(Dicentrarchus labrax) during migration

Preliminary observations of the behaviour
of sea bass (Dicentrarchus labrax )
during migration
V. A. Quayle and D. Righton
Centre for Environment, Fisheries & Aquaculture Science, Lowestoft
www.cefas.co.uk
The highly similar patterns of vetical movement of two
bass tagged and released together off Guernsey provide
evidence of shoaling for the first two weeks at liberty
(Figure 3a) after which they appear to have separated
(Figure 3b). One of the bass was caught three weeks
later, over 400km from its release location, the other
was caught four weeks later, 240km from release. Whilst
they were shoaling, both fish occupied shallower water
during the night than during the day (Figure 3a).
Introduction
The distribution of European sea bass (Dicentrarchus
labrax) changes seasonally as they migrate between
feeding and spawning grounds, yet their behaviour
during migrations is unknown. We used miniature
electronic data storage tags (DSTs) that record depth and
temperature to identify characteristic behaviours of sea
bass at a fine temporal scale during migrations from and
to spawning grounds within the North Sea and
English Channel.
a)
Date
10
20
30
30
40
No. of occurrences
Depth (metres)
12:00h
20
20
0-10
10-20 20-30 30-40 40-50 50-60 60-70 70-80
30
40
50
25
20
15
60
10
5
0
0-10
50
10-20 20-30 30-40 40-50 50-60 60-70 70-80
70
Average change (in metres) between successive
depth observations (calculated on a daily basis)
60
Day
12:00h
2
00:00h
Figure 3a and 3b: Vertical depth movements of two sea bass
released off Guernsey depicting (a) the first two weeks of their
liberty, when their depths were closely matched (r = 0.43) and (b)
the second two weeks of their liberty, when their depths were no
longer significantly related (r = 0.19).
30
12:00h
10
20
30
3
00:00h
No. of occurrences
Depth (metres)
31/3/06
10
10
Inshore
25
20
15
10
40
Conclusions
5
0
0-10
50
10-20 20-30 30-40 40-50 50-60 60-70 70-80
Average change (in metres) between successive
depth observations (calculated on a daily basis)
60
Figure 2: Behaviour patterns exhibited by bass. Left- Example depth
data for each type. Right- Frequency distribution of the average
change between successive depth observations (calculated on a
daily basis) for each behaviour type in the full dataset.
Table 1: Criteria used to classify daily behavioural patterns.
Behaviour patterns were first identified by eye, and summary
statistics extracted (see text). The percentage agreement between
visual classifications and numerical classifications is shown in the
final column.
Figure 1: Release and recapture positions of tagged bass.
Release positions shown by white crosses and recapture positions
shown by black circles.
29/3/06
30
3
00:00h
10
Behaviour
pattern
27/3/06
70
20
Average change (in metres) between successive
depth observations (calculated on a daily basis)
Day
1
00:00h
0
25/3/06
40
60
30
Diving
To date 10 bass (11%) have been recaptured, of which
four were at liberty between 55 – 79 days and migrated
more than 100km (Figure 1). The six remaining bass
were recaptured within 55 days and showed only limited
dispersal from release sites (up to 89km).
16/3/06
30
17/3/06
0
0
12:00h
23/3/06
14/3/06
40
60
Results
21/3/06
12/3/06
50
50
50
2
00:00h
19/3/06
10/3/06
20
60
40
1
00:00h
0
8/3/06
Date
12:00h
3
00:00h
No. of occurrences
Above: G5 tag
Above right: Tagged sea bass
12:00h
2
00:00h
6/3/06
b)
Mid-water
Day
1
00:00h
0
Depth (metres)
A total of 89 sea bass
(>36cm) were tagged in
the North Sea and English
Channel between
November 2005 and
October 2006. Bass were
anaesthetised with
2-phenoxy-ethanol
(0.5ml-1) before tagging
either internally with a
G5 tag (Cefas Technology
Limited
www.cefastechnology.co.uk)
or externally with a
LTD1400 (Lotek marine
technologies
www.lotek.com). Tags
were programmed to
record temperature and
depth every 10 minutes.
4/3/06
10
Depth (Metres)
Method
2/3/06
0
Depth (Metres)
During their time at liberty, bass spent the majority of
their time at shallow depths. We identified three distinct
daily behaviour patterns (Figure 2; Table 1) that could be
distinguished numerically on the basis of comparisons
between the minimum depth, depth range and an index
of activity calculated as the standard deviation of depth
divided by mean depth. The most common behaviour
pattern was ’mid-water’, during which bass resided
well below the sea surface and made frequent ascents
and descents (mean depth of 27m ± 6.8 m). The second
most common behaviour pattern was ‘diving’, during
which bass undertook many large return dives (up to 70
m) to and from very shallow water, often just below the
sea surface. The final behaviour type we identified was
‘inshore’, during which bass remained in very shallow
water (2 m ± 0.9 m) and during which depth changes
often closely resembled the rise and fall of the tide.
Minimum
depth
Depth
range
Activity
Occurrence
(days)
Success of
numerical
classification
(%)
Inshore
< 3m
< 7m
< 0.55
24
83
Mid-water
> 3m
> 7m
< 0.55
96
93
Diving
< 3m
> 7m
> 0.55
76
81
In summary, our study is the first application of
electronic tags to investigate the behaviour of sea bass.
Our results show that, overall, bass prefer to occupy the
upper layers of the water column, but that behaviour (as
evidenced by depth changes and depth selection) can
change dramatically as bass migrate between spawning
and feeding grounds. The recapture data are consistent
with previous studies that suggest bass migrate annually
between the North Sea and western English Channel.
Suggested further reading
Pawson, M. G., Pickett, G. D., Leballeur, J., Brown, M. & Fritsch, M. 2007. Migrations,
fishery interactions and management units of sea bass (Dicentrarchus labrax) in
Northwest Europe. ICES Journal of Marine Science, 64: 332–345.
Pawson, M.G., Kelly, D. F. & Pickett, G. D. 1987. The distribution and migrations of
bass Dicentrarchus labrax L. in waters around England and Wales as shown by
tagging. Journal of the Marine Biological Association of the UK, 67: 183-217.
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© Crown Copyright 2007