Science & Conservation
How are tagging programs used in conservation efforts?
Tagging programs can provide us with information on movement patterns and growth, and are key to estimating population size.
Tagging fish at a recorded location and learning where and when they are later caught allows us to define large scale movement patterns. Of course, traditional tags (non-acoustic) can't tell us where fish are between the time of being tagged and being caught! However, when many tagged fish are caught and reported, it is possible to get a clear idea of how a species moves around a large area.
By finding out how much a fish has grown between original tagging and recapture, important information can be gained about seasonal and yearly growth rates.
Method 1 - "The Less Complicated, But Less Accurate Method": By using the ratio of tagged fish to those without tags, and some fairly simple math, population size can be determined. (Stay tuned, we'll be adding details about this math soon!) However, this only works based on identifiable 'sampling events', such as trawling for fish over a set period of time, or fishing using a weir. It is more difficult with recreational anglers to determine population sizes because anglers often don't report how many other fish they catch during a fishing event.
Method 2 - "The More Complicated, But More Accurate Method": By using fairly complicated math and statistical analysis called a Capture-Mark-Recapture or CMR analysis, estimates of population size, probability of capture, and mortality may be possible. The emphasis is on may because many capture histories from many fish are required, as well as the number of fish that are not tagged during each sampling event. Capture histories are based on individual fish being recaptured - one recapture is good, but getting recaptured several times is much, much better! This is one more reason to make every effort to return tagged fish to the water after recording tag information.