6. SURVEYS FOR BASIC FISHERY DATA

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.

6.1 SAMPLING IN SPACE AND IN TIME

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

• A census-based Frame Survey providing the raising factor F that expresses the total number of boats.
• An Active Days Survey to determine a time raising factor A expressing number of days with fishing activities.
• A sample-based Boat Activity Survey to determine the Boat Activity Coefficient (BAC) expressing the probability that any boat will be active on any given day.

Overall CPUE

• A sample-based Landing Survey to determine sample overall CPUEs (usually at the same time as other data on species composition, prices and average weight per species).

The generic formula for estimating catch is thus:

Catch = CPUE × [BAC × F × A]

• CPUE is estimated from a Landings Survey
• BAC is estimated from a Boat Activity Survey
• F is provided by a Frame Survey
• A is determined from an Active Days Survey

6.2 CENSUS IN TIME AND SAMPLING IN SPACE

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

• A census-based Frame Survey providing the raising factor F that expresses the total number of boats.
• A Census in time on selected sites to determine the total fishing effort and mean effort AverF, which expresses the average number of boat-days for a single boat. Based on AverF, BAC is computed as AverF/NC where NC is the number of calendar days in the month.

Overall CPUE

• A sample-based Landing Survey to determine sample overall CPUEs (usually at the same time as other data on species composition, prices and average weight per species).

The formula for estimating catch remains:

Catch = CPUE × [BAC × F × A]

• CPUE is estimated from a Landing Survey.
• BAC is computed as AverF/NC, with NC = number of calendar days.
• F is provided by a Frame Survey.
• A is set to NC.

6.3 CENSUS IN SPACE AND SAMPLING IN TIME

This approach was discussed in Section 3.2. The Frame Survey component has been removed. Its survey requirements are:

Fishing effort

• A survey at all homeports to determine the total fishing effort and average fishing effort per day, AverE. Since all homeports are visited at least once during the month, the total number of boats F is known. BAC is computed as AverE/F.
• A survey (or exercise) to determine a time raising factor A expressing number of days with fishing activities.

Overall CPUE

• A sample-based Landing survey to determine sample overall CPUEs (usually at the same time as other data on species composition, prices and average weight per species).

The formula for estimating catch remains:

Catch = CPUE × [BAC × F × A]

• CPUE is estimated from a Landing Survey.
• BAC is computed as AverE/F.
• F is always known on a monthly basis.
• A is determined from an Active Days Survey.

6.4 CENSUS FOR EFFORT AND SAMPLING FOR LANDINGS

This approach was discussed in Section 3.1. Its survey requirements are:

Fishing Effort

• A census conducted every day at all homeports to enumerate fishing effort expressed in total number of boat-days.

Overall CPUE

• A sample-based Landing Survey to determine sample overall CPUEs.

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.

6.5 BRIEF DISCUSSION ON THE FOUR GENERIC SURVEYS

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:

• It must be conducted to cover all homeports, all fishing boats and gears and in accordance with pre-set survey standards and categories. It is thus a census-based approach.
• It should be conducted as often as possible to record any fundamental changes to the distribution of boats and gear, but in practice it may only be conducted on a yearly basis.
• In addition to the data required for the estimation of fishing effort it can provide much other useful information on the socio-economics of fishing communities and also for planning field data collection operations, such as periods of landings, standard days of little or no activity, sequential or concurrent use of gears, fishing grounds, etc.

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:

• It is formulated by using the calendar days of a month and subtracting days (or fractions of days) for which it is known or assumed that little or no fishing has taken place.
• It does not account for individual variability of boat activities (this is the role of BAC). It refers to days for which there are no reasons to assume that fishing activities are below normal level.
• Examples of not active days are periods of bad weather, national or religious holidays, standard non-working days such as Fridays, Saturdays, Sundays, market days, etc.
• Active Days can be area-specific and boat/gear-specific and are formulated separately for each combination of minor stratum and boat/gear type. For example, in the same area bad weather may affect boats using purse seine and not the beach seines. Or, bad weather may affect gillnet fishing in one area but not in another.
• Determining Active Days is simplified if sampling in time (landings and/or effort) is frequent enough to cover 12-15 days of the month, which gives sufficient days for low or zero activity to enter the samples. In this case the total number of days in the month is used as raising factor.

6.5.3 Boat Activity Survey

The sole objective of this sample-based survey is to formulate the Boat Activity Coefficient (BAC). Boat Activity Survey characteristics are:

• This survey is always conducted at homeports. BAC is formulated separately for each boat/gear category and in accordance with survey standards.
• BAC accounts for the individual variability of boat activities and is determined by examining an appropriate number of boats and finding out how many have been active on a given day.

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.

Example 1

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;

• BAC = 30/150 = 0.2, which is the probability that any boat will be fishing on any day.

The number of boats expected to be fishing on any day is

• 0.2 × 5 = 1, a fact that is immediately verified by the diagram.

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 boat-days will be:

E = BAC × F × A = 0.2 × 5 × 30 = 30 boat-days, a result that can also be confirmed by the diagram.

In this example days 1, 8, 18, 19 and 30 were non-fishing days due to bad weather.

Formulation of the population-specific BAC focuses only on days with fishing and has resulted in the same BAC as before, that is

• BAC = 25/125 = 0.2

In estimating total effort the time raising factor A is now set to A = 25, thus resulting

• E = 0.2 × 5 × 25 = 25 boat-days, a fact confirmed by the diagram above.

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:

• It is always conducted at landing sites and may record landings of boats operating from different home port locations.
• Landings are reported separately for each boat/gear category and in accordance with survey standards.
• It requires skilled staff for species identification and accurate recording of fish weights.

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 Frame Survey Active Days Survey Boat Activity Survey Overall CPUE and secondary landing data Landing Survey