Download our poster about cod

NE Atlantic cod stocks are in serious decline and the long-term sustainability of cod fisheries is of immediate concern.  The paucity of data on migration and distribution of cod in relation to key life history events such as spawning, at appropriate spatial and temporal scales, presents an obstacle to assessing the suitability of current assessment methodologies and the efficacy of management and conservation measures such as closed areas.  CODYSSEY (Cod spatial dynamics and vertical movements in European waters and implications for fishery management) was a 4-year EU-funded R&D project that involved nine European research institutions (click on logos at top of page) in eight different countries and was funded jointly by national governments and the EU between 2002 and 2007. The principal aim of CODYSSEY was to improve the biological and ecological understanding of cod stocks in four ecosystems (the Barents Sea, the North Sea, the Baltic Sea and the Icelandic/ Faroe plateau) by combining individual-based behavioural data collected from electronic data storage tags (DSTs) with environmental data to understand the processes that underpin horizontal and vertical movements in different ecosystems. In all, over 3,000 tags were released between 2002 and 2005 in the Barents Sea, the North Sea, the Baltic Sea and the Icelandic and Faroe Plateau. Over 850 tags have been returned by fishermen to date, and more than 130,000 days of data were collected – equivalent to 350 years of data. The longest dataset was returned from a cod tagged in Icelandic waters, which remained at liberty for nearly three years before recapture.

The key findings of the study were as follows:

1. Cod did not always migrate between feeding and spawning grounds; many were resident in the same locale for the period of their liberty, and some showed evidence of using both strategies.  Examples of resident cod were found in all ecosystems.  Migratory cod often travelled hundreds of km as they moved from spawning grounds in spring to summer feeding grounds.  Migrations of cod at liberty close to, or over, a year could be most accurately described as homing migrations.  Spawning migrations of cod tagged in the Skagerrak provided some evidence that cod can exhibit natal homing.  Stock (and sub-stock) separation was most evident at spawning time, when the area occupied by each stock contracted as cod sub-stocks congregated on spawning grounds.  At other times, the occupied areas of cod sometimes overlapped as individuals dispersed onto feeding grounds.  Complex spatial relationships between sub-stocks were therefore an emergent property of the migratory system.
2. Overall, cod inhabited depths between 10m and 850m and, in general, mean depth was greater in the summer and autumn months that it was in winter and spring.  Cod experienced temperatures as low as -1.5°C as they foraged on pelagic prey in polar fronts in deep water off the Icelandic coast or in the Barents Sea, or as high as 21°C as they rested close to the seabed in the southern North Sea respectively.  Resident cod experienced relatively small depth ranges relative to migratory cod, and also experienced the natural warming and cooling as the seasons progressed from spring through to winter.  In contrast, migratory cod experienced much larger depth ranges and, in the polar ecosystems, experienced cooler temperatures in frontal waters during the summer, than they did in coastal waters during winter.
3. Cod vertical movements were variable with ecosystem, migratory strategy and season.  The timing of vertical movements was also influenced by tidal and day/night effects.  The magnitude of vertical movements was proportional to residence depth, so cod were found closer to the seabed in shallower water than in deeper water.  During bouts of activity, cod tended to move in a ‘saw tooth’ pattern, with ascents balanced by descents and vice versa. 
4. The potential for otoliths or ‘ear stones’ to perform as ‘natural data loggers’ was investigated.  The structure of otoliths was related to temperature and food intake, although the relationship was not simple.  In consequence, it proved difficult to reconcile the patterns in otoliths returned with the data collected by electronic taga.  However, there were clear differences in otolith structure between different ecosystems, principally due to the degree of seasonality in temperature that cod experienced.  These differences were reproducible using an otolith growth model developed in the IBACS project.

In conclusion, the results of CODYSSEY have shown that Atlantic cod show a remarkable behavioural repertoire and environmental tolerance that permits individuals to occupy a wide range of marine habitats.  Vertical and horizontal movements appear to be regulated by the interaction between simple behavioural rules and habitat complexity. In consequence, complex spatial dynamics activity patterns were observed.  Individual-based models and otolith analysis show considerable promise as tools to further understand cod ecology at a population and regional level.  The CODYSSEY project has therefore resulted in a number of insights into cod behaviour and ecology that have relevance to fisheries management at national and international level.