Solent Bivalve Survey 2019 Report
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Solent Bivalve Stock Survey Spring 2019 This report has been produced by the Southern Inshore Fisheries and Conservation Authority. A copy of this report is available on our website at www.southern-ifca.gov.uk or from the Southern IFCA Office at: Unit 3, Holes Bay Park Sterte Road West Poole Dorset BH15 2AA 01202 721373 [email protected] !1 Contents 1. Introduction 3 1.1. The Fishery 3 1.2. The Solent 4 1.3. Current Management 5 2. Methodology 5 2.1. The Survey 5 2.2. Equipment 6 2.3. Data Analysis 6 3. Results 6 3.1. Southampton Water 7 3.2. Portsmouth Harbour 9 3.3. Langstone Harbour 11 4. Discussion 13 4.1. Southampton Water 13 4.2. Portsmouth Harbour 14 4.3. Langstone Harbour 15 4.4. Cockle 16 4.5. American Hard-Shelled clam 17 5. References 17 6. Annex 18 6.1. Annex 1 18 6.2. Annex 2 21 !2 1. Introduction The following report details the bivalve 1 surveys carried out in Southampton Water, Portsmouth Harbour and Langstone Harbour during March - April 2018, October 2018, and March 2019. The report will assess the distribution and abundance of clam and cockle species over time to evaluate the population health and stability of commercially important species for the dredge fishery. In addition, the outcomes from the survey will provide data which can be used as a baseline on which to monitor 2 future trends and potential changes to populations which will feed into the development and monitoring of local management strategies. 1.1. The Fishery The fishery for bivalves is seasonal in the Solent, to allow protection of important features, with prohibition on the use of a dredge between March 1st and October 31st each year (Solent Dredge Fishing 3 Byelaw 2016). The main commercial species fished via dredging is the Manila clam (minimum size 35 mm) (Ruditapes philippinarum), as it is abundant and sells for a high price (Figure 1.1). Other species also taken when in suitable quantities are the common cockle (minimum size 23.8 mm) (Cerastoderma edule), American Hard- Shelled clam (minimum size 63 mm) (Mercenaria mercenaria) (Figure 1.1) and, occasionally, the Grooved Carpet Shell or Figure 1.1 - Image displaying (1) native clam (Ruditapes decussatus). The Manila Clam (Ruditapes dredge fishery uses a type of mechanical philippinarum), (2) Common Cockle dredge, commonly referred to as a ‘box (Cerastoderma edule), and (3) clam dredge’ (Figure 1.2). The dredge is American Hard-Shelled Clam towed from the stern of the vessel, (Mercenaria mercenaria). supported on the seabed by skis. A front !3 opening consists of a row of metal teeth, which dig into the sediment as the dredge is towed, collecting buried bivalves into Portsmouth Harbour the metal basket. The basket is designed Southampton Water so that sediment, debris and smaller sized bivalves are washed through whilst Langstone Harbour retaining the sizeable bivalves. Figure 1.3 - The Solent and adjacent estuaries A showing the three locations for the survey. The solid black lines indicate the boundaries of the B Southern IFCA District. 1.2. The Solent The Solent is a strait of water with adjacent C estuaries which separates the mainland of England from the Isle of Wight (Figure 1.3). It is a highly protected area with the wider D E Solent containing two European Marine Sites (EMS), the Solent EMS and the Solent and Isle of Wight Lagoons Special Figure 1.2 - a box clam dredge used in Area of Conservation (SAC). The Solent Southampton Water; A) metal teeth defining EMS is a complex site encompassing both the front opening of the dredge, B) skis to Special Areas of Conservation (SACs) support the dredge on the seabed, C) dredge basket, D) spring-loaded opening to designated under the EC Habitats Directive remove contents onto E) sorting table. and Special Protection Areas (SPAs) designated under the EC Birds Directive. The bivalve beds of commercial importance The EMS is made up of the Solent to the Solent exist primarily in Southampton Maritime SAC, the Solent and Water, but there are also fisheries in the Southampton Water SPA, Chichester and areas of Portsmouth Harbour and Langstone Harbour SPA and Portsmouth Langstone Harbour (Figure 1.3). These Harbour SPA. The Solent EMS is unique in areas form three distinct bivalve Europe with regard to the complexity of the management areas and, as such, these marine and terrestrial habitats present areas are all sampled as part of this survey, including mudflats, saltmarsh, eelgrass allowing data on the stocks of commercially (Zostera spp.) and natural shoreline important bivalve species to be gathered transitions to adjacent coastal habitats and compared across the three main areas including grazing marsh, saline lagoons of importance for the local dredge fishery. and reedbeds (SEMS, 2017). This variety !4 of habitats supports key species of birds 21st March & 4th April 2018, 23rd & and form rich grazing, breeding and 25th October 2018, and 18th & 20th nursery grounds. The mudflat habitat in March 2019. Each survey was carried particular, a key habitat under both the out across three defined bivalve SAC and SPA designations, supports a rich management areas (Figure 1.3) using a invertebrate fauna which in turn provides a chartered fishing vessel which routinely food source for internationally important operates in that area: populations of migratory species and an • Southampton Water, using vessel internationally important assemblage of ‘Benjamin Guy’ wildfowl (Gravestock, 2016). • Portsmouth Harbour, using vessel ‘Solent Star’ 1.3. Current Management There are a number of management • Langstone Harbour, using vessel measures currently in place regulating ‘Sand Julie’ bivalve dredge fishing within the Solent: For each area a number of beds were • Fishing Season - (Solent Dredge surveyed, which were defined based on Fishing Byelaw 2016). fishers’ knowledge, covering the major • Gear Type (Bottom Towed Fishing Gear bivalve beds present. For each bed, 3 2016 byelaw, The Solent European dredge tows were conducted for a Marine Site (Prohibition of Method of duration of 1 minute, with the aim of Dredging) Order 2004). keeping the vessel speed as constant • Vessel Size (‘Vessels Used in Fishing as possible throughout the survey. A 2012’ IFCA byelaw). waypoint was created for the start and • Minimum Size (Council regulation (EC) end of each tow, and for each waypoint, no. 850/98), (Southern IFCA Fishing for time, vessel speed, and GPS positioning Cockles byelaw and American Hard- were also recorded. A box clam dredge Shelled Clams - Minimum Size byelaw). was used to collect bivalve samples More information regarding fishing throughout each tow. Upon tow regulations around The Solent can be completion the dredge was brought found on the Southern IFCA website at inboard and the contents emptied onto a sorting table. A photograph was then http://www.southern-ifca.gov.uk/byelaws taken of the dredge contents. Next, the sediment types present in the dredge 2. Methodology were recorded based on presence/ absence, and scored from 0-5 based on 2.1. Survey abundance of each sediment type This report analyses data collected over 3 present. Bivalve species were then survey periods at the start and end of the picked out of the dredge and separated fishing seasons currently set under the into buckets. For each species, Solent Dredge Fishing Byelaw 2016. individuals were measured along the These survey periods were between the widest axis using Vernier callipers to !5 give a length measurement (mm). A 2.3. Data Analysis maximum of 100 individuals per species Data recorded on log sheets was inputted were measured, with the remainder into an Excel spreadsheet, before being counted. Manila clam species were analysed and compared both spatially further separated into groups above and and temporally across areas and beds. below minimum size (35mm) before being Averages and statistical analyses for weighed. Bivalves were then returned to significant differences between datasets the tow location from which they were were calculated using SigmaPlot. To caught, to ensure that they were returned compare temporal differences between to the appropriate classification area for bivalve areas, t-tests and Mann-Whitney bivalves as classified according to health U tests were used depending on whether standards set out in “European the data were normally distributed. Community Regulations 853/2004 and 854/2004”. To improve comparison of data between surveys, data was standardised. Firstly data was converted to ‘catch per metre of 2.2. Equipment dredge per hour’ based on either count or The dredge used for all three surveys has weight, depending on the data available. spacing between bars large enough to This gave a measure of catch per unit minimise retention of undersized bivalves. effort (CPUE). Weight per metre of This means that the dredge displays a dredge per hour was used for Portsmouth degree of bias towards retention of larger Harbour and Langstone Harbour, size classes, and so the survey data will however for Southampton Water weight not reflect the full spectrum of bivalve data from Spring 2018 was not available populations. In contrast, the use of this due to poor weather conditions type of dredge also allows for a more preventing the scale from working representative sample of what a properly, so comparisons between survey fisherman would catch, meaning that the periods had to be made using count data. results of this survey will provide useful information on which to monitor the fishery and to base management 3. Results decisions. Finally, the data does not A full summary of the Spring 2019 data factor in dredge efficiency. The efficiency from all three areas can be found in of the box clam dredge can vary from 2% Annex 1.