Not to be cited without prior reference. to the ~uthors, .
Internatio'nal Council for the
Exploration of the Sea
C1\I1996/, 5:21
ShelfEdge Current and its
Effects on Fish Stocks
HYDROGRAPHY AND l\fACKEREL DISTRIBUTION ON THE
SHELF EDGE WEST OF THE NORWEGIAN DEEPS
D G Reid, 1\1 Walsh and W R Turrell
•
SOAEFD Marine Laboratory
PO Box 101, Victoria Road
Aberdeen, AB119DB
, Scotland, UK
SUMMARY
A combined acoustic, fishing and hydrographie survey was carried out in the Viking Bank
area ofthe North Sea in December 1995. The aim ofthe survey was to locate and map the
distribution of the western mackerel/in the area prior to the start of their spawning
migration und to determine the hydrographie background. The maekerel were loeatcd in
high concentrations in greater than 200 m ofwater on the western edge ofthe Norwegian
Trench. The area was defined with cooler waters to the east, south and west, suggcsting
that the distribution was controlled by water temperature. Data from CTD sections, sea
surface temperatures and eurrent meter moorings are presented and the implications for
the control of distribution and migration of the western mackerel are discussed.
INTRODUCTION
Recent work on the migration of scombrid fish has suggested that water temperature is one
ofthe major environmental parameters controlling direction and speed ofmigration (Walsh
and l\fartin, 1986; Castonguay et al., 1992; Walsh et al., 1995; Reid et al., in press). The
winter migration ofthe western mackerel (Scomber scombrus) from feeding grounds in the
North and Norwegian Seus to spawning areas south and west of Ireland occurs in the
. months of December to March. The migration path follows the shelf edge for most of its
route, with the fish being found generally between the 100 and 250 m contours (Walsh et
al., 1995). Walsh et al. showed that the migration around the north of Scotland appears to
followa track which coincides with a tongue ofwarmer water transported norlhwards up
the shelf edge by the shelf edge current (SEC). Observations made during an acoustic
survey in January 1995 (Reid et al., in press) indicated that when the migrating mackerel
encountered an intrusion of unusually warm water onto the shelf the fish stopped their
active migration and adopted different schooling behaviour. This led us to hypothesise that
the spawning migration ofthis species may be influenced by "enviroregulation". This is a
process by which the fish select their immediate environments by behavioural means (Neill,
1984). If the fish find themselves in some "non-preferred" temperature, they may swim
1
1
faster or deeper in an attcmpt to gain more preferred temperatures. For example, Olla et
al., (1975) showed that mackerel swam faster at water temperatures below 7°C. :l\ligration
may be triggered when water temperature drops below a threshold, and the subsequent
migration route constrained by the narrow tongue of warm water derived from the
northward flowing current.
Analyses of commercial catch data have shown that prior to the start ofmigration, the bulk
ofthe western mackerel appear to assemble along the 200 m eontour in the north eastern
North Sea (east ofViking Bank, approximately 60 0 N 3°30'E, see Fig.l). The fish are known
to arrive in this area in late autumn (from Oetober) and remmn in the general area until the
start of aetive migration (Skagcn pers. eomm.). This area lies within the Norwegian
eontrolled sector ofthe North Sea and is not open to maekerel fishing for UK fishing vessels.
As a result, from December on, the UK fishing vessels tend to wmt in UK waters until the
fish start their migration (Masson, Simpson pers. eomm.). Given aecurate information from
these vessels, it is then possible to determine the timing of the start of migration fairly
aecurately. In recent years migration has usually commenced in early to mid December.
If enviroregulation is constraining the distribution and migration of the mackerel, then it
would be expeeted that immediately prior to the start ofmigration, the fish would tend to
eoncentrate in the warrnest areas ofthe North Sea, leaving those areas which cool earlier
or faster. As these areas of aggregation also eooled migration would commence. The shelf
edge area adjacent to Viking Bank is likely to be one area where the water will stay warmer,
longer due to Atlantic inflow along the shelfmargin. The present study was designed to use
a eombined acoustie, hydrographie and fishing survey to:
•
•
•
confirm anecdotal information on the pre-spawning aggregation ofmackerel
to determine the hydrographie strueture in the area of mackerel aggregation and
whether this matched the predietions based on enviroregulation
if possible, observe the mackerel at the start of their migration and determine the
eues for that start
l\IATERIALS AND l\IETHODS
A combined aeoustic, fishing and hydrographie survey was conducted from the Seottish
Office research vessel Scotia from 1-15 December 1995. Seven overlapping acoustic surveys
were completed. The first ofthese was condueted as a pilot survey (Fig. 1), eovering the area
between 59°15'N and 61 °30'N and along the 200 m eontour (from 64 km west ofthe contour
to 32 km east). This area was chosen on the basis of information from commercial. The
subsequent six surveys concentrated on a smaller area which eontained all the observed
mackerel echotraces. This covered an area from 59°15' to 60 0 40'N (Fig. 1), and within
16 km ofthe 200 m contour to the west and 8 km to the east. All the surveys followed a zigzag track design. The first survey used a baseline transect spacing of 16 km. The
subsequent surveys used a baseline transect spacing of 3 km.
Acoustic data were colleeted using SIl\IRAD EK500 38 kHz and 120 kHz split beam
echosounders. The transceivers were mounted in a catamaran towed body which was towed
from a boom alongside the vessel at a depth of approximately 4 m. The echosounder range
was maintained at 250 or 500 m throughout the survey with a pulse interval of 1.5 or 3 msec
respeetively. Output was recorded eontinuously as hard-copy and in digital form. Tbe hard
copy ofthe echogram was printed out in eolour using a Hewlett-Packard PmntJet interfaeed
2
•
...
to the echosounder. Digital data \vas transferred by ethernet to a SUN SPARC ii>c
computer and recorded trarismission by transmission in 0.5 in depth sampies on DAT tapes.
Echo integration (MacLerinari and Siminonds, 1991) ,vas caiTied out over 15 min intervals
(25 'mni at 10 knots) by the echosounder and recordcd on tho. priiüout. The system was
calibrated usirig a standard target prior to the sUrVey (Foote et al;, 1987).
Fishirig was carded, out on fish concentrations identified [rom the echogram, using a
strengthened pelagic trawl. All fish (or a subsampie for largo. eatches) were scored for
, species, h~ngth, weight, sex and stornach contents. Otoliths were taken from subsampies of
the maekerel und herring for subsequent age determination in the laboratory.
e
",
Sen. surface temperature and salinity \vere eontinuously recorded throughout thc survey
using an OeeanData model TSG 103 thermosaliriograph connected to the vessels non-toxic
sea water supplY. Data were recorded with for subsequent arialysis on a PC inicroeomputer.
Simultaneous navigation data \vere recorded from a GPS navigation system connectE:~d to
a PC rimning a plotter (MicroPlot - SIS Systems, Aberdeen). Aseries of CTD arid XBT
'stations were carried out duririg the sUrVey, and a CTD section aeross the 200 m eontour
\vas completed. Observations were obtriincd using a SeaBird 25 sealogger CTD.\Vith rosette
water sarripler. Salinity calibrations ,vere performed using in situ water sampies analysed
on a Guildline Poitasal salinometer.
Four current meters (Anderaa Rcil-7) were deployed aerosEl thEi shelfbieak. Positions etc
are presented in TabIe 1. The teinperature and eurrerit measurements obtained from the
three moorings were Iow passed filtered in order to remove the tidal component. Tbe
prinCipal direction derived from tho bottom eurrent meter was used as the loeal along
isobath direetion; and velocities -\vere resolved into aIong and across isobath directions.
RESULTS
Th~ initial task ofthe survey was to deteimine the inain Ioeation ofrriaekerei aggregation.
The map plotted in Figure 1 shows the general area ofthe survey. Thc pilot surveynrea
was carried out to 10caIise the main concentrations ofmaekerel. The bulk ofthe maekerel
'seen on this pilot survey were eorieentrated in a rectanguID.r area defined by 59°30' to
60 0 30'N and 3°00' to 3°30'E. Twelve suceessful trawlirig operations were eamed out
(Fig. 2). Five ofthese \vere doininated by maekerel, five by hemng, and two eoritriined other
species, mainly Nonvay }:lout, blue ;,vhiting and other gadoids. On the basis ofthese trawls,
eehotraees seen during the sUrVey were assigned as mackerei, herring or others. In same
eases it ,vas not possible to give adefinite assigilment. Figure 3 is a c1ose-up viewofthe .
mairi survey area sho\ving the locations i:>f those eehotraees whieh could bei positively
assigned to herring or maekerel. It was apparent that the bulk ofthe mackerel i.verc found
in areas ofwater depth greater thari 200 m, and the hemng, generally, in shallower waters.
The hydrographyofthe area was exaniined in i:i number of\vays., Figure 4sho\vs a eontour
plotofthe sea surfaee temperature (SST) recorded during the pilot survey. The area \vas
charactcdsed by warnier water in the riorlh and west, whieh extended iri an are towards the .
main area ofmaekerel coneentratiori, in tho lo\ver right hand corner. The area whcire the
fisn were eoncentratEid had cooler surfaee waters to the east, south' and west. FigUre 5
sho\vs the evolution of SST in the main survey area as eontour plots for threo time penods:
4-6 Deeember (Fig. 5a), 7-9 Deceinber (Fig. 5b) and 9-12 December (Fig. 5e). These plots
confirm the preseriee of a tongue of warm water sUITounded by cooler surfaee waters.
3
Additionally, the survey area appears to have bccome slightly warmer during thc survcy
pcriod."
Thc CTD seetion was carried out to determine the vertical structure in the study area. A
total of12 dips were carried out. The results ofthese dips are presented in Figures 60. and
6b, along with the locations ofthe mackerel and herring aggregations. The shelf edge area
where the mackerel were concentrated was characterised by the wurmest ,vater across the
section. To the east, the surface waters were cooler an"d less saline~ owing to the presence
ofthe Norwegian Coastal Current. Tbe deeper arcas to the east, in the Norwegian trench
were made up of cold, saline ,vater - Norwegian trench b()ttom water. To the west of the
shelfbreak, there was litUe or no stratification, hut the water was again cooler than at the
shelf break. All the evidence from both horizontal and vertical profiles of temperature
" suggest that the mackerel were concentrated in the area ofthe Norwegian Trench with the
highest temperatures. This interpretation is confirniE~d by the TS plot for the section in
Figure 7.
Prior to the survey, four current meter moonngs were deployed across the shelfbreak in the
north of the study area, the positions and other information are given in Table 1. The
low-pass filtered current components and low-pass filtercd temperatures are shown in
Figure 8a-c. At the two most easterly moorings, it was evident that the decay of
temperature stratification during the recording period occurred as aseries of events, most
probably wind-driven (Figs 8a and b). After complete vertical mixing had occurred, some
events reintroduced stratification, by rripid cooling of the bottom waters. These cooling
cvents were not clearly associated with reversals in tho along isobath components, hut may
have been more dependent on an increased on-shelf flow of cold bottom water from the
deeper Norwegian Trench. At mooring 543 stratification was much more persistent
(Fig. 8c), with cooler waters remaining at the bottom until the end of the observational
period. In order to clearly understand the results and to identify the controlling
mechanisms responsible for the maintenance or erosion of thermal stratification on the
Viking Bank, and the creation of cooling events, further comparative work is required
. relating the current meter results to one another, arid to wind observations. This work is
. '
In progress.
Figure 9 shows stick plots for the three moorings which sUrVived to be collected in
December. Mooring 542 was furthest to the cast, at 3°13 I E,just cast ofthe 200 m contour.
The plots show that the flow was generally from the NNW and stronger at the decper
current meter. There was some evidence of reversal in the flow at meters 541 and 542
around the 18 November and again around 28 November. The westemmost moonng, which
was over 30 mHes [rom the 200 ni contour, showed weaker transport and a much more
~ariable pattern, rind was probably dominated by ,Vind efTects.
DISCUSSION
The hypothesis being investigated in this study was that the distribution ofmackerel is, at
least in part, controlled by imviroregulation (Neill, 1984). 1fthis hypothesis was true, the
mackorel would be expected to move from the areas of low ,vater temperature rind to
concentrate in the oreas of highest ,vater temperature. Anecdotal information from
fishermen have suggested that the mackerel concentrate in such an areri ofthc North Sea
every year prior to migration. "The present study has confirmed this and also shows that the
mackerel are, as hypothesised, concentrated in the warmest cireas. This needs to be
4
.,.
e
~
qualified. Enviroregulation assurnes that below a certain water temperaturo tho mackoz.ol
will start to move faster, when they arrive in warmer water their movement will slow down
and they will tend to concentrate there as long as the water temperature is suitablo.
Therefore we assurne that the temperature in this area during tho survey period was high
enough that the fish were not stimulated to swim further.
.
Examination ofthe temperaturc records from the current meters shows that the deep water
temperature in the area of mackerel concentration is relatively stable from September to
Docomber, and actually rises slightly over the period. This is prosumably due to the mixing
ofthe warmer surface waters as the stratification breaks down. Further onto the shelfarea
ofthe North Sea tho deep wator temperature is consistently lower «8°C) throughout the
deploYment period. Tho deeper water in tho Norwegian Trench is characterised as generally
being cold water «8°C). So the mackere1 are essentially constrained between two colder
water bodies. Tho curront metors show thrit tho flow into the area along tho 200 m contour
is consistently from tho north, and this should transport tho warmer Atlantie water of tho
Slope Curront into tho aroa. This is eonfirmed by tho surfaeo temperaturo plot in Figuro 4.
Further sotith tho wator temperatures are also eolder (see Fig. 5) and it may bo assumed to
eonstrain migration in that direction.
There was no evideneo from this study thrit the maekerol had eommenced their migration.
The fish aggregations wero seen repeatedly at tho same loeation, and wero usually in
contact with tho seabed. Fishing vessels in the Scottish sector reported no sightings of
maekerol sehools, entering their wators. Wo would hypothesise that the migration would
bo likoly to start when tho aroa tho fish were oeeupying during the survey had eooled below
a eritieallovel. From the eurrent meter temperaturo reeords, this would bo likely to oeeur
oither through slow eooling with the onset ofwinter, or more rapid eooling due to reversal
of the normal eurrent direetion allowing the cooler southern waters to enter the area
oeeupied by the fish. Both these scenarios may initiate migration, and the two possibilities
may explain the vadability in inset ofmigration.
ACKNOWLEDGEMENTS
The work presented here was carried out as part ofthe SEFOS projeet,jointly supported by
tho European Commission AIR programmo and SOAEFD. We would liko to thank the
offieers and crewofFRV Scotia for their help. Wo would also liko to thank Mr P Copland,
Mr R Adams and l\lr R Payne for their support during the survey.
REFERENCES
Castonguay, M., Roso, G.A. and Legget, W.C. 1992. On movcments of Atlantie mackerel
(Scomber scombrus) in tho northern Gulf of St Lawroneo: assoeiations with windforcod advoctions of warmed surfaee waters. Can. J. Fish. Aquat. Sei.; 49, 22322241.
Foote, K.G., Knudsen, H.P., Vestnes, G., MaeLennan, D.N. and Sirrimonds, E.J. 1987.
Calibration of aeoustic instrumonts for fish density ostimation: a practieal guido.
lCES Cooperative Research Report, 144.
5
MaeLennan, D.N. and Simmonds, E.J. 1991. Fisheries Acoustics. Chapman and Hall,
London and New York. 325pp.
Neill, W.H. 1984. Behavioural enviroregulation's role in fish migration. In: Meehanisms
of Migration in Fishes, McCleave, J.D., Arnold, G.P., Dodson, J.J. and Neill, W.H.
(eds), pp61-66. Plenum Press, New York.
OHa, B.L., Studholme, AL., Bejda, AJ., Samet, C. and Martin, A.D. 1975. The effeet of
temperature on the behaviour of marine fishes. In: Combined Effeets ofRadioaetive,
Chemieal and Thermal Releases to the Environment. International Atomie Energy
Agency, Vienna, Austria.
Reid, D.G., Turrell, W, Walsh, M. and Corten, A Cross-shelfprocesses north ofScotland in
relation to the southerly migration of western mackerel. lCES J. Mar. Sei. (in
press).
Rose, G.A and Legget, W.C. 1988. Atmosphere-ocean eoupling in the northern Gulf of
St Lawrence: frequency-dependent wind-foreed variations in near sea temperatures
and currents. Can. J. Fish. Aquat. Sei., 45, 1222-1233.
Walsh M. and Martin J.H.A 1986. Reeent changes in the distribution and migrations of
the western maekerel stock in relation to hydrographie changes. ICES CM
1986/H:17 ppl-7, 2 Tabs, 9 Figs. (mimeo).
Walsh, M., Reid, D.G. and TurreH, W.R. 1995. Understanding mackerel migration off
Scotland: tracking with echosounders and commercial data, and including
environmental correlates and behaviour. lCES J. Mar. Sei., 52, 925-939.
6
TABLE 1
Current meter details
Record
No
5411
....................
5412
5421
.........................
5422
Sounding
(m)
Valid data starts
Valid data ends
27
60 26.83'N 02°51.86'E ......................
95
106
1100 hrs
22 September 1995
0900 hrs
11 December 1995
30
60 28.55'N 03°13.02'E ............................
195
206
1500 hrs
22 September 1995
0900 hrs
7 December 1995
120
1500 hrs
24 September 1995
1500 hrs
24 September 1995
Position
0
0
31
5431
..................
5432
Depth
(m)
0
60 30.17'N 02°8.95'E
.....................
109
.
' e
Figure 1. Survey areas antpositions of mooringsand erD Une - December 1995
63
..
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...............
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Pilot survey area
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Currenf meter mooring
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Figure 2 Haul positions and composition December 1995
63
Mackerel - few herring
....
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o
62
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Figure 3. Positions eherring and
mackerel aggregations4 • 13 December 1996
(
3.0
)
3.5
e
e'
Figure 4~ Surface temperature contours-Viking'Bank Area December 1995,
61.50-....,---------~-:---~~-~-:--~---~~-------....-..-
61.00-
Latitude
I
1'.00:
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1.50
2:00
2.50
L.ongitude
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Figure 5a~ Seä sujface temperatures
'/""
4 ~ 6 Decemoor 1996
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Sea sunace temperatures
7 ~ 9 Decemoor 1995
60.
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-0
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59.9-
59.7-
59.6-
3.4
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-
Longitude
Figure Sc. Sea Surface Temperatures
9 - 12 December 1996
•
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-c
::J
....,
.-
60.
~
59.
•
59.
59.
59.
3.0
3.1
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3.4
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-
-
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-----
---~-
.
Figure Ga. Tenftjerature contours Jrld:fishl positions
along erD line - Decemb,er"1995
•
l
.
..............
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···8····8··················:·.
.
.
50
100
----9.2;-- - - - - I
150
200
300.00E
12-12-1995
Latitude 59
55.00N;
320.00E
longitude'
I
340.00E .
I
Figure, 6b. ~linity/contours anaetish: positions"
along Cl1Ddihe:- D~c~mber:"1995'
50
•
<=>" - - - - - - 35.2~
100
150~
200
Depth (m);
3~00.00E
12-12-1995
Latitude 59,
55.00N;
3
40~OOE
I
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Figure 7. Temperature & salinity plot at erD Section
9.5
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Figure 8a.
Along isobath current component and temperatures at the
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