Re: NANFA-- exotic species impact
Thu, 27 May 2004 14:45:05 +0000

> A surprisingly simple question came up this morning. Have there been any
> native species that have become endangered or threatened by the release of
> an introduced species, whether through the aquarium trade or otherwise?

Among North American fishes, the answer is yes, especially in the West and in spring environments. In fact, some ichthyologists believe that it's the spread of exotic fishes, rather than loss of habitat, that's accelerating if not driving many Southwest fishes towards extinction.

In small, isolated habitats, where native fishes may not have evolved adaptations to withstand competition, the impact of an exotic can be swift and catastrophic. The Grass Valley speckled dace (Rhinichthys osculus reliquus) is a case in point. This dace, which had a distinctively speckled lower lip and a generally unspeckled body, occurred in one spring-fed creek in a grassy meadow in eastern Lander County, Nevada. It was collected once, in 1938, and never seen again. Brook and rainbow trout, which had been stocked in the creek, ate the dace into extinction.

The Banff longnose dace (Rhinichthys cataractae smithi) was declared extinct in 1987. This diminutive version of the eastern longnose dace was restricted to a small marsh fed by two hot springs in Banff National Park in Banff, Alberta. With water temperatures between 17-24C (63-75F), the area was popular with bathers, who enjoyed the public spas near the springs. Unfortunately, these spas sometimes dumped sewage into the dace's habitat. Human waste, however, was just one threat facing the dace. The Banff springs were also an ideal place for aquarists to release unwanted tropical fishes. Guppies and sailfin mollies, as well as introduced mosquitofish, reproduce year-round in the marsh, whereas the dace only spawned once a year. Eventually, the exotic fishes took over, out-competing the dace for food, and preying on the dace's eggs. The last remaining Banff longnose dace hybridized with the eastern longnose dace from the nearby Bow River. Since the Banff subspecies' unique gene
tic structure is now irreversibly mixed with another subspecies (termed introgressive hybridization), it is considered extinct.

In 1989, the High Rock Spring tui chub (Siphateles bicolor spp.) slipped out of existence six years after the spring's owner was granted an aquaculture permit by the California Department of Fish and Game to farm Mozambique tilapia (Oreochromis mossambicus), an exotic cichlid. The voracious tilapia escaped from its holding facility and destroyed the chub, as well as various undescribed invertebrates that were also unique to this miniature ecosystem. In this case, however, the landowner wasn't totally-in-fault. The agency that granted the permit did not recognize the unique taxonomic status of the High Rock Spring tui chub, nor did it consider the potential consequences of the tilapia's inevitable escape.

In most cases, though, exotic fishes is not the sole cause of imperilment, but a major contributing factor. Dams, dewatering, habitat destruction, etc., may be pushing species to the edge, but exotics are finishing them off.

Chris Scharpf
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