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The effect of spatial scale on growth of the Chilean sea urchin: thinking of tomorrow

The sea urchin (Loxechinus albus) fishery is the largest marine invertebrate fishery along the Chilean coast, contributing almost 65% of the world supply. Overfishing and closures have occurred in recent years, and sequential depletion (harvesting in one area until stocks are gone, then moving on to another area) is believed to be a primary cause. Sea urchin populations are highly likely to be structured as metapopulations – local population groups of sedentary adults connected by a planktonic larval stage. The complexity of the spatial dynamics of the sea urchin and its fishery (especially in relation to serial depletion) represents a challenge for assessment, management and conservation of this system. While metapopulation modeling to account for spatially explicit stocks is a long term goal for sea urchin management and conservation, necessary population data at several spatial scales are lacking. This project supported two years of work by a Chilean graduate student to (1) determine the relationship between length and age at multiple field sites and (2) use several growth models to evaluate the spatial variability in growth rates at different scales.

An extensive sampling program was carried out at numerous southern Chilean sites to collect sea urchin samples and to obtain information of size and age through an exhaustive age-reading procedure. Samples were collected by the student and other researchers, but were also obtained in collaboration with the urchin fishery, an important step in building relationships that allow for more collaborative management of resources. Body size measurements and the genital plates of urchins were collected to compare size and age (determined from the plates) for growth modeling. Flores found a linear relationship between size and age, a finding that is consistent with other recent research suggesting that traditional non-linear growth models may not always be applicable to long-lived species.

While the linear growth pattern was evident for all sites samples, there still was variability in the relationship between size and age. To make this information more useful to metapopulation modeling, Flores explored various spatial scales to determine their relationship to the age-size pattern. Results indicate that micro (within individual sampling/fisheries sites) and macro (latitudinal gradient) scales do not explain any variability in growth patterns, while the meso (exposed coast versus protected bays and fjords) scale explains a significant amount of variation in sea urchin growth. These results and the relationships they reveal are critical inputs to fishery management models and approaches that are trying to take into account harvestable sizes of individual urchins, proportions of harvestable individuals within fishing sites, and the levels of harvest among sites across larger regions of the resource.