Gila Topminnow

Dominant breeding males aggressively defend individual territories, which females enter when ready to spawn (4). Gila topminnows are able to mate all year round where the water temperature is constant, but in waters that are subject to seasonal temperature fluctuations, the breeding season is concentrated in the spring and summer period (7). Females typically give birth to 10 to 15 live young per brood, depending upon the fluctuating habitat conditions and size of the adults, with larger broods produced during the summer (4) (8). Females often carry two broods at a time, one at a far more advanced stage of development than the other (4). Additionally, females are capable of storing sperm for later fertilisation of eggs, and can produce up to ten broods after separation from males (4) (7) (9). The life-span of this fish is around one year, but it appears to be linked to the age at sexual maturation, which is dependent upon the time of year in which individuals are born (4). Those born in warmer waters in summer can reach sexual maturity in as little as six weeks, although it may take up to 11 months for others (4) (9) (7). This omnivorous fish has a wide-ranging diet, consisting of bottom debris, vegetable matter and small crustaceans (3). This species will also voraciously feed on aquatic insect larvae, especially mosquitoes, when abundant (3) (4).

Several habitats where this tiny fish is found are legally protected, including the portion of the Rio Yaqui basin in the San Bernardino Wildlife Refuge, Arizona, where the rarer Yaqui topminnow (P. o. sonorensis) subspecies occurs (4). The Gila topminnow was re-introduced to its native rivers from stock populations as early as 1936 for the purposes of mosquito control, and many re-introductions have since occurred for the purposes of conservation of the species, both into man-made and naturally occurring habitats. In particular, large-scale re-introductions began after a Memorandum of Understanding was established in September 1981 between the U.S. Fish and Wildlife Service, the U.S. Forest Service and the Arizona Game and Fish Commission. However, this re-introduction programme had limited success, with the majority of populations disappearing almost immediately, or surviving only for a few short years. Failure was often due to replacement by invading mosquitofish or subsequent habitat destruction or alteration. Most chosen re-introduction sites were too small to resist environmental changes, and were isolated, limiting genetic flow and repopulation by other sub-populations. As the poor results became increasingly apparent in the late 1980s, conservation effort switched its focus to the protection of natural and re-established populations, and re-introductions into better quality areas (9). Additional conservation initiatives have included the propagation of the species for restocking purposes at Dexter National Fish Hatchery and Technology Centre in New Mexico and Arizona State University (9). In an attempt to protect the species and its habitat, road closures, livestock enclosures, recreation management, fish barrier construction, closure of areas to fishing, and habitat restoration have all also been implemented (9). Provided suitable habitat is preserved, protected and restored in this way, and mosquitofish can be effectively controlled, it is hoped that the Gila topminnow's high reproductive rate will allow it to rapidly repopulate habitats and flourish once more (7).

The Gila topminnow is a small, silvery-tan to olive-coloured fish that is darker above, and yellowish to white on the belly (3) (4). Scales on the back are darkly outlined, extending down the sides to the upper belly as black speckles, and a dark stripe extends along each side of the body. The elongated body has a slightly curved back, ending in a rounded to almost square caudal fin (4). Males are significantly smaller than females, and dominant breeding males are dark to jet-black in colour, with a few small golden areas (3) (4). Males can also be distinguished from the more robust females by their very long gonopodium, which is a modified anal fin used as a sexual organ for internal fertilisation. The Yaqui topminnow (P. o. sonorensis) subspecies can be distinguished from the Gila topminnow (P. o. occidentalis) subspecies by a longer snout and a shorter lateral stripe. Additionally, the pigmentation of blackened breeding males is somewhat paler in the Yaqui topminnow than the Gila topminnow (4).

This fish inhabits headwater springs and vegetated margins, pools and backwater areas of creeks, streams and rivers (2) (3). Both subspecies are usually found in shallow, warm water (2), congregating in areas of moderate current, below riffles and along the margins of flowing streams in accumulated algae mats (3).

The Gila topminnow (P. o. occidentalis) once occupied aquatic habitats below 5,000 feet elevation throughout the Gila River system of Arizona (US), New Mexico (US), and northern Sonora (Mexico), but has been extirpated from much of its former range (3) (5). Currently, populations are found in the Rio Concepcion in Mexico (6), and also in several localities in the Gila River system of Mexico and Arizona, and one locality in the Bill Williams River drainage in Arizona, with several of these containing re-introduced populations (4). The Yaqui topminnow (P. o. sonorensis) is found in the Yaqui River system in south-eastern Arizona and northern Mexico and also in the Sonora, Matape, and Mayo rivers in Mexico (6).

Classified as Lower Risk / Near Threatened (LR/nt) on the IUCN Red List 2007 (1). There are two subspecies: Poeciliopsis occidentalis occidentalis known as the Gila topminnow, and Poeciliopsis occidentalis sonorensis known as the Yaqui topminnow, which is classified as Vulnerable (VU) on the IUCN Red List 2007 (1).

The main threats to the Gila topminnow are habitat loss and competition and predation from introduced non-native fishes (8). Man-made alterations to water sources, including redirection and groundwater pumping have affected essential habitat by changing spring and stream flows, often causing droughts or the loss of natural floods, which in turn affects the aquatic vegetation on which this species seems to rely (8) (9). In recent decades, however, it is the introduction of non-native fish, principally the mosquitofish (Gambusia affinis), that has been the major cause of demise for the Gila topminnow, causing rapid declines in population size through competition and direct predation (6). Mosquitofish, along with several other alien species, were purposefully introduced into waters in the Gila River basin to control mosquitoes and have become ubiquitous within the river system (9).

Formerly occurred from Frisco Hot Spring, New Mexico, to near the mouth of the Gila River: it was perhaps more widespread in spring-fed creeks and marshes of southern Arizona between 900 and 1500 m above sea level. Inhabits vegetated springs and margins, pools and backwaters of creeks and small to medium rivers. Usually found in shallow, warm water.

The Gila topminnow or charalito (Poeciliopsis occidentalis) is a species of fish in the family Poeciliidae. It is found in Mexico and the United States.