Exotic crayfish in a brown water stream: effects on juvenile trout, invertebrates and algae

Abstract

SUMMARY 1. The impact of the introduced omnivorous signal crayfish (Pacifastacus leniusculus) on trout fry, macroinvertebrates and algae was evaluated in a brown water stream in southern Sweden using in situ enclosures. We also examined the gut content of all surviving crayfish in the enclosures. Two crayfish densities in addition to a control without crayfish were used in replicate enclosures (1.26 m2) in a 1‐month experiment. Additionally, 20 trout fry (Salmo trutta) were stocked in each enclosure to assess the effects of crayfish on trout survival and growth. 2. Detritus was the most common food item in crayfish guts. Animal fragments were also frequent while algae and macrophytes were scarcer. Crayfish exuviae were found in crayfish guts, but the frequency of cannibalism was low. 3. Trout survival in enclosures was positively related to water velocity but was unaffected by crayfish. 4. Total invertebrate biomass and taxon richness were lower in crayfish treatments. The biomass of all predatory invertebrate taxa was reduced but only three of six non‐predatory taxa were reduced in the crayfish treatments. 5. Epiphytic algal biomass (measured as chlorophyll a, on plastic strips) was not related to crayfish density, whereas the biomass of epilithic algae (measured as chlorophyll a) was enhanced by high water velocity and high crayfish density. The latter was possibly mediated via improved light and nutrient conditions, as active crayfish re‐suspend and/or remove detritus and senescent algal cells during periods of low water velocity. 6. We conclude that the introduced signal crayfish may affect stream communities directly and indirectly. Invaded communities will have reduced macroinvertebrate taxon richness and the signal crayfish will replace vulnerable invertebrate predators such as leeches. In streams that transport large amounts of sediment or organic matter, a high density of crayfish is likely to enhance benthic algal production through physical activity rather than via trophic effects.

The long-term effects of invasive signal crayfish (Pacifastacus leniusculus) on instream macroinvertebrate communities

“…The class Hirudinea is widely cited as being severely affected following crayfish invasion, typically demonstrating significant reductions in the abundance of taxa following the establishment of large crayfish populations (Stenroth and Nyström, 2003;Crawford et al, 2006;Ruokonen et al 2014). Results from this study provide further evidence to support this with G. complanata displaying significant reductions in occurrence following crayfish invasion for both spring and autumn survey periods.…”

“…The most widely cited taxa affected by invasive crayfish colonisation are gastropods and bivalves, with reductions in species richness, abundances and biomass widely reported (Weber and Lodge, 1990;Nyström et al, 1996;Stenroth and Nyström, 2003;Dorn, 2013;Ruokonen et al, 2014). Results from this study indicate a significant effect of crayfish on Radix spp.…”

“…A number of reasons have been speculated as potential mechanisms to account for the reduction of Hirudinea taxa following crayfish invasion including the predatory nature of leeches and direct competition for resources. The life history of a number of Hirudinea species is strongly associated with gastropod populations as a function of their ectoparasitic nature (Elliot and Mann, 1979;Ruokonen et al, 2014), although their low mobility and soft bodies most likely results in preferential predation by P. leniusculus (Stenroth and Nyström, 2003).…”

“…Studies documenting the effect of invasive crayfish in stream ecosystems have identified significant reductions in the biomass and richness of aquatic macrophytes Nyström et al, 2001), and aquatic invertebrates Stenroth and Nyström, 2003;Crawford et al, 2006;Nilsson et al, 2012;Ercoli et al, 2015b).…”

“…Despite the widely acknowledged threat that invasive crayfish pose, much of the research to date has been focused on lentic ecosystems, whilst the impact on stream communities has been poorly quantified in many regions (Stenroth and Nyström, 2003;Pysek et al, 2008;Lodge et al, 2012). The majority of studies have investigated the short-term effects of invasive crayfish at site or reach scales via small-scale in-situ enclosure experiments or through the use of ex-situ laboratory mesocosms typically over one season (e.g., Parkyn et al, 1997;Stenroth and Nyström, 2003;Magoulick, 2014;see Twardochleb et al, 2013 for a meta-summary of all published invasive crayfish studies). Other studies have employed in-situ sampling over a limited duration (1 -12 months) where invading crayfish populations are already well established (e.g., Crawford et al, 2006).…”