Sampling plankton at sea in discrete layers over the entire water column while collecting the corresponding environmental data implies important and costly logistics issues. It is also often impossible to determine with sufficient precision the vertical position of a taxon or a species in order to answer specific ecological questions. The difficulties become even more important when trying to document the distribution and growth conditions of fish or benthic invertebrate larval stages. In many cases, for a given species, the various stages are in very low concentrations and the development period is spread over several weeks.

As part of a research project on the larval ecology and recruitment of the Northern shrimp, Pandalus borealis, it was necessary to determine the distribution and the preferred thermal habitat of the different developmental stages, from the emergence of the first Zoea stage until the time of settlement on the bottom of the first post-larval or young juvenile stage.

Life cycle: larval stages

The duration of each larval stage varies with temperature. In the Northern Gulf of St. Lawrence, there is about a 3-month period (May to July) from the emergence of the larvae (I) until the settlement on the bottom (VI). Adapted from: S.A Pedersen, Danish Institute for Fisheries Research, Denmark.

To supplement the at-sea sampling effort and to help with the interpretation of field data, it was necessary to develop and construct a small-scale (1/15) model of the typical three thermal layers of the water column of the Northern Gulf of St. Lawrence. It was then possible to study the distribution and the behaviour of each shrimp larval stage in the presence of a strong thermocline and a layer of cold water.