Swimming depth of sea trout

3. Processing and presentation of data

The results presented are based on DST data in which the whole sea migration was recorded from the home estuary to the sea feeding grounds and back into the home estuary when they entered the river prior to spawning and/or overwintering. The main patterns presented on the migration behaviour are based on uniform sampling throughout the whole sea migration. The results from the subsampling periods are also presented for comparison in relation to the overall depth distribution of the sea trout, to give extra insight into behaviour and to check whether the sampling rates may have influenced various aspects of the results.

The aim of the data analysis was to identify the major patterns in vertical distribution of sea trout. The main task was to compile the data in a way that gave a clear picture on how much time the sea trout spent within any given depth zone. That task was possible to implement comprehensively because of the large number of recordings on the monitored fish and the repeated monitoring in different years.

In addition to the presentation of the main patterns of distribution, detailed data on the variation found within the data sets are also presented. This was carried out in various ways. Firstly, by graphs showing how the individual depth of the sea trout varied with time and habitats, in some instances using the corresponding measurements of temperature and salinity. Secondly, the overall mean fish depth and corresponding confidence limits are presented for all the individuals used in the data processing of sea trout swimming depth. The possible variation of sea trout swimming depth in relation to light levels according to the sun position (day vs. dusk/dawn vs. darkness) is also explored. Following that, I examine, on a weekly basis, whether the swimming depth changes during the sea migration period and to what extent. For this purpose, I use data from 1997. That year, salinity was also measured which gives in parallel the opportunity to view by the same weekly means the variation in salinity as well as temperature experienced by the sea trout throughout the sea migration. Then the difference in salinity and temperature experienced by sea trout in 1997, within the given depth zones is presented. The final step in examining the data was to look at the data collected in 2004 when measurements were taken at 5 seconds interval.