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The American signal crayfish Pacifastacus lernusculus, an invasive species widely introduced throughout Europe, is a major threat to native European crayfish species and is causing increasing concern because of its wide impact on aquatic ecosystems. This thesis investigates the within catchment expansion of signal crayfish populations in two upland rivers and the spatial ecology and movement of the introduced signal crayfish and the indigenous white-clawed crayfish Austropotamobius pallipes. Populations of signal crayfish are established and expanding on the upland rivers Wharfe and Ure. On the Wharfe the signal crayfish population is well established and now occupies about 30 km of river and is currently expanding at a rate in excess of 2 km year(^-1). On the Ure the signal crayfish population is younger and currently occupies 1.6 km and is currently expanding at about 0.5 km year(^-1). The range expansion is biased towards downstream in both rivers, by a ratio of about 3:1 (downstream:upstream).The movements and dispersal of white-clawed and signal crayfish was studied utilising a combination of radiotelemetry and internal and external Passive Integrated Transponder (PIT) tags. Radiotagged adult signal crayfish were capable of substantial active movements (maximum movement 790m in 79 days). The level of movement of adults suggests they may have the potential to be responsible for the observed rates of population expansion. Although the movements of radiotagged adult signal crayfish within main river channel were equally distributed upstream and downstream, in-stream barriers both natural and artificial were found to limit the upstream movements of PIT tagged crayfish and this may contribute to the observed downstream bias of signal crayfish population expansion. The movements and dispersal of PIT tagged white-clawed crayfish within a small upland high gradient stream were strongly biased towards downstream. Maximum movement of radiotagged adult signal crayfish occurred during midsummer. Temperature appeared to be a major factor influencing the timing and extent of movements between tracking periods although there was a large variation between individuals. All significant downstream movements made by crayfish were active movements and not the result of passive movement during periods of high discharge. There were no sex or size differences in the dispersal and movement of radiotagged and PIT tagged signal crayfish whilst in PIT tagged white-clawed crayfish size, sex, injuries and duration of tracking influenced extent of movement. The expansion of the signal crayfish population in the River Wharfe appears to lead to the progressive loss of white-clawed crayfish populations where they come into direct contact. Limited differences in the microhabitat utilised by the two species were found where the species were syntopic, suggesting the potential exists for direct competition between the two species. In addition signal crayfish showed greater movement and dispersal than white-clawed crayfish. This may contribute to the ability of signal crayfish to colonise rivers rapidly and may also offer a competitive advantage over white-clawed crayfish thus contributing to the observed replacement. The results are discussed in the context of the conservation and management of crayfish and the ecology of invasive species.

Ectothermic animals may respond to environmental hypoxia by moving to cooler environments, exhibiting hypoxia induced behavioural hypothermia (HIBH). One benefit of hypothermia is the concomitant lowering of O2 demand, preserving internal partial pressure of O2 (P02) and delaying internal hypoxia. However, thermal acclimation may remove any such benefits. The macro-invertebrate Pacifastacus leniusculus was used as a model organism in which to clarify the role of haemocyanin in mediating responses to hypoxia. Quantifying the functioning of He is fundamental in understanding its importance in transducing environmental hypoxia. The role of temperature in influencing He 02-affinity was examined in crayfish acclimated to SOC, 13°C or 20°C. Increasing the temperature decreased the 02-affinity of He, regardless of acclimation history of the crayfish. The response of He Oy-affinity to variations in L-Iactate, urate, calcium and magnesium concentrations in the haemolymph were quantified as regulators of 02-binding. Effects of modulators in vitro were modelled to aid in the interpretation of He function in vivo; L- lactate was only found to potentiate He O2 binding at YC, however, urate, Ca and Mg all potentiated He 02-affinity, although this was temperature dependent for Ca. The in vivo functioning of He was assessed by determining the arterial and venous content and partial pressures of O2 and CO2, along with pH of haemolymph from crayfish exposed to progressive hypoxia of 9 mm Hg for 24hrs. Crayfish exhibited a tolerance to sustained hypoxia for 24hrs. The behavioural response of P. leniusculus inside a hypoxic thermal gradient was also investigated and found that it did not exhibit HIBH. However, evidence shows P. leniusculus was utilising anaerobic respiration demonstrated by increased haemolymph L-Iactate and glucose concentrations measured after hypoxic exposure. Therefore the response of P. leniusculus to hypoxia was first hyperventilation resulting in respiratory alkalosis, followed by anaerobiosis. ii

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Non-native invasive species are major drivers of biodiversity loss and ecosystemlevel modification. The signal crayfish (Pacifastacus leniusculus) is a highly successful invasive species and demonstrates traits often seen in keystone species, including top-down predatory effects, a high degree of omnivory, and an ability to physically modify its habitat. From field surveys, and in situ and artificial channel experiments, I show that signal crayfish have direct and indirect impacts on the benthos, as well as ecosystem process rates, in lowland, chalk stream ecosystems. Furthermore, I show that these effects are often dependent on crayfish life stage. I demonstrate that two native fish species (chub, Leuciscus cephalus and bullhead, Cottus gobio) may be affected positively, as well as negatively, by signal crayfish invasion. In addition, population genetics reveals overall high levels of genetic diversity in populations of signal crayfish in the UK.

The European lobster (<i>Homarus gammarus</i> L.) fishery in all coasts of Scotland records total annual landings of nearly 900 metric tonnes and total value of about £7 millions.  Two Scottish lobster populations were investigated, the Hebrides and Southeast with data from early 1960’s to late 1990’s. Diverse sources of information were used and when possible comparisons between the populations were carried out at varied spatial and temporal scales emphasising on two components, the sub-legal and legal lobsters.   The lack of accessibility to the Hebrides fishery limited the comparisons between the populations.  More studies on the southeast lobster population were possible. Varied methods and traditional and innovative techniques applied to the data showed dissimilarities between the populations. Differences in the size-structures, estimates of fecundity, size at the onset of sexual maturity (SOM), sex ratio, fishery duration, influence of environmental variables on the catch of undersized and legal size lobsters, and respective exploitation rates were found. The Southeast lobster population showed high historical exploitation levels but recruitment levels indicated strong resilience of this population to environmental or fisher-related processes.   Increasing landings with time might be related to an increase in temperature at large and small spatial scales. Habitat type and shelter availability affect the size-structure of this population.  Exploitation rate estimates indicated a relatively healthy state of this fishery but risks of growth over-fishing might be considered. Fluctuations of the Hebrides fishery were strongly related to density-independent processes.  The historical patterns of exploitation and the little evidence of good recruitment levels questioned the resilience of the population.  Assumptions of over-fishing risks and recruitment failure were weak.  The results contribute to the knowledge of the European lobster fishery in Scottish waters and for other commercially important lobster fisheries.

The aim of this research was to examine various aspects of the reproductive biology of male Nephrops norvegicus. Due to differnces in the reproductive behaviour between the sexes males suffer greater fishing mortality. A reduction in the numbers and size of males within a fished population may have effects on the reproductive output of the population. The structure and functioning of the testes of N. norvegicus were examined histologically. The testis appeared to be made up of many acini, which drain into convoluted collecting tubules that carry the developing spermatozoa to the vas deferens. Each acinus appears to have an independent cycle of production by which secondary spermatophores are produced. There is a pronounced seasonal development of the ovaries in female N. norvegicus, however, no such seasonality has been observed in the testes. This study used histological and biochemical analyses to determine any changes in the level of reproductive output of the testes over the course of the year. There were no observed changes in the structure of the testes. There was no thickening of the tubule walls which has been observed during the breeding season in other decapods. There was no change in the amount of sperm produced over the course of the year.