EFFECTS OF PHOTOPERIOD ON THE REPRODUCTIVE
SUCCESS OF FATHEAD MINNOWS (Pimephales promelas)
JOHN BUCHANAN CLAYTON, B.S.
The length and sequence of day and night cycles was manipulated to determine the
effects of photoperiod on reproductive success of fathead minnows, Pimephales
Fathead minnows appear to
require a long photoperiod of at least 14 hours of light, placing them into the category
of long-day animals. Exposures to skeleton photoperiods show the possibility of a
"daily" rhythm of sensitivity of light. Through this sensitivity rhythm, fathead minnows
are most likely able to synchronize their reproduction with favorable environmental
conditions. As interpreted day length grew longer, overall egg production increased.
Results indicate that the longer the "day" the higher the reproductive success,
especially if there is some period of darkness associated with it.
Fish, like all animals, reproduce to maintain survival of the species. They must
not only reproduce they must reproduce when maximum reproductive success is
possible. By monitoring various environmental factors, such as temperature,
water quahty, and photoperiod, organisms are able to predict when condhions are
favorable for maximum reproductive success
. In temperate and high latitudes, where
favorable conditions will generally occur only once a year, reproduction occurs in
response to seasonal cycles in almost all plants and animals
such as amphibians, mammals ,birds, reptiles and
aquatic organisms .
Seasonal reproduction is influenced by abiotoic factors such as temperatures,
water quahty, and photoperiod and biotic factors such as food availabilhy, predators,
and pathogens being at optimum condhions . These biotic and abiotic factors define the environment for
most fish and do not remain at the appropriate levels for maximum success throughout
the year. Instead, biotic and abiotic factors commonly vary in diurnal, lunar, and annual astronomical cycles.
Fathead minnow biology
The fathead minnow is a small, typically riverine species that is dark, olive-green
or brown on its dorsal surface, with silvery or brassy reflections on hs sides, and
silvery white on hs ventral surface. Aduhs generaUy range from 3.0 to 9.0 cm in
length. They have a sharply triangular head and shallowly forked caudal fin. There is
one anal fin containing 7 rays, two pelvic fins each with 8 rays, and two pectoral fins
with each usually containing 15 or sometimes as many as 17 rays; less than 15 or more
than 17 rays in the pectoral fin is rare (Scott and Crossman 1990). Their diet consists
of algae, organic detritus, aquatic insect larvae, and zooplankton . In warm, food-rich waters, growth is rapid, and in all populations the males
grow faster and larger than the females . The fathead
minnow typically has a life span of slightly longer than 1 year and rarely lives beyond
Fathead minnows have a very widespread distribution through most of central
North America, from the Rocky Mountains to the Appalachian Mountains and from
northern Mexico to northern Canada. The fathead minnow predominates in silty pools of intermittent streams where
it tolerates high temperatures and low levels of dissolved oxygen , but it exists in a wide variety of habitats . It
is also able to tolerate a wide range of pH (Mount 1973). It exists in muddy dhches,
warm brooks, stained acid waters of ponds and small lakes, alkaline lakes, and saline
lakes in excess of 10,000 mg/L salinity. Fathead minnows
will generally be the most abundant species present when in its preferred habitat.
The fathead minnow derives hs common name from the physical appearance of
the breeding male. As a secondary sexual characteristic, the male will develop an
elongated fleshy or spongy pad that extends from the nape of the neck to the origin of
the dorsal fin. This dorsal pad gives
the appearance of an enlarged or "faf head. Other secondary sexual characteristics of
the male include well-developed tubercles on the snout, alternating light and dark
banding on the side and increased aggression . Breeding males
have even been known to attack the hand of a person that gets too close to the nest site.
Spawning of fathead minnows is prolonged and may last from spring to late
summer; it will generally begin when water temperatures reach higher than 15 C. Species that have prolonged spawning seasons are
generally multiple spawners, producing several clutches of eggs during the spawning
season (fractional spawning) . By utilizing this type of
spawning, fish can minimize the effects of a short-term environmental upset . In the event of an environmental change, an entire generation would
not be lost because some of the eggs would not have been released. Fractional
spawning also allows a fish to increase its fecundity by enabling h to produce more
eggs throughout the season than could be held in hs coelomic cavhy at one time . The fecundity of the fathead minnow is higher than has been
believed . It was thought that fecundity was as low as 300 to
400 eggs per female per season, but Gale and Buynak (1982) have shown that
fecundity may average about 8,000 eggs per female per season. This translates to 1.7
to 4.0 million eggs per kg offish.
Spawning has been reported to occur primarily in the morning with as much as
73.8% of the eggs spawned between the hours of midnight and 10:00 a.m.. Eggs are deposited on the underside of
submerged or floating objects. During spawning, the male will seek out the female and
herd her into the spawning poshion below the nesting she.
The male will then align himself on the left side of the female, and, after a
considerable amount of circling, the male nudges and lifts the female, whh his back,
until she is on her side below the nesting surface. Females need not be "ready,"
endogenously, to deposit their eggs or engage in spawning behavior. Instead, female
sexual behavior can occasionally be triggered by male generated stimuli. Similariy, the male does not appear to require that the female be "ready" for the male to exhibh sexual behavior . Once the adhesive
eggs are deposited and fertilized, the male drives the female out of the nesting site.
The male will then guard, clean, and aerate the eggs until they hatch . Although male parental care is common among organisms, there is a
survivorship cost to the parent, and the likelihood of surviving to the next breeding
season is reduced . Intmders into the nesting site are "head
butted," with the tubercles, by the guarding male .
Daily spawns can range from only a few to over 1000 eggs, and time between
clutches can range from less than 1 to more than 16 days (personal observation). The
eggs range in size from 1.15 to 1.3 mm and will generally hatch in 4 to 6 days
depending on the water temperature). The newly hatched
fry average about 5 mm in length and have a white transparent color