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Appendix 2 provides an introduction to some types of recording forms for the collection of fishery data for a variety of purposes in the fisheries sector. A wide variety of types of data for a range of different analyses are required from time to time in the fisheries sector, and although this publication can only touch lightly on each of these, it is useful to take a broad perspective on the likely information needs early on in planning a statistical data base, to avoid having an excess of information on one aspect at the expense of a complete lack of data on other aspects.

6.1 Type and Quantity of Catches and Landings

The main types of commercial landings need to be categorized and properly codified at the preparatory stage of planning a statistical system. Each category will require a somewhat different approach to sampling and some will contain a variety of species or commercial classes that need recording separately. Some typical categories recommended to the Commonwealth of the Bahamas (Caddy 1981) were:

 (i) live fish (held in live wells) and sold piecemeal,

(ii) whole iced fish landed in boxes,

(iii) frozen fish or crawfish tails in “40-1b” bags,

(iv) fish fillets frozen in bags,

(v) conch (in shell, shucked before sale).

Similar categories have to be chosen for each island (group) that reflect the different types of resource locally available and important for data collection. These categories each need to be sampled for sizes, ages and species composition in order to adjust up to the total landing estimated for the main species of concern.

The various approaches to estimating total catch from a fishery and the consequent level of landings are given in Bazigos (1974) and FAO (1981). Two approaches are commonly used:

(a)  find the total catch from a known fraction of all fishing operations for which catch figures are known. As noted later, this is only possible if the entire sampling frame has been defined, and sampling carried out randomly;

(b)   attempt to reach all sales transactions going through fish processing plants and authorized buyers by a scheme such as that given in Appendix 3 (a Census Approach).

6.2 Conversion Factors

The above example indicates one of the special problems found in many fisheries; the spiny lobster from spear fisheries are likely to be butchered at sea, and only tails landed. This may or may not be the case for the trap fishery. If not, then carapace length may be available for one fishery and tail lengths only for the other. Similar problems will be found in combining gutted or dried weights of fish with fresh weights in estimating total fish landings. For such conversions, a conversion factor will be needed for each processed type to express it in terms of the conventional measurement adopted: usually either total or carapace length, or whole fresh weight. Conversion factors can be established by measuring/ weighing a sample of some 200 labelled individuals of the full range of sizes before processing them in a commercial fashion, and reweighing and remeasuring them afterwards. Conversion factors can then be obtained by regression analysis (see FAO (1980) for more details). This is time consuming, but only needs repeating when commercial methods of treatment change.

6.3 Main Types of Gear/Vessel in Use

Especially for biological sampling, it is important that the fishing gear category be reflected on the sampling forms. Some typical categories for the Bahamas were:

 (i) spear fishing (crawfish tails, whole or filleted fish),

(ii) handlines (whole or filleted fish),

(iii) seines (whole or filleted fish),

(iv) traps (crawfish tails, whole or filleted fish),

 (v) bully nets (crawfish tails).

Obviously, again, these categories have to be adjusted to local practice and properly codified.

6.4 Identity of the Resource

Prior to setting up a sampling or monitoring scheme for resource assessment purposes, it is necessary to establish what are the species groups being fished before deciding on meaningful resource categories.

(1)   Species identification: proper species identification is a necessary prerequisite to detailed assessment of most resources, and this is a problem faced by most tropical countries where species diversity is high. Identification of fish and invertebrates in the WECAFC area is possible by means of the FAO Species Identification Sheets (see e.g., fishing area 31: the Western Central Atlantic: Fischer, 1978). For actual identification of fish in landings it is of course impractical for the statistical worker to carry with him a 7-volume compilation including of the order of 1 000 potential and actually commercial species. In fact, the situation is usually made more manageable by first extracting those sheets for species of commercial importance by the country in question. The situation is simplified for several of the more important species (e.g., lobster, conch in the Caribbean) where identification is not usually a problem. For several others, especially the trap fishery for reef fish, these are often sold in mixed form after removal of species of high price; as such, their individual categorization may not be a top priority, i.e., a category such as “mixed reef fish” may be necessary. Alternatively, or in addition, one or two key species of reef fish that are easily recognizable, may be selected for statistical sampling as “indicators” of the state of reef resources. Someone familiar with the resources of the area and their commercial categorization will need to design a simple “guide to the commercial fisheries” based on the FAO Identification Sheets, and an appropriate sampling form (e.g., forms F and G in Appendix 2) for each subregion (where possible, coordinating this between adjacent island states).

(2)   Data on species composition: Market surveys at regular annual intervals are useful to note the proportion of the key species being offered for sale, and their relative prices, and can offer indirect evidence of changes in relative abundance of species, since the price of “substitute” or former “trash” species tends to rise as the preferred species become less abundant and too expensive for most consumers. For a more accurate idea of species composition, sampling at landing points is the recommended approach, or if discarding at sea is the case (it usually is) at sea sampling on commercial boats will be needed.

(3)   Stock identity: In resource assessment, it is important first to consider what are the “unit resources” or stocks of a given species that are being exploited, and design the statistical system in such a way that information from different unit resources can be separated. For example, while it may be reasonable and appropriate to consider managing all of the coral reef resources of a small island as a unit, in the case of two systems of coral reefs separated by a considerable distance, it may be more appropriate to treat the fish stocks of these two reefs as individual resources.

On the contrary, for migratory, especially small neritic pelagic resources, (e.g., herring anchovies), the stock of a series of islands sharing a common shelf will need to be managed in a unit. For oceanic species (e.g., tuna, flying fish), it is likely that the islands of an archipelago will be sharing a common resource, even if there is not a common shelf together with fisheries from adjacent continental areas. Some of the problems in dealing with these types of common resources are advanced in Gulland (1981) and Caddy (1982), but from the point of view of assessment of these last two types of resources, it is important that all data from a unit stock be considered together, and kept separate from adjacent stocks.

6.5 Direct Approaches to Fishery Assessment: Mapping and Surveying Shelf Resources

The direct approach to estimating the potential productivity of island shelves is to construct a map of shelf habitats (reef, sand, mangrove, turtle grass, and mud, etc.), stratified by depth, and use this as the basis for a resource survey to determine the overall biomass of different commercial species, or of the resources as a whole (a multispecies assessment), using either a research or chartered commercial vessel.

The funds and personnel to carry out a proper resource survey may not always be available, and a map of the main habitats and fishing grounds may have to be constructed from interviews with fishermen, or in more detail, from LANDSTAT images and aerial photographs (see Caddy and Piaggesi (1982) for details of the methodology).

As discussed earlier, thematic maps, however simple, allow fishing grounds to be outlined and their area estimated by planimeter. The population biomass for a species is then given by (area of grounds x average biomass per unit area). A first rough estimate of potential yield from this biomass requires for each species a knowledge of the fishing mortality rate F (given by fishing yield/average biomass) and natural mortality rate (M), which is obtained from biological research, or by using the value obtained for the same species in adjacent areas. The yield tables of Beverton and Holt (1966) can then be used to estimate what will be the level of fishing and size at first capture that gives the maximum yield per recruit.

6.6 Indirect Approaches to Resource Analyses

Estimating the potential yield from a resource indirectly by analysis of commercial data collected, supplemented of course by information on the biology plus distribution of the resource, is the indirect approach to estimating potential productivity. This approach should however, be the main one adopted when the resource is at, or close to, the maximum yield, since it permits modelling of the relationship between fishing effort and the yield and economic return from fishing in a way that it is difficult or impossible to do from surveys alone. In any case, this is less expensive and also provides information for analysing the economic performance of the fishery. This is not to minimize the importance of resource surveys, but for smaller island countries these need only be carried out at longer (e.g., three to five-year) intervals, except in special circumstances, if statistics are being collected and analysed.

6.7 Catch and Effort Analyses

It is important and basic to nearly all types of assessment, that administrators have a knowledge of the total catch of a given species category, or at least an indication of trends in the levels of landings. This information can be obtained in a number of ways (and these categories generally apply for other types of information discussed later).

(1)   Total landings: a full-coverage system which may be a cost-effective approach to collecting landing data (as an alternative to a continuously operating and costly sampling surveys) is the use of compulsory sales slips for those resources passing through accredited fish dealers and merchants. Such a scheme is outlined in Appendix 3 and will need to be modified in the light of local conditions. This scheme has obvious attraction in cases where resources are sold through middlemen. It is also of value when a valuable resource is exhausted and overfished, and should be under strict resource conservation to allow stock recovery (e.g., lobster, conch). Under these circumstances, licensing of a limited group of fishermen, or the granting of territorial use rights (TURFS) (Christy, 1982) will be necesary to ensure stock recovery. Counter indications for the use of a full coverage system is where there are many and diffused landing sites, a high proportion of bartering or direct sales to consumers, a low degree of literacy of vendors or if little surveillance is possible.

Statistically, the main problem with the data collected from this approach is a significant chance of bias (e.g., consistent under-reporting), and the role of the experienced fishery officers here is to estimate from other sources, what are the likely magnitudes of landings not being reported this way.

The other main alternative to estimating total landings and effort (especially for day boats) is to determine the mean catch rate for a given resource, and to obtain an estimate of total landings by combining the results of the fisheries survey (estimation of the effective fleet size using an up-to-date boat registry), plus special information on the average number of days fished per survey period by boat from a given gear on a given type of resource.

For a given uniform type of vessel and fishing method, the landings in a month J of species A on a given ground can then be obtained from:

Types of information (a) and (c) come from interviews with fishermen or company records, (b) by combining the results of a fisheries survey (estimation of the coefficient of fishing activity) with an actively maintained boat registry. Monthly landings by community can be added by vessel type and month to give annual, regional, or national totals as required. Care will have to be taken that the vessels picked as typical of a given class are not those likely to have a higher catch rate than average. In replacing one individual fishing boat with another, it is desirable to replace it with another with a similar performance.

(2)   Fishing effort: given one or more uniform boat types fishing the same fishing gear, one category can be chosen as standard, and the total effort exerted by all vessel categories per month can be obtained from an estimate of its landings per month, as follows:

This figure can be obtained either for a given resource A, or for the whole (multi-species) resource, and of course added over communities and months to give annual figures.

6.8 Size Frequency and Age Composition Data

The size range of a species being landed in a given area is a sensitive indicator of events happening to the resource. Thus, a decline in mean size in the catch with an accompanying drop in catch rate is usually an indicator of overfishing, while if accompanied by an increase in catch rate of juveniles, it will be a good indication that better than average recruitment (compared with recent years) is occurring. Conversely, an increase in mean size and catch rate may indicate that effective effort has declined in recent years; however, if accompanied by a decline in catch rate, it may indicate that recruitment levels have declined to lower than average levels.

The distribution of size in a population is a function of its age composition, its growth rate and of the mortality rates exerted on it by natural causes and by fishing. All of these rates can be estimated from carefully planned sampling procedures, and accompanying biological information. Although the classical approach has been to combine sampling for sizes with an age-reading function to determine growth and mortality rates directly (e.g., Holden and Raitt, 1974), there are now a variety of methods (see Jones, 1985; Pauly, 1982) to estimate those important parameters directly from the size frequencies. Although these approaches require a good understanding of the methods being used, they provide most of the basic information needed for fish population analysis other than catch rate and landings.

The approach to data gathering most suited for entering reliable estimates of population parameters from size frequency data, is sufficiently time-consuming that it would be advisable to concentrate for the first few years on a maximum of half a dozen species of particular interest that are in need of conservation. The importance of avoiding mixing similar species in this type of work must be emphasized. Good initial candidates for sampling for size frequencies in the Caribbean are:

SpeciesMeasurementSpecial features to note
(1) Spiny lobster(carapace length)-record separately by sex
(2) Conch(shell height)-record thick and thinlipped shells separately
Plus, fisheries consisting predominantly of one species, e.g.:
 Total length 
(3) Flying fish" 
(4) King mackerel or dolphin" 

and reef fish, e.g.:

(5) Grouper, goat fish or known species of reef fish (e.g., parrot fish).

Other species, or groups of species will have to be priorized for other areas.

Sampling should be for all major gear types capturing the species in question, weighted by the proportion of the total catch they constitute. Ideally, small samples from a larger number of sources is much preferable to measurement of the same number of individuals from one large catch. Thus, if the fleet catching spiny lobster consists of:

(a)  12 boats trap-fishing, each taking roughly 100 kgs/day, and fishing 10 days/month (i.e., 12 x 100 x 10 kgs = 12 000 kgs/month);

(b)  8 boats, spear-fishing, each taking roughly 50 kgs/day and fishing 15 days/month (i.e., 8 x 50 x 15 kgs = 6 000 kgs/month).

Then say, a total of 10 boxes of 50 kgs each of lobsters should be measured from 5 to 10 of the trap-fishing boats, and 1 box from each of 5 of the spear-fishing boats per month; before beginning to combine the size frequencies to obtain monthly or fortnightly totals.

Although not required for all analyses, size frequencies can be “weighted up” to give the estimated size composition for the whole monthly commercial catch of 18 000 kgs. This is done, size category by size category, as follows (for example, the spiny lobster length group 75–80 mm carapace length):

An illustration of this procedure is given in Figure 3 for a fishery having three gear categories. Over a period of a minimum of one year, regular samples of a species integrate over gear type in the above fashion should permit estimates to be made of growth and mortality rates.

Figure 3a

Figure 3a Monthly size frequencies from sampling 3 commercial gear/vessel categories Where:

Ail = Number per size group l measured from gear i.

Bi = Sample weight (gear i) in kg.

Ci = Weight caught in time interval (kg) by gear i.

Figure 3b

Figure 3b Overall monthly size frequency (all gear)

In certain fisheries supplying fish processing plants, it may be possible to obtain data on commercial size categories which can be used to save a great deal of effort in obtaining the overall size frequency, if they can be made available. Thus, if three commercial categories for which known weights have processed, the overall size frequency can be obtained by sampling each size category separately, they can be combined as described above for the separate commercial categories.

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