Calling into question the accuracy of the ‘Standard Otter Survey’

We are required to survey otters across Europe as the species has been designated as being of conservation concern, under the EU Habitats and Species Directive. This means that under European Law, we are obliged to monitor their presence to give information on the conservation status of the species i.e., find out how well our populations are doing. This is easier said than done for animals like otters, as they are secretive and not easily observed. They require indirect observation such as detection of their signs including spraints (faeces), and other signs of otter activity such as muddy slides that otters use to enter a river from the bank, footprints on sand or mud, and holts (an otter den or burrow). We’ve been using a combination of these clues to build up a case of otter activity and presence, and such clues have become commonplace indicators for the ‘Standard Otter Survey’, used across Europe to monitor otter distribution, and to file reports of the status of our otter populations to Europe every few years.

From top let to right: Otter spraint and surveyors looking for otter signs under a bridge. Lower left to right: Otter footprints, possible otter track and an otter sign heap.

From top let to right: Otter spraint and surveyors looking for otter signs under a bridge. Lower left to right: Otter footprints, possible otter track and an otter sign heap.

Indirect monitoring of any animal by its very nature can be a haphazard adventure. In places like Ireland where we experience more rain than sunshine, otter signs such as spraints are often washed away, or surveyors may not have access to an area likely to have signs of spraints, if water levels are higher than usual. Humans too are prone to error, and some surveyors may simply be better than others, or have more interest in pursuing the trail than those who simply want to get the job done and move on. All of these inaccuracies and possibilities can result in the creation of ‘false negatives’ (non detection of a present animal) and even ‘false positives’ (misidentification of animal signs that may belong to another species, or the detection of a transient otter). And to top these problems all off, otters may spraint more around bridges and confluences, and consequently, display greater territoriality behaviour, possibly resulting in some areas being perceived as having a greater otter presence than others.

Due to a lack of any better, or at least an affordable alternative, the inadequacies of our current technique have now been addressed and scrutinised in a new study by Neil Reid and co-authors from Queen’s University Belfast and The National Parks and Wildlife Service, and published in the European Journal of Wildlife Research. The results showed that rainfall (and the number of days after rain), surveyor variation, the number of bridges and confluences all significantly negatively impacted the detection of otters. Statistically analysing these criteria across over 1200 survey sites, Reid et al. estimated that otters were incorrectly recorded as absent from 22% of the sites.

The implications of these finding are very important. Over the last number of years (based on Standard Otter Survey calculations) Ireland has reported a decreasing trend in our otter distribution. However, Reid et al. are now challenging the overall value of the ‘Standard Otter Survey’ to assess the conservation status of otters across Europe. Reid et al. also question the trends that have been calculated from previous surveys since the 1980s and  highlight the point that those surveys too probably contained the same level of errors that were seen in this current study. Reid also says that future surveys need to be readjusted to account for biases and errors and occupancy modelling  should be employed to properly account for false negatives as well as site revisits that were negative after the first survey. Taking biases into consideration, Reid et al. found that the otter in Ireland is in ‘favourable’ conservation status, with about 94% occupancy, placing the Irish otter population as one of the healthiest in Europe.

Neil Reid presented this paper at the recent European Otter Workshop, held in Kinsale last April. Paul Chanin (a well respected otter expert) believes that when the ‘Standard Otter Survey’ was designed in 1980, it was used to assess otter distribution in Britain, where otters had declined to very low numbers. At that time, the protocol adequately addressed conservation concerns, but now that otter numbers are increasing in Britain, and remain high in Ireland, it may be time to rethink how we monitor otter distribution given the inadequacies found in our current strategy. Chanin also pointed out that the survey was not initially designed as a basis to derive the conservation status of otters across Europe, but due to lack of a more suitable, cost effective alternative, this technique was adopted for that purpose.

This study has taken something we all took for granted, a standardised survey that has been used routinely for decades, the results of which, are littered (possibly erroneously in light of this study) throughout the literature. This study reiterates that we should never become complacent about science, and we need to continue to question what we consider to be the ‘standard’.

ResearchBlogging.orgReid, N., Lundy, M., Hayden, B., Lynn, D., Marnell, F., McDonald, R., & Montgomery, W. (2013). Detecting detectability: identifying and correcting bias in binary wildlife surveys demonstrates their potential impact on conservation assessments European Journal of Wildlife Research DOI: 10.1007/s10344-013-0741-8

  • Great post Denise! xx

    • Thanks Duckie! I think it is quite interesting how a standardised technique could prove to be so inaccurate. I wonder how much more stuff we thought we knew about otters is incorrect too?

  • Great post Denise! xx

    • Thanks Duckie! I think it is quite interesting how a standardised technique could prove to be so inaccurate. I wonder how much more stuff we thought we knew about otters is incorrect too?

  • Thanks for the blog. Glad it got published. Hopefully, another to come…

    • Thanks for reading Neil! It is a nice paper with a strong argument.

  • Thanks for the blog. Glad it got published. Hopefully, another to come…

    • Thanks for reading Neil! It is a nice paper with a strong argument.

  • Glen Power

    Good work, Denise. Are there any other survey techniques that can be implemented in Ireland? I know they’re expensive but what about motion cameras in areas where suspected otters exist like holts, culverts and under bridges?
    .

    • Thanks Glen. Yes, camera trapping can work, although in our experience at least, it can be quite difficult to capture otters as they emerge from the water as the cold body temperature doesn’t always trigger the camera.

  • Glen Power

    Good work, Denise. Are there any other survey techniques that can be implemented in Ireland? I know they’re expensive but what about motion cameras in areas where suspected otters exist like holts, culverts and under bridges?
    .

    • Thanks Glen. Yes, camera trapping can work, although in our experience at least, it can be quite difficult to capture otters as they emerge from the water as the cold body temperature doesn’t always trigger the camera.

  • Great post, I had a similar problems when surveying Red Squirrels!

    • Thanks Rachel. I think surveys for lots of species experience similar problems, most of which go unaccounted for in the results.

  • Great post, I had a similar problems when surveying Red Squirrels!

    • Thanks Rachel. I think surveys for lots of species experience similar problems, most of which go unaccounted for in the results.

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