Description of the IMR Standard Light Trap and the Vertical Distribution of Some Decapod Larvae (Homarus and Nephrops)

Abstract

The construction of different versions of a cheap, robust, and easy to operate light trap for catching various aquatic organisms is shown. The trap can be used to > 300 m depth and meets a number of criteria. Small-scale vertical distribution of decapod larvae was investigated during trap trials. The traps (6-10) were set for 24 h at different depths, once a week, between 25 July and 28 September 2006, within the Kåvra lobster reserve at the Swedish west coast. This is an area with low salinity in the surface water during summer due to outflow of water from the Baltic Sea. The larvae of the European lobster Homarus gammarus (stage I) and Norway lobster Nephrops norvegicus (stages I-III) were found within and below the thermo- and haloclines. No larvae were found within the upper 2 m. This finding may have important bearings on the larval transport by currents and increase the possibility for retention of larvae, but was not tested in
this study. The highest catches of both H. gammarus and N. norveigicus were obtained during August. The trap appears to be well suited for investigating small-scale vertical distribution during the dark period, and for collecting animals in good condition. However, the trap did not
catch all larval stages, and the relation between light intensity (both natural and trap light) and catch ability is unknown.