There are 2 main methods:
1. Measure the population size, say, every 10 years using line transects and see how it changes. Getting adequate sample sizes is very expensive, making adequate seasonal sampling costly.

2. Measure the trend in acoustic detections at a sample of sites inclduing different habitats and see how the trend is evolving. This approach can get huge sample sizes from all seasons. Effort is needed to get spatial sampling cheaply.

The ECOMMAS study by Marine Scotland has produced valuable detail on trends.

Very substantial differences exist in the distribution of click rates used by different species.
Study of these differences has been limited to relatively small samples of click train data.

The F-POD gives access to very large samples of click train data and allows much deeper and more detailed analysis of click-rate profiles.

The graphs here show a typical diffefrence between porpoises and dolphins - in this case the dolphin is the Bottlenose dolphin which characteristically produces some very low click rate trains = trains with long inter-click intervals.

Large differences between night and day in acoustic detections are commonly seen and demonstrate a major limitation of visual surveys.

The F-POD app allows very quick inspection of these patterns, and associated patterns in the frequency, the click rates etc associated with these diel patterns.