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

Worldwide, there are over 60 species of seagrass which provide many ecosystem functions, including supporting diverse benthic assemblages, providing carbon to the estuarine food web, structural support for other primary producers and habitat for juvenile salmonids and other fish species. A few seagrass species, however, are considered to be invasive outside their native range. Zostera japonica, in particular, has been reported to have several negative ecological consequences.

In Willapa Bay, WA, USA, coverage of the upper intertidal zone by Z. japonica has increased to the level where it is affecting the livelihood of the shellfish industry. Studies to document the impact of Z. japonica on shellfish, however, have been limited and have not addressed the economic impact to the industry. In this paper, the authors conducted studies 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). Results were analyzed from each experiment using plot mean in a paired T test, with a two-tailed P value.

The results revealed that across all sites, and two years of assessment there was no effect of Japanese eelgrass removal on the density of Manila clams that settled into the treated area within that season of treatment. 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.

In summary, the negative economic impact of Z. japonica to the shellfish farmer can be dramatic, particularly for Manila clams. Management of Z. japonica on shellfish beds through the use of the herbicide imazamox can help mitigate for the expected crop losses. Additional studies that assess large-scale economic and long-term environmental impacts of Z. japonica and its potential management are warranted.

Article by Kim Patten, from Washington State University, Long Beach, WA, USA.

Full access: http://mrw.so/5198vD

Image by Eldan Goldenberg, from Flickr-cc.