Aim. Survey Methodology

Transcription

1 The fish that smells of cucumber: conservation of smelt Osmerus eperlanus in the Thames Estuary Interim Report on results of 2015 survey work Joanna Barker Project Manager, Europe Conservation Programme December 2015 Aim Identify important habitats for smelt in the Thames Estuary, in particular the spawning ground, to inform a Smelt Management Plan for the region. This interim report provides initial results of the 2015 survey data, completed to inform the design of the 2016 survey season. Full analysis of data will be completed at the end of the 2016 survey season. Survey Methodology Seine netting was conducted at low tide once every other week from 1 March 2015 to 31 July The seine net has 1.5-mm knotless mesh and measures 23.1 m long by 2.8 m high with rope wings of 18.3 m on each end. Depending on catch rates, between three and five sites were sampled on the two-hour slack period before and after low tide. Ichthyoplankton netting was conducted every other week to alternate with seine netting between 1 March and 31 July (Fig.1). The ichthyoplankton sampler consists of a metal frame suspended between two 2.5 m long floating pipes spaced 90 cm apart. The frame supports a 30 cm square steel collar that is lowered 40 cm below the surface and forms the opening into the fry net. The 250 mm mesh net has a square pyramid shape narrowing into a collection bottle with a 1 m total length. The 1 L collection bottles are modified with 1 mm mesh panels to allow water to pass through while retaining fish larvae. A digital flow meter is attached to the steel collar and suspended in the water column. To minimize fry disturbance, the floats are grey in colour and the collection net is placed forward of the floats. Additionally, the sampler is situated to the side and front of the 3.5-m inflatable boat so as to avoid interference from shadows and the wake. Depending on catch rates, between two and five sites were sampled on the two-hour slack period before and after low tide.. Figure 1: Ichthyoplankton netting equipment used in the 2015 survey

2 Chiswick Hammersmith Fulham Battersea Putney Hurlingham Wandle Battersea Reach Figure 2: Location and survey effort at eight sampling sites where seine netting and ichthyoplankton netting was completed in the 2015 smelt survey period.

3 Survey Effort In total, 18 surveys were completed on a weekly basis between 1 March 2015 and 31 July 2015, except for 13 May and 1 July where surveys were cancelled due to poor weather conditions. An equal number of ichthyoplankton and seine netting surveys were completed: ichtyoplankton surveys were conducted on 17 March, 2 April, 9 April, 21 April, 7 May, 29 May, 18 June, 16 July and 30 July; seine netting surveys were conducted on 11 March, 25 March, 13 April, 27 April, 20 May, 10 June, 24 June, 8 July and 22 July. In addition, 2 public seine net demonstrations to increase awareness of biodiversity in the Thames were completed on 6 June and 11 September. Any smelt captured in these events were recorded. The majority of surveys were completed at three sites, Putney, Wandle and Battersea, but survey effort varied each week due to tide times and the amount of catch that had to be processed (Fig. 2). Additional sites were sampled at the beginning of the survey season, when different areas were tested for their suitability, and at the end of the survey season, when the experience gained allowed for greater efficiency at each site (Fig.2) Preliminary Results Comparison of Smelt Catch per Unit Effort (scpue) In total, 284 smelt fry were caught during the 2015 survey period. Smelt were caught at every site except for Hurlingham and Battersea Reach, however only one survey was completed at each of these sites. Smelt Catch Per Unit Effort (scpue) for ichthyoplankton netting was calculated by dividing the number of smelt caught by the volume of water trawled during each ichthyoplankton trawl, then multiplying by Seine netting scpue was simply calculated by dividing the number of smelt caught by the number of seine netting events completed. Due to the difference in methodology, the scpue for seine netting and ichthyoplankton netting cannot be directly compared. The greatest scpue for ichthyoplankton netting and seine netting was at the Wandle site (Ichthyoplankton = 0.38 scpue, Seine = 4.14) (Fig. 3). Interestingly, the seine netting scpue was high at the westernmost sampling locations, Chiswick and Hammersmith, and the easternmost sampling locations, Wandle and Battersea, but low at Putney and Hurlingham (Fig.3). In 2016, it would be beneficial to sample further East and further West to investigate whether there is a gradient in scpue in the Tidal Thames. The ichthyoplankton scpue was greatly influenced by a single, large catch of smelt on 2 April 2015 (see Analysis of 2 April Catch ). When smelt catch was analysed over different months, April was identified as the greatest scpue for Ichthyoplankton netting (scpue = 0.32) while June was identified as the greatest scpue for seine netting (scpue = 3.67) (Fig.4). The temporal difference in peak scpue for seine netting and ichthyoplankton netting can be explained by the selectivity of the different gear types (see Analysis of smelt fry size ).

4 Figure 3: Comparison of Smelt Catch Per Unit Effort (scpue) at each site using different sampling methodology Figure 4: Comparison of Smelt Catch Per Unit Effort (scpue) over each month using different sampling methodology Analysis of 2 April catch The largest catch was on 2 nd April 2015, when 230 smelt fry were caught during two trawls West of Wandsworth Bridge (Site name = Wandle ) (Fig. 5). These fry ranged between 4 mm and 7 mm long and due to their ontogenetic staging (straightened body, large but elongated yolk sac, poorly developed mouth and gill structure), were estimated to be between 2 and 5 days old (Adrian Pinder pers. comms.). Based on this information, the smelt hatching date was estimated at between 28 March and 31 March In 2016, it would be beneficial to start sampling at the beginning of March, to increase chances of catching recently hatched smelt fry and more accurately identify spawning location.

5 Start trawl 2 End trawl 2 End trawl 1 Start trawl 1 Figure 5: The start and end location of two ichthyoplankton trawls completed on 2 April 2015 where 230 smelt fry were caught (Trawl 1 caught 200 fry and trawl 2 caught 30 smelt fry). Analysis of smelt fry size The smallest smelt measured 4 mm in total length and was caught using the ichthyoplankton net on 2 April 2015 at Wandle. The largest smelt measured 73 mm in total length and was caught using the seine net on 22 July 2015 at Wandle. The total length for smelt increased from April to July, with an average growth rate of 0.48 mm per day (Fig. 6). Ichthyoplankton netting only caught smelt fry under 20 mm in length and seine netting only caught smelt fry above 20 mm in length, thus a combined approach using these two methodologies was appropriate and should be repeated in Interestingly, smelt fry caught using the ichthyoplankton net on 29 May 2015 and 16 July 2015 were much smaller than other smelt fry caught in the same time period by seine netting (circled red in Fig. 6). This may suggest that there are two spawning locations and/or two spawning times for smelt in the Tidal Thames, which needs to be closely investigated in Figure 6: Size of smelt fry caught during the 2015 survey period. The points circled in red could suggest there were two spawning locations and/or timings.

6 Sampling by citizen scientists Trained citizen science volunteers conducted kick netting and foreshore assessment of catch at 9 sites between Hammersmith and Greenwich, once per week at low water from 16 March 2015 to 28 July Throughout this survey period no smelt eggs or smelt fry were identified. Significant changes to the citizen science survey methodology will be made in 2016 to increase chances of identifying smelt eggs and fry based on this experience. Challenges during 2015 During the 2015 survey season, fish fry identification was a major challenge for the team, especially distinguishing between the different species of coarse fish. The help of Steve Colclough at the Institute of Fisheries Management, Tom Cousins and Pete Grey at the Environment Agency and Adrian Pinder at Bournemouth University was much appreciated when trying to identify fry and a fry ID guide will be developed for the 2016 survey using the photographs taken in the 2015 survey (Fig. 8). The second major challenge was that ZSL s survey boat was not robust enough for the survey work, especially the ichthyoplankton netting. Sampling sites were restricted to areas close to access points, so that the ichthyoplankton net could be disassembled between sites. This reduced the scope of the survey and the number of sites able to be sampled each week. We are currently fundraising to buy a new aluminium boat for the 2016 survey season, so that we can increase survey effort and the number of sites sampled each week. Collaboration with HR Wallingford Ltd. ZSL and HR Wallingford have started a collaborative project to investigate how juvenile smelt fry use the Thames Estuary and identify important habitats for their conservation. ZSL will provide data collected during the 2015 survey season for HR Wallingford to use in their TELEMAC flow model of the Thames Estuary in combination with their Individual Behaviour Model (IBM). The collaborative project will focus on the following objectives: Investigate whether the 230 fry surveyed on 2 April 2015 originated from a single spawning ground. Identify the probable location of the spawning ground. Investigate the likelihood that the larger fry found in subsequent surveys came from this cohort. Investigate how juvenile smelt fry use the estuary and identify important habitats for their conservation. Probability kernels will be produced for the probable spawning location and distribution of smelt fry by December 2015 and these will be used to help inform the 2016 survey location and methodology. 7 May

7 2 April 10 June Figure 8: Photographs of smelt fry showing development during the 2015 survey period Acknowledgements We would like to thank Steve Colclough at the Institute of Fisheries Management; Tom Cousins and Pete Grey at the Environment Agency; Adrian Pinder at Bournemouth University; Tanya Ferry at Port of London Authority; Jenn McCard and Jacob Ainscough for all their support in the 2015 Smelt Survey. We would also like to thank the 67 citizen scientists who volunteered to be part of the project and the Heritage Lottery Fund for funding the project.