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LANCASTER ENVIRONMENT CENTRE GRADUATE INDUSTRY PROJECT 26 Figure 9. Map showing the underlying geology of the Don catchment, with signal crayfish populations shown in CPUE _________________________________________________________________ LANCASTER ENVIRONMENT CENTRE GRADUATE INDUSTRY PROJECT 27 5. CRITICAL ANALYSIS 5.1.DISCUSSION/CONCLUSION The distribution of signal crayfish was seen to have changed little from the previous records in the Don catchment (Figure 3 & 4), with five distinct populations extant on the Moss Brook, Shire Brook, Blacka Dike, River Rother and River Rivelin at Walkley Bank Tilt. Due to the extensive stretches of connecting waterways between these locations where no trace of crayfish was found, the findings of this study strongly contradict the hypothesis that all populations eminated from a single point. There is anecdotal evidence that signal crayfish were introduced to BirleyHay pond on the Moss Brook as a crude form of aquaculture in the late 20th Century. Whilst inferences can be drawn regarding the origins of other populations, it is unlikely that they can ever be confirmed due to the high level of anthropogenic activity involved and the anonymity of those responsible. Signal crayfish are dispersing at a rate of 0.96km per year in a downstream direction, with no movement upstream recorded. This is significantly less than could be expected based on previous studies in other catchments, where a conservative estimate of 1.5km per year in either direction could be anticipated. A clear bias was observed for woodland rivers, though grassland flanked river channel also provided suitable habitat for signal crayfish. Watercourses that were heavily channelised and flowed through urban landscapes were seen to free from infestation, though the role of human preference in determining the presence or absence of signal crayfish is unquantifiable. Catch per unit effort of crayfish provides a useful point of comparison between isolated populations within the catchment. A strong linear trend was identified between the age of the population, based on previous records, and the number of individuals at each site (Figure 6). Signal crayfish have been found in the Moss Brook since 1995, and this study found it to consistently have the highest CPUE of any waterway. A method of dating a population was proposed based on CPUE, which appears to be accurate in the context of this study. Whether it will prove effective in other watercourses, or stand up to robust analysis of larger sample sizes, remains to be seen. Crayfish size and water depth is a well documented relationship [57][58] that is supported by the findings of this survey. Larger rivers such as the Rother consistently yielded larger crayfish than the smaller tributaries such as the Moss and Shire Brooks. Size of crayfish appears to be independent of the age of the population, as the two most recently colonised watercourses, the Rivers Rother and Rivelin, supported the largest individuals. Size of indivual may influence CPUE, though whilst the aforementioned rivers had both low CPUE and high indivual size, no trend was identified across all watercourses in the catchment. LANCASTER ENVIRONMENT CENTRE GRADUATE INDUSTRY PROJECT 28 Weirs provide neat ‘bookends‘ to the extent of the signal crayfish population on the River Rother between the Woodhouse Mill regulator and Beighton Weir (Figure 8), though elsewhere their efficacy as barriers cannot be evaluated as there are no crayfish in close proximity. Rivers such as the Don have frequent and large weirs but no crayfish at this time, or any records of their previous inhabitance. It could be speculated that the lack of crayfish is a result of the prohibitive number of barriers, though this cannot be supported. The lack of similar studies regarding crayfish dispersal and barrier efficacy is an obvious gap in our knowledge that would have considerable conservation implications. 5.2.PROJECT DESIGN As mentioned previously, crayfish trapping is inefficient (thus requiring high population densities) and is biased towards larger adult individuals within a population [48] All possible methods have their disadvantages, and given the range of environmental conditions encountered at various sites, the author believes that trapping was the most appropriate surveying technique. Due to the low efficiency of 2-4% estimated by Peay [48], sites where crayfish were absent in this study cannot be considered free of infestation with complete certainty. In some cases anecdotal evidence exists of sightings where this study found negative results, most notably around the Salmon Pastures nature reserve on the River Don and the Catcliffe Flash nature reserve on the River Rother. It is possible that signal crayfish do exist in these locations but at densities too low to be detected by trapping. As neither case was reported to CAS, the EA or any other body, no official records exist. In both of these instances, river conditions are too deep to allow manual searches, which are not as reliant on high population densities and so they can be neither confirmed or disproved. The use of un-baited artificial refuge traps may be beneficial in cases such as this, as their bias towards individual size is less pronounced. Unfortunately the cost of using such traps was prohibitive in this study. 5.3.ABNORMAL WEATHER The summer months of June and July in 2012 were subject to exceptionally adverse weather conditions, which resulted in several environmental conditions falling outside the paremeters that would be expected for this time of year. Rainfall for June in Sheffield was recorded at 182.3mm; substantially greater than the average of 68.3mm since 1955, and the average temperature was 13.4oC, against the average of 14.3oC4. These climatic abnormalities have resulted in far greater volumes of water in the rivers and at correspondingly higher flow rates. Previous studies have found 4 http://www.sheffieldweather.co.uk (Accessed 3rd August 2012) LANCASTER ENVIRONMENT CENTRE GRADUATE INDUSTRY PROJECT 29 conflicting effects resulting from such high flow events, with Bubb et al [16] finding them to be of little concern whilst numerous other papers have observed significant displacement of individuals [61] and mortality [62] from heightened flow. Crayfish have been observed to show reduced levels of activity at lower temperatures, both in the field [16] and in laboratory conditions [63]. It is therefore necessary to consider the effects of the abnormal weather conditions experienced during the study period when interpreting the results of the survey. 5.4.TAMPERING OF TRAPS As mentioned previously, one trap was removed from the River Rother immediately downstream of the weir at Beighton. Every care was taken during fieldwork to place traps out of sight and at times when bystanders would be kept to a minimum, in order to reduce the likelihood of tampering. Whilst one trap was removed from the site, the other was left untouched but yielded a negative result. Due to the placement of the second trap around a corner from the one that was removed, the author believes that the second trap was overlooked, rather than tampered with and its contents removed. It is impossible to say with certainty that no other traps were tampered with, but each was checked for this possibility prior to removal from the watercourses, and in no instance was there evidence to suggest the contrary. A trap placed on the Moss Brook (SK426800) was also found to be damaged; a hole was created that would allow crayfish to escape. The bait pouch was also damaged and all of the bait was removed, suggesting that this was the result of a wild animal rather than human interference. Due to the fine nature of the threads that compose the trap, it is possible that the damage was caused by a particularly aggressive signal crayfish, though the possibility of another animal, such as a rat, causing the damage cannot be ruled out. 5.5.FUTURE WORK The findings of this study are strongly contrary to the hypothesis that the Don catchment contains a single population (or meta-population) of signal crayfish due to the isolated nature of the extant populations, and the absence of crayfish in the connecting river segments. The identification of several likely sources of introduction (Walkley Bank Tilt, Birleyhay pond) and anecdotal evidence further strengthen this argument. The possibility remains however, that all populations originated from a single introduction, but have fractured and become isolated over time. This could be tested more robustly in two ways. Firstly, the use of more sensitive equipment such as artificial refuge traps may detect populations at lower densities in the areas where this study found no crayfish. The presence of signal crayfish in the River Don would indicate the possibility that all populations spread from a single point, as it would need to have been navigated at some point. Secondly, the ancestral lineage could be traced by examining the genetic similarities between populations within the catchment. A study of this nature was performed on LANCASTER ENVIRONMENT CENTRE GRADUATE INDUSTRY PROJECT 30 populations of Austropotamobius italicus in the Iberian Peninsula, using mitochondrial DNA to trace their lineage [63]. The level of expertise and cost of such of work was obviously beyond the scope of this project, but it would prove a fascinating and useful project for future work. Some information regarding the capabilities of weirs and other barriers in limited crayfish movement was found during the initial literature search for this study, though very little in the way of empirical evidence. Fully understanding the efficacy of weirs in this respect, and the effect that design and location has on them, would allow land managers to implement barriers to greatest effect and exert maximum level of control of the dispersal of crayfish populations. It is therefore suggested that this line of research be prioritised for any future studies.