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We examined the effect of sample size on the modeled proportion of dead NZMSs by adding assumed sample sizes of 50 and 150.
Although the fitted GLM predicted values agreed well with the observed laboratory saltwater trial data, the predicted proportion of dead NZMSs under various salinity exposure regimes, based on the laboratory data, are slightly out of agreement with the mortality rate observed in the field.
The chosen submodel for the laboratory saltwater trial uses all three predictor variables (time, salinity, and status of the experiment), each of which is highly significant (P < 0.001 ), to predict the proportion of dead NZMSs (Fig.
Results from the laboratory saltwater trial conducted with NZMSs sampled 5 days after the backflush showed a remarkable difference in salinity tolerance when compared with those NZMSs sampled prior to the backflush.
More NZMSs were killed in response to the backflush than was predicted by GLM.
The increased salinity tolerance of NZMSs collected from Capitol Lake after the backflush was likely the result of acclimatization, and it is noteworthy that several live and actively crawling juvenile snails were observed in the postbackflush sample at a salinity of 27 after 216 h (all other trials were terminated at 120 h).
Among the concerns to managers responding to the Capitol Lake NZMS infestation is the potential for spread into the low-salinity waters of adjacent southern Puget Sound, and the threat that NZMSs may pose to the marine ecosystem there, including effects on the distribution and abundance of native littorinid snails.
(2008) documented the occurrence of NZMSs in the diet of juvenile Chinook salmon (Oncorhynchus tshawytscha) and determined that they could pass through the alimentary canal alive.
A pilot-scale study conducted by the Washington Department of Ecology using benthic samples acquired pre- and postbackflush showed that although the overall abundance of macroinvertebrates (including NZMSs), and the species diversity decreased after the backflush, the proportion of live NZMSs to the overall benthic macroinvertebrate community increased, and NZMS remained among the top 5 dominant species.
By incorporating predictions of maximum achievable salinities and durations, our GLM results can be used by managers to make informed decisions about the potential efficacy of eradicating or controlling localized infestations of NZMSs.
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