Trophic niche and detection of the invasive signal crayfish (Pacifastacus leniusculus) in Scotland

Abstract

Aquatic invasive species are a major threat to native freshwater biodiversity. The North American signal crayfish Pacifastacus leniusculus was introduced to Great Britain during the 1970s and is now widely distributed throughout England, Wales and Scotland. First recorded in Scotland in 1995, P. leniusculus is now established at more than twenty sites. The only other introduced crayfish species present in Scotland is the white-clawed crayfish Austropotamobius pallipes. A. pallipes is restricted to only two locations in Scotland, Loch Croispol and Whitemoss Reservoir. P. leniusculus negatively impacts macrophytes, invertebrates and fish though ecological and physical processes. Additionally, P. leniusculus has displaced A. pallipes throughout much of its native range within Great Britain due to competition and disease. Consequently, the two A. pallipes populations in Scotland have a high conservation value. This PhD study aimed to improve understanding of P. leniusculus invasion success by examining trophic dynamics and to develop methodologies that could improve the detection and control of P. leniusculus populations in Scotland. Stable isotope analysis was used to determine the diet composition, trophic position and whether an ontogenetic dietary shift occurs in the Loch Ken population of P. leniusculus. Bayesian mixing models indicated that P. leniusculus in Loch Ken do exhibit an ontogenetic dietary shift. Additionally, individuals of all sizes occupied the trophic position of a predator in Loch Ken suggesting that invertebrates and fish constitute an important component of P. leniusculus diet. Stable isotope analysis was used once again to compare the isotopic niche width and diet composition of P. leniusculus populations from Loch Ken and A. pallipes populations from Loch Croispol and Whitemoss Reservoir. At the species level, A. pallipes exhibited a larger niche width than that of P. leniusculus. At the population level, the isotopic signatures of the A. pallipes populations were considerably different from each other suggesting an overestimation of A. pallipes’ niche width at species level. Results showed no dietary overlap between species and Bayesian mixing models suggested P. leniusculus and A. pallipes were consuming different resources, indicating there would be no direct competition for food resources if they were to co-occur. A plus-maze study was used to determine if P. leniusculus exhibited a preference for one of four food attractants (Oncorhynchus mykiss, P. leniusculus, beef or vegetation), which could be used to improve trapping efficiency. In the maze system, P. leniusculus exhibited no preference for any food attractant presented. This would suggest that either the maze was not a good model or food attractants would not improve trapping efficiency of P. leniusculus. Additionally, a comparative investigation into the use of gill nets as a method to control P. leniusculus was conducted. Results showed that the net type and the presence of fish entangled in the net influenced the number of P. leniusculus caught. Finally, environmental DNA (eDNA) was used and evaluated for detection of P. leniusculus. A robust quantitative Polymerase Chain Reaction (qPCR) assay and DNA extraction protocol were developed. Using the developed qPCR assay, P. leniusculus eDNA was detected in controlled aquaria conditions but not in environmental water samples collected from the field. Furthermore, the quantities of P. leniusculus eDNA declined in aquaria conditions while individuals were still present suggesting the mechanisms for eDNA release by P. leniusculus are complex. Stable isotope analysis indicates that P. leniusculus exhibit an ontogenetic dietary shift, and in each life stage, P. leniusculus function as an omnivore but occupy the trophic position of a predator. Niche width analysis revealed that the diet of P. leniusculus was less general than that observed in A. pallipes and thus diet of P. leniusculus may not be responsible for invasive success. Food attractants will not enhance trapping efficiency but nets may present a potential new method to control P. leniusculus. Similarly, eDNA presents a promising new method for rapid detection of P. leniusculus. It will not be possible to eradicate P. leniusculus in Scotland but the findings of this PhD may help prevent establishment of new populations. These results should be incorporated into future management strategies for P. leniusculus populations in Scotland and may have broader applications in Great Britain and Europe.