Do we need to jump in? A comparison of two survey methods of exotic ascidians on docks

Erin Grey

doi:10.3391/ai.2009.4.1.8

Abstract

Many ports and marinas contain exotic ascidians, ostensibly because they offer adequate physical habitat and harbour many boats or ships, which are considered important vectors of exotic ascidians. Standardized quantitative sampling of these habitats would be useful to assess exotic ascidian presence and abundance, as well as for more detailed ecological studies. In this paper two methods for sampling exotic ascidians are compared using data from nine marinas along the Olympic Peninsula and Upper Puget Sound, Washington, U.S.A. One method is considerably less expensive, drier and easier: it consists of laying a simple grid over the side of the floating dock from the surface, with species identity and abundance being measured on the spot. The other method involves swimming underneath marina floating docks, taking standardized digital photographs and analyzing the photographs later. Differences between these two methods might be expected because the environment sampled, the underneath versus the sides of floating docks, is slightly different, especially with respect to light. However, this study finds that both methods observed almost identical presence/absence patterns for exotic ascidians and, at least for the two most common exotics Botrylloides violaceus and Diplosoma listerianum, observed similar abundance trends across sites. Further analyses show that, while there are differences in the overall communities observed by the two methods, community patterns are correlated. Overall, this study finds that the easier surveys of the sides of floating docks are as effective for rapid assessment of exotic species presence/absence and relative abundance as those of the undersides of floating docks.

Highlights

Docks and pilings found in ports and marinas often harbour populations of one or more exotic ascidian species
Possible reasons for this include the availability of suitable hard substrate provided by these establishments, designs that often result in the retention of locally-produced propagules (Floerl and Inglis 2003) and the traffic of boats with hull fouling, which is considered a major vector of exotic ascidians (Wonham and Carlton 2005; Floerl and Inglis 2005 and Lambert 2007)
Given the ubiquity of exotic species in ports and marinas, and the fact that ports and marinas are becoming increasingly numerous as human populations expand along coastal areas (Gray 1997), it is unsurprising that floating docks have been sites for many experimental studies investigating exotic ascidians (e.g., Agius 2007; Blum et al 2007)

Summary

Introduction

Docks and pilings found in ports and marinas often harbour populations of one or more exotic ascidian species. Regular surveys of dock fouling communities could help to rapidly detect new introductions, track fluctuations in abundance over time and investigate important invasion pathways. They would be valuable to many types of ecological studies aimed at disentangling factors influencing exotic ascidian establishment and success across space or time. Standardizing such surveys would facilitate data exchange between investigators, enabling comparisons and conclusions to be drawn across a broad geographic scale

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