Direct and indirect competition between invasive signal crayfish (Pacifastacus leniusculus) and native fish species : a long term study on behavioral and ecological effects in the laboratory and field

  • Direkte und indirekte Konkurrenz zwischen dem invasiven Signalkrebs (Pacifastacus leniusculus) und heimischen Fischarten : eine Langzeitstudie verhaltensbiologischer und ökologischer Effekte in Labor und Freiland

Vaeßen, Susanne; Hollert, Henner (Thesis advisor); Schäffer, Andreas (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019


Non-native crayfish are spreading rapidly throughout Germany and Europe and have pushed native crayfish species to the brink of extinction acting as a vector for the crayfish plague (Aphanomyces astaci), being immune themselves. The North American signal crayfish (Pacifastacus leniusculus) is the most problematic species, as it is very large and aggressive, rapidly multiplying and - in contrast to other alien crayfish species - able to tolerate cool temperatures. Therefore it is able to penetrate even the last retreat of the native crayfish - the upper reaches of rivers. It is already clear what catastrophic effects this has on native crayfish, but the question arises as to whether such a big omnivore, occuring in large numbers, will affect the entire ecosystem and thus the native fish species as well. As according to the Water Framework Directive more and more barrier structures are removed from rivers for better continuity, which also supports crayfish spread, it makes sense to check whether this does not result in negative consequences for our rivers as a whole. To answer this, two streams that are currently colonized by signal crayfish were examined. Fixed sampling sites with consistent physical characteristics located along the current distribution limit of the crayfish allowed monitoring of the spread of the population and their impact on the ecosystem. For this purpose, the stock of signal crayfish, macrozoobenthos (invertebrates, mainly small crustaceans and insect larvae) and fish (trout, bullheads, loaches, etc.) was recorded regularly for three years. In addition, the behavior of crayfish (direct effects) in the laboratory was monitored by keeping brown trout (Salmo trutta fario) and Atlantic salmon (Salmo salar) together with varying densities of signal crayfish, and permanently monitoring their interaction with underwater cameras. This caging experiment was repeated in the spring of 2015 with native crayfish (Astacus astacus) to see if they are indeed less aggressive than their North American relatives. Signal crayfish were found to multiply and spread in both streams, but there were system-dependent differences in speed and established population density. When a stretch of river is reached and populated, a population maximum is established, which settles at different levels, depending on the conditions the animals find and, most likely, also on whether the population is regularly harvested by anglers or not. Where the invasion travelled downstream, the crayfish spread much faster than if they had to move against the current. Again, however, even aginst the current individual animals overcame large distances from the main population within a short time. Macrozoobenthos was not always negatively affected by the presence of crayfish. Groups that had a relatively high proportion of total numbers were always decimated the most. This effect changed greatly depending on the seasons, and other factors also affected the impact of signal crayfish effects. There was evidence that the decimation of one invertebrate group sometimes had a positive effect on the proliferation of another. Some groups showed no reaction at all. In each case, however, species composition changed due to the different reactions of the individual groups. The crayfish had a negative impact on total numbers of macrozoobenthos. However, this effect only became significant when the crayfish population reached a critical point. Population densities similar to those found in native crayfish were apparently tolerated better by the invertebrate community. The examination of the fish population in the field showed that bottom-dwelling, nocturnal species that use benthic invertebrates as the main food source, such as bullheads (Cottus gobio) and loaches (Barbatula barbatula), are impacted the most by signal crayfish presence. Fish species of the open water, such as trout and minnows (Phoxinus phoxinus), were less affected, but still showed negative tendencies. Brown trout appeared to be only slightly negatively correlated to signal crayfish density in both streams. However, it has been observed in the laboratory that both brown trout and Atlantic salmon as open water species were displaced from shelters by signal crayfish. Trout were more affected, as they tended to use provided shelters more than salmon did. Since many negative effects of the signal crayfish occurred only in conjunction with high population densities, their ability to reach higher densities in less time than native species is ultimately more crucial to the ecosystem than the higher metabolic rate and aggression of the individuals. Up to a certain density, crayfish were tolerated well by the ecosystem - just as well as the native species. Large differences in signal crayfish effects between the two streams further showed that their impact on an ecosystem is unpredictable, but dependent on many factors and regional differences. The extent of their impact may vary for every ecosystem. Therefore, it seems to be particularly important for future studies to investigate the proliferation and invasive migration behavior of various non-native crayfish species.


  • Department of Biology [160000]
  • Institute for Environmental Research [162005]
  • Chair of Environmental Biology and Chemodynamics [162710]