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Population dynamics of the sea urchin Heliocidaris erythrogramma on the east coast of Tasmania


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Pederson, HG 2003 , 'Population dynamics of the sea urchin Heliocidaris erythrogramma on the east coast of Tasmania', PhD thesis, University of Tasmania.

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Formation of sea urchin 'barrens' has been documented widely in temperate regions
of both northern and southern hemispheres. While the phenomenon has been reported
worldwide, the mechanism(s) underpinning barren ground formation are poorly
understood. Elucidating the mechanism(s) of barrens formation is important, not the
least reason being because it may provide options for management to minimise the
establishment and or spread of urchin barrens. One potential mechanism is through
increases in urchin population density as a result of reduced predation mortality
because of exploitation of predators. We examined whether fishing of rock lobster
(Jasus edwardsii) and fish predators on rocky reefs in Tasmania could account for
population increases of the sea urchin Heliocidaris erythrogramma to the point where
overgrazing of macroalgae may lead to urchin barrens.
Large-scale surveys (over > 100 km of coastline) of abundances of urchins and
putative predators revealed a significant negative relationship between urchins and
lobster abundances, but not between urchins and demersal fishes. At smaller scales,
both large lobsters and demersal fish are significantly more abundant inside no-take
marine reserves than in equivalent adjacent habitat subject to fishing. These
observations are consistent with results of small-scale experiments in which the rate of
urchin mortality was ea. 23 times greater inside two marine reserves than in adjacent
exploited habitats. Caging experiments also showed that predation by rock lobsters on
sea urchins is highly size-specific. Juvenile lobsters are not capable of predating
urchins with test diameter (TD) > 60 mm, while medium-sized lobsters preyed on
urchins up to 80 mm TD, and large urchins >81 mm TD were only at significant risk of predation mortality from large lobsters. Demersal fish were notably less important
predators of urchins between 40-100 mm TD than were rock lobsters. Inside reserves,
if urchins were tethered so that they could not seek shelter, then juvenile urchins were
predated more frequently than larger ones. However, when tagged urchins were free to
move, mortality of adult urchins was significantly higher than that of juveniles,
because juveniles were able to shelter in small crevices.
I also examined the possibility that higher urchin population densities on barrens
could be the result of sporadic large recruitment events. In mark-recapture
experiments, three distinct urchin populations were used to construct individual-based
growth models to generate size-at-age relationships. Age frequency distributions of
urchin populations in algal beds and adjacent barrens habitat were estimated using this
relationship and examined for the presence of dominant cohorts which may indicate
prominent recruitment events. Significant differences were found in the age
frequency distributions of urchin populations in barren and algal bed habitats at two
separate locations, with dominant peaks in the age structures of urchins in barrens
populations unmatched by those in adjacent algal beds.
Estimates of age-specific mortality rates and age frequency distributions derived for
several distinct urchin populations enabled construction of matrix models of
population growth. Using the empirical estimates of size-specific per capita rates of
predation by lobsters on urchins, I estimate the effect of exploitation of lobsters in
regulating urchin population density by comparing model projections for urchins
based on lobster populations inside and outside marine reserves. The modelling
suggests clearly that the reduction in biomass of legal-sized lobsters on the east coast of Tasmania due to fishing is sufficient to account for increases in populations of H.
erythrogramma to the point where barrens formation may occur.

Item Type: Thesis - PhD
Authors/Creators:Pederson, HG
Keywords: Sea urchins
Copyright Holders: The Author
Copyright Information:

Copyright 2003 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references

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