Ref. Ares(2018)3458869 - 29/06/2018
Ref. Ares(2019)2387732 - 04/04/2019
Note for the Ministry of Foreign affairs of Denmark
Institut for Kemi og Biovidenskab Fredrik Bajers Vej 7H
9220 Aalborg
Phone. 9940 9940
www.bio.aau.dk
Discard survival in Danish set-net fisheries
1Department of Chemistry and Bioscience - Section for Environmental Technology, Aalborg University, Aalborg,
Denmark.
2Aalborg Zoo, Aalborg, Denmark.
3Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark.
Abstract
A discard ban in the reformed European Common Fisheries Policy include the possibility of exempting species
with proven high discard survival from landing obligation. European plaice (Pleuronectes platessa) is a key
species for commercial and recreational fishing in the North Sea, Skagerrak, Kattegat and Baltic Sea
. Discard
survival in P. platessa from beam and otter trawls range from 68% to 0%, but survival data on trammel fished
plaice is absent from the literature. To address this issue we estimated discard survival in 118 plaice from 13
different net chains, conducted by two fishing vessels over seven fishing days in three different periods in
November 2017, January and February 2018. Individuals were caught using trammel nets having sea bed
temperatures at 2–7°C; salinity, 11-14 ppt; depth, 7–18m; soaking time, 24–48h, and kept 4-10 days in livewells
within local harbours for observation of post-capture survival rate. Fish were individually tagged and the vitality
assessed via catch-related injuries and reflex impairment,
was determined after capture and at the end of the
predetermined observation periods. All individuals were alive at the end of the observation periods. Reflexes
were limited affected by the capturing process, and retained at the end of the observation periods. In conclusion,
our study show the vitality of trammel net fished P. platessa is limited affected after capture, and document a
high short term discard survival.
Background
Discards refer to captured fish that is returned to the sea during commercial fishing and may exceed 50% of the
total catch for some fisheries (Uhlmann
et al., 2014). During netting and handling, captured fish are exposed to
a range of physical (abrasion, crushing, scale loss), physiological (exhaustion, air exposure), and environmental
(light, pressure, salinity and temperature changes) stressors (Davis, 2002
; Tenningen
et al., 2012;
Uhlmann and
Broadhurst, 2015
; Benoît
et al., 2010, 2013
; Davis and Olla, 2002
; Giomi
et al., 2008). Consequently, many
discarded fish dies within days of being returned to the sea; either directly because of catch-related trauma, or
indirectly because reduced vitality increase their susceptibility to predation (Broadhurst
et al., 2006; Wood,
1991; Wood
et al., 1983; Wilson
et al., 2014
; Benoît
et al., 2013
; Davis and Olla, 2002; Depestele
et al., 2014).
These fish represent a waste both commercially and environmentally (Depestele
et al., 2016
; Diamond and
Beukers-Stewart, 2011
; Heath
et al., 2014; Jensen and Vestergaard, 2002).
The Common Fisheries Policy (CFP) of the European Union and has enacted a landing obligation, prohibiting
the discard of quota regulated fish species (Official Journal of the European Union, December 28th, 2013).
Page 1 of 7
Discard survival may be substantial for some species or fishing methods, making the landing obligating a burden
on both the fishers and the environment (Condie
et al., 2014a, 2014b; Guillen
et al., 2014). Consequently, the
regulation includes the possibility of exemption from landing obligations for “
species for which scientific
evidence demonstrates high survival rates, taking into account the characteristics of the gear, of the fishing
practices and of the ecosystem” (Article 15, paragraph 4b). The regulation does not provide an exact definition
of “
high survival rate”, stating that exceptions must be based on scientific evidence, and may be allowed if
“
survival levels are assessed to be sufficiently high”.
Discard mortality is typically assessed via tagging or captivity experiments. Tagging experiments allow
survival to be assessed after the fish have been discarded back into the ocean, thereby providing a more
‘realistic’ estimate of survival rate. However, for many researchers, the high cost of tagging equipment limits
the number of fish that can be assessed using this method (Capizzano et al., 2016). In captivity experiments,
the fish are transferred to livewells and monitored for predetermined periods of time, or until mortality has
subsided. This approach allows a relatively large number of individual fish to be monitored closely on a regular
basis (several times a day), thereby providing a more ‘exact’ estimate of the survival rate. Furthermore, the
ability of fish to recover from sub lethal trauma can be assessed by determining the vitality of the fish after
capture and at the end of the monitoring period. Vitality is assessed based on the degree of injury sustained by
an individual and impairment to its reflexes, which individually and jointly have been found to be good
predictors of survival (Benoît et al., 2010, 2012; Davis, 2010;
Davis and Ottmar, 2006). Injuries are scored of
using a catch damage index (Benoît et al., 2010, 2012) and impairment of reflexes is determined via the reflex
action mortality predictor (RAMP) method, (Davis and Ottmar, 2006;
Davis, 2007, 2010). The RAMP method
consists of a suite of species-specific observations of the voluntary behaviour, stimuli response, and clinical
reflexes of the fish (Kestin et al., 2002;
Uhlmann et al., 2016), and has been used on a wide number of teleost
species, including several species of flatfish (Davis, 2010; Humborstad et al., 2016; Uhlmann et al., 2016).
European plaice (
Pleuronectes platessa) is a key species for commercial fisheries in the North Sea, Skagerrak,
and Kattegat area (Feekings
et al., 2012; Madsen
et al., 2013). Discard survival in P. platessa from beam and
otter trawls range from 89% to 0%
(Depestele et al. 2014
, Methling et al., 2017; Van Beek et al., 1990; Kaiser
and Spencer, 1995),
but data on discard survival from set-net fisheries is absent from the literature.
In general,
a lower discard mortality could be likely in fish caught with set-net, because mortality inducing stressors such
as crushing and exhaustion are minor relative to fisheries using trawls (Depestele et al., 2014
; Mehault et al.,
2016; Uhlmann et al., 2016). Furthermore, fish caught with set-net experience only short periods of air
exposure, because unwanted fish are released from the net and subsequently discarded, almost immediately
when the netting is hauled back on the vessel. This lack of prolonged air exposure would also be expected to
reduce discard mortality, because the duration of air exposure prior to discard is identified as a principal factor
contributing to discard mortality (Neilson et al., 1989,
Parker et al., 2003
; Benoît et al., 2010, 2013
; Castro et
al., 2003
; Macbeth et al., 2006). On the other hand fish might be gilled in the net for several hours and subjected
to stress and injuries. To test this hypothesis, we assess discard survival and vitality in
P. platessa commercially
caught with set-nets in the autumn 2017 and winter 2018.
Materials and Methods
Fish capture and housing
Experimental trials were conducted in the Western Baltic Sea in ICES subsquare 23 (The Sound) and 22 (Belt
Sea) from commercial gill-netters fishing with commercial trammel nets targeting plaice and cod (Table 1).
Table 1 Gillnet information
Abbreviation Code
Standard Abbreviations
ISSCFG
Set gillnets (anchored)
GNS
07.1.0
Trammel nets
GTR
07.5.0
Page 2 of 7
Combined gillnets-trammel nets
GTN
07.6.0
Gillnets and entangling nets
GEN
07.9.0
A total of 118 fish from 13 chains were assessed for discard survival. Chains were collected over 7 fishing days
between 25 November 2017 and 10 February 2018, using two different vessels (Table 2).
Table 2 Vessel information
Name
H 32 - Fuglen
NF 76 - Duddi Krog
Captain/Owner
Jacob Pind
Jonny Krog
Harbour
Sletten, Denmark
Langø, Denmark
Crew
1
2
Overall length
9,79 m
12,6 m
Width / draft
3,60 m / 1,60 m
4.60 m / 2.10 m
Motor
145 HP
175 HP
Gross register tonnage
9,6 t
11,5 t
Trammel net consists of three layers of net: an inner wall of fine-meshed net, with an outer wall of large meshed
net on each side. The outer walls traps fish when they encounter the inner wall and attempts to escape.
Consequently, the mesh size of the inner walls determines the size of fish trapped in the Trammel net. In
general, a larger mesh size traps larger fish, while allowing smaller fish to escape. For the experimental trials,
the nominal full mesh sizes for the inner walls was 150 mm for chain 1-10, and 170 mm for chain 11-13. The
measured inner mesh sizes was 148±0.4 mm for chain 1-10, and 168.4±0.5 mm for chain 11-13. Measured mesh
sizes were determined based on 10 randomly chosen meshes, using a ruler and light hand force to stretch the
mesh. All other technical parameters of the nets also corresponded to commercial practice.
Fishing procedures were conducted commercially and not influenced by the experimental sampling. When
the netting was hauled back onto the vessel, netting and fish passed through a 2x10 kg net hauler (NET-OP 125).
Through the net hauler, the netting and fish were placed on a steel table while the fish are untangled manually
by the fishermen. Untangled fish can be released back into the ocean over the railing (
i.e., discarded) or placed
in a number of plastic containers specifically assigned to individual species. The duration of time from netted
fish exited the sea until they were untangled was very short (often less than 1 min). All of the captured fish were
cleaned manually within 30 min of the last fish being untangled. Because a limited number of plaice were
captured by the fishing vessels, all captured fish with a total length (TL) ≤40 cm were used for experimentation.
Although previous experiments on plaice captured using otter trawl did not find any statistically significant sized
related mortality (Methling et al., 2017), it was deemed important to collect over a size span to detect any
possibly size related mortality in plaice captured using trammel net.
During experimentation, fish marked for potential discard was placed in 45L plastic tanks with oxygenated
seawater. Here, reflex impairment (described below and in Table 3) was assessed and the total length (TL)
measured, after which the individual fish were tagged in the dorsal fin with a 1cm plastic tag and transferred to
90L lidded boxes, submerged in a 300L lidded holding tank with oxygenated seawater. A fifth of the water in
the holding tank was exchanged with fresh seawater every 15 min, ensuring the temperature did not rise above
sea surface temperature. For individual fish, the duration of time from the fish exited the ocean until it was
placed in the holding tank did not exceed 5 min. For chain 1-4, the experiments were performed from a smaller
motorboat following the fishing vessel, rather than on the vessel. In these experiments, the untangled fish were
collected into a knotless net and moved to a 45L plastic tank on the motorboat.
All fish was brought to shore with 3 hours of being placed in the holding tank. Within an hour of arriving at
the dock, catch-related injuries (described below and in Table 3) were assessed and the individual fish randomly
assigned and transferred to 360L livewells within the harbour. The livewells (W120 x D80 x H38 cm) were
constructed of wood, and the sides equipped with holes to facilitate continues exchange of water between the
Page 3 of 7
harbour and the inside of the livewells. The top was equipped with a lid to shield the fish from birds, and the
bottom was covered by a 1-cm layer of sand to simulate the fish’s natural environment. A maximum of 10 fish
was assigned to each livewell.
Post-capture survival rate
Following transfer to the livewells, the post-capture survival rate was monitored every 6 hour for a period of 4-
10 days (Table 4). Individual fish were identified as dead if they exhibited a lack of visible operculum movement,
loss of equilibrium, or were unresponsive to a gentle nudge on the caudal peduncle. Fish identified as dead
were removed from the livewell, euthanized in an overdose of 2-phenoxyethanol, and sacrificed by spinal
transection. Once a day, water temperature, dissolved oxygen, and salinity was measured in each livewell, and
in the surrounding water. Salinity was measured using an EC300 Conductivity Meter (VWR, 1, 100 Matsonford
Rd #200, Radnor, PA 19087, USA). Temperature and dissolved oxygen was measured using a MULTI 3420 D.O.
Meter (WTW, Dr.-Karl-Slevogt-Straße 1, 82362 Weilheim, Germany). At the end of the monitoring periods,
surviving fish were transferred from the livewells to 45L tanks with oxygenated seawater, for a second
assessment of reflex impairment and injuries (Table 4). After this assessment, the fish were
sacrificed via an
overdose of 2-phenoxyethanol and spinal transection.
Reflex action mortality predictor (RAMP) and catch damage index
Assessment of reflex impairment for RAMP (see Table 3 for description of reflexes) was performed in 45L tanks
with oxygenated sea water. Reflexes were selected from 12 candidate reflexes tested on 20 healthy plaice,
caught by seine (Depestele et al., 2014; Methling et al., 2017). Assessment of catch damage index was
performed on wet towels (see Table 3 for description of injuries). Injuries were selected from Methling et al.
(2017). All assessments were completed within 10 min. For RAMP and catch damage index, a score of 1 was
given if a reflex or injury was present and a score of 0 if it was absent.
Table 3 Stimulus and responses of the reflex action mortality predictor (RAMP) test, and description of injuries
assessed with the catch damage index. For RAMP, individuals was scored 1 if the response was completed as
described within 5 seconds of the stimulus, or 0 if the response was absent (
i.e., not completed within 5
seconds). For catch damage index, individuals was scored 1 if the damage was present, or 0 if the damage was
absent.
Reflex
Stimulus and responses
Evade
Swims toward the bottom when released at the surface.
Righting
Righting itself when turned upside down under water.
Tail grab
Struggle or tries to escape when tail is held between two
fingers.
Injury
Description
Abrasion (<10% / 10-50% / >50%):
Bruises and discoloration (both sides).
Fin fraying:
Shredding of the thin skin between the fins.
Blood dot:
Red dots (r ≈ 1 mm) on the downwards facing side.
Blood clot:
Blood clots visible through the skin.
Wounding (head / body):
Shallow cuts or punctured skin.
Deep wounding (head / body):
Deep cuts or punctured skin, often with Bleeding.
Protruding intestine:
Intestines visible through the anus.
Net-mark:
String cuts from net contact.
Results
Page 4 of 7
All plaice were alive at the end of the observation periods (Table 4). The assessments of reflex impairment for
RAMP showed that >90% of the 118 fish completed all three responses (Evade, Righting, Tail grab) when
stimulated, both after capture and at the end of the observation periods (Figure 1). After capture,
approximately 2/3 of the fish had <10% abrasion and 1/3 of the fish had 10-50% abrasion. At the end of the
observation periods, approximately 1/10 of the fish had recovered sufficiently to have their score downgraded
from 10-50% to <10% abrasion. Only a few percent of the fish had >50% abrasion after capture, and at the end
of the observation periods. Approximately 1/2 of the fish displayed 'Fin fraying, Blood dots and Net marks after
capture. For most fishes, these injures were unchanged at the end of the observation periods. The number of
fish with Blood clots declined from 13% after capture to 5% at the end of the observation period. Less than five
percent of the fish suffered Deep Head or Body wounding, or Head Wounding. A few fish appeared to have
sustained Body Wounding during the observation periods (Figure 1).
Figure 1 Number of fish (out of a total of 118) scoring 1 on the reflex action mortality predictor (RAMP) test,
and the catch damage index. Show are number of fish scoring 1 after capture (light grey), number of fish scoring
1 at the end of the observation periods (dark grey), and number of fish scoring 1 both after capture and at the
end of the observation periods (black).
Table 4 Summary of fishing conditions and fish survival.
Soaking
Deck
Bottom
Average
Length
Observation
Chain
Depth
Survival
Date
time
temp
temp
length
range
Numbers
period
number
(m)
(%)
(hrs)
(°C)
(°C)
(cm)
(cm)
(days)
1
25-11-17
7
23
4.0
7.1
32.8±1.9
(30-40)
5
10
100%
2
25-11-17
9
26
4.0
7.0
31.6±0.6
(28-35)
10
10
100%
3
26-11-17
7
24
5.0
7.0
33.8±0.6
(31-37)
8
10
100%
4
26-11-17
7
25
5.0
7.0
32.6±3.6
(22-39)
4
10
100%
5
18-01-18
11
24
6.0
3.8
35.0±0.0
(35-35)
3
7
100%
6
18-01-18
11
25
6.0
4.1
33.0±1.4
(25-37)
10
7
100%
7
19-01-18
9
23
4.0
3.7
34.5±1.3
(25-40)
11
6
100%
8
19-01-18
11
24
4.0
4.0
31.8±1.1
(26-35)
9
6
100%
9
20-01-18
8
24
4.7
3.4
33.5±1.1
(28-36)
6
5
100%
10
20-01-18
10
24
4.7
3.6
33.0±2.0
(27-39)
5
5
100%
11
09-02-18
16
46
0.0
2.3
33.1±0.5
(30-38)
16
5
100%
12
09-02-18
17
47
0.0
2.1
33.9±0.5
(31-37)
13
5
100%
13
10-02-18
18
19
-0.1
2.3
35.4±0.8
(30-40)
18
4
100%
On individual days, during the observation periods, the differences in water parameters (temperature, dissolved
oxygen, and salinity) between the livewells and the harbour was negligible. The dissolved oxygen level was
Page 5 of 7
normoxic, the salinity around 10 ppt and the temperature decreased from ~6°C in November 2017 to ~1°C in
February 2018 (Table 5).
Table 5 Water parameters during the observation periods.
Start
End
Temperature (°C)
Dissolved oxygen (%)
Salinity (ppt)
26-11-17
05-12-17
6.1 ± 0.1 (5.4-7.0)
91.8 ± 0.4 (87.0 - 97.0)
10.7 ± 0.1 (9.8 - 11.9)
18-01-18
26-01-18
3.1 ± 0.1 (3.0 - 3.2)
93.9 ± 0.1 (92.0 - 94.0)
12.2 ± 0.1 (12.0 - 12.4)
09-02-18
14-01-18
1.0 ± 0.1 (0.5 - 1.5)
96.5 ± 0.1 (95.2 - 97.4)
10.5 ± 0.1 (9.7 - 11.1)
Acknowledgement The research was funded by the European Maritime and Fisheries Fund (EMFF) and the Ministry of Environment
and Food of Denmark (Grant number: 17-4010-000155). The authors wish to thank the Society for Sustainable
Coastal Fisheries (Foreningen for Skånsomt Kystfiskeri) for cooperation, and the fishermen Jacob Pind and
Johnny Krog. The experiments were approved by The Animal Experiments Inspectorate (Permit Number: 2017-
15-0201-01297) and methods were performed in accordance with the relevant guidelines and regulations.
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