Benthic invertebrates’ by-catches in Ifremer bottom trawl surveys in the English Channel and Southern North Sea: 2006-2018 observations

Bottom trawling mostly targets fish and cephalopods while
mega-zooplancton and most other benthic invertebrates are considered
as by-catches of this fishing technique. The mesh size of bottom
trawl, generally decreasing from opening to cod-end, and the ground
gear used (usually only scratching the seabed surface) result in
mostly mega-(more than 2 cm) epifauna being captured. Although bottom
trawl is seldom recognised as a valid sampling device for benthic
invertebrate species, such observations are nonetheless believed to be
particularly relevant as 1) they represent the benthic fauna fraction
the most likely affected by bottom fishing 2) they integrate
assemblages’ composition over large areas (3-4 km long and 10-20 m
large) and are more representative of larger scale habitat structure
and 3) they are particularly suited to collect over-dispersed or
motile species. IFREMER contributes to the collection of basic
biological data in the English Channel and North Sea through its
annual bottom trawl surveys, the CGFS (Channel Ground Fish Survey,
carried out in October since 1988 on board of the RVs Gwen Drez and
later Thalassa, Coppin and Travers-Trolet, 1989) and the IBTS
(International Bottom Trawl Survey, undertaken in January/February
since 1970 on board of the RV Thalassa, Verin, 1992). Since 2006, all
megabenthic invertebrates captured in the trawl have been identified,
counted and weighted. Additionally, in September 2014, IFREMER carried
out a bottom trawl scientific survey, CAMANOC (Campagne Manche
Occidentale, Travers-Trolet and Verin, 2014), on board of the RV
Thalassa in the western English Channel, where megabenthic
invertebrates caught in the trawl were also identified. The CGFS had a
fixed sampling design while IBTS and CAMANOC had a random stratified
sampling strategy but with varying intensity depending on the covered
survey area. Their data merged together cover the entire English
Channel and southern part of the North Sea although there are much
more observations and longer time series in the eastern part of the
Channel than in the western part. For all three surveys, the sampling
gears used were all Very High Vertical Opening (VHVO) bottom trawls
(or GOV), well adapted for catching demersal species (in particular
fish and cephalopods), with a 10 mm mesh size at the cod-end for
catching juveniles. The sampling strategy was using standard 30
minutes hauls at 4 knot speed during daylight, evenly distributed over
the whole study area. Demersal species and megafauna/epifauna caught
in the bottom trawl were sorted, identified, counted and weighed
(ICES, 2017). The present dataset focuses on benthic invertebrates and
is composed of two tables. The haul table gives information about each
operation (survey, date, location, gear type, swept area in km2). The
catch table gives information on species catch raised as total number
or total weight (g) in the haul. It is strongly recommended to
standardise these values per km2 as the swept area may vary vastly
from one observation to the next, due to both current speed and
difference in gear size. When abundance or weight were not evaluated,
the value -1 is used and marks the presence of the species in the
catch. Colonial species are not generally counted. Bottom trawling
mostly targets fish and cephalopods while mega-zooplancton and most
other benthic invertebrates are considered as by-catches of this
fishing technique. The mesh size of bottom trawl, generally decreasing
from opening to cod-end, and the ground gear used (usually only
scratching the seabed surface) result in mostly mega-(more than 2 cm)
epifauna being captured. Although bottom trawl is seldom recognised as
a valid sampling device for benthic invertebrate species, such
observations are nonetheless believed to be particularly relevant as
1) they represent the benthic fauna fraction the most likely affected
by bottom fishing 2) they integrate assemblages’ composition over
large areas (3-4 km long and 10-20 m large) and are more
representative of larger scale habitat structure and 3) they are
particularly suited to collect over-dispersed or motile species.
IFREMER contributes to the collection of basic biological data in the
English Channel and North Sea through its annual bottom trawl surveys,
the CGFS (Channel Ground Fish Survey, carried out in October since
1988 on board of the RVs Gwen Drez and later Thalassa, Coppin and
Travers-Trolet, 1989) and the IBTS (International Bottom Trawl Survey,
undertaken in January/February since 1970 on board of the RV Thalassa,
Verin, 1992). Since 2006, all megabenthic invertebrates captured in
the trawl have been identified, counted and weighted. Additionally, in
September 2014, IFREMER carried out a bottom trawl scientific survey,
CAMANOC (Campagne Manche Occidentale, Travers-Trolet and Verin, 2014),
on board of the RV Thalassa in the western English Channel, where
megabenthic invertebrates caught in the trawl were also identified.
The CGFS had a fixed sampling design while IBTS and CAMANOC had a
random stratified sampling strategy but with varying intensity
depending on the covered survey area. Their data merged together cover
the entire English Channel and southern part of the North Sea although
there are much more observations and longer time series in the eastern
part of the Channel than in the western part. For all three surveys,
the sampling gears used were all Very High Vertical Opening (VHVO)
bottom trawls (or GOV), well adapted for catching demersal species (in
particular fish and cephalopods), with a 10 mm mesh size at the
cod-end for catching juveniles. The sampling strategy was using
standard 30 minutes hauls at 4 knot speed during daylight, evenly
distributed over the whole study area. Demersal species and
megafauna/epifauna caught in the bottom trawl were sorted, identified,
counted and weighed (ICES, 2017). The present dataset focuses on
benthic invertebrates and is composed of two tables. The haul table
gives information about each operation (survey, date, location, gear
type, swept area in km2). The catch table gives information on species
catch raised as total number or total weight (g) in the haul. It is
strongly recommended to standardise these values per km2 as the swept
area may vary vastly from one observation to the next, due to both
current speed and difference in gear size. When abundance or weight
were not evaluated, the value -1 is used and marks the presence of the
species in the catch. Colonial species are not generally
counted.Bottom trawling mostly targets fish and cephalopods while
mega-zooplancton and most other benthic invertebrates are considered
as by-catches of this fishing technique. The mesh size of bottom
trawl, generally decreasing from opening to cod-end, and the ground
gear used (usually only scratching the seabed surface) result in
mostly mega-(more than 2 cm) epifauna being captured. Although bottom
trawl is seldom recognised as a valid sampling device for benthic
invertebrate species, such observations are nonetheless believed to be
particularly relevant as 1) they represent the benthic fauna fraction
the most likely affected by bottom fishing 2) they integrate
assemblages’ composition over large areas (3-4 km long and 10-20 m
large) and are more representative of larger scale habitat structure
and 3) they are particularly suited to collect over-dispersed or
motile species. IFREMER contributes to the collection of basic
biological data in the English Channel and North Sea through its
annual bottom trawl surveys, the CGFS (Channel Ground Fish Survey,
carried out in October since 1988 on board of the RVs Gwen Drez and
later Thalassa, Coppin and Travers-Trolet, 1989) and the IBTS
(International Bottom Trawl Survey, undertaken in January/February
since 1970 on board of the RV Thalassa, Verin, 1992). Since 2006, all
megabenthic invertebrates captured in the trawl have been identified,
counted and weighted. Additionally, in September 2014, IFREMER carried
out a bottom trawl scientific survey, CAMANOC (Campagne Manche
Occidentale, Travers-Trolet and Verin, 2014), on board of the RV
Thalassa in the western English Channel, where megabenthic
invertebrates caught in the trawl were also identified. The CGFS had a
fixed sampling design while IBTS and CAMANOC had a random stratified
sampling strategy but with varying intensity depending on the covered
survey area. Their data merged together cover the entire English
Channel and southern part of the North Sea although there are much
more observations and longer time series in the eastern part of the
Channel than in the western part. For all three surveys, the sampling
gears used were all Very High Vertical Opening (VHVO) bottom trawls
(or GOV), well adapted for catching demersal species (in particular
fish and cephalopods), with a 10 mm mesh size at the cod-end for
catching juveniles. The sampling strategy was using standard 30
minutes hauls at 4 knot speed during daylight, evenly distributed over
the whole study area. Demersal species and megafauna/epifauna caught
in the bottom trawl were sorted, identified, counted and weighed
(ICES, 2017). The present dataset focuses on benthic invertebrates and
is composed of two tables. The haul table gives information about each
operation (survey, date, location, gear type, swept area in km2). The
catch table gives information on species catch raised as total number
or total weight (g) in the haul. It is strongly recommended to
standardise these values per km2 as the swept area may vary vastly
from one observation to the next, due to both current speed and
difference in gear size. When abundance or weight were not evaluated,
the value -1 is used and marks the presence of the species in the
catch. Colonial species are not generally counted.