`~ This paper not to be cited without prior reference to the

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This paper not to be cited without prior reference to the author
International Counci1 for the
Exploration of the Sea'
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C.H.1970/B:ll
Gear and Dehaviour Committee
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'Fish Concentration;and' Water'Temperature'
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by
I1.v.Seyd1itz *)
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Observations about the relationship between the occurence of
commercial fishes and the temperature may he1pto save scouting
and fishing time in deep sea fisheries. German trawlers observe
the water temperature by ~eans of the thermosonde (v.Seydlitz,
C.N.1969/D:6 Gear and Behaviour Committee). It is hoped that in
this way more secured data about the temperature range preferred
,by the fish can be found.
In 1969 data were collected during the herring fishery with midwater trawl in the Irish Sea, off the French coast and in the
Gulf of Maine. In Apr~l 1970 data from experiments with the bottomtrawl were collected during fishing for cod near the Cape Breton
(ICNAF Area 4Vn). Results of these observations, as weIl from
commercial trawlers as from the FRV "Walther llerwig" are reported
in this paper.
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I1erring observations in 1969:
In September 1969 German trawlers fished successfully for herring
in.the IrishSea ...The surface temperature was 14-15°C. During
various scouting hauls the temperature in the openin of the trawl
measured by the thermosonde varied bet,~een 4 0 and 11 8C. The best
herring catches were obtained between 7 0 and 8°c. In c01der water
(less than 6°C) herring could not be found., In warmer water (more
than 10°C) horse mackereis (Trachurus trachurus) were caught as
by catch. The histogram (Fig.l) shows the average catch per hour
during fishing time.
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In the Irish Sea the best herring catches were obtained during the
night time. The photo (Fig.2) shows echo traces of the ship's echo
recorder at mid-night and echo traces of the netsonde. The midwatertrawl with an opening of 16-18 mtrs was towed near to the bottom.
Dy using the thermosonde it could be observed that the black traces
didnot move into the upper layers with 9-l0 0 C water temperature.
It seemed that this temperature layer was a boundary for the fishes.
For this reason the trawl was operated successfully near to the
bottom only.
*). H. v. Seydli tz , Dipl..-Diol.
Institut für Fangtechnik
2 Hamburg 50
Palmaille 9
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2
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La~erin
the year German trawlers were fishing on the her~ing
spawning gro~nds off the Freneh eoast. In Deeember sueeessrul
fishing was possible in a temperature range 01' 10.5,- 110C~'
A German trawler fishing in the Gulf of Naine in July 1969, (ICNAF
Area 5) obtained good herring eatehes when temperatures of 7_8°C
were reeorded by the thermosond~ in the opening 01' the trawl. Iri
areas with temperatures higher than 8°C,and lass than 7°C the
mean 01' the eatehes was only the half' of' the eatehes as in the
areas wi~h 7-8 0 C.In areas with temperatur~s lass ~han 6°c na
herring was f'ound (Fig.3).
Cod observations in 1970:
The FRV "Walther IIerwig" operated in April 1970 in the Cabot-Street
and f'ished eod f'or mesh-seleetion experim~nts. During seouting and
f'ishing echo sounder and temperature were observed. It was f'ourid
that good eatehes of' eod eould be made, when, echo traees were
f'ound in the temperature range pref'erred by thc ead at that time.
So it was use1'ul to observe the temperature as asecond parameter
because the echo traces were not only caused by ~he cod, they were
also caused by small f'ood f'ishes. For instance: black traces were
found in the: west 01' lvoolfall-Dank ,,,here the ~~mperature ,w~s 0 0
0.5°C. According to f'ishing experiments the traces originated fro~
eapelin (Nallotus villosus)j f'or eod the maximum catch per ho ur
consisted of' 67 fishes only (Fig.4).
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Near to the St.Pierre-Dank water temperatures between 1 and 2 C
were observed in the mouth of the net. The echo recorder sho,,,ecl
small traces. In the ~eshes of' the net sandcel~ (Ammodytes spee.)
were f'oundj in the same time a ~aximum of' 228 ead ~er one'tiour
fishing was obtained (Fig.5).
Echo traces were f'ound also o1'f' to Cape Dreton~ In the meshes of'
the net herring and small erabs were observed. The thermosande
recorded 2-3.9°Cnear to the bottom. The maximum catch per hour
was 6,808 eod and the average cateh of' 25 bottom hauls was al~ast
2,500 eod. The traces and the 1'ishes could be observed'only iri the
'vater layer with temperatures warmer than 2 0 C. The warm water layer
fu~whieh the cod concentrations oecured, were eovered by cold surface layers with' a temperature 01' 0 0 to 1 0 C (Fig~6) •
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On the Cape Dreton 1'ishing grounds American plaice
Jlippoglossoides
platessoides) were observed as by catch in the lower temperature
ranß'e. 1Vhen the tempera ture exceeded 3 0 C wi t'oh (Glyptocephalus
cynoglossus) and haddock (Nelanogrammus aeglef'inus) were caught.
In the deeper water ,,,i th temperatures higher than 5 0 C ,,,hi te hake
(U~hycis spec.) replaced the cod and the haddock.
ThcOfrequeney distribution whieh illustrates the relation between
the water temperature and the quantity 01' eod caught on the Cape
Dreton f'ishing grounds demonstrates that the best average eatehes
could be obtained between 2 and 3.9 0 C (Fig. 7).
Conelusion
Dy analysis of the temperature observations carried out in the
Institut f'ür Fangteehnik it could be demonstrated that for commercial 1'ishing vessels temperature represents an important 1'actor
.! • . f
- 3 for finding herring and cod during the prespawning and spawning
time. The histograms which illustrate the relation between the
water temperature and the quantity of fish caught by the commercial trawlers and the FRV "Walther IIerwig" in 1 hour fishing time
are similar to anormal frequency distribution.
Since the summer 1970 more and more trawlers are being equipped
with the combined echo-thermo-netsonde •
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•
lolh
5
4
3
2
---- .... _--.
6-6,9
42,8-44,5
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7-7,9
44,6-46,3
00000
000001 • • • • • / • • • • • \
'-8,9
46,4 -48,1
9-9.9
48,2-49.9
10-10.9
50,0-51,7
11-11,9 ·e
51,8-53.4 ·F
Fig.l
Average catches per hour of a German midwater trawler
in the Irish Sea (13 to 18 September 1969) in relation
to water temperature at the net.
Plain white = herring; 0 = mackerel; + = horsemackerel
Ship's sounder eehogram
Netsonde Echogram
Fig.2
Ship's sounder and netsonde eehograms of midwater trawlin$
for herring in the Irish Sea (18 September 19b9, 23.00 h),
As shown in Fig.l, herring stayed in lower water layers of
7 to SOC, so that the midwater trawl was towed with the
footrope very elose to the bottom.
r-----------------
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to/h
1S
10
S
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6 -6.9
42.8 - 44.5
7-7.9
44,6-46.3
8 -8.9
T Co
46,4-48.0 T FO
Fig. J
Average catches per hour of a German midwater trawler
in the Gulf.of Maine in July 1969 in relation to the
wat~'r temperature at the net •
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Fig. 4
Echotraces in the west of the Woolfall-Bank
on the 10th
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of April 1970. The temperature ne ar the bottom was 0 C.
A 60 minutes ground trawl haul yielded 65 cod and capelin.
It seemed ~hat the echotraces were given by the capelin •
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Fig.5
Echotraces near the St.Pierre Bank on the 28th of April
1970. The temperature near the bottom was 1-1.5°C.
A 60 minutes grouhd trawl haul yielded 115 cod and
ammodytes.
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Fig. 6
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Eehotraees on the Cdpe Breton fishing grounds on the
14th of April 1970 (125-130 mm). The thermosonde registered3.1 - 3~20C near the bottom. A 60 minutes haul
with the ground trRwl yie1ded near 6,800 eod.
Stück
Stund e
2500
200 0-
--
1500
---
100 0
---
50 0
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0- 12- 3- 4- 5- T
Co
0,90 1,90 2,90 3,go 4,go 5,90 emperatur
~ljaufang bei Car:>e Breton
ynd auf der Neufundland-Bank
vom 7.4.- 28.4.1970.
Mittlerer Stundenfang von 66
Hols auf Temperaturbereiche
aufgeteilt.
Fig. 7
The average ~ateh of .eod in the Cape Breton fishing
area(7 to 28 April 1970). The ealeulated mean is
shown as eod eaught by hour in relation to the water
temperature.