Abstract
Visual searches are a common method of detecting invasive species in coastal waters, but the statist ical properties of search methods have rarely been evaluated. Understanding the error rate (especially false negatives) and effective detection distance of searches can improve survey design, and quantify the uncertainty in risk assessments used to inform invasive species management efforts. An experiment using artificial tunicates (“decoys”) was conducted in Hillsborough Bay, Prince Edward Island, to determine the effectiveness of SCUBA divers conducting underwater visual searches for the vase tu nicate, Ciona intestinalis (Linnaeus, 1767). Single decoys and clusters of three decoys, constructed from water -filled, ivory-coloured balloons 5-6 cm in length, were placed at a blue mussel, Mytilus edulis (Linnaeus, 1758), aquaculture site on buoys, lines and mussel socks. The probability of detecting tunicate decoys on a mussel sock in the experiment is 89.8% (±SD 7.1), known in this paper as a true positive. The probability of not detecting tunicate decoys actually placed on a mussel sock in the experiment is 10.2% (±SD 7.1), known in this paper as a false negative. Divers detected 79.2% (±SD 7.1) of single decoys and 94.0% (±11.4) of clusters. Divers were able to detect single decoys from a measured horizontal distance of 2.7 m (±0.8), and clusters from 2.8 m (±0.9). The typical detection distance for real C. intestinalis estimated by divers was, on average, 2.1 m (range 1 - 3 m), and tunicates of lengths ≥ 2.9 cm (range 1-4 cm) could be detected.
Highlights
Fouling of surfaces in the marine environment by non-indigenous tunicates (Chordata: Ascidiacea) has become a global problem in the past few decades (e.g., Lambert 2007; Locke and Carman 2009; Locke 2009)
Severe fouling by tunicates on suspended blue mussel aquaculture equipment in Prince Edward Island (PEI), Canada, has increased production and processing costs due to competition between tunicates and mussels for food and space, the expense of tunicate control measures, and logistical and personnel requirements related to the increased weight and volume of equipment fouled by tunicates (Thompson and MacNair 2004; Locke et al 2007)
A two-sample paired t-test verifies there was no difference in detectability of single versus cluster decoy C. intestinalis
Summary
Fouling of surfaces in the marine environment by non-indigenous tunicates (Chordata: Ascidiacea) has become a global problem in the past few decades (e.g., Lambert 2007; Locke and Carman 2009; Locke 2009). Since its first reported occurrence in PEI in 2004, the vase tunicate, Ciona intestinalis (Linnaeus, 1767), has rapidly increased in abundance to become the nuisance species of greatest concern to bivalve aquaculture. A quarantine approach was implemented in PEI, prohibiting (1) transfers of mussels from tunicate-infested to uninfested estuaries, and (2) processing of harvested mussels from infested estuaries in fish plants adjacent to uninfested estuaries. This approach successfully contained the occurrence of C. intestinalis to three estuaries adjacent to its original point of introduction for three years (Locke et al 2009)
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