World Class Science for the Marine and Freshwater Environment
Common skate survey
Annual Report 2017
ELECTRA MF6001: Workpackage Task 1.4
Victoria Bendall, Patrick Jones, Rose Nicholson, Stuart Hetherington and Gary Burt
Issue date: 15th December 2017
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Cefas Document Control
Sarah Jones, Catherine Bell and Jamie Rendell Submitted to:
15th December 2017 Date submitted:
Suzanna Neville Project Manager: Victoria Bendall, Patrick Jones, Rose Nicholson, Stuart Hetherington Report compiled by: and Gary Burt.
Joanne Smith, Sophy Phillips, Jim Ellis Quality control by:
Suzanna Neville 15th December 2017 Approved by and date:
V3 Version:
Version Control History
Author Date Comment Version
Victoria Bendall, Patrick 29th November 2017 First draft V1 Jones, Rose Nicholson, Stuart Hetherington and Gary Burt.
Victoria Bendall, Joanne 29th November – 12th December 2017 Editing & comments V2 Smith, Sophy Phillips, Jim Ellis, Rose Nicholson, Stuart Hetherington and Patrick Jones
Victoria Bendall & Joanne 12th December 2017 Final V3 Smith
Suzanna Neville 15th December 2017 Final submitted V3
Project Title: Common skate survey Annual Report 2017 (ELECTRA MF6001: Work Package Task 1.4)
Defra Contract Managers: Sarah Jones, Catherine Bell and Jamie Rendell
Funded by: Department for Environment, Food and Rural Affairs (Defra)
Department for Environment, Food and Rural Affairs (Defra) Marine Science and Evidence Unit Marine Directorate Nobel House 17 Smith Square London SW1P 3JR
Authorship:
Victoria Bendall, Patrick Jones, Rose Nicholson, Stuart Hetherington and Gary Burt
Cefas, Lowestoft
Disclaimer: The content of this report does not necessarily reflect the views of Defra, nor is Defra liable for the accuracy of information provided, or responsible for any use of the reports content.
This report can be cited as:
Bendall, V. A., Jones, P., Nicholson, R., Hetherington, S. J., & Burt., G., (2017) Common skate survey Annual Report (ELECTRA MF6001: Work Package Task 1.4) Cefas. 39 pp.
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Table of contents
Executive Summary……………………………………………………………………………………………3
Introduction……………………………………………………………………………………………………..5
Aims and Objectives…………………………………………………………………………………………..7
Materials and Methods………………………………………………………………………………………..8
Results………………………………………………………………………………………………………….14
Summary of results…………………………………………………………………………………………..32
Concluding points...…..……………………………………………………………………………………...34
Future Work………..…………………………………………………………………………………………..34
Acknowledgements……………………………………………………………………………………………………….……………34
References………………………………………………………………………………………………………………………………….35
Appendices…………………………………………………………………………………………………………………………………37
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Executive Summary
A fifth common skate survey was undertaken in 2017, thereby extending the time-series (2011 and 2014 – 2017), for surveying common skate (Dipturus batis ‘complex’) in the Celtic Sea (ICES Divisions 7.e–h). The commercial fishing vessel FV ‘Govenek of Ladram’ was chartered to undertake the field survey (25th October – 1st November 2017) using fixed trammel nets. In all, field data were collected for 641 blue skate (Dipturus batis; 339 females, mean total length
(Lx̅ ) = 115 ± 20 cm; 297 males, Lx̅ = 111 ± 18 cm; five unsexed) and seven flapper skate
(Dipturus intermedius; four females, Lx̅ = 105 ± 4 cm; three males, Lx̅ = 146 ± 41 cm).
Male blue skate (D. batis; n = 297), ranged from 57–135 cm in total length (LT), and in all 180 specimens (102–136 cm LT), were classed mature based on external examination of the claspers. Female blue skate (D. batis; n = 339) ranged from 63–149 cm LT, with 29 specimens
(116 – 131cm LT) classed active and at the ‘egg-laying stage’. In addition, 196 females (115-
149cm LT), were observed with their cloaca to be very enlarged and swollen but with no egg cases protruding, and were therefore classed as unknown due to no internal examination possible to confirm maturity stage. Flapper skate (D. intermedius; n = 7), ranged from 100–
193 cm LT, with one male specimen (193cm LT; 33% of total male catch), classed mature, and four females (100-108cm LT; 100% of total female catch) and two males (117-128cm LT; 67% of total male catch) classed immature.
Of the 648 common skate (D. batis ‘complex’) captured during the 2017 survey, a total of 551 D. batis (309 female and 242 male) were tagged with external mark identification (ID) tags (button sure-tags), and a further 20 (10 female and 10 male) were also fitted with electronic data storage tags (DSTs). All seven D. intermedius caught were also tagged with external ID tags. From previous surveys (2011, 2014–2016), a total of 2,378 D. batis (1,168 females and 1,210 males) have been tagged and released with mark ID tags and 95 with DSTs, of which 46 (1.9%) mark ID tags and 20 (21%) have been recovered to date. Recaptures were from 4–180 km from the release position, and were at liberty for 24–2,098 days.
Other skates caught were 25 cuckoo ray (Leucoraja naevus; eight females Lx̅ = 62 ± 4 cm;
17 males, Lx̅ = 64 ± 4 cm), 18 spotted ray (Raja montagui; nine females Lx̅ = 67 ± 5 cm; nine males, Lx̅ = 64 ± 4 cm) and shagreen ray (Leucoraja fullonica; two males, 81–83 cm).
Five shark species were caught; comprising 14 spurdog (Squalus acanthias; 13 females, Lx̅ =
111 ± 6 cm; one male LT 73 cm), eight blue shark (Prionace glauca; all female, Lx̅ = 198 ± 21 cm), four porbeagle (Lamna nasus; three males, Lx̅ = 192 ± 28 cm; one unsexed), three bluntnose six-gill shark (Hexanchus griseus; all females, Lx̅ = 169 ± 41 cm) and lesser-spotted dogfish (Scyliorhinus canicula; four males Lx̅ = 62 ± 4 cm) were caught.
The 2017 field survey fished 23 sets across 14 prime stations, with an average catch per unit effort (CPUE) of 46 D. batis per 1,000 panel-hours (12–137 D.batis per 1,000 panel-hours). New fishing stations undertaken during the 2017 survey provided evidence that the distribution of D. batis also extends further north than sampled in previous surveys. The highest CPUE occurred at stations towards the North of the survey transect, although not at the Northern extremity of the transect. In contrast, abundance was lower towards stations towards the South-west of the transect, in deeper waters.
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Whilst the fishing gear and fishing depths (101–150 m) were as used in commercial fishing operations, soak times (23–27 hours) were shorter than normal fishing practices for the area. On-deck vitality (the proportion alive when brought on board) of D. batis (n = 599) was 93%. Data were more limited for other species, and was 100% for D. intermedius (n = 7), L. naevus (n = 25) and P. glauca (n = 8), 94% for R. montagui (n = 18) and 75% for S. acanthias (n =14). All four L. nasus were dead.
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Introduction
Elasmobranchs (shark, skate and rays) are sensitive to fishing pressure, in part due to their large size, low population productivity and aggregating nature (Ellis et al., 2017). Current stock assessments are hampered by limited data and, consequently, some precautionary management measures have been introduced. There is a strong need to gain a better understanding of elasmobranchs in UK waters, including distributions, life history traits, by- catch levels and discard survival, to better inform processes such as the landings obligation and regional management.
Once thought to be a single species, common skate (Dipturus batis complex) is now known to comprise two distinct species: the larger-bodied flapper skate (Dipturus intermedius) and the smaller-bodied blue (or grey) skate (Dipturus batis; previously referred to as Dipturus cf flossada, Iglésias et al., 2010). It is generally thought that the flapper skate has a more northerly range, occurring predominantly in the waters off the west coast of Scotland and in the northern North Sea, whilst the blue skate is more common in the Celtic Sea. The two species overlap to varying extents.
Under EU fisheries legislation, the common skate complex are prohibited species, meaning that it cannot be targeted, retained, transshipped or landed. However, previous surveys have shown that they are susceptible to by-catch, particularly in Celtic Sea trammel net fisheries (Bendall et al, 2012, 2017; Ellis et al, 2015; Hetherington et al., 2016), and the prohibition on landings can result in high discard levels.
Fishermen operating in the Celtic Sea (ICES Divisions 7.e–h) consider the prohibition on landing common skate complex to be an ineffective management measure, as they believe high levels of blue skate by-catch indicate high local abundance. The Cornish Fish Producers Organisation (CFPO) suggest that the blue skate should not be listed as a prohibited species, due to its abundance in parts of the Celtic Sea, and that it should possibly be included under the current Total Allowable Catch (TAC) for ‘skates and rays’.
Rationale and Purpose
Stakeholder consultations show that the by-catch and (dead) discards of blue skate, spurdog (Squalus acanthias) and porbeagle (Lamna nasus) are a concern to the south-west fishing industry and UK policy makers. South-west commercial net fisheries can seasonally have a high proportion of elasmobranch by-catch (Bendall et al., 2012, 2017; Ellis et al, 2015; Hetherington et al., 2016) and so may experience an economic impact, due to the associated gear damage and extended periods of down time from clearing nets of unwanted by-catch, with no associated economic return. Such seasonally high levels of discarding are also not in line with the principles of Defra’s sustainable use of the marine environment or the reformed Common Fisheries Policy (CFP), e.g. opposing wasteful discarding when supported by scientific evidence.
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Following an industry request to Defra to repeat the 2011 Fisheries Science Partnership (FSP) common skate survey, additional Defra funds were made available to support a specific common skate survey between 2014–2018. This fishery-dependent survey is in place to collect data to better inform upon the most appropriate future management of the common skate complex.
Informing policy
The purpose of the common skate survey is to build a time series of the spatial and size distribution of the common skate complex in the Celtic Sea (7.e–h), and to provide an indicator of stock size. Such information is required to allow scientists and policy makers to develop practical and more pragmatic management measures. For example, the larger-bodied flapper skate is arguably of higher conservation concern and should remain a prohibited species, whilst the smaller-bodied blue skate, where by-catch and discarding can be high in some commercial fisheries (Ellis et al., 2015), may be better included within regional fisheries management.
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Aims and Objectives
The overall aim of the common skate survey is to improve our knowledge of abundance, distribution, biology and catches of the common skate ‘complex’ in the Celtic Sea using information collected via a fishery dependant survey.
The specific objectives of the common skate survey 2017 were:
1. Return to the broad survey areas surveyed in the 2011, 2014–2016 common skate surveys, repeating prime stations along a predefined transect, to provide a time series of catch rates, with a view to developing an indicator of stock size.
2. Collect biological information, including the species, sex and size composition, and biological maturity.
3. Determine levels of on-deck vitality (survival) for blue skate and other elasmobranchs in the trammel net fishery (based on health state score of skate prior to discarding), including tag and release studies.
4. Extend the survey area northwards to better understand the spatial distribution of the common skate complex
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Materials and Methods
Overview
Using fixed trammel nets, the field study was designed to collect data on catch rates, distribution, size and sex composition, maturity and on-deck vitality of common skate in the Celtic Sea (ICES Divisions 7.e–h). Due to significantly poor weather the 7-day survey had to be postponed by two weeks compared to previous years, but was still conducted over neap tides. The survey returned to the previous survey areas of 2011 and 2014–2016, repeating prime stations in each of the areas to continue the development of a time series index. In addition, the survey area was extended slightly to the north.
Fishing vessel and gear
The FV ‘Govenek of Ladram’ (Figure 1), is a 22.7 m LOA gillnetter operating out of Newlyn, Cornwall. The skipper has fished in the Celtic Sea and south-west Approaches for ca. 20 years and has considerable experience of common skate in this area.
Fixed trammel nets were used throughout the field survey, each panel 100 m long and 2.5 m deep, and nets consisting of 24 panels. Most (65%) survey stations used nets of 262 mm mesh size nets, with 35% of stations using nets of 300 mm mesh size. Soak times were 23– 27 h, which is less than standard commercial practices (usually 48–72 h or more), in order to reduce mortality and maximise coverage of the survey area.
Figure 1. Vessel used during the project, FV ‘Govenek of Ladram’
Survey design, spatial coverage and timing
The common skate survey began in 2011 under a Fisheries Science Partnership (FSP) project (Bendall et al, 2012) providing information on common skate by-catch in the UK trammel net
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fishery in the Celtic Sea (ICES Divisions 7.e–h). Building on this initial work, the common skate survey has become increasingly standardised (temporal and spatial coverage, fishing gear and soak time) through four subsequent surveys, beginning in 2014 to the present survey in 2017, reported here. Multiple years and standardisation of the common skate survey have therefore worked towards developing a stock size indicator for blue skate.
The design of the 2017 common skate survey was modified to incorporate as many repeat stations from previous surveys as possible, whilst further extending the survey area to the north to identify the spatial extent of the population in the Celtic Sea. In all ,12 prime stations were identified, each with a two nautical mile range around a centre location. Two new exploratory stations north of previous surveys (C13 and C14; Figure 2) were also fished in 2017. Wherever possible two sets of nets were deployed at each prime station within the boundary of the two nautical mile range.
Figure 2: Survey area for 2017, showing prime survey stations and shot and haul positions for 2017 together with all previous surveys, 2011 and 2014–2016.
The 2017 survey was conducted over seven days at sea during the second neap tide of October (25th October to 1st November 2017). The survey station, location, depth, fishing gear and soak time were recorded. During the survey a total of 14 prime stations were fished, with 23 sets of trammel nets soaked for 23–27hrs (Table 1). In all, 12 stations were repeated from
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previous (2011–2016) surveys, with two new survey stations undertaken to the north of the 2016 survey area..
Set Prime Shot Haul Soak time Depth Shot Latitude Shot long ICES No. station Date Date (hr:min) (metres) (Degrees.Min) (Degrees.Min) Division 1 12 26/10/2017 27/10/2017 24:15 150 48°58.5’N 8°6.5’W 7.h 2 11 26/10/2017 27/10/2017 25:00 146 49°5.5’N 7°56.0’W 7.h 3 9 26/10/2017 27/10/2017 24:30 132 49°16.2’N 7°40.4’W 7.h 4 10 26/10/2017 27/10/2017 25:30 137 49°11.8’N 7°48.9’W 7.h 5 8 27/10/2017 28/10/2017 24:00 130 49°21.6’N 7°34.5’W 7.h 6 8 27/10/2017 28/10/2017 24:45 128 49°21.0’N 7°33.0’W 7.h 7 7 27/10/2017 28/10/2017 25:01 122 49°29.0’N 7°26.5’W 7.h 8 7 27/10/2017 29/10/2017 26:15 123 49°27.0’N 7°27.0’W 7.h 9 6 28/10/2017 29/10/2017 23:50 117 49°35.0’N 7°19.6’W 7.h 10 6 28/10/2017 29/10/2017 25:00 117 49°33.0’N 7°21.0’W 7.h 11 5 29/10/2017 30/10/2017 25:00 115 49°43.5’N 7°13.0’W 7.h 12 5 29/10/2017 30/10/2017 25:45 115 49°45.3’N 7°10.9’W 7.h 13 4 29/10/2017 30/10/2017 25:45 109 49°52.2’N 7°0.2’W 7.h 14 4 29/10/2017 30/10/2017 26:30 108 49°53.6’N 6°57.5’W 7.e 15 2 29/10/2017 30/10/2017 25:45 102 50°6.3’N 6°48.0’W 7.f 16 2 29/10/2017 30/10/2017 27:00 102 50°4.4’N 6°50.0’W 7.f 17 3 30/10/2017 31/10/2017 24:00 104 49°57.0’N 6°53.4’W 7.e 18 3 30/10/2017 31/10/2017 25:45 102 49°58.4’N 6°52.4’W 7.e 19 14 30/10/2017 31/10/2017 23:00 104 50°6.4’N 7°1.0’W 7.g 20 13 30/10/2017 31/10/2017 23:45 104 50°10.7’N 6°59.0’W 7.f 21 13 30/10/2017 31/10/2017 24:30 102 50°13.5’N 7°0.5’W 7.g 22 1 30/10/2017 31/10/2017 25:15 101 50°10.8’N 6°49.3’W 7.f 23 1 30/10/2017 31/10/2017 25:15 101 50°10.2’N 6°47.2’W 7.f
Table 1. Fishing locations undertaken for the common skate survey 2017.
Catch Per Unit Effort (CPUE)
For the 2017 survey, CPUE was based upon the number of blue skate per net panel per hour, using the following characteristics number of panels and panel length (total net length) and soak time. The CPUE was then calculated as follows (based upon Musick et al., 2005):
Unit effort: Total soak time (hours) x total number of panels fished = No. panel hours
CPUE = 1000 x Number of common skate caught = No. skate per 1000 panel hours No. of panel hours
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Collecting field data
Catch information were recorded by species for each survey location. All common skate were identified to species, and data collected for total length, disc width, sex and maturity (males and active females only; Table 2). Due to female immature and mature stages being unable to be recorded without internal examination of reproductive organs (which was not feasible during this survey), all elasmobranch catch data was summarised by published length at maturity data, to allow for a representative understanding of elasmobranch catch compostion within the Celtic Sea region (7.e -7h).
On-deck maturity scale (external descriptors) Sex A (Immature) B (Developing) C (Mature) D (Active) Male Claspers Claspers longer than Claspers longer Clasper reddish and undeveloped, shorter posterior margin of than posterior swollen, sperm than extreme tips of pelvic fin, their tips margin of pelvic present in clasper posterior margin of more structured, but fin, cartilaginous groove, or flowing if pelvic fin. claspers soft and elements pressure exerted on flexible and hardened, and cloaca. cartilaginous claspers stiff. elements not hardened. Female Egg-case(s) being It is not possible to record these stages without internal exuded from the examination of reproductive organs cloaca.
Table 2. Maturity scale used to record maturity information during the 2017 survey (adapted from McCully et al., 2012)
On-deck vitality was recorded (see section below) and live individuals tagged and released. Dead common skate were sampled to determine sexual maturity and examine stomach contents. All other elasmobranchs caught were identified to species, and where possible measured, sexed and scored for on-deck vitality.
Vitality assessment
Every effort was made not to alter typical commercial fish handling practice, so that the elasmobranch experienced as typical capture and handling events as possible It should be noted, however, that soak times were shorter than normal commercial practice, with all nets hauled within 23–27 h to ensure maximum coverage of the survey area, repeating as many prime stations as possible.
The on-deck vitality (health) of each common skate was assessed using a semi-quantitative reflex and injury scoring method. A reflex action and injury was scored as present or absent against a predefined list (such as spiracle closure, body flex and startle response; Table 3) for skates and rays that had been developed at the Cefas laboratory for the Defra-funded project MF1234 (Catchpole et al., 2015). A reflex action and injury were scored as ‘present’ (1) when clearly observed, or ‘absent’ (0) when it was not present, or there was doubt of its presence. In addition, a second semi-quantitative vitality assessment was made of the vigour of each common skate, at the point of discard. The vigour assessment was based on four defined
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vitality classes from ‘Excellent’ through to ‘Dead’ which was adapted from several previous studies (based upon Benoit et al., 2010; Table 3).
Vitality Description
Excellent Vigorous body movements; no or minor a external injuries only
Good/fair Weak body movements; responds to touching/prodding; minor a external injuries No body movements but some movement of spiracles ; minor a or major b external Poor injuries Dead No movement of body or spiracles (no response to touching or prodding) a Minor injuries were defined as ‘minor bleeding, or minor tear of mouthparts or wing (≤10% of the diameter), or minor surface abrasion. b Major injuries were defined as ‘major bleeding, or major tear of mouthparts or wing, or major surface abrasion.
Table 3: Description of the categories used to score the pre-discarding vitality of individual skate for the semi-quantitative activity method (adapted from Benoît et al., 2010).
Mark ID tag-and-release programme
Where possible, common skate were tagged with an external mark ID tag (those with significant damage or wounding and judged unlikely to survive were not tagged with an external mark ID tag). Individuals suitable for tagging were identified to species, measured (total length and wing width), sexed and a vitality assessment (reflex impairment and injury) made. External ST1 button suretags were placed directly onto the wing. Immediately prior to, and at the point of release (discard), a second vitality assessment of the vigour of each individual was made.
Electronic tag-and-release programme
Electronic archival tags or Data Storage Tags (DSTs) record and log record fine-scale behaviours and movements (swimming movements, depth and temperature measurements at 10 minute intervals). In recent Cefas studies, buoyant Cefas G5 DSTs have been shown to record post-discard mortality (Bendall et al., 2012; Catchpole et al., 2017). Tagging live discards with DSTs can help quantify capture-induced changes in behaviour and mortality that may be associated with the discarding process.. G5 DSTs are brightly coloured and carry a reward label and return address. Physical recovery of the tag is necessary to retrieve archived data from the tag. As a prohibited species, there is no fishing opportunity on blue skate, so reliance on the fishery to capture tagged common skate and return the DSTs could not be assured. For this reason, buoyant Cefas G5 DSTs were used (Figure 3), where if they become detached (either through a weak link attachment of the tag, removal by fishers, or if they detach from a dead individual), they float and can be washed ashore and recovered from the coastline.
G5 DSTs were attached to a ST1 button sure-tag and attached externally to blue skate (n = 20) through the wing. For the purpose of the study, the Cefas G5 DSTs (www.cefastechnology.co.uk) were programmed to record depth and temperature at 10 minute intervals.
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Figure 3: Blue skate (Dipturus batis), tagged with a ‘fixed’ buoyant Cefas G5 DST.
Reward scheme
Physical tag recovery can be achieved through the fishery on capture of a tagged common skate or porbeagle. For common skate, if the tag is shed from the host, tags have the chance of drifting to shore to be recovered from a beach by a member of the public.
To encourage the recovery of information from the recovered DSTs, a reward scheme was established throughout the commercial fisheries of the UK and the EU, with the intention of raising the profile of Cefas’ discard survival tagging work within the local fishing industry and detailing how to return tags back to the Cefas Laboratory. Reward posters were distributed widely to the local offices of the Marine Management Organization (MMO) and Inshore Fisheries and Conservation Authorities (IFCAs). Tag-return information can be supplied via the tag-reporting hotline, by post, or on the internet (see www.cefas.co.uk/fishtagreturns).
Tag returns
For all tags returned, as much recapture information as possible is obtained, including the receiving port, tag number, vessel name and nationality, gear type, date, capture position (latitude and longitude), and any other relevant information. When all the recapture information has been processed, a reward of £10 for ID marker tags and €100 for DSTs is paid along with any postage costs. A project summary is sent to the returnee with a letter of thanks.
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Results
Elasmobranch by-catch and sex composition
A total of 726 elasmobranchs were captured during the 2017 survey. The main species caught was blue skate (D. batis; n = 641, 89%), comprising 339 females (mean length (Lx̅ ) = 115 ± 20 cm), 297 males (Lx̅ = 111 ± 18c m), and five unsexed, with a relatively even sex compostion of 1.1:1 female to male ratio (Table 4). Seven flapper skate (D. intermedius), comprising four females (Lx̅ = 105 ± 4 cm) and three males (Lx̅ = 146 ± 41 cm) were also caught, with a relatively even sex compostion of 1.3:1 female to male ratio. Overall, the common skate ‘complex’ in the study area was dominated by D. batis (98.9% by numbers), with only 1.1% being D. intermedius (Figure 4).
The other elasmobranchs caught comprised of cuckoo ray (Leucoraja naevus; n = 25; eight female Lx̅ = 62 ± 4 cm; 17 male Lx̅ = 64 ± 4 cm) with a male sex composition bias of 0.5:1 female to male ratio; spotted ray (Raja montagui; n = 18; nine female Lx̅ = 67 ± 5 cm; nine males Lx̅ = 64 ± 4 cm), with an even sex composition of 1:1 female to male ratio; spurdog
(Squalus acanthias; n = 14; 13 females Lx̅ = 111 ± 6 cm and one male LT 73 cm), demonstrating strong sexual female bias (13:1 female to male sex ratio); blue shark (Prionace glauca; n = 8; all female, Lx̅ = 198 ± 21 cm); porbeagle (Lamna nasus; n =4; three males Lx̅ = 192 ± 28 cm and one unmeasured, as it fell out of the net during retrieval); bluntnose six-gill (Hexanchus griseus; Lx̅ ; three females Lx̅ = 169 ± 41 cm); shagreen ray (Leucoraja fullonica; two males LT
= 81–83 cm); and lesser-spotted dogfish (Scyliorhinus canicula; four males Lx̅ = 62 ± 4cm).
Excluding retained elasmobranch catch (e.g. blue shark, spurdog, cuckoo and spotted ray), the majority of elasmobranchs were returned back to sea alive.
Elasmobranch maturity based upon published maturity at total length (LT) data
Blue skate (D. batis), showed a relatively even composition of LT at maturity (based upon
Iglesias et al., 2010; males LT >115cm and females LT >123cm), with 193 females (57%) and 183 males (61%) classed mature (Table 4). In contrast, only one of seven D. intermedius, was classed mature, a male LT 193cm (based upon Iglesias et al., 2010; males LT >186cm and females LT >198cm), with the remaining four females and two males classed immature.
For the other batoids the majority of spotted (R. montagui), cuckoo (L. naevus), and shagreen
(L. fullonica) ray caught were classed mature (based on length at 50% maturity (L50), of McCully et al., 2012, and visual inspection of male L.fullonica).
For shark species; spurdog (S. acanthias; based upon L50 of Silva and Ellis 2016). catches were all classed mature with 13 females and 1 male,; blue shark (P. glauca; based upon L50 of Megalofonou et al., 2005) and bluntnose six-gill (H. griseus; based upon length at length at maturity of Ebert 2002), catches were all female and classed immature, with the exception of one blue shark out of eight captured classed as mature; Porbeagle (L. nasus; based upon length at maturity by Natanson et al., 2002), and lesser spotted dogfish (S. canicula based upon length at maturity by Henderson & Casey 2000), catches were all male and classed mature.
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d) female d) female blue skate;
-
a) a) male largest flapper
; ; c
)
Dipturus Dipturus batis
(
) captured in fixed trammel nets during 2017 survey;
‘complex’
)
b
Dipturus Dipturus batis
, , total 193 length b) cm; from iris observed characteristic a blue skate
)
us
Example Example of common skate (
Dipturus intermedi Dipturus
(
a)
Figure Figure 4. skate cases. egg skate blue female e) 15
Total elasmobranch catch Female Male Sex Length Length Length Total Total Total Species Ratio Range (L ) Range (L ) *Immature *Mature Range (L ) *Immature *Mature No. R No. R No. R F:M cm cm cm Blue skate 641** 1.1:1 57 - 149 339 63 - 149 146 193 297 57 - 135 114 183 (Dipturus batis) Cuckoo ray 25 0.5:1 55 – 72 8 55 - 67 2 6 17 57 - 72 17 (Leucoraja naevus) Spotted ray 18 1:1 54 – 74 9 60 - 74 9 9 54 – 68 3 6 (Raja montagui) Spurdog 14 13:1 73 - 118 13 103 - 118 13 1 73 1 (Squalus acanthias) Blue shark 8 8:0 171 - 228 8 171 – 228 7 1 (Prionace glauca) Flapper skate 7 1.3:1 100 - 193 4 100 – 108 4 3 117 - 193 2 1 (Dipturus intermedius) Lesser-spotted dogfish 4 56 - 66 4 57 - 66 4 (Scyliorhinus canicula) Porbeagle 4*** 166 - 221 3 166 - 221 1 2 (Lamna nasus) Bluntnose six-gill (Hexanchus griseus) 3 128 - 209 3 128 - 209 3
Shagreen ray 2 81 - 83 2 81 - 83 2 (Leucoraja fullonica) Total No. 726 384 162 222 336 120 216 * Based upon published sexual maturity data for relevant species (ref: blue & flapper skate, Iglesias et al., 2010; cuckoo, spotted and shagreen ray, McCully et al., 2012; spurdog, Silva & Ellis 2015; blue shark, Cooper, unknown; porbeagle, Natanson et al., 2002; bluntnose six-gill, Ebert 2002. ** five blue skate of unknown sex; *** one porbeagle of unknown sex.
Table 4: Summary of maturity (based upon published data), sex composition and length range for captured elasmobranch species in fixed trammel nets during the 2017 survey.
Common skate (Dipturus batis ‘complex’) on-deck maturity and length frequency
Male blue skate (D. batis; n = 297) ranged from 57–135 cm LT, with 180 specimens (114-
136cm LT; 61% of total male catch), classed as sexually mature (n=23, 8% of total male catch) and active (n=157, 53% of total male catch), based upon external examination of the claspers
(Table 5; methods detailed in Table 2), with 114 specimens (57-122cm LT; 39% of total male
catch), classed immature (n=76, 26% of total male catch).
Female blue skate (D. batis; n = 339) ranged from 63–149 cm LT, with 29 specimens (116–
131cm LT), classed sexually active and at the ‘egg-laying stage’. Out of 310 females (63-
149cm LT; 73% of total female catch), recorded as unknown, 196 females (115–149cm LT ; 49% of total female catch), were observed with cloaca to be very enlarged, reddish in colour and swollen. However no egg cases were observed protruding from these specimens and without an internal examination maturity was unable to be determined.
The smallest mature male blue skate (D. batis), examined was 91 cm LT, and the largest
immature male was 124 cm LT. The smallest female observed with egg-cases exuding (‘active’
maturity stage) was 116 cm LT.
16
Total Common skate Female Male (Dipturus batis 'complex')
Total No. Total No. Unknown *Active Total No. Unknown Immature Developing Mature *Active Species (LR cm) (LR cm) (LR cm) (LR cm) (LR cm) (LR cm) (LR cm) (LR cm) (LR cm) (LR cm)
Blue skate 641** 339 310*** 29 297 3 76 38 23 157 (Dipturus batis) (57-149) (63–149) (63-149) (116-131) (57-135) (60-117) (57-124) (91-122) (114-132) (102-136)
Flapper skate 7 4 4 3 2 1 (Dipturus 0 0 0 0 (100-193) (100-108) (100-108) (117-193) (117-128) (193) intermedius) Total No. 648 343 314 29 300 3 78 38 23 158 (LR cm) (57-193) (63-149) (63-149) (116-131) (57-193) (60-117) (57-128) (91-122) (114-132) (102-193)
LR = length range in cm; Maturity stage based on external examination of female cloaca and male claspers, detailed in methods section, Table 2; * Active = Females at egg-laying stage with egg case protruding from cloaca; Males stiffened clasper, running
with milt; ** five blue skate unsexed; *** includes 196 females (115 – 149cm LR), observed with cloaca enlarged & swollen but with no egg cases protruding.
Table 5: Summary of on-deck maturity stage and length range for common skate (Dipturus batis ‘complex’), captured in fixed trammel nets during the 2017 survey.
Flapper skate (D. intermedius; n = 7), ranged from 100–193 cm LT, with one male specimen
(193cm LT; 33% of total male catch), classed mature, and four females (100-108cm LT; 100%
of total female catch) and two males (117-128cm LT; 67% of total male catch) classed immature.
Length frequency for blue skate (D. batis), captured during 2017 survey revealed peaks in LT
for both sexes (Figure 5). Females recorded as unknown revealed two peaks at LT 82cm and
LT 127cm, while females classed as active and at the egg laying stage showed one peak at
LT 126cm. Males recorded as immature peaked between 81–84cm, with developing at 101cm, mature at 119cm and active at 124cm respectively. Length frequency for flapper skate (D.
intermedius), was data limited, with four females observing a relatively constrained LT (100-
108cm LT), compared to males (117-193cm LT).
)
)
e d
17
30 a)
25
20
15 Frequency
10
5
0
71 73 75 97 99 63 65 67 69 77 79 81 83 85 87 89 91 93 95
101 121 123 125 147 149 103 105 107 109 111 113 115 117 119 127 129 131 133 135 137 139 141 143 145 Total Length (cm)
Females: Unknown Females: Active (Egg Cases) Females: Unknown (Extended Cloaca) b)
20
18
16
14
12
10 Frequency
8
6
4
2
0
71 73 91 93 57 59 61 63 65 67 69 75 77 79 81 83 85 87 89 95 97 99
109 111 113 129 131 101 103 105 107 115 117 119 121 123 125 127 133 135 Total Length (cm)
Males: Immature Males: Developing Males: Mature Males: Active
Figure 5. Blue skate (Dipturus batis), total length (LT) frequency by on-deck maturity stage for a) females (n = 339) and b) males (n = 297; three males were excluded here due to unknown maturity), captured in fixed trammel nets during the 2017 survey.
18
For 297 male blue skate (D. batis) captured during 2017 survey, it was possible to calculate the proportion of sexually mature and active (n=180) males within total male catch (Figure 6).
Lengths at 50% maturity (L50) were observed to range between 102-115cmLT. Further analysis is required to calculate observed L50 values for males from all common skate surveys combined, which will be included in the final report (due 2018). Unfortunately it will not be possible to calculate for females due to being unable to confirm maturity stages by internal examination.
1.00
0.75
0.50 Proportion mature Proportion 0.25
0.00 55 65 75 85 95 105 115 125 135 145 Total length (cm)
Figure 6. Proportion of male blue skate (Dipturus batis), classed on-deck as sexually mature or active by length recorded, captured in fixed trammel nets during the 2017 survey.
The relationship between disc width and total length was a straight line (Figure 7).
19
120
Females: y = 0.6982x + 1.7008 Males: y = 0.6893x + 2.5002
100
80
60 Females
Males Wing width (cm) width Wing
40
20
0 0 20 40 60 80 100 120 140 Total Length (cm)
Figure 7: Total length (TL) vs wing width measurements of male and female blue skate (Dipturus batis), captured in fixed trammel nets during the 2017 survey.
Spatial distribution of common skate (D. batis ‘complex’) catches
The spatial distributions of male and female blue skate (D. batis; by maturity stage (based on the L50 of Iglesias et al., 2010) are shown in Figure 8. Mature individuals were observed predominantly in two distinct areas, set numbers 8–10 (prime stations C06–C07, near the mid- point of the survey transect) and also to the north east of the transect at set numbers 13–18 (Prime stations C01–C04), where 35 females were observed to be spawning (egg-cases protruding from cloaca; Figure 8). The immature stages of blue skate (D. batis) were found to be relatively evenly distributed throughout the survey transect. A detailed review of how these results compare to previous surveys will be reported in the final report (due in 2018).
20
Figure 8. Spatial distribution of ‘immature’ and ‘mature’ male and female blue skate (Dipturus batis; based upon L50 of Iglesias et al., 2010) captured in fixed trammel nets during the 2017 survey.
21
Seven flapper skate (D. intermedius) were captured during the 2017 survey. Based upon published 50% maturity ogives (Iglesias et al., 2010), the majority were classed as immature and captured either mid-way along the survey transect at prime station C07 or to the north at prime stations C01 and C03 (Figure 9). One 193 cm LT male was actively mature (with milt running from the claspers) and was caught at prime station C03.
Figure 9. Spatial distribution of ‘immature’ and ‘mature’ male and female flapper skate
(Dipturus intermedius; based upon L50 of Iglesias et al., 2010) captured in fixed trammel nets during the 2017 survey.
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Health and vitality assessement of blue skate (D. batis)
Out of 641 blue skate (D. batis) caught within fixed trammel nets (23–27 h soak time), 598 (93%) were observed alive prior to discarding, and 43 (7%) were dead. All dead skate had been heavily scavenged by sea lice, predominantly at prime stations C02–C04)(Figure 10). At these sites, large parts of the catch were scavenged by lice once captured in the nets, as lice entered the body cavity via either the gills or cloaca. Target species such as hake (Merluccius merluccius) and anglerfish (Lophius piscatorius), together with blue skate, were scavenged by lice from the inside out, so leaving the skeleton. On one occasion, an entire porbeagle (Lamna nasus) was completely scavenged by lice with just the skin and cartilage remaining, all within less than 27 h soak duration (Figure 11).
Figure 10. Prime stations with blue skate (Dipturus batis), heavily scavenged by lice, captured in fixed trammel nets during the 2017 survey.
23
c), c), and a
–
) d
heavily heavily scavenged by lice (a
Dipturus Dipturus batis) (
)
c
(d).
) of of observed catches of blue skate
s
Example
nasus Lamna
( . .
porbeagle Figure 11
)
) b a
24
A total of 636 (339 females and 297 males) blue skate (D. batis), were assessed for vitality prior to returning back to sea. In all, 543 (85%) were in excellent or good condition (i.e. lively), 55 (9%) in poor condition (i.e. sluggish), and 38 (6%) were recorded dead and scavenged. Overall, vitality prior to release appeared to be consistently high for both males and females with no obvious difference in vitality between sexes (Table 6).
Blue skate by-catch in fixed trammel nets ‘Lively’ ‘Sluggish’ Dead (soak time: 23 – 27hrs) Dead Sex Excellent Good Poor Total length (scavenged) composition range (cm) Sex No. % No. % No. % No. % No. %
Males 297 47% 57 - 135 164 55.2% 79 26.6% 26 8.8% 28 9.4%
Females 339 53% 63 - 149 211 62.2% 89 26.3% 29 8.6% 10 2.9%
Unsexed 5 5 100%
Total 641 100% 57 - 149 375 59.0% 168 26.4% 55 8.6% 43 6.7%
Table 6: Sex composition, length range and on-deck vitality of blue skate (Dipturus batis) in fixed nets of short soak duration (23 – 27hrs), captured in fixed trammel nets during the 2017 survey.
A sub-sample of 475 blue skate were also scored for visible signs of natural reflex reactions to touch (such as spiracle closure, body flex and startle response; Table 7), together with any observed impairments (such as bleeding, abrasion, wounding and net marks; Table 8). Blue skate, observed to be in a ‘lively’ health state (either ‘excellent’ or ‘good’ condition), generally scored highly (>85%) in all reflex reactions. While those observed in a ‘poor’ health state also scored highly (>85%) for spiracle closure and startle touch reactions, however for females in poor condition only 63% observed body flex reaction, potentially highlighting an impaired reflex response that may help gauge discard survival chances. For visible injuries, virtually all blue skate (D. batis) were observed to have net marking (>90%), regardless of health state, with abrasion also observed across health states (>45%), most notably in blue skate in poor health states (females 54%; males 71%).
25
Female blue skate reflex reactions ‘present’ Male blue skate reflex reactions ‘present’
Out of 475 Female No. No. No. Male No. No. No. Normal reflex blue skate blue skate Assessed Assessed Assessed blue skate Assessed Assessed Assessed
reactions observed observed Excellent Good Poor Excellent Good Poor
Total % Total % No. % No. % No. % Total % No. % No. % No. % No. No. No.
Body flex 421 89% 222 88% 145 90% 62 91% 15 63% 199 90% 131 92% 51 86% 17 81%
Spiracle closure 475 100% 253 100% 161 100% 68 100% 24 100% 222 100% 142 100% 59 100% 21 100%
Startle touch 459 97% 245 97% 159 99% 64 94% 22 92% 214 96% 139 98% 57 97% 18 86%
Total No. examined 475 100% 253 100% 161 100% 68 100% 24 100% 222 100% 142 100% 59 100% 21 100%
Table 7: Blue skate (Dipturus batis), assessed on-deck for reflex reactions against vitality, captured in fixed trammel nets during the 2017 survey.
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Female blue skate injury ‘impairments’ Male blue skate injury ‘impairments’
Out of 475 Female blue No. No. No. Assessed Male No. Assessed No. Assessed No. Assessed blue skate skate Assessed Assessed Poor Excellent Good Poor Injury ‘impairments’ observed Excellent Good blue skate observed Injury ‘impairment’
Total Total Total % % No. % No. % No. % % No. % No. % No. % No. No. No.
Bleeding 69 15% 35 14% 16 10% 13 19% 6 25% 34 15% 19 13% 5 4% 10 48%
Abrasion 218 46% 108 43% 64 40% 31 46% 13 54% 110 50% 60 42% 35 59% 15 71%
Wounding 62 13% 29 11% 12 7% 15 22% 7 29% 33 15% 9 6% 12 20% 12 57%
Net marks 432 91% 228 90% 145 90% 63 92% 20 83% 204 92% 127 92% 57 97% 20 95%
Total No. 475 100% 253 100% 161 100% 68 100% 24 100% 222 100% 142 100% 59 100% 21 100%
Table 8: Blue skate (Dipturus batis), assessed on-deck for injury ‘impairments’ against vitality captured in fixed trammel nets during the 2017 survey.
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Catch Per Unit Effort (CPUE) for blue skate (D.batis)
The number of blue skate (D. batis) caught per 1,000 panel-hours (for fixed trammel nets) was calculated for each survey station and plotted with the repeat stations from 2011, 2014 - 2017 (Figure 12).
Figure 12. The number blue skate (Dipturus batis), caught per 1,000 panel-hours (fixed trammel nets), calculated for each survey station and plotted to show repeat stations comparisons from 2011 and 2014 - 2017 surveys.
The level of standardisation of the survey has increased from 2014 - 2017, demonstrated by panel hours of effort at each prime survey station and the consistency of the same annual average CPUE between years 2015 - 2017 (Table 9). For 2016 and 2017 surveys, net length (2400m) and condition were the same at all prime survey stations, although it is worth noting that the 2017 survey took place approximately one month later in the year compared to the 2016 survey.
Survey Mesh Size range No. of Panel hrs of Mean panel hrs of effort Mean No. skate per 1,000 Year (mm) panels effort (± SD) panel hrs of effort (± SD) 2011 267 – 329 1010 41474 1975 (± 716) 41 (±32) 2014 262 – 300 898 22091 884 (± 516) 43 (±40) 2015 300 444 13042 767 (± 248) 46 (±31) 2016 262 – 300 492 11946 629 (± 60) 46 (±38) 2017 262 – 300 552 13808 600 (± 23) 46 (± 33)
Table 9: Number and hours of effort by each trammel net panel for each common skate survey undertaken between 2011 - 2017.
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In the 2017 survey, the lowest average blue skate abundance observed for a prime station was 13 individuals caught per 1,000 hours panel effort at station C01, only 5-10 nautical miles from the highest CPUE in 2017, 91 blue skate caught per 1,000 panel hours effort at station C02. Neither of these stations had been consistently visited in previous years. There was a sharp contrast in CPUE between stations C01, C13 and C14, at the North-east end of the transect, and station C02, where results of this year’s survey suggest similarities with stations C03 and C04, with relatively high abundance of blue skate. Two nets were hauled at all of these stations except C13, which increases confidence in these findings.
Blue skate abundance at prime stations C03 and C04 markedly higher in 2017 compared to 2016, reversing the apparent declining trend at these stations at the North-east end of the survey transect, in relatively shallow waters (depths range 100-110m), where mature females and egg cases were found in 2017. The apparent increase in abundance at station C07 is offset by an apparent decrease at the adjacent stations C06 and C08. CPUE calculated for stations in deeper waters (below 130m) at the South-west of the transect were equivalent or slightly lower compared to the previous year, with the second lowest CPUE, 14 blue skate per 1,000 hours panel effort, occurring at 150m depth at station C12. Whereas the maximum CPUE in the 2015 and 2016 surveys (102 and 170 blue skate per 1,000 hours panel effort respectively) occurred at station C09 at 132m depth, the CPUE in 2017 for this station, where only one net was hauled, was marginally above the average for the survey, at 51 individuals per 1,000 hours panel effort compared to an average of 46.
Mark ID and DST tagged common skate ‘complex’
Out of the 648 common skate ‘complex’ captured during the 2017 survey, a total of 551 blue skate (D. batis; 309 female and 242 male) and seven flapper skate (D. intermedius; four female and 3 male) were tagged with external marker identification (ID) tags (button sure-tags) (Table 10). A further 20 blue skate (D. batis; 10 female and 10 male) were fitted with electronic data storage tags (DSTs).
Total Total Common skate No. No. Length range (cm) Excellent Good Poor Grand Total ‘complex’ & Sex Mark ID DSTs Blue skate 551 20 57 – 149cm 364 154 33 571 (Dipturus batis) Females 309 10 63 – 149cm 208 81 20 319 Males 242 10 57 – 136cm 156 73 13 252 Flapper skate 7 100 – 193cm 7 7 (Dipturus intermedius) Females 4 100 – 108cm 4 4 Males 3 117 – 193cm 3 3 Total No. 558 20 57 – 193cm 371 154 33 578
Table 10. Summary of blue (Dipturus batis) and flapper (Dipturus intermedius) skate externally tagged and released during the 2017 survey.
29
Common skate ‘complex’ tag recaptures from previous surveys
A total of 2,378 blue skate (D. batis; 1,168 females, 1,210 males), mark-ID tagged under the Defra-funded common skate surveys between 2011–2016, 46 (2%) have been recovered to date (Figure 13; Appendix 1). These recaptures were reported from between 4–170 km from the release position and had been at liberty for between 24–2,098 days prior to recapture.
In addition, 95 blue skate (D. batis; 51 females, 44 males), and 14 flapper skate (D. intermedius; 11 males, three females) were tagged externally with a floated Cefas G5 archival tag (Appendix 2). In total, 20 (21%) blue skate (D. batis) and one flapper skate (D. intermedius; 7%) have been returned to date (as of November 2017), yielding >5,000 days of data (Figure 14; Appendix 2). Of the 20 blue skate (D. batis) returns, 12 were males at liberty between 103–494 days, with eight females at liberty between 1–295 days. The vitality of all DST tagged skate upon release was recorded as either excellent or good (17 ‘excellent’ and three ‘good’), with DST depth data showing typical swimming behaviour and long-term discard survival.
A detailed account of all external mark ID and electronic DSTs recaptures from the common skate surveys 2011 and 2014–2017 , will be reviewed and reported upon in the final report (due 2018).
30
), ),
intermedia
Dipturus cf. cf. Dipturus
) & one flapper skate ( skate flapper one & )
Dipturus batis Dipturus
)
b
).
Dipturus batis Dipturus
Release and recapture locations of a) 19 DST tagged blue skate ( skate blue tagged DST 19 a) of locations recapture and Release
.
)
a
Figure 13 Figure ( skate blue tagged ID mark 46 b) and
31
Summary of results
The common skate survey 2017 sucessfully met its primary objectives, returning to previous survey stations of 2011, 2014 - 2016 adding data to the valuable time-series of blue skate (Dipturus batis), abundance and distribution within the Celtic Sea. The survey area was also expanded to the north-west and south-west of the survey transect to collect further data on the spatial extent of blue skate (D. batis). On deck vitality (survival) of D. batis in fixed trammel nets was observed to be high (>90%), similar to that recorded for previous common skate surveys (Bendall et al, 2012 & 2017; Hetherington et al., 2016). Blue skate captured exhibited a wide size range, relatively even sex composition and evidence of sexual maturity with the majority (53% of total male catch) of mature males in active breeding condition, and 29 females (9%), in active egg-laying stage with egg cases protruding from cloaca.
In all a total of 551 blue (D.batis) and seven flapper skate (Dipturus imtermedius), were mark ID tagged with a further 20 blue skate DST tagged prior to release during the 2017 survey. From previous surveys (2011, 2014–2016), a total of 2,378 D. batis (1,168 females and 1,210 males) have been tagged and released with mark ID tags and 95 with DSTs, of which 46 (1.9%) mark ID tags and 20 (21%) have been recovered to date. Recaptures were from 4– 180 km from the release position, and were at liberty for 24–2,098 days. The tag ata compiled from all five common skate surveys undertaken will inform our understanding of the spatial and temporal distribution of the common skate ‘complex’ in relation to environment and vulnerability to commercial fisheries within the region, and will be reported upon in full within the final report (due 2018).
Elasmobranch catch composition
During the 2017 survey, scientists were able to examine shark, skate and ray species typically captured in commercial offshore trammel net fisheries off Cornwall (ICES Division 7e-h), in an area where common skate (Dipturus batis ‘complex’) appear sufficiently abundant (locally and/or seasonally) and are often caught as by-catch (Bendall et al., 2012; Ellis et al., 2015). Of the 14 prime stations fished, elasmobranch catch compositions were predominantly made up of blue skate (D. batis; 89%) with a relatively even sex composition and a length frequency of between 57-149cm. Seven flapper skate (D. intermedius), were captured during the 2017 survey, having not been observed at all during 2015 and 2016 surveys, with the majority captured classed immature, highlighting an overlap in spatial distribution with the more commonly observed blue skate (D.batis) species. Other elasmobranch catch was distributed more widely across the survey area, with the majority returned back to sea alive.
Blue skate (D. batis) maturity composition
Out of 641 blue skate (D. batis) captured, 29 females, (9% of total female catch), and 157 males (53% of total male catch) were in ‘active’ breeding condition (females at the egg laying stage with egg cases protruding; and males with stiffened claspers running with milt). In addition,196 females (115-149cm LT), were observed with cloaca to be very enlarged and swollen but with no egg cases protruding, and therefore still classed as unknown due to no internal examination possible to confirm maturity stage.
32
Females observed in ‘active’ breeding condition with eggcases protruding from cloaca, were aggregated to the north of the survey transect highlighting evidence of potential spawning grounds for blue skate (D. batis) within this region. This is however speculative at this stage and will require further analyses of all data collected from the common skate surveys to date, which will be followed up in the final report (due 2018).
Total length recorded at maturity for male blue skate ranged between 114 – 136cm LT , and for active females at the egg laying stage 116 – 131cm LT, and therefore in keeping with published length at 50% maturity (L50) data of Iglesias et al., 2010 where L50 was observed for males LT >115cm and females LT >123cm.
Vitality and reflex impairment of elasmobranch catch
While vitality rates were high, reflex and injury assessments indicated that 47% of female blue skate (D. batis) recorded in a ‘poor’ health state showed impairment to their body reflex reaction. Virtually all blue skate catch were also observed to have net marking (>90%), regardless of health state, with abrasion observed across health states (>45%), most notably in blue skate (D. batis) in a poor health state.
On deck vitality for the majority of elasmobranchs caught was consistently high prior to being retained aboard or discard back to sea. However for porbeagle shark (L. nasus), mortality rates were high, with all three dead.
Catch Per Unit Effort (CPUE) for blue skate (D.batis)
The 2017 survey observed that blue skate (D. batis) remain locally abundant (average CPUE of 46 blue skate (D. batis) per 1,000 panel-hours) across the whole survey area, supporting previous surveys and fisher’s anecdotal information that blue skate by-catch and discarding can be high. The highest CPUE occurred at stations towards the North of the survey transect, although not at the Northern extremity of the transect, with a possible increase in abundance compared to previous years. In contrast, abundance was lower towards stations towards the South-west of the transect, in deeper waters. Standardisation of survey design between 2011, 2014 and 2015 (net length, soak time, condition etc.) means that the historic CPUE data should be interpreted with caution. However increased standardisation during both the 2016 and 2017 surveys can allow for greater confidence in the resulting CPUE data.
Flapper skate (D. intermedius) catches
While blue skate (D. batis) appeared relatively abundant throught the survey transect, conservation concerns arguably remain for the larger flapper skate (D. intermedius). Seven individuals were captured during the 2017 survey, the majority immature with only one observed to be actively mature, a male (with milting claspers) at 193cm in total length.
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Concluding points
• From five dedicated common skate surveys undertaken in September 2011, 2014 – 2016 and October 2017, a time series index is being developed, along with standardisation, to provide evidence of seasonal and regional abundance of blue skate (D. batis) within the Celtic Sea (7.e – 7h).
• The 2017 survey found that blue skate (D. batis) remain abundant (average CPUE of 46 blue skate per 1,000 panel-hours) across the whole survey area, supporting fisher’s anecdotal information that blue skate (D. batis) by-catch and discarding can be high.
• New stations undertaken within the 2017 survey provide evidence of blue skate (D. batis) spatial distribution northwards, highlighting a wide spatial extent.
• On deck vitality of blue skate (D. batis) in fixed trammel nets was high (>90%) for short soak times between 23 – 27 hrs.
• A wide variety of elasmobranch by-catch exists within the southwest net fisheries, including porbeagle shark that showed high on-deck mortality and remains a commercial and conservation concern.
Future Work
Increased standardisation of the Common skate survey (temporal and spatial coverage, fishing gear and soak time) from 2011 to 2017 has provided a fishery-dependant time series of common skate in the Celtic Sea. The annual survey will continue to build the time series to increase the value and meaningfulness of the data for future fisheries management within the region and to manage both common skate species separately.
Acknowledgements
Special thanks to the skippers and crew of the fishing vessel ‘Govenek of Ladram’ and to Paul Trebilcock and colleagues from the CFPO, whose support, participation and contribution to data collection has furthered our understanding of the common skate complex in the Celtic Sea.
34
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Appendix 1: Summary details for all external mark-ID returned blue skate (Dipturus batis), released during common skate surveys 2011–2016.
wing Gear Release Recapture TL Health Distance Days at Recovery Tag ID Sex width Soak Date Date (cm) state (km) liberty (method) (cm) (hrs) E020495 20/09/2014 24/06/2015 M 81 63 Poor 28 39 24 FRA (Trawl) E017095 24/08/2011 06/10/2011 M 128 88 Excellent 18 9 44 UK (Net) E022041 21/04/2014 24/11/2014 F 53 NR Excellent NR 18 66 FRA (Trawl) E021737 17/09/2014 24/11/2014 F 130 90 Good 18 55 69 FRA (Trawl) E016804 21/08/2011 01/11/2011 F 113 90 Excellent 26 23 73 FRA (Trawl) E020470 20/09/2014 04/12/2014 F 83 60 Good 28 68 76 FRA (Trawl) E017012 24/08/2011 10/11/2011 F 129 93 Excellent 18 35 79 FRA (Trawl) E021623 17/09/2014 20/12/2014 M 124 90 Poor 22 29 95 FRA (Trawl) E021344 19/09/2014 20/01/2015 M 126 91 Excellent 14 170 124 IRE (Trawl) E020336 21/09/2014 08/04/2015 M 110 80 Excellent 18 17 199 FRA (Trawl) E023451 24/09/2015 15/04/2016 F 136 98 Excellent 10 116 204 FRA (Trawl) E017058 24/08/2011 19/03/2012 F 128 97 Excellent 18 12 209 FRA (Trawl) E016850 21/08/2011 21/03/2012 M 102 75 Excellent 26 10 214 FRA (Trawl) E030053 26/09/2016 04/05/2017 M 89 65 Good 25 48 220 UK (Trawl) E012690 17/09/2014 25/04/2015 F 125 90 Excellent 22 37 221 IRE (Trawl) E020499 20/09/2014 02/05/2015 M 81 58 Poor 28 54 224 FRA (Trawl) E021208 20/09/2014 10/05/2015 M 66 46 Excellent 27 30 232 FRA (Trawl) E017015 24/08/2011 16/04/2012 F 129 90 Excellent 18 47 237 UK (Net) E016912 22/08/2011 07/05/2012 M 116 86 Excellent 49 45 259 FRA (Trawl) E021742 17/09/2014 16/06/2015 M 121 84 Good 17 46 272 FRA (Trawl) E020311 22/09/2014 29/06/2015 M 124 84 Excellent 22 28 280 FRA (Trawl) E021681 17/09/2014 13/08/2015 F 122 91 Good 6 27 330 UK (Trawl) E016812 21/08/2011 28/08/2012 M 111 80 Excellent 26 41 373 FRA (Trawl) E029645 26/09/2016 30/10/2017 F 113 80 Poor 22 70 399 UK (Net) E016915 22/08/2011 27/09/2012 M 106 78 Excellent 46 13 403 FRA (Trawl) E021456 23/09/2015 07/12/2016 F 99 71 Excellent 11 56 441 RUS (Trawl) E016848 21/08/2011 07/11/2012 F 100 69 Excellent 26 NR 445 FRA (Trawl) E024024 24/09/2015 06/05/2017 M 121 87 Good 6 68 590 UK (Trawl) E023629 24/09/2015 21/05/2017 M 105 75 Excellent 10 107 605 FRA (Trawl) E021362 18/09/2014 20/05/2016 M 117 82 Excellent 41 59 610 UK (Net) E021281 19/09/2014 14/06/2016 M 125 90 Excellent 17 82 634 UK (Net) E020592 23/09/2015 26/06/2017 F 121 86 Excellent 12 61 642 FRA (Trawl) E022022 19/01/2014 27/10/2015 F 121 92 NR NR 18 646 UK (Net) E017025 24/08/2011 04/06/2013 F 121 87 Excellent 18 23 650 UK (Net) E024837 24/09/2015 10/08/2017 M 97 72 Excellent 10 38 686 FRA (Trawl) E016845 21/08/2011 27/08/2013 M 125 89 Excellent 15 46 737 UK (Trawl) E023453 24/09/2015 29/10/2017 M 112 77 Excellent 7 109 766 UK (Net) E016864 21/08/2011 28/04/2014 M 131 89 Excellent 15 41 1004 FRA (Trawl) E020465 20/09/2014 29/10/2017 M 118 85 Excellent 28 4 1135 UK (Net) E016936 22/08/2011 14/10/2014 M 102 74 Excellent 46 81 1150 FRA (Trawl) E017069 24/08/2011 19/05/2016 F 133 94 Excellent 18 45 1730 UK (Net) E016872 22/08/2011 19/05/2016 M 118 88 Excellent 49 111 1732 UK (Net) E016904 22/08/2011 19/05/2016 F 87 62 Excellent 43 125 1732 UK (Net) E016972 22/08/2011 20/05/2016 M 100 70 Excellent 43 103 1733 UK (Net) E016955 22/08/2011 19/06/2016 F 89 62 Excellent 43 105 1763 UK (Net) E016911 22/08/2011 20/05/2017 M 120 82 Excellent 46 99 2098 UK (Net)
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Appendix 2: Summary details for 19 returned DSTs from blue skate (Dipturus batis), and one flapper skate (Dipturus intermedius), released during common skate surveys 2011–2016.
wing Gear Days Release Recapture TL Health Recovery Recovery Species Tag ID Sex width Soak at Date Date (cm) state (km) (method) (cm) (hrs) liberty
D. batis A05545 21/08/2011 04/03/2012 M 124 85 Excellent 26 96 197 UK (Net)
A10233 24/09/2015 27/12/2016 M 120 87 Good 9 225 460 UK (Beach) 10 218 52 UK (Beach) A10270 24/09/2015 15/11/2015 F 118 81 Good A10873 28/09/2016 31/10/2016 F 125 87 Excellent 27 41 33 UK (Beach) 23/09/2015 Excellent 5 5 226 UK (Beach) A11399 06/05/2016 M 126 91 10 180 197 UK (Trawl) A11404 24/09/2015 08/04/2016 F 131 91 Good A11422 23/09/2015 30/07/2016 F 123 91 Excellent 5 266 311 UK (Beach) 10 372 73 UK (Beach) A11436 24/09/2015 06/12/2015 NR NR NR NR A11449 18/09/2014 07/04/2015 M 124 86 Excellent 26 104 202 UK (Beach)
18/09/2014 Excellent 16 121 295 UK (Beach) A11456 10/07/2015 F 128 91 A11466 19/09/2014 31/12/2014 M 125 87 Excellent 14 239 104 UK (Beach)
19/09/2014 15 204 1 UK (Beach) A11469 19/09/2014 F 147 106 Excellent A11470 19/09/2014 21/12/2014 F 126 90 Excellent 17 107 94 UK (Beach)
20/09/2014 23/04/2015 27 41 215 FRA (Trawl) A11478 M 121 80 Excellent A11488 21/09/2014 25/10/2015 M 123 89 Excellent 20 259 399 UK (Beach) A11489 20/09/2014 13/03/2017 F 120 91 Excellent 422 905 UK (Beach) A11498 20/09/2014 27/01/2016 M 118 84 Excellent 26 299 494 UK (Beach) A12974 26/09/2016 25/02/2017 F 130 84 Excellent 29 242 152 UK (Beach) A13031 26/09/2016 15/07/2017 M 117 83 Excellent 25 168 292 UK (Beach) D. intermedius A11480 20/09/2014 25/04/2015 M 150 111 Excellent 26 195 217 UK (Beach)
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