water temperature, and densities of snails and birds (Sandland et al. 2001).
The fathead minnow is an omnivore, readily eating both plant and animal material. The diet of the fathead minnow, which includes combinations of invertebrates, algae, and detritus, provides flexibility in its choice of foods. Invertebrate prey include rotifers, cladocerans, copepods, amphipods, ostracods, and chironomid and ceratopogonid insect larvae (Held and Peterka 1974; Price et al. 1991; Duffy 1998). Price et al. (1991) found size- and gender-related differences in the types of invertebrate prey consumed prior to the breeding season, likely related to differences in habitat use and activity levels. Duffy (1998) estimated that invertebrate prey consumption can be high, approaching or exceeding estimates of invertebrate production in prairie wetlands. Landry et al. (1995) found that turbulence in the water column can influence ingestion rate of invertebrate prey by fathead minnow larvae, suggesting that the diet of this species may be partially related to the physical characteristics of the water body in which it resides. Detritus can contribute up to 93% of the diet of fathead minnow (Litvak and Hansell 1990). Although detritus in diets is typically viewed as low quality food used primarily when higher quality food is scarce, fathead minnow whose invertebrate diet was experimentally supplemented with detritus showed greater growth compared to fish fed invertebrates alone (Lemke and Bowen 1998). Fathead minnow have a long digestive tract that likely contributes to efficient processing of detritus and thereby permits the extensive use of this poor but abundant food source (Gerking 1994).
Food consumption in fathead minnow can be affected by the risk of predation and interspecific competition among other members of small-bodied fish assemblages. In a laboratory study, Abrahams (1994) found that in the absence of yellow perch, fathead minnow consumed more food than brook stickleback, suggesting that the fathead minnow has a competitive advantage over other small-bodied fishes when the risk of predation is low. However, feeding rates of fathead minnow declined in the presence of yellow perch (brook sticklebacks were unaffected) such that the rates of the two species were similar. Danylchuk, Tonn, and Paszkowski (unpublished data) examined the food habits of co-occurring populations of fathead minnow, brook stickleback, and finescale dace in small boreal forest lakes in Canada and found a quantitative partitioning of resources. Fathead minnow focused more on detritus, brook sticklebacks on smaller invertebrates, and finescale dace on larger invertebrates. This, in addition to observed seasonal changes in resource partitioning, suggests that the diet of species in small-bodied fish assemblages may be partially attributed to interspecific competition (Danylchuk, Tonn, and Paszkowski, unpublished data). Overlap in diet can also occur between fathead minnow and juvenile waterfowl, indicating another source of competition that may influence feeding patterns of fathead minnow (Duffy 1998).
The fathead minnow is most often encountered with other small-bodied fish species (Scott and Crossman 1973; Nelson and Paetz 1991). In southwestern Wisconsin streams, the fathead minnow is closely associated with the brook stickleback and southern redbelly dace (Lyons et al. 1988). In the northeastern United States, fathead minnow are often found with species such as bluntnose minnow, eastern blacknose dace (Rhinichthys atratulus), common shiner, central mudminnow, white sucker, brown bullhead, and black bullhead (Cooper 1983). In the northern portions of its range, the fathead minnow most commonly co-occurs with brook stickleback, ninespine stickleback, finescale dace, northern redbelly dace, and pearl dace (Harvey 1981; Tonn and Magnuson 1982; Robinson and Tonn 1989; Danylchuk et al., unpublished data).
Although small in size, the fathead minnow has major importance to people. Ecologically, the fathead minnow can play a key role in the structure and function of aquatic ecosystems, especially where it occurs in high abundance (Scott and Crossman 1973). For example, Zimmer et al. (2001, 2002) found that the presence of fathead minnow in prairie pothole wetlands increased turbidity, total phosphorus, and chlorophyll a and decreased the abundance of some macroinvertebrates and salamanders. Eaton et al. (2005) found that the abundance of young-of-year wood frogs increased dramatically following winterkill of small-bodied fishes, including fathead minnow, in the frog's breeding lakes in Alberta. Correspondingly, the presence of fathead minnow caused nearly complete mortality of wood frog tadpoles in experimental ponds (Eaton and Paszkowski, unpublished data). Given that wood frogs recruit from lakes to riparian and upland areas where they become part of the boreal forest food web, variability in the density of fathead minnow could affect terrestrial systems as well. Fathead minnow can also act as forage for piscivorous birds (Gingras and Paszkowski 1999; Paszkowski et al. 2004), potentially contributing to higher trophic levels; however, few studies have assessed the ecological importance of the fathead minnow beyond the boundaries of aquatic systems (e.g., Janowicz 1999; Zimmer et al. 2001, 2002).
The fathead minnow is important as both a natural prey of and bait for game fish, such as largemouth bass and walleye (Cross 1967; Becker 1983). In Wisconsin in the early 1940s, the intensive collection of fathead minnow from natural systems to be used as live bait began to put pressure on minnow populations and also resulted in the bycatch of young-of-year game fish (Thomsen and Hasler 1944). As a result, a call was made for "everyone who fishes to realize that minnows are game fish food and that they are, therefore, the foundation upon which fishing is built." Thomsen and Hasler (1944) and other authors emphasized the need for artificially propagating fathead minnow (e.g., Williamson 1944). In the decades that followed, the commercial production of fathead minnow expanded rapidly to meet the demands of a lucrative recreational fishing industry (Bailey and Allum 1962; Davis 1993).
Often called 'tuffy' by minnow dealers because of its ability to withstand extensive transport and bait bucket conditions, the fathead minnow has become one of the most valuable baitfish in North America (Williamson 1944; Brown 1971; Davis 1993; Etnier and Starnes 1993; Jenkins and Burkhead 1994; Meronek et al. 1997). The capability of the fathead minnow to mature quickly, its willingness to use artificial substrates for spawning, and its short incubation period have all contributed to the fathead minnow's extensive use in bait aquaculture (Williamson 1944; Benoit and Carlson 1977; Davis 1993). For example, in 1982 the fathead minnow ranked just behind the golden shiner in importance to the aquaculture industry in Arkansas, with over 400,000 kg being produced; this production was valued at nearly two million dollars wholesale value (Robison and Buchanan 1984). In a 1992 survey of the bait industry in six states in the north-central United States, including Wisconsin, the fathead minnow was the most common bait minnow in Wisconsin, with nearly 100,000 gallons of fathead minnow sold with a minimum retail value of over twenty-six million dollars (Meronek et al. 1997). Most fathead minnow sold in Wisconsin were produced in ponds or tanks at fish farms within the state, but some were caught from the wild and others were imported from farms in other states such as Arkansas. The Wisconsin Department of Natural Resources produces and also purchases large volumes of fathead minnow each year to feed gamefish in state fish hatcheries (Al Kass, Wisconsin DNR, personal communication, 2008).
Fathead minnow use as bait has led to the establishment of fathead minnow into numerous water bodies in which they are not native (Bell 1956; Cooper 1983; Robison and Buchanan 1984; Sublette et al. 1990; Schade and Bonar 2005). Non-native fathead minnow populations have the potential to alter the balance of natural aquatic ecosystems (Goodchild 1999; Zimmer et al. 2001, 2002), and in some cases have reduced the abundance of native fishes (e.g., Markle and Dunsmoor 2007).
The ease with which fathead minnow can be cultured and maintained has led to its widespread use in aquatic toxicology and other types of laboratory evaluations. Numerous studies assessing fathead minnow responses to various chemicals or waste water effluents are conducted each year throughout the United States (e.g., Denny 1988; Weber 1993; Brazner and Kline 1990; Alados and Weber 1999; Siwik et al. 2000; Weber and Bannerman 2004). In Wisconsin, the fathead minnow is one of a handful of standard aquatic species used by the State Laboratory of Hygiene in bioassays to monitor the quality of surface and waste waters (Amy Mager, State Laboratory of Hygiene, personal communication, 2009).
The fathead minnow also has other human uses. For example, fathead minnow have been stocked into sloughs, ponds, and ditches for mosquito control, and in Madison, fathead minnow were found to be effective in reducing mosquito densities in storm water drainage channels and ponds, including mosquito species implicated in the spread of West Nile virus (Irwin and Paskewitz 2009). Fathead minnow have also been used in sewage treatment ponds to convert high concentrations of nutrient and plant material into usable biomass (Becker 1983). They have also been propagated for the aquarium trade, with the development of an ornamental red color morph known as the "rosy red" (Robison and Buchanan 1984), which is also sold for bait.