RAUK - Archived Forum - Number of clear trap days

This contains the Forum posts up until the end of March, 2011. Posts may be viewed but cannot be edited or replied to - nor can new posts be made. More recent posts can be seen on the new Forum at http://www.herpetofauna.co.uk/forum/

Forum Home

Number of clear trap days:

This is Page 1

Author Message
Matt Harris
Senior Member
Joined: 03 Jun 2003
No. of posts: 196


View other posts by Matt Harris
Posted: 25 Sep 2006
We are discussing how many clear trapping days (days where no animals caught)are needed to achieve 'reasonable effort' for removing slow-worms from a site using ACOs. Our SNCO has advised 10 clear visits, I gather the norm for road schemes is 5 clear visits, but we have been advised that HWM states that 7 is the minimum. I can't find this anywhere in HWM, so what do other people use as the number of clear trapping days, and based on what published advice?

Many thanks.
Gwent Amphibian and Reptile Group (GARG)
Vicar
Senior Member
Joined: 02 Sep 2004
No. of posts: 1181


View other posts by Vicar
Posted: 25 Sep 2006

Matt,

As far as I am aware, there is no clear statistical evidence supporting a recommended number of CTDs. I've been doing some similar stats for HCT re Smooth snake, and its fairly complicated (factors such as meteorology and time tin has been in position etc all affect the results). Of course the factors are different across species.

I think the quoted figures have been published in a worthy attempt to 'standardise' survey approach, based on semi-empirical, and possibly subjective evidence.

Once NARRS has been running for about 5 years we will probably have sufficient data to estimate these figures with some degree of confidence. Sorry not to be more help.

(Wasn't it Froglife who published probable non-presence figures?)


Steve Langham - Chairman    
Surrey Amphibian & Reptile Group (SARG).
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 25 Sep 2006

5 clear capture days probably originated from the GCN mitigation guidleines. Though I have also heard it quoted in conjunction with Highways projects in reference to reptiles.

The advise I've been given by EN in the past is that ultimately it is the decission of the consultant as to whether sufficient capture effort has been applied.

I agree with this stance. For example I could feel that ACO capture had served its purpose and judge that the site should be destructively searched - should one wait for 5 - 10 days to carry on pointlessly checking cover objects if it is strongly felt that capture effort by ACO is no longer effective?

Be very interested to hear others views on this and also if there is any published reference for reptiles.


Gemma Fairchild, Independent Ecological Consultant
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 25 Sep 2006
Also, with ACO capture clear days should be defined as 'no observations' not 'no captures' technically - again a difference to pitfall trapping of GCN which are either in the trap or they are not - certainly not always the case with ACO capture work where a proportion will always be on the 'another day' list
Gemma Fairchild, Independent Ecological Consultant
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 25 Sep 2006

Perhaps of some help, this is my preferred method of keeping a handle on reptile mitigation works. From an area of a current mitigation we are working on.

Temperatures have been reasonably constant during the capture period, there are a couple of obvious days where it rained heavily etc but I think in all it is pretty clear what is happening from the graph. We expect a new peak tomorrow (showers today, sun out tomorrow with max 18 c)- this will be very informative as conditions should be ideal for ACO obs.. certainly if the decline in observations in this area continues we will have a good indication we are nearing a point of diminishing return.

The more I think about this the  more I think a set number of CTDs is very arbitary for reptiles. Should we be looking towards using another criteria for 'reasonable effort'? Is it reasonable effort for example if in 10 days all that is observed is a single juvenile slow-worm? Can it be reasonable to carry on for a further 10 days just in case there is one more? A very interesting topic indeed.

PS we are not quite as bad at catching things as the graph might suggest, this is raw data that includes a number observations of juvenile lizards on ACOs in warm conditions so the same animals appear over and over as observations as opposed to captures - their day will come


Gemma Fairchild, Independent Ecological Consultant
Matt Harris
Senior Member
Joined: 03 Jun 2003
No. of posts: 196


View other posts by Matt Harris
Posted: 26 Sep 2006
Ok Thanks,

As I thought, it doesn't seem to be written down anywhere. I agree that a set number of CTDs for reptiles is arbitary and subjective, but it would be helpful to have a guideline for this figure, as there are for other survey parameters.

Is there anything mentioned in Foster and Gent's Reptile Survey Methods? We don't have a copy here.
Gwent Amphibian and Reptile Group (GARG)
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 26 Sep 2006

I don't recall ENS No.27 addressing mitigation only survey methodology and can find no reference to CTDs briefly looking through the publication.

I don't always think these guidelines are always a good thing; I will be honest about this. I don't think it is good to be told I need to carry out site survey 20 times when I have made as full an assessment as I'm ever likely to in 7 visits just because someone happened to find the Froglife Advice Sheet 10 and decided to quote it without really understanding the content.

These decisions should be left to the consultant i.e. what is 'reasonable survey effort' and what is 'reasonable capture effort'. The real issue here is that more consultants working with reptiles ought to get a handle on the animalĘs basic ecology, perhaps then we won't need arbitrary rules.

(rant over )

PS we got the new peak for our graph at 9 animals for this area all lifestages/Af & Lv - an indication that capture effort is turning the tide but it ain't over yet..


Gemma Fairchild, Independent Ecological Consultant
Matt Harris
Senior Member
Joined: 03 Jun 2003
No. of posts: 196


View other posts by Matt Harris
Posted: 06 Oct 2006
I think guidelines are helpful as long as they are treated as just that; guidelines and not rules carved in stone. We have to deal all too often with consultants who have, as you say, found a Froglife advice sheet and quote it without understanding the content.

I can't find any published reference to CTDs either. Turns out in this case that far from the HMW citing 7 CTDs as the norm, as the consultants claimed, they were in fact referring to the KRAG advice sheet (protected reptiles and dvlt) which says that there should be 7 survey visits to determine presence or likely absence. In other words, equating CTDs for reptile removal, to recommended site visits for initial presence/absence survey.

In this particular instance, as I expect is commonplace, the consultants are under considerable pressure from the developer to 'sign off' the site for the bulldozers. The impetus to complete surveys and translocations asap is sometimes at odds with following established methodology. Or at least, this pressure results in the consultant trying to exploit any ambiguity, latitude or contradiction in guidelines to complete the job with minimum time and effort, whereas we would prefer things to be done properly.

Gwent Amphibian and Reptile Group (GARG)
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 06 Oct 2006

Hi Matt, yes I know what goes on all too well.

I think the more usual approach for reptiles by most consultants is to book x amount of visits. Something between 60 and 80 being the norm. At the end of this time it is usual to regard that appropriate capture effort has been applied, rather than using CTDs.

I would like to hear from a few of our other members who are consultants as to their methods of reaching 'sign off'. My own experiences of working for other consultants in the past showed 'various' approaches.

The M+G policy is to book more than sufficient sessions, if we capture a site out before we complete all of them, so be it we charge the client for the completed sessions and the job is done. Many consultants will take the opposite approach of booking as few as possible when tendering for a contract in hope of getting the work, only then to book more or to run into problems with the number of animals on the site/size of the receptor area/client cost and time.. well you probably know the score!

It interests me a lot as a subject as I strongly believe that adequate initial survey is the key to avoiding all these problems on a site and it should be perfectly within the consultants ability to book an appropriate number of capture sessions based on site complexity/species/relative density etc.

Without being able to reach these conclusions from survey data how on earth can an appropriate receptor area be identified or appropriate capture effort appled? In other words how can the site be effectively mitigated at all?


Gemma Fairchild, Independent Ecological Consultant
sussexecology
Member
Joined: 30 Sep 2010
No. of posts: 37


View other posts by sussexecology
Posted: 20 Mar 2011
I realise that this is a bit out of date, but we tend to use a basis of 10 clear days in suitable weather conditions, followed by destructive searches. This worked on one project last year when we had more than 10 days that were clear and the destructive searches were negative.

GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 20 Mar 2011

Steve, it's 5 years on, any chance of the NARRS data shedding any light?

 

 


Gemma Fairchild, Independent Ecological Consultant
Vicar
Senior Member
Joined: 02 Sep 2004
No. of posts: 1181


View other posts by Vicar
Posted: 21 Mar 2011
Not from NARRS no. NARRS have gone more the way of a regional occupancy model, but some of the supporting DICE research might be applicable.

But, from SARG stats...yes (sort of). This is a version of probable absence, as used for a pre-cursor to reintroductions.

We can generate detectability stats for month of year and met conditions. Probable absence can be calculated from the number of negative results, accounting for the 'minimum' detectability.

'Minimum' is a challenge. For non-mitigation sites, we use the lowest detectability value for viable sites, as we believe this represents the smallest or lowest density populations. I'd guess you'd use the same approach here.

Basically, each survey is not treated the same, but is weighted according to met conditions and month of year. This 'weight' is removed from 1.0 (certain presence) until a threshold for absence is reached.

So far, we can't see any clear correlation between detectability and refugia density, which is counter-intuitive. Probably, the variation in our refugia densities isn't wide enough.

A near-term goal is to write up the approach in a simple manner, which can be exploited in the field (rather than using a computer model - although it could easily be a web-based application).

You also need to determine the target confidence level (nothing is absolute in stats!). For re-introductions, NE have mandated a confidence of absence of 95%, which is really high.

The number of clear trap days to determine probable absence also varies with species. But as a guide, for snake species, at C95%, you're probably looking in excess of 20 clear surveys!

I'd have thought that a slightly lower confidence level was more practicable, and would decrease the number of negative surveys required.



Steve Langham - Chairman    
Surrey Amphibian & Reptile Group (SARG).
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 21 Mar 2011

Thanks Steve very interesting indeed.


Gemma Fairchild, Independent Ecological Consultant
Scale
Senior Member
Joined: 05 Dec 2010
No. of posts: 83


View other posts by Scale
Posted: 21 Mar 2011
gScale40626.3023032407
Vicar
Senior Member
Joined: 02 Sep 2004
No. of posts: 1181


View other posts by Vicar
Posted: 21 Mar 2011
Have been thinking more on this, and I can't see an easy way of incorporating all the Met variables into a simple table. Also, some met variables affect some species more than others. E.g. Vb are more sensitive to cloud cover than Af.

I think a half-way house would to create a table of expected detectability (per hour of survey) per species by month of year. This would assume average met data for the month, but could be assured by the met conditions for each survey being reported in an annex of any report. (How I'd love a reptile survey reporting standard!).

So, for Ca, surveys in August would carry a high weighting (the peak detectability month), compared with surveys in March (when only half the snakes have emerged from hibernation). So, you'd need fewer clear trap days if surveying in August, and more if the survey was in March.

This feels like a better solution than a flat number of clear surveys, and although not perfect, would at least be evidence based.

Exactly the same process could be used for initial presence/absence surveys.



Steve Langham - Chairman    
Surrey Amphibian & Reptile Group (SARG).
sussexecology
Member
Joined: 30 Sep 2010
No. of posts: 37


View other posts by sussexecology
Posted: 21 Mar 2011
[QUOTE=Vicar]Have been thinking more on this, and I can't see an easy way of incorporating all the Met variables into a simple table. Also, some met variables affect some species more than others. E.g. Vb are more sensitive to cloud cover than Af.

I think a half-way house would to create a table of expected detectability (per hour of survey) per species by month of year. This would assume average met data for the month, but could be assured by the met conditions for each survey being reported in an annex of any report. (How I'd love a reptile survey reporting standard!).

So, for Ca, surveys in August would carry a high weighting (the peak detectability month), compared with surveys in March (when only half the snakes have emerged from hibernation). So, you'd need fewer clear trap days if surveying in August, and more if the survey was in March.

This feels like a better solution than a flat number of clear surveys, and although not perfect, would at least be evidence based.

Exactly the same process could be used for initial presence/absence surveys.


[/QUOTE]

This is the thing though that I was trying to point out last night (although I didn't have much time to post the message). The number of clear trap days depends on what species you have present and the time of year that you are doing this.

Tended to find that if you are dealing with slow worms, the number of clear trap days will be lower. 

There is some guidance in the ARG Guidance on reptile translocations which outlines the standards in best practice.



Vicar
Senior Member
Joined: 02 Sep 2004
No. of posts: 1181


View other posts by Vicar
Posted: 21 Mar 2011
It probably also depends on met conditions at the time of survey, habitat type, tin material & size and density of laydown....and probably other factors too....such as time since last rainfall.

Steve Langham - Chairman    
Surrey Amphibian & Reptile Group (SARG).
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 21 Mar 2011

Absolutely Steve, this is why I have big problems with an arbitary figure.

ACO are only one tool in capture works.

By graphing the capture results I get a high degree of confidence that the site is actually being cleared. One can look back over months and see the capture rate reducing until eventually sightings are down to single animals, sooner or later we simply know who's left onsite using ACO and can gauge ACO effectiveness is over. Time for destructive search.

Compare to a sample mitigation where the consultant caught 30 slow worms in a day then no sightings for a week. 7 CTDs, must be time for sign off? Nope, just shows they had a particularly good day and intercepted the proportion of the population using ACO at the time in one hit.

Next time it rains, they'll be more again. It could be 10 or 20 days or more before it rains. If though guidelines just say 10 days to sign off, well you know what's going to happen. Chances are next time the peak will be 20 animals, the next 10 and so on. This is how I work.

I've a feeling we should be looking at the data from capture works to form a model for a degree of confidence. Again though it's not site clearance, it's just the end of ACO effectiveness. So I'm really thinking here CTDs should never be a criteria for sign off. However convenient it might appear. Before we know it clients are telling us that 5 days without sightings means it's over, nope it's over when I say it is

It did though prompt me to look at presence/absence surveys we did in the past.

9 - 18 visits to establish adder presence (not visual spring survey)

5 - 8 visits to establish grassies

3 visits to establish slow worms

1 visit for Zv

It would be kind of crazy though to check ACO for another 20 days at the end of a mitigation when the vegetation is mostly gone and any remaining animals are not using ACO in any case.


Gemma Fairchild, Independent Ecological Consultant
Vicar
Senior Member
Joined: 02 Sep 2004
No. of posts: 1181


View other posts by Vicar
Posted: 21 Mar 2011
[QUOTE=GemmaJF]It would be kind of crazy though to check ACO for another 20 days at the end of a mitigation when the vegetation is mostly gone and any remaining animals are not using ACO in any case.[/QUOTE]

Agreed. This is why I think you wouldn't need a 95% confidence level. Although...you might want to put a dent in the population prior to manipulating the habitat.

I worry a bit about artificially increasing the population density through manipulating habitat, in terms of predator/prey balance, territorial disputes and the available food chain.

Hopefully it's common practice to remove some of the population, then manage the habitat, avoiding unduly high pop densities ?

Steve Langham - Chairman    
Surrey Amphibian & Reptile Group (SARG).
GemmaJF
Admin Group
Joined: 25 Jan 2003
No. of posts: 2090


View other posts by GemmaJF
Posted: 21 Mar 2011

It should be Steve, but I'm hearing more and more about avoiding fencing and manipulating habitat in early stages. To me it just seems like carnage for the reasons you have given. God only knows we moan enough when conservation organisations do this kind of thing!

We don't manipulate habitat until ACO are practically not producing, certainly I would want all gravid animals off the site and most if not all the adults. Then we strim between the ACO leaving little islands. By then though I would be looking at just a few individuals left onsite and just a few weeks before calling an end to ACO capture work. Sometimes we find grass snake get 'flushed' under the ACO. Been there all the time not using the ACO but when the covers gone, they hide under them. 

I can't imagine why anyone would think it was a good idea to reduce available habitat early during the works for animals that are going to be stuffed in a bucket and moved. But didn't Jon post something from NE a while back saying this is the way they wanted to go to reduce client cost and time??? (Hopefully I misunderstood it)


Gemma Fairchild, Independent Ecological Consultant

- Number of clear trap days

This is Page 1

Content here