Sixty data storage tags were analysed with respect to depth and ambient temperature and for relationships between depth, ambient temperature, and growth. The fish were tagged and released in April 1996–1999 on the spawning grounds at the southwest coast of Iceland. Profiles of depth and ambient temperature suggest that cod (Gadus morhua L.) that spawn in the study area select between two alternatives in foraging strategies, i.e., deep- or shallow-water migrations. The shallow-water fish appear to follow the seasonal trend in temperature characteristic for the shelf waters. The deep-water fish, however, migrate to deeper and cooler waters outside the spawning season and increase their vertical movement. A significant positive relationship between depth and ambient temperature was observed for the shallow-water fish, and a significant negative relationship was observed for the deep-water fish. Daily variation in depth and ambient temperature showed increasing trends with increase in depth, in particular for deep-water fish. Growth was negatively related to depth and positively related to ambient temperature. This indicates higher growth rate of cod that forage in shallow waters versus deeper waters. Supply of food, for which depth may be a reasonable proxy, offers a more likely explanation for this growth pattern than environmental temperature.

Barbaro, A., Einarsson, B., Birnir, B., Sigurðsson, S., Valdimarsson, H., Pálsson, Ó. K., Sveinbjörnsson, S., and Sigurðsson, Þ. 2009. Modelling and simulations of the migration of pelagic fish. – ICES Journal of Marine Science, 66: 826–838. We applied an interacting particle model to the Icelandic capelin stock to reproduce the spawning migration route for three different years, successfully predicting the route for 2008. Using available temperature data and approximated currents, and without using artificial forcing terms or a homing instinct, our model was able to reproduce the observed migration routes from all 3 years. By a sensitivity analysis, we identified oceanic temperature and the balance between the influence of interaction among particles and the particles' response to temperature as the control parameters most significant in determining the migration route. One significant contribution of this paper is the inclusion of orders of magnitude more particles than similar models, which affects the global behaviour of the model by propagating information about surrounding temperature through the school more efficiently. To maintain the same dynamics between different simulations, we argue a linear relationship between the time-step, radii of interactions, and the spatial resolution, and we argue that these scale as N−1/2, where N is the number of particles.

We investigated the behaviour patterns of coastal (relatively stationary in shallow waters) and migratory frontal (offshore) types of Atlantic cod Gadus morhua in Icelandic waters using information from data storage tags (DSTs). Consistent and repeated patterns of migration were observed for both coastal and frontal cod, with characteristic DST profiles repeated from year to year. The migration timing in 2 successive years was close to being synchronous, suggesting that the onset of migration was consistent from year to year. A tidal location model suggested that the feeding migrations of cod in Icelandic waters were undertaken in groups or shoals during the whole year. The overall stability of the behaviour patterns of both types suggests that coastal and frontal individuals use different behavioural strategies which might either be related to food availability/ competition or genetic control.

A model is proposed for investigating the statistical properties inherent in data on fish stomach contents of cod (Gadus morhua) taken on board Icelandic research vessels. The model explicitly incorporates components to describe the spatial distribution of prey species. It is found that combined delta-gamma (a presence-absence model and an abundance where present model, respectively) generalized additive models are useful in the analysis and explain a considerable portion of the variability in this type of data set, but a fairly large number of degrees of freedom is required to describe the behaviour of the data. A feature of the proposed model is the ability to combine information on the stomach contents from several predator length groups into a single analysis. Analytical results indicate that sampling procedures that base the analysis on the contents of individual stomachs are preferable to those based on combining the contents of several stomachs into aggregate samples.Résumé : Un modèle est proposé pour étudier les propriétés statistiques inhérentes aux données sur le contenu stomacal de la morue (Gadus morhua) capturée par des navires de recherche d'Islande. Le modèle incorpore explicitement des éléments pour décrire la distribution spatiale d'espèces proies. On a constaté que les modèles additifs généralisés delta-gamma (un modèle présence-absence et un modèle abondance là où il y a présence, respectivement) sont utiles pour l'analyse et expliquent une bonne part de la variabilité dans ce type d'ensemble de données, mais un nombre de degrés de liberté assez élevé est nécessaire pour décrire le comportement des données. Une caractéristique du modèle proposé est la capacité de combiner l'information sur le contenu stomacal de plusieurs groupes de longueurs de prédateurs dans une seule et même analyse. Les résultats obtenus indiquent que les méthodes d'échantillonnage qui fondent l'analyse sur le contenu d'estomacs individuels sont préférables aux méthodes fondées sur la combinaison des contenus de plusieurs estomacs pour former des échantillons regroupés.[Traduit par la Rédaction]

Pálsson, Ó. K., and Björnsson, H. 2011. Long-term changes in trophic patterns of Iceland cod and linkages to main prey stock sizes. – ICES Journal of Marine Science, 68: 1488–1499. Data on the diet composition of Iceland cod (Gadus morhua) were collected in March of the years 1981–2010 and in autumn of the years 1988–2010. Capelin, northern shrimp, and euphausiids dominate the diet in all years and may be classified as the stable food of Iceland cod. Overall, total consumption by the smallest cod (20–29 cm) remained stable over the three decades, whereas that of larger fish has declined since the mid-1990s. This decline may explain the reduced growth rate of cod in recent years. Long-term, prey-specific patterns were identified in consumption, and significant trophic links were found between cod consumption and stock sizes of capelin and northern shrimp. In March, the correlation between cod consumption on capelin and capelin stock size was highly significant, a type I functional feeding response, but not significant in autumn. The correlation deteriorated in the early to mid-1990s and in the early 2000s. Increased inflow of Atlantic water into north Icelandic waters, and associated changes in capelin distribution, may have contributed to this trend. The interaction between cod consumption on northern shrimp and shrimp stock size showed a highly significant type I functional feeding response in both seasons.