ICES CM 2015 /Q:11
Net displacement and time at large: Adult migrations probably do not contribute to
whole Baltic scale redistribution of cod (Gadus morhua L.)
Stefan Neuenfeldt*, Christian Möllmann
* Author to whom correspondence should be addressed: Stefan Neuenfeldt, Technical University of Denmark, National
Institute of Aquatic Resources, Jægersborg Alle 1,, DK-2920 Charlottenlund, Denmark, Tel.: +45 3396 3402; email:
[email protected].
Extended abstract
Introduction
Process-based understanding of changes in commercial fish spatial distributions, and to disentangle the role
of natural drivers and various anthropogenic impacts, is a challenging research topic with high relevance to
resource management.
Especially in time of locally high recruitment, it is important to understand beforehand how the local
recruitment is translated to stock recovery over the whole assessment area. In case of Baltic cod, there has
been an increase in abundance in one of the Baltic basin (ICES Sub-division 25), resulting in densitydependent decrease of weight at length (Svedäng 2014). However, the specimen did apparently not migrate
eastwards out of the basin in search for better feeding conditions.
Material and Method
Based on this more anecdotal though a bit surprising observation, we collected a database of historical
tagging experiments on Baltic cod in order to investigate the hypothesis, that only a very small proportion of
the tagged population actually undertakes ‘off-standard migrations’. The term ‘off-standard migrations’
includes those migrations which obviously are not related to either the ‘standard’ feeding and spawning
migrations, which are with the exception of exchange between the Bornholm (ICES Sub-div. 25) and Arkona
Basins (ICES Sub-div. 24), limited to within-basin migrations, but long-distance migrations typically longer
than 100 km.
Data from 1236 cod recaptures from traditional tagging programs and 602 records from data storage tags,
comprising depth, temperature and salinity every 10 minutes while at large, have been collected in the
INSPIRE tagging database. Recapture rate represents <10 % of the actually tagged cod. Hence, to recapture
the total of 1236 cod, more than 12 000 specimen had to be tagged.
Results and Discussion
A preliminary analysis of the traditional tagging data is presented here. 565 tag-recapture data-points
indicate that cod in general do not perform long-distance migrations, but that only a small fraction (<10%) of
the tagged population is conducting trans-basin migrations (Figure 1 A). Furthermore, the net displacement
is independent of the time at large (Figure 1 B). There is hence no diffusion-like process at work. This means
that adult cod migrations probably do not contribute to whole Baltic scale re-distributions of this species.
Furthermore, it implies that regional stock recovery might not lead to recovery of cod in the whole Baltic Sea,
but rather to regional regulation of stock size due to density-dependent processes.
A
B
Figure 1: Net displacement data from a sub-set of 565 recapture (Panel A); Net displacement verus days at
large (Panel B)
References
Svedäng, H., and Hornborg, S., 2014. Selective fishing induces density-dependent growth. Nature Communications, 5:
pp -.