The Impacts of Nonnative Japanese Eelgrass (<i>Zostera japonica</i>) on Commercial Shellfish Production in Willapa Bay, WA

Abstract

Eelgrass species worldwide are valued for the ecosystem service they provide to estuarine and marine habitats. One species, Zostera japonica, however, has some negative impacts outside its native range and is considered invasive. In Willapa Bay WA, USA, the nonnative eelgrass has expanded to the level where the shellfish industry is concerned about its potential impacts on its livelihood. Studies were conducted using paired plots, Z. japonica controlled with the herbicide imazamox vs. untreated controls, to assess the effects of Z. japonica on Manila clams (Ruditapes philippinarum) and Pacific oysters (Crassostrea gigas). Recruitment of new Manila clams was not affected by Z. japonica. The growth of young clams, total commercial clam harvests, clam quality and clam harvest efficiency, however, were greater on plots where Z. japonica was chemically controlled than where it was not treated. The response of oysters to Z. japonica control varied by site; there was no effect at one site, while the other sites had a 15% increase in shucked meat with Z. japonica control. The potential economic impact of a Z. japonica infestation of a shellfish bed was ~$47,000 ha −1 for Manila clams and $4000 ha −1 for oysters for each crop harvest cycle.

Large-Scale Chemical Control of Smooth Cordgrass (Spartina alterniflora) in Willapa Bay, WA: Towards Eradication and Ecological Restoration

“…It is utilized by 10% of the world’s population of short-billed dowitcher ( Limnodromus griseus caurinus ), red knot ( Calidris canutus roselaari ), and dunlin ( Calidris alpina pacifica ) and hosts between 200,000 and 300,000 individuals of 43 shorebird species during annual migrations. Willapa Bay is also home to a robust (>$60 milllion yr –1 ) shellfish industry that supplies ~17% of the nation’s oysters (Patten 2014). Monitoring studies from 2004 to 2016 were conducted throughout the bay but focused mainly at numerous locations within the Willapa National Wildlife Refuge.

Figure 1 Map of Willapa Bay, WA, showing locations of major study areas and infestation of Spartina alterniflora (TS, Tarlatt Slough; PP, Porter Point; PR, Palix River).

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Ecological effect of a nonnative seagrass spreading in the Northeast Pacific: A review of Zostera japonica

“…No significant effect was found on waterfowl and Chinook salmon (Oncorhynchus tshawytscha). The below-ground architecture of roots and rhizomes likely reduced abundance of native species Neotrypaea californiensis (burrowing shrimp) burrows and Praxillella gracilis (polychaete), and the nonnative fisheries-cultured Ruditapes philippinarum (Manila clam) had reduced meat weight, growth and recruitment (Berkenbusch et al, 2007;Harrison, 1987;Patten, 2014;Tsai et al, 2010), while the whole infaunal community increased in diversity and abundance Cole, 2011, 2012;Posey, 1988). Posey (1988) also tested abundance of specific Polychaeta, Oligochaetes, Crustacea and Diptera infaunal and epifaunal species.…”