by Piter Kehoma Boll
Human activities have been introducing, either deliberately or accidentally, several species in areas outside of their native range. Many os these species, when they reach a new ecosystem, can have devastating effects on the local communities.
One common practice is the introduction of exotic fish for food production or recreation. Although the impact of exotic fish species can be severe, there are several factors that modulate this severity. However, one situation in which it can have catastrophic outcomes is when fish are introduced in water bodies that were originally fishless.
Mountain streams and lakes are usually fishless because of physical barriers, especially waterfalls, as they prevent fish from moving upstream. But fish have been introduced in many mountain lakes to provide a local food stock or for sport fishing.
One place that was plagued this way is the Gran Paradiso National Park in the Western Italian Alps. During the 1960s, the brook trout, Salvelinus fontinalis, a fish that is native from North America, was introduced in several of the park’s high-altitude lake. Later, when the area became proteced, fishing was prohibited.
From 2013 to 2017, a fish erradication program was conducted in four lakes of the park, namely Djouan, Dres, Leynir and Nero. Fish were captured using gillnetting and electrofishing. Since the trouts had colonized the streams that are connected to the lakes, they had to be removed from there as well.
The communities of organisms living in the lakes and streams were monitored to assess their recovery after the fish removal. The lakes showed a remarkable resilience, reaching a community structure similar to that of lakes where fish were never introduced. The streams, on the other hand, did not show a great difference before and after fish removal. The reason, however, was not that streams have low resilience. On the contrary, streams showed a great resistance to fish invasion. Trouts did not seem to have affected the macroinvertebrate communities of streams that much. But why is it so?
One hypothesis was that macroinvertebrates constantly colonize the streams by passive dispersion, coming from upstream waters. However, this is not applicable to streams that drain the lakes, as lake and stream communities are very different. Lower predation by trouts is not an option either, because it was shown that stream trouts actually eat more than lake trouts. Maybe stream invertebrates reproduce more quickly than lake ones? No! Studies have shown than this is similar in both environments.
The reason why stream invertebrates are less affected by the introduction of fish is still a mystery. One possible explanation is that streams present more microhabitats that are not explored by the trouts, providing refuges for the invertebrates. We need more studies to understand what is going on.
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References:
Tiberti R, Bogliani G, Brighenti S, Iacobuzio R, Liautaud K, Rolla M, Hardenberg A, Bassano B. (2019) Recovery of high mountain Alpine lakes after the eradication of introduced brook trout Salvelinus fontinalis using non-chemical methods. Biological Invasions 21: 875–894. doi: 10.1007/s10530-018-1867-0
Tiberti R, Brighenti S (2019) Do alpine macroinvertebrates recover differently in lakes and rivers after alien fish eradication? Knowledge & Management of Aquatic Ecosystems 420: 37. doi: 10.1051/kmae/2019029
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