Fowey 2011 Semi-‐Quantitative Electro-‐ Fishing Survey
Total Page：16
File Type：pdf, Size：1020Kb
River Fowey Electrofishing report WRT/FRA Fowey 2011 Semi-Quantitative electro- fishing survey Report prepared for the Fowey Rivers Association by the Westcountry Rivers Trust; Giles Rickard, Adrian Dowding, Bruce Stockley and Nick Paling. Report date: 23.12.11 Fowey site 2 Westcountry Rivers Trust Kyl Cober Parc Stoke Climsland Callington Cornwall PL17 8PH Copyright © 2011 by the Westcountry Rivers Trust 1 River Fowey Electrofishing report WRT/FRA Context Survival to the end of the first summer is known to be poor for salmonid fry. Up to 90% of the alevins that emerge from the redds will not survive. Even in good quality habitat with a rich food supply, high densities of fish will undergo strong competition for resources with each fish trying to gain a profitable feeding station. The fry index surveys are used as a coarse measure of fry numbers/abundance at each particular site. For each single year it also gives a broad indicator of salmonid spawning success across a catchment. Site Selection and Permissions Sites were selected at regular intervals throughout the catchment as agreed in 2010 (Figure 1). Electrofishing permission was obtained from the Environment Agency. Access to sites was arranged by telephone or cold-calling on landowners. A shallow riffle section was chosen at, or as near as possible to, each selected site. The sites marked in Figure 1 represent the actual sites fished, rather than the sites initially identified for sampling. The completion of the walkover survey of the Fowey will prove particularly valuable in the future and will help identify fry habitat in the catchment, which may lead to the revision of some sites. Permission to access previously non-electrofished sites was time consuming although the response was positive with permission gained for all new sites where contact was achieved. Sites that were not fished by the Trust fell into Three main categories a) contact/permission was not identified in time, b) permission was refused and alternative sites may need to be considered for next year, c) the site had been fished by the Environment Agency in 2011. Field sampling Each site was electro-fished by a two person team using an E-fish 500W single anode backpack. The unit was predominantly fished at the same settings of (50Hz at 250v), although the voltage was increased to 300v where the conductivity was found to be particularly low, as identified in the headwaters of the main river Fowey. The operatives fished continuously in a downstream direction for a standard five minutes over suitable fry habitat without the use of stop nets. The fishing area was variable because only the length of fishing time was fixed. Fish were collected in a net and placed into a holding bucket. Copyright © 2011 by the Westcountry Rivers Trust 2 River Fowey Electrofishing report WRT/FRA All Salmonids were length measured with habitat features such as land use, substrate type and shading recorded at each site. A photograph of each site was also taken. Sites were classified on the basis of fry numbers recorded at each site. This classification scheme is taken from the original fry index produced by Crozier & Kennedy, 1994 (Table 1). Table 1. Semi-quantitative abundance categories and their relationship to quantitative electro-fishing densities (Crozier & Kennedy, 1994). Density Classification Semi-quantitative Quantitative equivalent (n5min -1) (n100m-2) A (excellent) >23 >114.7 B (good) 11-23 69.1-114.6 C (fair) 5-10 41.1-69.0 D (poor) 1-4 0.1-41.0 E (absent) 0 0 Six species of freshwater fish were recorded in the Fowey catchment during sampling including; bullhead, eel, lamprey, salmon, sea trout and brown trout. The results of the electro-fishing survey are classified according to the methodology of Crozier and Kennedy, as seen in table 1 below, with each site being given an equivalent density classification compared to quantitative monitoring. The results with the classification for each site can be seen in Table 2 in the appendix. The fry that escaped during electro-fishing were assigned to either trout or salmon groups depending on the percentage of each species already recorded at the site Copyright © 2011 by the Westcountry Rivers Trust 3 River Fowey Electrofishing report WRT/FRA River Fowey electrofishing 2011 Figure 1 Copyright © 2011 by the Westcountry Rivers Trust 4 River Fowey Electrofishing report WRT/FRA Results Salmon Three ‘A’ classifications were found this year which maybe considered an improvement to 2010 where no ‘A’ class sites were found. There was also a tangible increase in fry present on the Cardinham Water and the St Neot where full 2010 and 2011 surveys could be compared like for like. Salmon fry presence in the upper Fowey (Trenant confluence and above) was relatively similar if not slightly down in 2011. These results may potentially be related to autumn flows and successful negation of Golitha Falls. A comparison of 2010 and 2011 data for the middle and lower fowey is more difficult as there is not a full overlap of data between years. Crude assessment however would suggest a similar or improved density of fry in 2011. Warleggan The assessment of the Warleggan in 2011 was of particular interest as only two sites were electro fished the previous year. Although the very bottom site near the confluence was not fished, the distribution was very linear with salmon numbers steadily increasing from the headwaters near Temple new Bridge down river. The results here suggest that the Warleggan is a significant tributary for salmon fry and spawning with two ‘B’ classification sites found along with two ‘C’ classifications. The Warleggan was stocked at two sites in 2011 so some of the results from the lower sites may need to be assessed with caution and compared to a baseline year where no stocking has taken place. Upper Fowey By contrast the upper fowey catchment (Above Draynes Bridge) was for the second year relatively poor given the size of the river and assumed good habitat with no fish found above Westerlake Farm with the Trust survey, although a very low density of fry were found with the 2011 Environment Agency survey at Harrowbridge. Six sites were assessed above the bridge with low numbers found in two (‘D’ class sites) and no salmon found in four sites (‘E’ class). It would be particularly interesting to fully understand why the classification here is not higher given the extent of habitat work improvement already undertaken here. This maybe related to the identified difficulty in salmon navigating Golitha falls, particularly when there are low flows in the Autumn or siltation of spawning sites from sand further up the river. An assessment of historical Environment Agency electrofishing data, suggests that few fry or par have been recorded above Palmers Bridge since the mid 1970’s (Pilcher, 2000). Copyright © 2011 by the Westcountry Rivers Trust 5 River Fowey Electrofishing report WRT/FRA It would be useful to conclude whether the low fry density in this area is due to a lack of adult fish reaching high up the catchment or whether it is down to poor spawning and egg survival, due to the aforementioned sand deposition causing the siltation of spawning habitat. Trenant Stream All sites on the Trenant Stream were fished in 2011 with the Park pit site being fished by the Environment Agency in 2011. Three other sites on the Trenant Stream were also fished by the Environment Agency in 2011 that provides very useful additional data. Of the six sites surveyed only the two lowest sites (1 WRT, 1 EA) both below Periock Wood recorded salmon fry and both at low densities ‘Class D’. Both WRT electro fished sites fish in 2011 produced identical classifications to 2010. A higher salmon density may have been predicted as it is a relatively significant tributary to the Fowey and is not known to have major water quality issues as when compared to the St Neot. The Trenant is notably smaller however and this may deter salmon from traveling further up it or indeed it may make it flow dependent. Also there were no salmon classifications above ‘C’ found above the Trenant confluence, which may indicate that salmon densities are starting to reduce this far up the catchment. Cardinham Water The distribution of salmon in the Cardinham Water was generally linear with increasing numbers found moving down the catchment, other than the lowest site. However the lowest site (Upstream of Dreasonmoor Wood confluence) does not contain ideal fry habitat and maybe a candidate for site change after the walkover has been completed here. The site above at Fletchers bridge was found to have a class ‘A’ density and represents an improvement on 2010 and indicates highly successful spawning above here. There were another three ‘C’ class sites above here including a very small minor tributary (Deviock Wood) which can be considered a notable nursery area for fry. When compared to the 2010 survey all bar one site in the 2011 survey was equal to or higher than the previous years survey, suggesting a clear improvement in salmon spawning/ fry survival. Copyright © 2011 by the Westcountry Rivers Trust 6 River Fowey Electrofishing report WRT/FRA Plate 1: Site 46 Below Fletchers Bridge Lower Fowey The 2011 results for the lower Fowey were mixed with five sites below the Trenant confluence achieving good results (A and B class) and four relatively poor results (D class). The majority of sites fished in both 2010 and 2011 were relatively similar although the Ashford Bridge site changed significantly from class D (2010) to class A in 2011. However it is understood that fry have been stocked out near Treverbyn Bridge, which may explain the significant increase in abundance, which therefore makes the result somewhat unreliable.