The Fathead Minnow is a species that thrives in areas where other fish have trouble establishing. Fathead Minnows can withstand alkalinities higher than 2,100 mg/L, dissolved oxygen levels lower than 1 mg/L and temperatures between 7°C and 33°C. Most commonly this means Fathead Minnows settle in pools of small, muddy streams or in ponds with limited interspecific competition but they can also excel in intermittent streams, vernal pools, and other temporary water bodies. To endure these harsh environments, Fathead Minnows are very opportunistic eaters, taking whatever filamentous algae, diatoms, small invertebrates, and even loose organic matter they can find on the bottom using their long intestine and grinding teeth to digest the difficult materials. In areas where predation is a risk Fathead Minnows will stay close to aquatic vegetation beds and use their strong sense of smell to detect potential threats.
Spawning age is quite variable, but in California an individual's first summer, when temperatures are between 15°C and 32°C, is a common maturing point. Males are territorial, aggressively defending and carefully cleaning a submerged board, stone, root mass, old tire, or any other surface that eggs can be hung underneath. Interestingly, the same head pad that males use to clean the nest surface is also used to secrete a mucus on the developing embryos that is believed to improve the young’s chances of survival. Once males have established their territories, females will leave their school to judge the male’s nest and courtship display. If the male is successful the female will attach her adhesive eggs to the underside of the nest object where the male will then fertilize them. Females can hold 600-2,300 in their body at once but at any given time only a third of those eggs will be ripe. Fathead Minnows mate multiple times throughout the summer however, and females will potentially lay more than 4,100 eggs by the end of the season. Both sexes have several partners during the spawning season leading to nest sites that have more than 12,000 embryos in various stages of development under a single Male's care. Eggs hatch 4-6 days later at 25°C and the 4.8 mm-long larvae will stay under the nest for several more days after hatching. Their growth will be influenced by a variety of factors, including population size, temperature, and food availability, but in general they will reach 84 mm in length by the end of one year. Most do not live past this age and adults usually die within 30-60 days of spawning. The maximum age and length recorded is 3 years and 109 mm respectively.
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