Using 'transect' vs 'transit' data for DSM

[Abigail Schiffmiller]

Hi All,

I have line transect data for cetaceans in SE Alaska. There are the 'transect' lines (and associated data) which are from the actual survey design with even coverage probability over the sample area, but there is also sighting data along 'transit' lines between transect end points that were also 'on effort' so the extra sighting data could be used to get better detection functions. 

I know that, once the detection function is determined, you only use the 'transect' data to get the abundance estimate with regular distance sampling.  BUT, I know that a benefit of DSM is that you can use data from platforms of opportunity and uneven coverage. What I can't remember/figure out  is if I can use both my 'transect' and 'transit' segments/data in the same DSM from the get go. Many 'transit' lines cross 'transect' lines, so I don't know how those areas that are crossed twice get incorporated/counted.

It would be great if I could use both in the same DSM because it gives me about 50% more data/power for my DSM model (which I really need in this super complex study area!!).

Can I use both and not worry about it? Is there some adjustment I need to make to use both? Can I only use one or the other in a given DSM model?

Thank you!!

-Abigail

[Eric Rexstad]

Abigail

When using design-based inference, off-effort sightings can be used to aide in fitting the detection function (under some conditions), but not in the calculation of encounter rate.  However, when using model-based inference, in my opinion, the off-effort sightings can be used not only for the detection function modelling (under those conditions), but also in assessing the manner in which encounter rates change as a function of environmental covariates.  You may get other points of view to your question.

The regions that are visited by transit and transect lines may have different numbers of encounters that might be explained if you are using dynamic covariates that change from one visit to another.

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Eric Rexstad
Centre for Ecological and Environmental Modelling
University of St Andrews
St Andrews is a charity registered in Scotland SC013532