6.1 SAMPLING IN SPACE AND IN TIME
6.2 CENSUS IN TIME AND SAMPLING IN SPACE
6.3 CENSUS IN SPACE AND SAMPLING IN TIME
6.4 CENSUS FOR EFFORT AND SAMPLING FOR LANDINGS
6.5 BRIEF DISCUSSION ON THE FOUR GENERIC SURVEYS
This section extends the concepts of catch estimation outlined in sections 2 and 3 by looking at the generic expression for estimating total catch using the CPUE and Effort parameters, and the four different survey schemes (in time and space) that may be used to estimate these parameters.
Supplementary guidelines are also presented, relating to commonly used basic fishery data collection systems, including:
(a) How approaches become more reliable (at a cost) by working on a generic survey design and removing survey components that are directly associated to assumptions and/or sampling errors.(b) Brief description of each of the four generic surveys.
The diagram above recalls the generic expression for estimating total catch discussed in Section 2. It also indicates that for the formulation of its two parameters (CPUE and fishing effort), a maximum of four surveys are required, of which three are associated with fishing effort and one with the CPUE.
The above diagram also corresponds to the most economical sampling approach outlined in Section 3.4. (All surveys and estimates are made within the estimation context or stratum.) This sampling in space and time approach consists of the following four surveys:
Fishing effort
Overall CPUE
The generic formula for estimating catch is thus:
Catch = CPUE × [BAC × F × A]Where:
This approach was discussed in Section 3.3. The component related to Active Days (time raising factor A) has been removed. Its survey requirements are now three types as follows:
Fishing effort
Overall CPUE
The formula for estimating catch remains:
Catch = CPUE × [BAC × F × A]Where:
This approach was discussed in Section 3.2. The Frame Survey component has been removed. Its survey requirements are:
Fishing effort
Overall CPUE
The formula for estimating catch remains:
Catch = CPUE × [BAC × F × A]where:
This approach was discussed in Section 3.1. Its survey requirements are:
Fishing Effort
Overall CPUE
This approach is directly derived from the generic formula 6.1 by removing all survey components relating to fishing effort, and catch is simply estimated as:
Catch = CPUE × Effort.
Catch estimation can be made through the use of between 1 and 4 different survey types, combined with census estimates where necessary.
6.5.1 Frame Survey
The objective of a Frame Survey is to provide total numbers of potentially operating fishing craft for each estimation context, which normally refers to a minor stratum, a calendar month and a boat/gear category. Frame Survey basic characteristics are:
6.5.2 Active Days Survey
This is usually carried out at the end of the month, when all sampling has finished and estimates are about to be produced. It provides a time raising factor for estimating total fishing effort. Active Days Survey characteristics are:
6.5.3 Boat Activity Survey
The sole objective of this samplebased survey is to formulate the Boat Activity Coefficient (BAC). Boat Activity Survey characteristics are:
6.5.4 Objectives of Active Days and Boat Activity Surveys
Boat Activity surveys examine the individual level of boat activities and aim at determining the probability that any boat of a specific boat/gear category will be active on any one day. Active Days Surveys, on the other hand, aim at determining a time raising factor expressing the number of days in a month that are potentially days of fishing, that is excluding days of no fishing in a uniform manner.
These concepts can be illustrated by the following two examples.
The above diagram illustrates fishing activities of a hypothetical fishery of five boats A, B, C, D and E. Fishing is indicated by a shaded area, non fishing is blank. The BAC for this group of boats is formulated by considering the entire dataset of boat status indicators and finding out how many elements represent boats fishing.
In this case it is evident that the data population of boat status indicators consists of 5 boats × 30 days = 150 elements, of which 30 days represent fishing. Thus;
The number of boats expected to be fishing on any day is
In this example all days in the month are potentially fishing days; i.e. there is no reason to assume that any day should be different from another in terms of activity level. Thus the time raising factor A is set to 30 and the resulting fishing effort in boatdays will be:
E = BAC × F × A = 0.2 × 5 × 30 = 30 boatdays, a result that can also be confirmed by the diagram.Example 2
In this example days 1, 8, 18, 19 and 30 were nonfishing days due to bad weather.
Formulation of the populationspecific BAC focuses only on days with fishing and has resulted in the same BAC as before, that is
In estimating total effort the time raising factor A is now set to A = 25, thus resulting
6.5.5 Landing Survey
The main objective of sampling through a Landings Survey is to formulate the overall CPUE used in the generic formula for estimating total catch landings. Surveys can also provide secondary data on species composition, prices at landing, average weight by species and other data. Its basic characteristics are:
SUMMARY In this section survey requirements in the most commonly used data collection systems for basic fishery data were presented, including how catch estimates can become more reliable using the same generic formula by replacing sampling components with census approaches; that is replacing the cheaper sampling method with the more expensive, full data coverage method of the census. The following four generic surveys outlined were: Fishing effort
Overall CPUE and secondary landing data
