How to monitor harbour porpoises in a large cSAC?

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How to monitor harbour porpoises in a large cSAC?

The objective of this project was to determine the most appropriate approach, by full consideration of pros and cons, to year-round monitoring of harbour porpoise at the candidate Special Area of Conservation (cSAC) and wider southern North Sea and to subsequently design a sampling plan to monitor the site to inform assessments of site condition and development of site management.

Earlier this year SMRU Consulting Europe in collaboration with our Centre for Research into Ecological & Environmental Monitoring (CREEM) colleagues won a tender to design a harbour porpoise monitoring plan for the Southern North Sea candidate Special Area of Conservation (#7 in the below figure) of which the results are now available, published in this JNCC report.

The objective of this project was to determine the most appropriate approach, by full consideration of pros and cons, to year-round monitoring of harbour porpoise at the candidate Special Area of Conservation (cSAC) and wider southern North Sea and to subsequently design a sampling plan to monitor the site to inform assessments of site condition and development of site management.

In this project, there were two regions of interest: the cSAC, and the wider southern North Sea designated as a buffer region of 50 km around the cSAC and clipped to the EEZ (thus, splitting the buffer into disjunct areas). The areas of the cSAC and buffer were 36,951 and 41,381 respectively.

Our Approach

Location of the six candidate and single designated marine Special Area of Conservation sites in the UK territorial seas and Northern Ireland adjacent waters (provided by JNCC).

We conducted desk-based reviews of the published and grey literature to evaluate potential monitoring approaches, and used simulation studies to assess survey designs in the context of detecting trends in harbour porpoise population abundance.  We used existing reviews of survey methodologies to inform our assessment, providing a high-level evaluation of the pros and cons, resolution, utility (via power analyses), practicalities and cost of each of the survey methods.

Given the requirement for long term monitoring at the site relative to the detection scenarios specified by the Inter Agency Marine Mammal Working Group; we recommended that an array of static Passive Acoustic Monitoring (PAM) devices was the best approach for monitoring harbour porpoise.  Full details of our suggested monitoring design for the SNS cSAC can be found in the JNCC report.

Providing around the clock monitoring over extended periods of time is an advantage of static PAM as a medium term, large scale and cost effective monitoring option. This monitoring method should provide options for assessing short term displacement / shifts in distribution patterns (which is critical for a highly mobile, patch-exploiting species like the harbour porpoise) as well as exploring habitat use over annual, seasonal, diurnal or tidal cycles and providing a robust method for monitoring and assessing the status and range of harbour porpoise around the cSAC.

Power to detect declines

The ability of a study to detect change is quantified by a statistic called power. The power with which a change can be detected by the different approaches is a critical factor in assessing the suitability of different monitoring methods for harbour porpoise.  Some of the factors which can influence power include: the number of devices which are being used to conduct the survey, the number of years over which the survey, and the variation in the estimate of abundance.

Power against CV for different percentage decreases in abundance: 10% (solid line) and 25% (dashed).

Albeit a simple approach, the plot below illustrates that the power to detect a change declines substantially as the variability in the estimates (CV) increases, and the rate of change to be detected decreases, but the longer the study the higher the power to detect a change.

In the example given here, interest was in the power to detect a trend (decline or increase) of the harbour porpoise population over time, however, for the purposes of this project, the power to detect changes between regions will need to take other variables into account, such as seasonal changes. This study identified the levels of effort (in terms of PAM units deployed, duration of sampling and frequency of analyses).

Please explore the recently released JNCC report where you can find details on different monitoring approaches, and our recommended monitoring design for the Southern North Sea cSAC and what to consider to ensure a power sufficient enough to detect a meaningful change in porpoise abundance.

Wilson, L.J., Booth, C.G., Burt, L., Verfuss, U.K. & Thomas, L., (2019), Design of a monitoring plan for the Southern North Sea candidate Special Area of Conservation and wider area, JNCC Report 629, ISSN 0963-8091

About the Author:

Ursula is a Principal Scientist at SMRU Consulting Europe. Check out her bio under the "About Us" tab.

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