How and why are urchin size frequency surveys performed?

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Reef Check California — How and why are urchin size frequency surveys performed?

Sea urchins are an important herbivore on rocky reefs and as such, three urchin species are Reef Check California indicators. RC divers count the numbers of red urchin (Strongylocentrotus franciscanus), purple urchin (Strongylocentrotus purpuratus), and crowned urchin (Centrostephanus coronatus) on our invertebrate transect surveys. In addition to counting them on transects, once a year RC divers perform urchin size frequency surveys for two of these species, the red and purple urchins. These surveys are performed anywhere at a site and, unlike our other surveys, they are not associated with a transect. Because we have already estimated the urchin densities by counting them on transects, we do not need to measure the area over which we encounter the urchins we size. It is more important to get a large enough number of individuals to be able to estimate their size distribution. Divers aim to get a representative sample of the urchins at the site. Divers size the first 100 urchins of each species they find using calipers to measure, to the nearest centimeter, the test (or body) diameter of all urchins they encounter. To minimize bias, divers can gently clear small plots of urchins if it is needed to ensure that they accurately count and measure all urchins in the immediate vicinity.

Red urchins are commercially fished in California. The size distribution of urchins is an important parameter for their stock assessment and sustainable management. The size structure of a population can be affected by the level of exploitation and therefore can be an important indicator of the status of local populations. By counting the number of individuals in our invertebrate surveys, as well as calculating their size distribution throughout the site, we can use our data to describe the population density as well as size structure of these important detritivores in the rocky reef community. By identifying the size classes in a population we can understand if a population is comprised mostly of juveniles, adults or both. This information can be used to inform the management of the urchin fishery and help identify areas where fishing has to be regulated to maintain a sustainable use of this resource.