The next stop on our journey into the bowels of population estimation takes us to the deep blue sea, a placid lake or, if you prefer, a laughing river. You won’t find any deer in these places but if you throw out a line, you may find some fish.
It’s hard enough to estimate terrestrial populations. Imagine trying to estimate a population that you can’t “see” at all. Fisheries management has its own unique set of challenges for sure. But fish heads (as they are affectionately called in grad school) are smart. Catch-per-unit effort (CPUE) is widely used in fisheries management to estimate abundance. The basic premise is ratio of animals caught to the effort it takes to catch them is proportional to the number of animals in the population.
Changes in CPUE relate to changes in the abundance of the target species. More effort to “catch” the same number of animals (decreasing CPUE) signals a decrease in the population being sampled. One of the advantages of CPUE is that it does not interfere with routine harvest. Data are also easy to collect.
Assumptions for this technique include:
- probability of capture is constant for all animals for each sampling period
- population is “closed” between sampling periods (no births, deaths, etc)
- each unit of effort takes a constant proportion of the remaining population
It’s easy to see why this is used in fisheries. Control for gear, collect fishing effort, and amount of fish caught, and voila, a reliable index to estimate abundance for the population.
If it can work in water where you can’t even see the critters you are trying to estimate, surely it can be used on land. Hunter kill per hour, sightings per hour, and hunter success can all be used as indices of CPUE. Easy peasy, right?
Well…deer usually have other ideas especially when it means life or death. Behavioral changes and shifts in sex and age structure of the remaining herd makes them a bit more difficult to “catch” as the season wears on (assumption violation!).
Then you have to consider hunter behavior. If hunters can harvest either antlered or antlerless deer while out, then they are not pursuing them equally. This leads us to the other big issue – the “E” in CPUE. Defining effort is a challenge and a key to estimating the population with this method. If you cannot accurately quantify effort, then none of the assumptions even matter as you are missing the foundation on which CPUE is built!
CPUE was tried at Fort Indiantown Gap. It had potential, but it turns out that not enough deer were harvested to effect a change in kill/hour. There must be “change” in catch for the model to actually work. Another problem was hunter selectivity – even though they could harvest both antlered and antlerless deer, it was evident that hunters were hunting bucks early in the season and didn’t put effort towards harvesting antlerless until near the end of the season.
CPUE can be useful as an index. Number of hunters afield and permits used can provide a reasonable index of population trend over time. Trends are super useful to wildlife managers but as for CPUE providing a population estimate for deer…not unless we are fishing for them.
-Duane Diefenbach and Jeannine Fleegle
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This post is part of a series on Population Estimation
Fishing (you are here)