Biological control of Signal Crayfish using European Eel: examination of interactions under field and laboratory conditions

Invasive non-native species (INNS) represent one of the greatest threats to global biodiversity loss and can have serious negative effects on ecosystem structure, functioning and services. In the UK, the invasive Signal Crayfish (Pacifastacus leniusculus (Dana, 1852)) is widely recognised for its negative effects on native flora and fauna. In addition, its burrowing and foraging activities may modify river geomorphology and affect flood risk by promoting riverbed aggradation. Controlling the abundance and / or damaging behaviours of Signal Crayfish could reduce these impacts in UK rivers. Critically endangered European Eel (Anguilla anguilla (Linnaeus, 1758)) have the potential to act as an affordable and effective method of biological control (biocontrol) for Signal Crayfish, which may also provide longterm conservation benefits. This thesis examines the potential of using European Eel as a biocontrol agent for Signal Crayfish using three complementary studies. First, in situ experiments utilised a Before / After / Control / Impact (BACI) approach and Catch per Unit Effort sampling to assess the effects of reintroducing Eels on the abundance, health, and size characteristics of a resident Crayfish population. Results indicated that the presence of Eels may have had a limiting effect on Crayfish population growth and may have resulted in an increase in the amount of physical damage, such as missing claws, that Crayfish presented. Second, a series of ex situ mesocosm experiments were used to examine the interactions and feeding ecology of Eels and Crayfish. Eel were found to attack, sometimes kill, and consume small Crayfish (carapace length <27mm). Crayfish were observed to act defensively when inhabiting the same aquarium. Through direct and indirect observations, it was established that a total of 43.8% of juvenile Crayfish were killed by Eels and either consumed (20%) or left unconsumed (23.8%). Based on the population of Eels considered, larger individuals were likely to consume more Crayfish. Third, a series of olfactory experiments examined the behavioural responses of Crayfish that were naïve to Eel predation and those previously exposed to Eel predation. Establishing Crayfish past experience levels were based on communications with the Environment Agency and examination of historic environmental surveys. Experienced Crayfish displayed a stronger defensive response to Eel olfactory cues than naïve Crayfish, which suggests that naïve Crayfish may potentially be at a greater risk of predation by Eels in the short term. Using in-situ and ex-situ studies, this thesis has helped elucidate the potential of using European Eel as a biocontrol agent for Signal Crayfish in UK rivers. It also indicates some of the further benefits of continued Eel conservation efforts and restocking.