Behaviour of Atlantic salmon (Salmo salar L) recorded by data storage tagsin the NE Atlantic-implications for interception by pelagic trawls

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In a project carried out in the Nordic Seas (Northeast Atlantic) in 2002 - 2004, 413 Atlantic salmon (Salmo salar L.) were tagged with data storage tags (DST) either logging temperature and depth (StarOddi, Milli) or just temperature (Dallas semiconductor, iButton). The main aims were to study various marine life history traits of salmon, such as winter habitats, diurnal patterns of vertical migration, feeding activities. Another aim was to collect salmon behaviour data with relevance for issues related to by-catch of salmon in pelagic fisheries. In this paper we focus on possible implications of the diurnal depth distribution of salmon in relation to catchability of salmon in sampling trawls versus commercial fisheries. Five DSTs were returned from fishermen. Four of these were Milli-tags and are used in this presentation. Time from release to capture ranged from 48 - 128 days. The DSTs logged data at alternating intervals of 1, 5, 30, 60 and 120 min. almost 45000 recordings were retrieved. The data records were grouped in different phases: post-release; oceanic feeding & migration; coastal migration; estuarine migration; in-river phase. Salmon stay in the upper 5 m for 60% of the time during the oceanic phase, making deep dives (280 m) in between. The proportion of time the salmon stay at the surface (<2 m) decreases from 5.5 to 2.5% between 9 and 17 h.