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The red sea urchin, Strongylocentrotus franciscanus, is one of the three species of strongylocentroids found on the west coast of North America between Baja California and Kodiak, Alaska. It belongs to the phylum Echinodermata, and it is one of the largest species of sea urchins. Since 1970, red sea urchins have been fished commercially for their highly valued gonads. As a result, the abundance of harvestable size urchins has rapidly declined, and the fishing industry has suffered a dramatic decrease in landings in this relatively short history. Several management strategies have been proposed with the objective to restore the harvestable stock as well as to protect the species. Among these strategies, an upper-size limit has been promoted in addition to existing minimum size limit regulations. This alternative is based on the hypothesis that the recruitment and survival of juvenile urchins is enhanced by the presence of adults, which provide a spine canopy habitat that is utilized by juveniles as refuge against predators. However, the evidence in support of this hypothesis is circumstantial. To understand the ecological relationship between adult and juveniles as well as to identify if different size categories of adult maximize this relationship, I conducted laboratory and modeling investigations that tested the hypotheses that 1) recruitment and survival of juvenile urchins is enhanced by the presence of adult spine canopy, 2) the rate of survival of juveniles is related to the size of the adult and 3) that protection of juveniles by adults can affect population response to harvest. I found that juveniles look for refuge under adult spine canopies under the risk of predation and have higher survival in the presence of adults. However there is not a significant difference on survival based on the size of the adults. I conclude that the size of the adults is not important, but protection of adults may help restore and preserve a population under harvest conditions. Another potential benefit of implementing an upper size limit was explored since the fecundity of red sea urchins is related to body size. I used size-structured matrix models to further explore the importance of size-specific fecundity and the association between juveniles and adults. I assessed changes in population growth rate under different levels of harvesting that include lower and upper size limits. Under most scenarios, an upper size limit could benefit the recovery of populations of red sea urchins. However, in order to improve management strategies for the red sea urchins, more studies about the life history of red sea urchins should be conducted to reduce model uncertainties. |
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