Survival of sole (
Solea solea), turbot
Ref. Ares(2018)3458869 - 29/06/2018
Ref. Ares(2019)2387732 - 04/04/2019
(
Scophthalmus maximus), brill
(
Scophthalmus rhombus), thornback ray
(
Raya clavata) and spotted ray (
Raya
montagui) discards in North Sea pulse-
trawl fisheries
Author(s):
Edward Schram and Pieke Molenaar
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Contents
Summary
4
1
Introduction
6
2
Materials and Methods
7
2.1 Ethics statement
7
2.2 Experimental design
7
2.3 Sea trips
7
2.4 Experimental facilities
8
2.5 Col ection and assessment of test fish
8
2.6 Control fish
8
2.7 Monitoring of survival and experimental conditions
9
2.8 Data analysis
9
3
Results
11
3.1 Discards survival
11
4
Conclusions and recommendations
15
5
Quality Assurance
16
References
17
Justification
18
Annex 1
Title annex
19
Wageningen Marine Research report reportnumber | 3 of 20
Summary
4 of 20 | Wageningen Marine Research report reportnumber
Wageningen Marine Research report reportnumber | 5 of 20
1
Introduction
Demersal pulse-trawl fisheries in the North Sea is a mixed fisheries that mainly targets Dover sole
(
Solea solea) and plaice (
pleuronectes platessa). In addition to these main target species, various
bycatch species such as turbot, bril and rays are of economic importance to the fishermen.
Undersized fish and species with no market value are discarded. By 2019 this practise of discarding
wil be restricted for al quota regulated species by the implementation of a discards ban by the
European Commission. Fishermen will only be al owed to discard quota regulated fish species when
the discarded fish have high chances to survive.
Accurate estimates of discards survival are required to include the impact of the discard ban, which
leads to 100% mortality among discards, in fish stock assessments. Accurate and fisheries specific
discards survival estimates are also required if fishermen want to apply for high survival exemptions
on the discard ban.
Only one study previously assessed the survival of discards from pulse-trawl fisheries, resulting in
survival rate estimates of 15% (95%CI: 11-19%) for plaice, 29% (95%CI:24-35%) for Dover sole
and 16% (95%CI: 10-26%) for dab (Van der Reijden et al., 2017). For other species that are
commonly discarded by pulse-trawl fisheries such as turbot, bril , thornback ray and spotted ray,
discard survival has never been assessed. Reliable estimates of discards survival for these species are
thus non-existent for pulse-trawl fisheries. As a result the impact of the implementation of the discard
ban on the stocks of these species cannot be assessed nor can fishermen apply for ‘high survival’
exemptions on the discard ban.
This study therefore aimed to provide the first estimates of discards survival in 80 mm pulse-trawl
fisheries for turbot, bril , thornback ray and spotted ray. The survival of sole discards was assessed to
sharpen the first estimate made by Van der Reijden et al. (2017). We col ected these fish at sea
during nine trips with commercial pulse-trawlers and monitored survival for 15 to 18 days .
6 of 20 | Wageningen Marine Research report reportnumber
2
Materials and Methods
2.1
Ethics statement
The treatment of the fish was in accordance with the Dutch animal experimentation act, as approved
by ethical committees (Experiment 2017 D0012.002)
2.2
Experimental design
Al fish were col ected during nine sea trips with three commercial pulse-trawlers and three trips per
pulse-trawler. The numbers of fish col ected per species and trips are presented in Table 1. Sea trips
were spread out over the year (Table 1) to account for the potential effect of varying fishing conditions
throughout the year on discards survival (Van der Reijden et al., 2017). Within each sea trip, fish were
col ected from multiple hauls to account potential for variation in discards survival among hauls. The
typical number of hauls was 40 to 50 per sea trip and for each species the test fish were col ected
from three to five hauls. We aimed to col ect equal numbers of fish per haul but in practice this was
not always possible as for the unpredictable availability of species within hauls. During some trips
mortalities among test fish resulted in empty tanks in the monitoring units which were then utilized to
col ect additional test fish. Survival monitoring started during the sea trip and was continued for 14
days after the fish had been transferred to the laboratory. Total survival monitoring time ranged from
15 to 18 days after col ecting depending on the day of test fish col ection at sea.
Table 1
Overview of sea trips and fish sampling: total number of test fish sampled and control fish
deployed per species and sea trip.
Trip Vessel Year Month Week
Sole
Turbot
Brill
Thornback ray
Spotted ray
# Test #Control # Test #Control # Test #Control # Test #Control # Test #Control
1
UK33
2017 May
18
31
10
9
3
9
2
10
2
-
-
2
GO23 2017 May
21
30
10
11
3
12
3
11
2
-
-
3
TX3
2017 June
24
30
8
15
3
15
3
9
2
-
-
4
TX3
2017 July
28
30
10
8
3
9
3
9
2
-
-
5
UK33
2017 Sept
36
30
10
31
3
9
3
14
2
-
-
6
TX3
2017
Oct
44
30
15
12
3
9
3
14
2
-
-
7
GO23 2017 Dec
49
30
15
9
3
8
3
9
2
-
-
8
UK33
2018
Jan
4
30
10
9
3
10
3
10
2
14
2
9
GO23 2018 Feb
8
Total
241
88
104
24
81
80
86
16
14
2
2.3
Sea trips
Al fishery operations were conducted in the Southern North Sea according to the regular commercial
practices of the pulse-trawlers. Sea trips typical y started on Mondays around 0:00 and ended on
Fridays around 4:00. For each haul during a sea trip the operational and environmental conditions
were recorded by the skipper.
Wageningen Marine Research report reportnumber | 7 of 20
2.4
Experimental facilities
Fish sampled during sea trips were housed in four custom-built monitoring units instal ed on-board of
the vessels. Each unit consisted of a stainless steel framework in which holds 16 24 L tanks (60 cm L x
40 cm W x 12 cm H), resulting in a total capacity of 64 tanks on a vessel. Each tank was equipped
with an individual water supply. A central pump instal ed on the vessel continuously supplied surface
seawater to the tanks. Water flow rates to the tanks were instal ed at approximately one tank volume
per hour to maintain proper water quality. Tanks were covered with transparent lids to limit water
losses by sloshing while allowing for visual inspection of the fish. Upon return of the vessels in their
home ports, the entire units were off-loaded and transported to the laboratory by road in a
temperature control ed truck. Transport time ranged from one to three hours depending on the home
port of the vessel. During transport each unit was placed inside a pumping tank partly fil ed with
seawater and equipped with a submerged pump to supply water to each fish tank in the unit. Fish tank
discharged their effluents in the pumping tank al owing for recirculation of the water. Upon arrival at
the laboratory the fish tanks containing sole were manual y stacked in racks. Turbot, bril and rays
were, grouped by species, stocked in x m2 tanks. Al tanks were connected to a single water
recirculation system consisting of a X L pumping tank and a X m3 trickling filter. Total system volume
was approximately x m3 and continuously renewed with filtered water from the Oosterschelde at a
rate of X m3/d. Al tanks were placed in a temperature control ed room with its temperature set at the
actual North Sea surface water temperature at the time of test fish col ection. In the laboratory, al
tanks were supplied with coarse sand as bottom substrate and the fish were fed daily to visual y
observed satiation with polychaete worms (
Nereis spp) and uncooked brown shrimps (
Crangon
crangon). On-board, bottom substrate was not applied and the fish were not fed.
2.5
Collection and assessment of test fish
Test fish were randomly col ected from the end of the sorting belt. To account for potential effects of
processing time on discards survival (Benoit et al., 2013), fish were col ected as much as possible in
equal numbers at both the start and the end of the catch-sorting process of each haul. Col ected fish
were temporarily stored in 105L holding containers fil ed with seawater. During storage the seawater
in the holding containers was regularly renewed to maintain sufficient dissolved oxygen levels.
Upon completion of fish col ection, fish were sequential y taken from the holding containers to
measure total length (TL: in cm below) and for vitality assessment and tagging. Fish were taken
randomly from the holding containers in case more than the required number of fish had been
col ected. Vitality status of each individual fish was assessed by scoring vitality class, external damage
and reflex impairment as described by Van der Reijden et al. (2017) and summarized in Table 2. For
thornback and spotted ray the protocols for external damage and reflex impairment scores and were
adapted (Table 2). Individual fish were tagged with Trovan Unique glass transponders (type ID100) to
al ow for identification of individuals throughout the experiments. Transponders were injected
subcutaneously just behind the head using the injector IID100E. Upon completion of the vitality
assessment and tagging, live fish were placed in 24 L tanks (see
Experimental facilities) with a
maximum of five (sole), three (turbot and bril ), one or two (spotted rays) or one (thornback ray) fish
per tank. Fish that were found dead (defined as the absence of Head-complex, Table 2) at the
moment of vitality status assessment were recorded as dead at time zero. Dead fish were discarded
and not replaced by live individuals.
2.6
Control fish
In each of the nine sea trips and for each species tested, control fish of the same species were
deployed to separate potential effects of the experimental procedures on mortality from fisheries
induced mortality.
Control fish were obtained from commercial beam trawlers which had been requested to col ect least
damaged and undersized fish from short hauls. Control fish were also col ected during the sea trips
8 of 20 | Wageningen Marine Research report reportnumber
with the pulse-trawlers for use in subsequent sea trips. In both cases col ected fish were stored on-
board in 600L containers fil ed with surface seawater which was aerated and regularly exchanged to
maintain proper water quality. Prior to their use as control fish were kept in tanks in the laboratory for
at least three weeks. During this period, fisheries induced mortality levelled out while surviving fish
could recover from injuries and regain good condition. Fish were fed daily with live polychaete worms
(
Nereis spp) and dead, uncooked brown shrimps (
Crangon crangon) to visual y observed satiation.
Tanks with candidate control fish were inspected daily for mortalities which were removed upon
detection.
During each of the nine sea trips, control fish were taken on-board of the pulse trawler where they
were stored in 600L tanks with aerated and regularly renewed surface seawater. Only fish in visual y
observed good condition, wel fed and without visible injuries, were selected for use as control fish.
Control fish were exposed to the exact same experimental procedures as the test fish, including
vitality assessment, tagging and housing in the monitoring units throughout the experiments. The
number of control fish deployed was approximately 30% of the number of test fish per species (Table
1).
2.7
Monitoring of survival and experimental conditions
Monitoring of survival and experimental conditions started after the first fish had been placed in the
monitoring units. Al tanks containing fish were inspected twice daily on-board and once daily after
transfer to the laboratory. Tanks were inspected for mortalities through the transparent lid of the
tanks by visual observation of fish movement. In case any mortalities were suspected to be present,
these individuals were gently touched with a blunt plastic probe to provoke a behavioural response.
Fish that showed no response were manual y removed from the tank and dead was confirmed by
visual observation of the absence of the ‘head complex’ reflex (Table 2). Lethargic fish were not
removed. Dissolved oxygen concentration and saturation and water temperature were measured
(Hach Lange Multimeter XX). Water flows to the tanks were increased if oxygen saturation was below
60%.
2.8
Data analysis
The overal survival of test fish and control fish was estimated using the non-parametric Kaplan-Meier
estimator (Kaplan and Meier, 1958). PM.
Table 2
Description of criteria to score vitality status.
Vitality class – All
species
Class
Description
A
Fish lively, no visible signs of loss of scale or mucus layer.
B
Fish less lively, minor lesions and some scales missing, mucus
layer affected up to 20% of skin surface area, some point
haemorrhaging on the blind side.
C
Fish lethargic, intermediate lesions and some patches without
scales, mucus layer affected up to 50% of skin surface area,
several point haemorrhaging on the blind side.
D
Fish lethargic or dead, clear head haemorrhaging, major lesions
and patches without scales, mucus layer affected for more than
50% of the skin surface area, significant point haemorrhaging
on the blind side.
External damage scores – All species (Damages marked with * were not scored for
Wageningen Marine Research report reportnumber | 9 of 20
Thornback an Spotted ray)
Damage
Description (1 = present; 0 = absent)
Fin or wings
Fins are damaged or split (including tail fin). Wings in case of
rays.
>50%*
Damage to skin surface, scale or mucus layer at more than 50%
of the dorsal body surface.
Head haemorrhages*
Presence of a haemorrhage in the head of the fish
Hypodermic haemorrhages Presence of a hypodermic haemorrhage
Intestines
Intestines are protruding or are visible through damaged body
tissue of the fish.
Wound
Presence of a wound such that flesh is visible.
Reflex impairment scores – Sole, turbot and Brill Reflex
Description (1 = impaired; no (clear) response within 5 s
of observation; 0 = unimpaired; obvious response within
5 s).
Body flex
Fish is held on the palm of the hand with its ventral side up in
the air. Fish actively tries to move head and tail towards each
other or wriggle out of the hand.
Righting
Fish is held on the fingers of two hands with the dorsal side
touching the water surface. When released the fish actively
rights itself under water.
Evasion
Fish is held underwater in an upright position by supporting its
ventral side with the fingers and its dorsal side with the thumbs.
When the thumbs are lifted the fish actively swims away.
Stabilize
Untouched fish tries to find a stable position flat on the bottom
by rhythmic and swift movement of the fins and/or body.
Tail grab
Fish is gently held by the tailfin between the thumb and index
finger. Fish actively struggles free and swims away.
Head complex
Fish moves its operculum or mouth during 5 s of observation
while laying undisturbed under water.
Reflex impairment scores – Thornback ray and Spotted ray
Reflex
Description (1 = impaired; no (clear) response within 5 s
of observation; 0 = unimpaired; obvious response within
5 s).
Wings
Ray is held out of the water, dorsal side up with one hand
supporting the body at the head of the ray and the other hand
supporting the body at the start of the tail. The ray actively
flaps its pectoral fins (wings).
Eye retraction
While in the water the ray is gently tapped on the head just
behind the eyes with a blunt probe. The ray actively closes and
retracts its eyes.
Stabilize
While resting on the bottom, the ray is gently held by the tail.
When the tail is lifted, the observer notices more resistance
than caused by the weight of the ray; as if the ray sucks its
body to the bottom of the tank.
Tail grab
While resting on the bottom the ray is gently held by the tail.
When the tail is gently pul ed backwards, the ray struggles free
and swims away.
10 of 20 | Wageningen Marine Research report reportnumber
3
Results
DISCLAIMER The results presented in this draft report are preliminary and subject to change because at
the time of writing data collection nor data analyses has been fully completed. The overall
survival per species presented here as preliminary results are subject to change because
more data will be added (trip 9). The preliminary results presented below should therefore
only be considered as indicative for the final, yet to be established final result.
3.1
Discards survival
Mean discards survival per species and discards survival per sea trip and species are presented in
Table 3. Survival is presented as the number of fish alive after 15 to18 days of monitoring expressed
as percentage of the total number of fish col ected (test fish) or deployed (control fish).
The discards survival percentages reported for turbot, bril and thornback ray are based on a limited
number of observations per species (Table 1). Therefore these discards survival percentages should be
considered as indicative for the true discards survival percentages for these species in the 80 mm
pulse-trawl fisheries.
The discards survival percentages reported for sole are based approximately three times more
observations than the other species and therefore probably provides a more accurate estimation of the
true discards survival in the 80 mm pulse-trawl fisheries. To what extend the current estimate for the
sole discards survival represents the true discard survival percentages remains to be established.
The development over time of the survival of discards after col ection at sea is presented per species
in Figures 1 to 4. Mortality level ed out in al cases before survival monitoring was terminated showing
that survival monitoring periods were of sufficient duration.
Clearly discards survival varies among species as wel as sea trips. Environmental conditions such as
sea state and water temperature and fishing conditions such as catch composition varied among sea
trips (data not shown). The effect of these factors on discards survival is subject of further data
analysis and wil be reported in the final version of this report.
Survival among control fish was consistently high (mean survival over nine sea trips > 90%, Table 4)
for al species tested. This shows that it is unlikely that the experimental procedures caused any
additional mortality on top of the fisheries induced mortality.
Wageningen Marine Research report reportnumber | 11 of 20
Table 3
Mean (n=8) discards survival (%) per species and the discards survival per species and sea
trip for control fish and test fish. Survival is presented as the number of fish alive after 15 to18 days
of monitoring expressed as percentage of the total number of fish col ected (test fish) or deployed
(control fish).
Trip Vessel Year Month Week
Sole
Turbot
Brill
Thornback ray
# Test #Control # Test #Control # Test #Control # Test #Control
1
UK33 2017
May
18
45%
100%
44%
100%
33%
100%
40% 100%
2
GO23 2017
May
21
50%
100%
55%
100%
25%
100%
82% 100%
3
TX3
2017
June
24
40%
100%
40%
100%
7%
100%
65% 100%
4
TX3
2017
July
28
23%
100%
63%
100%
0%
100%
56% 100%
5
UK33 2017
Sept
36
3%
90%
17%
100%
0%
100%
57% 100%
6
TX3
2017
Oct
44
10%
100%
33%
100%
33%
100%
79% 100%
7
GO23 2017
Dec
49
0%
100%
22%
100%
0%
100%
44% 100%
8
UK33 2018
Jan
4
0%
100%
0%
100%
10%
67%
0%
50%
9
GO23 2018
Feb
8
Average of 8 trips
21%
99%
34% 100% 14%
96%
53% 94%
Fig. 1 Discards survival curves for sole per sea trip (r=1 to 8).
12 of 20 | Wageningen Marine Research report reportnumber
Fig. 2 Discards survival curves for turbot per sea trip (r=1 to 8).
Fig. 3 Discards survival curves for brill per sea trip (r=1 to 8).
Wageningen Marine Research report reportnumber | 13 of 20
Fig. 4 Discards survival curves for thornback ray per sea trip (r=1 to 8).
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4
Conclusions and recommendations
PM
Wageningen Marine Research report reportnumber | 15 of 20
5
Quality Assurance
Wageningen Marine Research utilises an ISO 9001:2008 certified quality management system
(certificate number: 187378-2015-AQ-NLD-RvA). This certificate is valid until 15 September 2018. The
organisation has been certified since 27 February 2001. The certification was issued by DNV
Certification B.V.
Furthermore, the chemical laboratory at IJmuiden has NEN-EN-ISO/IEC 17025:2005 accreditation for
test laboratories with number L097. This accreditation is valid until 1th of April 2021 and was first
issued on 27 March 1997. Accreditation was granted by the Council for Accreditation. The chemical
laboratory at IJmuiden has thus demonstrated its ability to provide valid results according a
technical y competent manner and to work according to the ISO 17025 standard. The scope (L097) of
de accredited analytical methods can be found at the website of the Council for Accreditation
(www.rva.nl).
On the basis of this accreditation, the quality characteristic Q is awarded to the results of those
components which are incorporated in the scope, provided they comply with al quality requirements.
The quality characteristic Q is stated in the tables with the results. If, the quality characteristic Q is
not mentioned, the reason why is explained.
The quality of the test methods is ensured in various ways. The accuracy of the analysis is regularly
assessed by participation in inter-laboratory performance studies including those organized by
QUASIMEME. If no inter-laboratory study is available, a second-level control is performed. In addition,
a first-level control is performed for each series of measurements.
In addition to the line controls the fol owing general quality controls are carried out:
Blank research.
Recovery.
Internal standard
Injection standard.
Sensitivity.
The above controls are described in Wageningen Marine Research working instruction ISW 2.10.2.105.
If desired, information regarding the performance characteristics of the analytical methods is available
at the chemical laboratory at IJmuiden.
If the quality cannot be guaranteed, appropriate measures are taken.
16 of 20 | Wageningen Marine Research report reportnumber
References
PM
Wageningen Marine Research report reportnumber | 17 of 20
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Justification
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the Management Team of Wageningen Marine Research
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Annex 1 Title annex
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Wageningen Marine Research
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T +31 (0)317 48 09 00
established to provide the scientific support that is essential for developing
E: xxxxxxxxxxxxxxx@xxx.xx
policies and innovation in respect of the marine environment, fishery
www.wur.eu/marine-research
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