SAMPLE BASED FISHERIES STATISTICS

Before continuing with the design of an ODK fisheries form, some basic statistical requirements of small scale fisheries data collection are explained. This as the requirements will set the prerquisites of a data collection form.

Details on small scale fisheries statistics and data collection can be found in:

de Graaf, G.J., Nunoo, F., Ofori Danson, P., Wiafe, G., Lamptey, E. & Bannerman, P. 2015.
International training course in fisheries statistics and data collection.
FAO Fisheries and Aquaculture Circular No. 1091. Rome, FAO. 134 pp.


Fisheries globally are dominated in numbers by large and dispersed small-scale fleet which can only be monitored with sample based surveys.

Landing surveys
Landing surveys are conducted at landing sites with the purpose of collecting sample data on total catch and species composition, associated effort, and other secondary data such as prices and in particular:

catch of all species;
associated fishing effort;
overall CPUE;
catch by species;
first-sale prices.

The primary objective is to estimate, on a sample basis, fishing days, overall CPUEs and species proportions for each minor stratum and fishing unit.


A major principle of sample based data collection is “Stratification”.

The major objective of stratification is to reduce the variability in the sampled data, which will improve the reliability of the collected data.

Stratification is done through;

Major strata: Subdivisions based on administrative criteria, which are imposed on the data collection programme for reporting purposes and are therefore not under the control of the survey designer. Normally Major strata are provinces, districts, etc.

Minor strata: Within a major stratum there are usually subdivisions based on criteria that are chosen by the designer for the sole purpose of increasing the accuracy of the derived estimates and the subsequent set up the sampling scheme. Estimation of total catch will take place at minor strata level. Estimates of population parameters are always calculated at minor stratum level. Totals at major stratum level are simply aggregations of estimates and counts from the minor strata involved.

Fishing units: A fishing unit is defined by vessel characteristics and its major gear, for example; i) Non-motorised dugout canoe with gillnets, ii) Motorised planked canoe with gill nets, iii) Motorised planked canoe with hook and line, etc. The selection of fishing units should be coherent with data available from frame surveys or vessel registers/licensing.

Estimation of total catch
With a sample-based approach, estimation of total catch would use the mean catch per fishing day from a landings survey and the mean number of fishing days per vessel from a vessel survey, which multiplied together would give the mean catch per vessel. The total catch can then be obtained by multiplying this by the total number of vessels (a raising factor) obtained from a frame survey or vessel register.

The generic formula for this estimation is shown below.

Generic formula estimation of total catchTo estimate total catch, we need to collect data on 3 variables:

  1. CPUE, from landings or a catch assessment survey (CAS);
  2. Total number, classification and distribution of the fishing units. In principle, this is the fishing capacity, and the data are collected through a frame survey or through registration/licensing systems;
  3. Active fishing days, the number of days the vessels actually go fishing, or the number of fishing trips made by the vessels each month. This information can be collected through a CAS (PAB-horizontal sampling) or through a separated effort survey (BAC-vertical sampling).

Estimation are of total catch is done monthly, for each fishing unit, and for each minor stratum and mathematical the generic formula can be described as;


Total monthly catch = Total number of fish units × average no of fishing days× average CPUE

The total monthly catch will be the sum of the monthly catches of all minor strata and fishing units and the annual catch will be the sum of all monthly catches.