Invasion dynamics of a non-indigenous bivalve, Nuttallia obscurata, (Reeve 1857), in the Northeast Pacific

Abstract

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 iii 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.