Scuba-studies on vertical distribution of Swedish rocky-bottom echinoderms. A methodological study

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Abstract
Divers using SCUBA censused test areas to determine the abundance of certain asteroids and echinoids. The studies showed the depths of the local maximum density of Asterias rubens, Psammechinus miliaris and Echinus esculentus. Differences in size and age of E. esculentus at different depths indicate that these animals migrate downward with age. Additional studies showed that not all animals present on a given test area were found by a SCUBA-equipped census-taker but comparable relative densities of the studied species were easily obtained. The upper limits of Porania pulvillus, Henricia sanguinolenta, So/aster papposus, Marthasterias g/acialis, and Echinus acutus during summer were determined by swimming along vertical transects recording depths and species observed. Some factors that may explain the distribution patterns found are discussed. Hydrographical factors were believed to be the most important.