2.1 A GENERIC FORMULA FOR ESTIMATING CATCH
2.2 SECONDARY ESTIMATES
In this handbook there is no discussion of complete enumeration (=census) approaches for determining total catch, such as compulsory logbook data. In most small-scale fisheries the amount of information on total landings, species composition, prices, etc., is often large, highly distributed and difficult to collect so that the use of census approaches is impractical and sampling techniques are nearly always employed. Some exceptions occur in estimating total fishing effort and a detailed discussion on alternative approaches is given in Section 3.
This section describes a generic approach for estimating total catch from basic fishery sample data. Such estimation can be performed against any reference (= estimation context), most commonly a combination of a) a geographical stratum, b) a reference period and c) a specific boat/gear category. The estimation of secondary data such as catch by species, values and average fish size are also presented on the basis of the estimated total catch.
Total catch can be estimated from sample CPUE multiplied by estimated effort.
refers to all species taken together and is usually computed within the logical context of a) a limited geographical area or stratum, b) a given reference period (i.e. a calendar month) and c) a specific boat/gear category.
is an overall average deriving from sampling and expressing how much fish (all species) is caught by a unit effort. Sampling context is the same as that for the estimated catch.
2.2.1 Catch by species
Once the total catch has been estimated, species composition is computed by means of the following simple formula:
is the estimated catch for each species within the estimating context described earlier.
is a fraction of the total catch corresponding to a species and is formulated from the proportion of a species found in the samples.
is the estimated total catch discussed earlier.
From catch by species and using the estimated effort, it is also possible to compute species-specific CPUEs.
2.2.2 Species value
Once the catch by species has been estimated, its value is computed by means of the following simple formula:
2.2.3 Estimated total value of landings
It is computed within the estimating context by simply adding up all estimated values by species.
2.2.4 Average weight per species
In addition to catch by species and prices, sample surveys usually provide also data relating to fish size (in weight units) on a sub-sampling basis. When this information is available, it is possible to produce estimates of average fish size for certain species.
2.2.5 Numerical example
The following theoretical example uses the formulae given above and illustrates a stepwise process for deriving primary and secondary estimates. For purposes of simplicity it involves only two species and the assumption that fishing effort is known.
A. Assumptions and sample data
Estimating context: Lake Volta, Area VII, February 2001, GillnetsB. Estimations
Estimated effort = 1,000 boat-days
Sample overall CPUE = 10 kg/boat-day
Species 1Proportion of species 1 in samples = 60%Species 2
Sample price of Species 1 = 5,000 Cedis/kg
1,000 fish found in sub-samples of 500 kgProportion of species 2 in samples = 40%
Sample price of species 2 = 6,000 Cedis/kg
1,000 fish found in sub-samples of 800 kg
Estimated total catch = 10,000 kg (from formula 2.1)
Species 1Catch of species 1 = 6,000 kg (from 2.2.1)Species 2
CPUE = 6 kg/boat-day
Value of species 1 = 30,000,000 Cedis (from 2.2.2)
Average weight of Species 1 = 0.5 kgCatch of species 2 = 4,000 kg (from 2.2.1)Total value of landings = 54,000,000 Cedis
CPUE = 4 kg/boat-day
Value of species 2 = 24,000,000 Cedis (from 2.2.2)
Average weight of Species 2 = 0.8 kg
At this point readers are familiar with the parameters involved in the estimation of total catch and other secondary basic fishery statistics. The following points have been emphasized:
(a) All estimations are performed within the context of a stratum, a reference period and a boat/gear category.No mention has so far been made as to the mechanics for collecting the data required for formulating the above parameters. This is discussed in more detail in the coming sections that deal with the operational aspects of sample-based fishery surveys.