Research Document 2018/010
Glass Sponge Reefs in the Strait of Georgia and Howe Sound: Status Assessment and Ecological Monitoring Advice
By Dunham, A., Mossman, J., Archer, S., Davies, S., Pegg, J., and Archer, E.
Abstract
Glass sponge reefs are structured biogenic habitats unique to the North East Pacific that support diverse biological communities and provide high levels of bentho-pelagic coupling. Over the past 15 years, nine glass sponge reef complexes have been mapped by the Canadian Hydrographic Service and the Geological Survey of Canada, in the Strait of Georgia and Howe Sound, using remote sensing techniques. In 2015 DFO protected these complexes via formal bottom-contact fishing closures extending 150 m beyond the reef footprints. In order to monitor the established fishing closures, characterization of the reef status and a monitoring plan must be developed. Glass sponge reefs largely occur beyond diving limits, restricting survey methods to remote visual survey platforms which can be resource-intensive and logistically challenging. The reefs require a monitoring program that uses relevant quantitative metrics at appropriate spatial and temporal scales and provides well-resolved time series data.
This paper is based on the results of two remotely operated vehicle surveys of nine sponge reef complexes and associated communities in the Strait of Georgia and Howe Sound completed in 2012 and 2013 prior to bottom-contact fishing closure implementation. We used an empirical, quantitative approach to assess the distinct and unique characteristics of the glass sponge reef ecosystem. First, we applied a suite of both novel and previously published quantitative indices for assessing biogenic habitats to a subset of reef imagery data. Indices were evaluated based on consistency, ability to distinguish between reefs of qualitatively different status, and data processing effort involved. Indices that demonstrated the most potential – characterizing sponge cover, condition, and distribution, as well as associated community structure and indicator taxa abundance – were subsequently applied to the full imagery dataset. Standardized summaries characterizing reef complexes were developed from a compilation of the most informative indices to serve as best available pre-closure reference of reef status for monitoring.
To support the development of a reef monitoring program, considerations for survey design, sampling methods, and data analyses are provided. A range of monitoring indices and associated sampling methods are collated to provide options for comparing reef status over time and space. We recommend that management decisions are based on trend analysis and consider proposed indices in combination, rather than in isolation. A diagnostic decision tree is presented to guide reef monitoring and inform adaptive management.
The methods developed in this paper can be applied to other reefs in the Strait of Georgia and Howe Sound and adapted for assessment of glass sponge reefs in other areas such as Hecate Strait and Chatham Sound.
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