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Abstract

Worrying trends such as deteriorating condition, decreasing growth rates and diminished distribution range have been observed in the eastern Baltic cod stock in recent years. However, the mechanisms underlying these changes are still poorly understood. In an effort to better understand the observed changes in this stock, since 2016 >23,000 eastern Baltic cod have been externally tagged with T-bar anchor tags and released into the Baltic Sea, as part of the international project TABACOD. In addition, 1260 cod have also been tagged using data storage tags (DSTs) which record high-resolution measurements of temperature and pressure. The conventional external tagging provides new data on the growth and movement of recaptured cod, but perhaps the most intriguing insights into Baltic cod ecology come from recaptured DST tagged cod. Although the number of returned DSTs to date is still rather low (N=23), each one provides a wealth of valuable new data. The unexpected results which have already been revealed by the returned DSTs include information on seasonal migration patterns which would not have been identified from the conventional external tagging alone. Additionally, we found examples of cod vertical movement patterns that aligned with moon phase, potentially following lunar-related changes in prey distribution. We have also gained direct evidence of the predation on one unlucky cod by a cormorant, through a DST recovered in a colony by an ornithologist. This DST indicates very shallow, inshore movements of Baltic cod. Such discoveries provide new pieces of the puzzle, which will help us to better understand Baltic cod ecology, formulate new hypotheses, and guide future research.
A) Overview of recaptures
• Days at liberty between 4-375 days
• Movement of the cod inuenced by season
and release posion (Fig. A)
• Geolocaon needed to reconstruct path
between release & recapture posions
B) Cormorant predaon
• Found by ornithologist on a cormorant
colony half a year aer death
• Captured in shallow, coastal water
(3m depth, Fig. B)
• Inuence on DST performance?
C) Shallow movements
• Scienc survey could miss cod making use of
shallow water habitats in quarter 1 & 4 (Fig.C)
• Upslope or pelagic movements?
• Potenal survey bias if interannual variability
D) Moon phase
• Extent of vercal migraon varies
with moon phase (Fig. D)
• Follow prey organisms in shallow water
at night
• Behaviour only observed with cod released
o the Swedish coast
The Balc cod –
An unexpected journey
Stefanie Haase1*, Kate McQueen1, Monica Mion2, Uwe Krumme1, Michele Casini2, Karin Hüssy3, Annelie Hilvarsson2,
Hans Jakob Olesen3
Conclusion
The DSTs from recaptured cod have already
revealed a surprising diversity in movement
and behaviour patterns of eastern Baltic cod,
both within and between individuals.
Use of shallow water habitats appears to be
more important than previously assumed.
This diversity should be considered when
attempting to understand changes in status
of the stock.
Figure A: Release (le) & recapture (right) posions of the 43 recaptured
DSTs-tagged cod.
Worrying trends such as decreases in stock size
and individual growth rates, and diminished
distribuon range, have been observed in the
eastern Balc cod stock in recent years.
However, the mechanisms underlying these
stock developments are sll poorly understood.
In an eort to beer understand the observed
changes in this stock, 1260 cod in the eastern
Balc Sea were tagged with data storage tags
(DSTs) which record high-resoluon measure-
ments of temperature and pressure. Each of the
43 recaptured tags provides a wealth of valuable
new data, revealing insights into Balc cod
ecology and their individual journeys through
the Balc Sea.
Figure C: Shallowest depth per individual and day between 8 am – 6 pm
(survey operaonal me) The vercal line shows the minimum opera-
onal depth of the scienc survey.
Figure B: Temperature and depth prole of DST-tagged cod which was
eaten by a cormorant. Arrows indicate predaon event.
Feb 15th 2018 Mar 1st 2018 May 1st 2018 May 15th 2018
Figure D: Temperature and depth prole of a DST-tagged cod released
by Sweden.
Nov 16 Dec 16 Jan 17 Feb 17 Mar 17 Apr 17 May 17
North
Sea
North Sea Balc Sea
18°E16°E14°E12°E10°EE
57°N
57°N
56°N
56°N
55°N
18
°
E16
°
E14
°
E12
°
E10
°
E8
°
E
55°N
54°N
54°N
0100 20050 Kilometers
Bornholm
*Stefanie Haase, stefanie.haase@thuenen.de
1 Thünen Instute of Balc Sea Fisheries
Alter Hafen Süd 2 18069 Rostock,
2 SLU, Swedish University of Agricultural Sciences, Lysekil
3 DTU, Technical University of Denmark, Lyngby
new moon full moon Temperature [°C]
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