This thesis describes how life history characteristics of the varnish clam (Nuttallia
obscurata), and interactions with the physical environment and other species, have
contributed to its successful invasion in coastal British Columbia. Lab and field
experiments were conducted to investigate varnish clam larval ecology (i.e. larval rearing
experiments), adult population dynamics (i.e. annual population surveys, mark-recapture
and length-frequency analysis, growth modeling) and ecological interactions with native
species (i.e. predator/prey preference feeding trials). Using these results, a matrix
demographic model was developed to determine which life history stage contributes the
most to varnish clam population growth.
Larval rearing experiments indicated that temperature and salinity tolerances of varnish
clam larvae are comparable to native species, however the planktonic phase is slightly
longer (3-8 weeks). Based on local oceanographic circulation, varnish clam larvae have
the potential to disperse throughout their entire geographic range in just one reproductive
season. Varnish clam population surveys revealed spatiotemporal variation in density and
size. No relationships were evident between varnish clam density and the number or
density of co-occurring bivalve species. Length-frequency analysis suggested that
recruitment varies among sites, with high post-settlement mortality coinciding with high
recruitment. The presence of similar recruitment pulses at geographically separate sites
indicates regional scale processes may influence recruitment. Individual growth rates
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varied among sites, with higher growth corresponding to lower population densities and
water temperature. Monthly survival rates ranged from 0.81 – 0.99 and were lower for
clams 10-30 mm. Predator/prey preference feeding trials showed that crabs prefer
varnish clams to local species when clam burial depth is limited. Crabs therefore have
the potential to influence varnish clam distributions, particularly on beaches where the
varnish clam is unable to bury deeply. Based on matrix demographic analysis, adult
survival (e.g. clams ≥ 40 mm) is the most crucial factor for varnish clam population
growth, and drives the observed population growth differences between sites. This study
of the varnish clam invasion demonstrates that its success lies in both species (e.g.
lengthy planktonic phase, high survival) and regional (e.g. favourable ocean circulation
patterns for rapid dispersal) characteristics. Measures to reduce introductions should be
targeted in areas where introductions are likely to have the furthest reaching impacts.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/54 |
| Date | 31 December 2005 |
| Creators | Dudas, Sarah |
| Contributors | Dower, John F |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | 1610541 bytes, application/pdf |
| Rights | Available for the World Wide Web |
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