Appendix G - Calculation of the consumer surplus
Calculating the consumer surplus value involved the following three steps:
Step 1: Obtaining the Trip Generation Function (TGF)
- Anglers were grouped into the following residence zones aiming for homogeneous distance groups and socioeconomic make up (although, according to Bennett (1995), neither of these can be fully achieved): 0-100km, 100-200km, 200-300km, 300-350km, 350-400km, 400-450km and more than 450km.
- The visitor rate (VR) was calculated for each zone as the proportion between the number of visits reported in the survey to the number of residents.
- The relationship between the visitor rate and average travel costs (TC) for each zone is estimated using linear, log and semilog regression functions. A general Trip Generating Function is VR=a+b·TC.
Step 2: Deriving the demand curve for angler fishing days
- The TGF obtained is valid under the status quo of zero access fees to the fishery. The TGF is first used to calculate the average quantity of angler fishing days from each zone under the status quo. The total number of expected angler fishing days is the sum of angler fishing days across zones.
- A set of hypothetical access fees to the fishery (from $20 to $200 in steps of $20) are included iteratively in the TGF to calculate the impact on the visitation rates. The TGP was used as follows: VR=a+b·(TC+p), where p stands for access fees or, more generally, prices. The visitation rates obtained are then converted to a number of angler fishing days using the residential population from each zone. The sum of angler fishing days across zones is used as the total number of angler fishing days under given access fees.
- The demand curve is then plotted as the relationship between access fees and the total number of angler fishing days. Three demand curve models were tested using level, log and semilog functional forms.
Step 3: Calculating the consumer surplus
The consumer surplus gives a measure of what all anglers would be willing to pay to access the fishery under different scenarios of access fees and beyond what they currently pay. This is calculated as the area under the demand curve and above the horizontal of current access fees, which is zero. The average consumer surplus per angler fishing day is the total consumer surplus divided by the number of current angler fishing days.