Desert Fishes Council, Death Valley, CA, November 2003.
Green sunfish impacts on Gila chub, a natural experiment thanks to a waterfall.
G.W. Knowles, P.J. Unmack & M.R. Baltzly
Gila chub (Gila intermedia) is a medium sized minnow restricted to an irregular patchwork of isolated populations in the Gila River drainage. They are typically isolated by dry or unsuitable habitat and/or downstream presence of either roundtail chub (G. robusta) or headwater chub (G. nigra). Their occurrence in smaller tributary streams has allowed them to avoid most of the effects of major hydrologic alterations so common in southwestern streams, such as dams and diversions, although minor alterations remain problematic in some populations. They have also somewhat avoided impacts from exotic fishes, as they are often isolated from other habitats by dry reaches or waterfalls. Despite their isolation, at least one population, Monkey Spring has been lost due to the introduction of largemouth bass (Micropterus salmoides). Green sunfish (Lepomis cyanellus) has also been implicated in their demise as populations of Gila chub have declined as green sunfish became more abundant. Recent rehabilitation efforts have narrowly saved two populations from impending extirpation due to green sunfish (Sabino Canyon, O'Donnell Creek). Natural waterfalls have also apparently saved several populations (Turkey Creek, Sycamore Creek, and Silver Creek). At these sites, Gila chub are usually abundant above the waterfalls in the absence of green sunfish, but are rare below when green sunfish are common. We set out to investigate this pattern by sampling fish populations above and below the waterfall on Silver Creek (Agua Fria drainage). We found only Gila chub above the waterfall, where they were abundant, with ~160 captured from four short seine hauls. Below the waterfall, green sunfish were the most abundant, and, despite significantly more sampling effort with a seine and electroshocker, Gila chub were considerably less abundant with only 23 individuals captured versus over 200 green sunfish. More striking however was the distribution of size classes. Above the falls, over half the population were less than 70 mm; this entire size class was missing below the falls, suggesting recruitment is either very low or nonexistent. This difference we attribute to the presence of green sunfish. We are planning additional work to further test this hypothesis, although on the basis of our initial results, it appears clear these two species cannot successfully coexist when green sunfish are abundant. Any populations of Gila chub that also have an abundance of green sunfish must be renovated or else Gila chub will eventually be extirpated as green sunfish invade further upstream.
For more details on this project see http://www.peter.unmack.net/fishes/az/silver/
For pictures of this project see http://www.peter.unmack.net/trips/ and scrol down to 2003 and click on the Silver Creek links.