Swimming depth of sea trout

2. Sampling methodology and data characteristics

2.1. Monitoring of migration behaviour and environment with data storage tags

The data used in this report are derived from work carried out by the author on sea trout in Icelandic waters in the period 1996-2008. The sampling of the electronic data storage tags followed a pre-programmed schedule. The tags logged information on the fish depth and ambient temperature in the sea and in the rivers before and after the sea migration. Some tags were equipped with salinity (conductivity) sensors enabling recording of this parameter. In addition, a few tags had sensors which measured the pitch and roll of the fish, although these data are not presented in this report. Data storage tags have to be retrieved in order to access the stored data. As the recaptures were mainly based on angling, only some of the tags were retrieved.

2.2. The sea trout and the study area

The sea trout studied were from two rivers in south east Iceland. These rivers, the Rivers Tungulaekur and Grenlaekur are well known for their strong sea trout stocks and are located in a part of Iceland where sea trout is the most abundant salmonid (Figs. 1 and 2). The rivers are mainly springfed and each of the rivers discharges at a mean rate of about 2 m³s^-1 during the summer. The River Tungulaekur runs into large glacial river (River Skafta) which has a mean discharge of about 120 m³s^-1 and the River Grenlaekur has a joint estuary with River Skafta. The section of Icelandic coast which is involved is unsheltered and there are extensive areas of sand which extend from coastal beaches well out to sea. Hydrographical data from the Marine Research Institute, Reykjavik, Iceland (http://www.hafro.is) shows that there is a thermocline in the area at approximately 20 m depth during summer 2 - 27 km from shore. The coastal area is influenced significantly for hundreds of kilometres along the coast by the massive inflow of fresh water from glacial rivers in the summer. It is therefore possible that in some areas the sea trout can experience low salinity close to shore without entering an estuary. There is some information on the prey of sea trout in this area. Sandeels are the predominant prey although herring is also important when available. The sea trout also eat smaller prey such as amphipods and polychaetes, but they are much less important quantitatively to the diet.

Fig. 1. Map of the coast of south Iceland, showing the area that the DST tagged sea trout are likely to have migrated through. The yellow circle shows the area where the two home rivers of the monitored sea trout are located (Tungulaekur and Grenlaekur). Also marked is the estuary Veidios where River Grenlaekur runs into sea as well as River Skafta, the glacial river which River Tungulaekur runs into. Red arrows indicate the feeding area of the sea trout closest to the estuary. The yellow line shows the coastal area that foraging sea trout from River Grenlaekur and River Tungulaekur utilise, from known outer limits of their distribution during sea migration (maximum of approximately 160 km east and west of Veidios) (Sturlaugsson, pers. observations).

Fig. 2. Map of the coast of south Iceland, showing water depth intervals by bottom depth isolines (100m, 200m etc.). The waters are relatively shallow along the coast with water of depth 20 m or less extending 1-2 km out from shore. The area where the Rivers Tungulaekur and Grenlaekur are located is shown.

2.3. Data sampling

The sea trout were tagged in the rivers, and included both immature and mature fish. Most were captured by rod fishing and tagged and released the same day. The great majority of fish were tagged in the spring prior to their annual sea migration, although a few were tagged at their spawning grounds in the autumn. Most of the DSTs were attached externally using a modified Carlin method although, in a few instances, the DSTs were implanted into the peritoneal cavity.

The manufacturer's declared accuracy of depth measurements for the DSTs used in the studies was +/- 0.4% of selected depth range, which was 50 m. The accuracy of the temperature measurements was +/- 0.1°C and the accuracy of salinity measurements +/- 1 psu.

The sampling rate differed between and within years, depending on the tag memory, tag type and the aim of the particular study. The densest (highest frequency) measurements were collected over shorter periods within the summer, when recording intervals of up to 5-10 seconds were used to obtain more detailed information on the vertical distribution and on the actual swimming between depth layers. Table 1 lists the number of fish from which data were obtained for each of the eight years and the number of recordings for corresponding measuring intervals. Table 2 gives the periods in each of these years when subsampling was carried out.

Table 1. Measurement intervals of DSTs used in given year are shown for 1996-2008. For the intervals involved in each year the number of fish that carried the tags during their sea migration and the number of recordings sampled from the sea migrations are given.

Table 2. Periods of subsampling during sea trout sea migration in the given year are shown for 1996-2008 for the relevant measuring interval.