Somatic and gonadal growth of the sea urchin Psammechinus miliaris (Gmelin) maintained in polyculture with the Atlantic salmon

Somatic (test) and gonad growth were recorded for the sea urchin Psammechinus miliaris (Gmelin) maintained in stocked salmon cages. Urchins of 11 to 16-mm test diameter maintained in pearl nets suspended in a stocked salmon cage showed a significantly greater increase in test diameter than urchins maintained in pearl nets away from the salmon farm. Urchins maintained in lantern nets suspended in stocked salmon cages showed enhanced-gonad growth as compared with local field populations. The gonad growth was rapid over the winter months when in the field the urchins' roe content was typically low. Uneaten salmon feed pellets seemed to be an important part of the diet in the urchin/salmon polyculture system. The location and mesh size of their enclosures will affect the availability of uneaten salmon feed pellets to the urchins.