Wednesday 22nd April, 2009
Sorry for the late post today, but I have a great excuse. I was out having dinner and beers with friends, talking about fishing and telling lies, and making great plans for the upcoming summer. I didn't really have time to prepare anything this week, so I'll hit you with some sciency stuff that I wrote back in grad-school.
Although they once inhabited huge native ranges including the lower reaches of many river systems, many native stream fishes are now primarily confined to small headwater streams. The replacement of native salmonids and other stream fishes by introduced, non-native species in lower reaches of many watersheds where native species were formerly abundant is now common worldwide. In the western United States, native cutthroat, bull, and redband trout have been isolated in headwaters above introduced brown, rainbow, and brook trout, while brook trout in the eastern US have been isolated above introduced populations of rainbow and brown trout. In Japan, the native charr have been isolated due to the introduction of North American native salmonids, and in Australia and New Zealand the native galaxids have been displaced and preyed upon by introduced brown and rainbow trout. Researches studying this phenomenon have suggested a wide range of theories to explain why the isolation of native species in headwater reaches has occurred. A discussion of several of these theories follows.
Inter-specific Competition and Predation
In many cases, the distribution pattern of native and introduced fish species can be explained, at least in part, to be the result of inter-specific competition and predation. There is a good deal of evidence that supports the claim that predation by brown trout has led to the elimination of native galaxids from the lower reaches of watersheds in Australia and New Zealand. In the United States, research suggests that brook trout and cutthroat trout are dominated by both brown and rainbow trout in lower stream reaches, and that brook trout are dominant over cutthroat trout species in most habitats (Fausch 1989). The dominance may be the result of several different factors including the territorial and aggressive nature of certain species, dominance of the young of year of one species over another, and the effectiveness of spawning at a certain time of year. Nevertheless, certain situations exist where brook and cutthroat trout are not displaced from all habitats by the introduced species. In these cases, other factors contribute to the situation and the brook and/or cutthroat trout some sort of advantage.
Stream Gradient, Stream Altitude, and Water Temperature
The effects of stream gradient, altitude, and temperature have all been proposed as explanations as to why native species have not been displaced in all portions of watersheds, including the headwater reaches. In these proposals, researchers suggest that one of these factors (or a combination of these factors) results in conditions in which the introduced species cannot exist, or the conditions tip the competitive scales in favor of the native species. A good example of this theory is the displacement of native cutthroat trout in the western US by introduced brook trout. Many populations of cutthroat trout now exist only above impassible barriers that exclude brook trout. However, there are several cases in which cutthroats exist in high gradient, headwater reaches that lack any significant barriers that could block upstream migration by brook trout. It has been suggested that velocity characteristics, food and habitat availability, and winter habitat in high gradient streams favor cutthroat trout. Fausch’s data correlating relative abundance of brook and cutthroat trout to gradient in Willow Creek, Colorado strongly supports theories that brook trout are not as competitive/effective in high gradient streams. Another study showed that brook trout biomass decreases with increasing gradient in a Wyoming stream that only contained brook trout, and Fausch claims that “it appears likely that brook charr either do not ascend streams with gradients higher than about 7%, or do not survive or reproduce once reaching there.” However, the distribution of introduced brook trout and native westslope cutthroat trout in several Idaho and Montana watersheds did not appear to be gradient dependent. Furthermore, brook trout in southern Appalachia have been excluded from all but the steepest stream reaches by introduces rainbow and brown trout, proving that they can exist in some very high gradient environments. Stream altitude and water temperature are often cited to explain the distribution of introduced rainbow and brown trout. It is suspected that warmer water temperatures favor brown and rainbow trout where they have been introduced over native brook trout populations, and that the same species tend to be limited by low water temperatures and/or high altitude in many of the same locations. However, after examining altitude and temperature limit data for distributions of brown and rainbow trout in North American streams, Fausch found that distributions are not consistent after being adjusted for latitude, and concluded that “the distributions of brown and rainbow trout… are not simple functions of either temperature or altitude.”
The replacement of native salmonids and other stream fishes by introduced, non-native species in lower reaches of many watersheds where native species were formerly abundant has also been attributed to the amount of time that the introduced species has been in the watershed. Simply stated, some believe that the only reason that introduced species have not invaded the entire watershed is that they have not had enough time to do it. This seems to be a plausible explanation where the introduction on non-native species has occurred fairly recently. However, in remote areas of Idaho, Montana, and Colorado, there exists streams with no major barriers to upstream migration where non-native brook trout have been present for up to 100 years. Still, the brook trout have not made their way into the highest reaches where native cutthroats still dominate.
Studying the causes of the replacement pattern of native salmonids and other stream fishes by non-native species in lower reaches of many watersheds may be more than just esoteric research. Determining the causes of such distribution patterns could be effectively utilized as a means for managing isolated populations of native fish species. The knowledge could also be used to predict the consequences of introducing a non-native species into a watershed where it has not been present before.
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