Techniques for census of hunting species

Last month, our colleague Iván Salgado gave the training course Hunting Species Census Techniques at the School of Regional Management, in Toledo. A training action on how to estimate abundance/density and population trend; produce quality information to manage hunting as a renewable natural resource. The event was attended by managers and technicians specialized in hunting and wildlife from the regional administration, hunting managers, environmental agents, hunting management teachers and other professionals related to the hunting sector.

Risks of hunting

To start the course, the risks of hunting were raised if it does not meet scientific-technical criteria; the overexploitation of animal resources and the degradation of the population due to trophy hunting. The objective of hunting management is to integrate hunting into the ecosystem as another predator-prey interaction; a profitable and sustainable hunting operation. Therefore, census data are essential information for managing and conserving game species.

Population dynamics

The fundamentals of the dynamics of hunting populations were then explained. The logistic equation, which describes the growth of a population (which limits carrying capacity). And the derivative function, which defines the recruitment rate, the productivity of the population, and therefore, the maximum sustainable performance.

The principles of sustainable and profitable hunting management were also discussed:

1. the population size is optimal between K and K/2; to ensure the viability of the population, maximize production and not degrade the ecosystem due to overpopulation.
2. the extraction rate does not exceed the recruitment rate so as not to overexploit the hunting resource.
3. catches are distributed between age groups and sex so as not to alter the structure of the population.

Hunting strategies were explained by fixed quota or capture effort and those with a capture threshold (e.g., constant escape of breeding individuals), which require monitoring of the population but ensure the sustainability and profitability of hunting.

Principles of sampling

We remember the principles of sampling design: the sample represents the population and the data are independent and random. In general, stratified random sampling is suitable for censoring animal populations because the spatial distribution of species is almost always aggregated. We also indicate the differences between census (to estimate population size/density) and abundance index (only to estimate population trends; if it grows, it is stable, or decreases).

How to use a digital field notebook was taught: students used mobile applications Obsmapp and Epicollect to record data in the field.
We explain the basis of distance sampling methods: the relationship between distance and probability of detection. And we teach how to use the DISTANCE program: how to adjust a detection function to distance data to estimate the proportion of unobserved individuals. We discuss how to choose the function and the setting, set the truncation distance, define the distance intervals, and calculate the effective bandwidth to describe the loss of detectability due to distance.

It is necessary to estimate the population trend in order, for example, to quantify the impact of an epizooty or an episode of abundance or scarcity of trophic resources and to adapt the hunting strategy. The TRIM program was used to process time series of counting data and estimate abundance indices and population trends, and also to analyze the effect of covariants (e.g., environmental factors; habitat or season) that explain changes in abundance.

As a field practice, an exercise was proposed to estimate the rabbit population density in the Finisterre reservoir, in Toledo. Distance data were recorded and rabbit latrines were counted and advantages and disadvantages of census methods were compared and we discussed how to adapt the sampling design to each case study. The data obtained were a rabbit density of 6.25 individuals/ha in the Finisterre reservoir; hunting specialists working in the region commented that the population density value was correct.

The technicians of Environmental Ideas prepare and provide specialized training in monitoring threatened and hunting species. And we also design species conservation and game management plans.

References

• Coltman DW, O'Donoghue P, Jorgenson JT, Hogg JT, Strobeck C & Festa-Bianchet M (2003) Undesirable evolutionary consequences of trophy hunting. Nature 426, 655—658.
• Introduction to Distance Sampling: Estimating Abundance of Biological Populations. ST Buckland, DR Anderson, KP Burnham, JL Laake, DL Borchers and L Thomas. Oxford University Press, 2001.
• Pannekoek J, Van Strien A (2005) TRIM 3 Manual (Trends & Indices for Monitoring Data). Statistics Netherlands, Voorburg
• Salgado, I (2018) Is the raccoon (Procyon lotor) out of control in Europe? Biodiversity and Conservation 27 (9): 2243—2256