Northern shrimp Pandalus borealis behaviour in the thermal gradient

Development, construction and tests on different prototypes were carried out from 2002 to 2003 followed, in the spring of 2004, by a series of successful observations of the behaviour of every developmental stages of the Northern shrimp. Two columns were used: a first column with a three-layer thermal profile and a second one where the temperature profile was homogenous (6°C) and which served as a control to compare the response of the larvae placed in the thermal gradient of the first column.

During slow swimming or when maintaining a specific position in the column, the larvae assume a vertical position; head (cephalothorax) pointing down. They move up and down by slow movements of the periopods (thoracic legs).

Sometimes, rapid movements (upward or across the column) are achieved with a rapid flexion of the abdomen.

Six larval stages of shrimp were investigated from mid-May to July 2004. The first results show interesting differences among the stages and also as a function of the presence or absence of a thermal gradient in the column.

In black  Observations in the column with a thermocline and a CIL. In red  observation in the control column (homogeneous temperature of 6°C).

Summary of the experiments conducted with the different larval stages of Northern shrimp in spring 2004. Each symbol represents the median position of 5 larvae after a 30 min. observation period. The continuous lines represent the trend in the mean position of the successive stages in each column. The shaded area represents the position of the thermal gradient in the column with a thermocline and a CIL.

It appears that the first stages prefer to stay in the upper portion of the water column, in warmer waters above the CIL. In the absence of a thermocline or of a cold layer, the larvae, as early as stage III, seem to move towards the deeper water, suggesting that under similar natural conditions that stage will occupy deeper water than the earlier two stages. When a CIL is present however, the larvae were more likely to remain within the column or more towards the surface up to stage V. All larvae at stage VI were found moving toward the bottom of the column, thermal gradient or not. These results compare to the distribution of larvae of the same developmental stages observed in the Northern Gulf of St. Lawrence during the spring of 2002 and 2003 (Ouellet and Allard, 2005), and will contribute to increasing the knowledge of the development conditions of the early stages of the Northern shrimp.

This experimental setup offers great possibility for future research projects such as studies about other species, the larval behaviour in the presence of preys, or the vertical movement as a function of day-night light cycle.