54

Rapp. P.-v. Réun. Cons. int. Explor. M er, 168: 54-57. Janv ier 1975.

F IS H IN G U N IT M E A S U R E S

A. I. T r e s c h e v

All U nion Institute of Fisheries and Oceanography, Moscow, USSR.

C U R R E N T L Y USED M EASURES

Fisheries science is a comparatively young branch of knowledge which has not yet developed unified methods of measuring even the most im portant quan ­tities such as the efficiency of fishing gear. I t was not until quite recently tha t attem pts to unify the inter­national fishing effort statistics were m ade by F AO (1968). According to this docum ent the efficiency of fishing gear is defined in terms of fishing time and fishing power. T he following measures are used :

Fishing time: N um ber of hours fished; num ber of hauls, num ber of drags or sets m ade; num ber of days fished; num ber of days on grounds; num ber of days absent from port and num ber of trips made.

T he num ber of hours fished is defined in different ways for different kinds of fisheries. For trawl fisheries it is the total num ber of hours during which the trawl was on the bottom and fishing. For dory fisheries it is the num ber of hours the dory fleet is absent from the m other vessel multiplied by the num ber of dories. For other fishing gear it is the num ber of hours the nets, seines, traps, dredges, etc., were used in the fishing operations.

The definition of fishing time is m uch too relative. T he num ber of days (24-hour periods) fished includes the time required to prepare the gear for fishing, the time spent on the main fishing operations such as the shooting (setting) and hauling in of the gear, the actual fishing and the removal of catch from the gear, as well as the searching time.

The num ber of days on grounds consists of the time spent on fishing and searching, as well as all the other time during which the fishing craft or gear was on the ground.

The num ber of days absent from port is counted from the day of departure to the day of arrival back in port and, where the fishing ground is a t a distance of more than 24 hours steaming from port, the day of departure and the day of arrival are counted as one day absent from port. W here the fishing ground is

close to the port, both the day of departure and the day of arrival are counted as days absent from port.

T he num ber of trips is defined as the num ber of voyages to fishing areas during which one or another gear was used for actual fishing.

Fishing power is measured or, to use a more correct term, characterised by the type of craft (length, gross tonnage, m ain engine horsepower) and the type o f gear (otter trawls, pair trawls, beam trawls, Danish seines, purse-seines (including ring nets), beach seines, drift gill nets, set gill nets, fixed gear, long lines, hand lines, dredges, harpoons and other gear). Nei­ther the size of the gear nor any peculiarities in its use are taken into account.

I t is easy to see tha t this system is nothing more than an arb itrary form for collecting statistical data. There is no scientific basis underlying this system of defining fishing effort. I t does not reflect the nature of fishing processes and cannot provide an adequate quan tita ­tive estimate of the effect of fishing resulting from the use of one or another fishing gear.

Some specific units for measuring the effect of fish­ing are used in m odern world fisheries, particularly when calculations are made for determ ining the effect of fishing on the stock. For example, the following units are used in trawl fisheries

the British un it: catch (tons)/100 hours fished x average tonnage

the G erm an unit: catch (tons)/num ber of days fished

the Soviet un it: catch (centners (100 kg))/num ber of hours trawling

the Norwegian unit: catch (tons)/24 hours fished x average tonnage.

O f these the British and Norwegian units are for­mally the most refined because they incorporate not only the fishing time bu t also the tonnage of the fishing vessel. However, even these units do not satisfy present requirements. All these units have the dis-

Fishing unit measures 55

Table 22. Scale of fishing unit measures

Nam e Purpose Abbreviation Dimension Fraction of basic unit

P ro m m ............................................... M easure pm10° m 3

24 hours1

D ec ip rom m ....................................... of dpm10s m 3

24 hours1:10

C en tip ro m m ..................................... fishing cpm10’ m 3

24 hours1:100

M illip ro m m ...................................... power m pm10° m 3

24 hours1:1000

P ro m u s............................................... M easure pu 109 m 3 1

D eciprom us....................................... of dpu 108 m 3 1:10

C entip rom us..................................... fishing cpu 107 m 3 1:100

M illip rom us...................................... effort m pu 106 m 3 1:1000

P rom ef................................................ Measure pet

109 m 31

D eciprom ef....................................... of dpet

10s m 31:10

C en tip rom ef..................................... fishing cpet

107 m 31:100

M illiprom ef....................................... efficiency mpet

106 m 31:1000

P ro m in ............................................... M easure P' dimensionless 1

D ecip rom in ....................................... of dpi dimensionless 1:10

C en tip ro m in ..................................... fishing cpi dimensionless 1:100

M illip ro m in ...................................... intensity mpi dimensionless 1:1000

advantage inherent in measures intended for a special limited purpose. They are not universal even within the group of fishing gear for which they are meant.

In m odern fisheries, vessels although equal in ton­nage m ay use fishing gear different in size and towed at substantially different speeds. This depends on the horsepower of the engines, the type of propulsion, the vessel design and construction. Hence the tonnage, and in general the dimensions of the vessel, may only act as an indirect indication of fishing efficiency.

T he units used in the trawl fisheries of other coun­tries are even less related to the fishing process.

For other m arine fishing methods the units of fishing efficiency have not been clearly defined at all. The modes of reporting fishing data differ not only between countries bu t even between areas of the same country.

From this it follows tha t a t present there is no uni­formity in measuring and comparing fishing efficiency. Different countries use different artificial conversion

methods depending on circumstances and on the p a r ­ticular applications. The chief cause of this seems to lie in the fact th a t attempts to work out the method of measuring the effect of fishing have so far been m ade without proper regard to the principles of fishing.

PR O PO SED NEW M EASURES

The problem of measuring fishing efficiency may be solved if, proceeding from a certain fishing gear clas­sification, two groups of units are established. The first group will include units intended for the technical description of fishing gear, while the second group will consist of units intended for determining their fishing efficiency. As the base value for the technical characteristic of the use of fishing gear let us adopt the concept of “ fishing power” by which we shall imply the zone of action of a unit gear in the process of

56 A. I. Treschev

D I A G R A MF I S H I N GG E A R

TR A W LS

PURSE SEINE a

DRIFT NETS

DRAGGED

FISHING

GEARS

v=vv2

Figure 24 a: Gears for which fishing power is defined directly.

F O R M U L A

V = a ß s

v= l ‘ q

An

V = a l s

Vf.a is

F I S H I N G

G E A RD I A G R A M F O R M U L A

FISH PUMPS

LIFTING NETS

D E T E R M I N E D

P A R A M E T E R S

RADIUS OF

ATTRACTION

FISHINGGEAR

D I A G R A M F O R M U L A B A S I S

SET GILL NETS

_ i 2\ / _ * • 1 n n n l 2 n a _ q CTcViPCv - 4 a

TRAPS - r V7— I 2

V = ^ - a Ttl a . q . V c r cU u - qCTC

HOOKED

FISHING

GEARS030 V - £ î T R 3n

3 ^ ïtRYi =Tî[2/na

C O EFFIC IEN T

OF F IS H IN G

CO N TIN U ITY

K = 1

Figure 24 b : Gears for which fishing power is defined experi- m enta'lv. t = duration of a cycle; t j = duration o f illumination

within a cycle.

fishing. T he fishing power for each gear group will be established in accordance with the classification of fishing gear proposed by Treschev (1958) based on the principle of action and specific features. W here the zone of gear action cannot be expressed directly, pro­portional values will be taken to estimate the fishing power.

We shall then express “ fishing effort” as the fishing power of the gear multiplied by the time of its action. T hen the fishing effort units which are different for different groups of fishing gear will be determ ined on the basis of units of fishing power.

Figure 24 c: Gears for which fishing power is defined by com­parison of catches. For C TC read set net, APC read drift net

and A read trap net.

Thus, “ fishing effort” and “ fishing power” in this system are not connected with the catch and charac­terise only the technical potentialities of the fishing gear. W ith the choice of appropriate units, they as­sume quite definite dimensional expressions charac­teristic of each class or group of gear of the same type. T he results of measurements obtained with the help of these units are universal w ithin each class (group) of gear, i.e. are independent of their design, size or m ethod of use.

As a unit for measuring fishing efficiency a unified measure m ay be taken for all classes and groups, namely the catch per unit effort. I f the catch is aver­aged over a sufficiently long period of time, the unit o f fishing efficiency for each class (group) of gear will incorporate all the peculiarities of the fishing process including fish behaviour and the organisation, tech­nique and tactics of fishing. Unlike measures such as catch per unit time, catch per unit tonnage, etc., which are not connected with the principle of fishing, the new measure of fishing efficiency will incorporate both the effect of fishing and the degree of perfection of the fishing technique. W ith unchanged fishing technique, the fishing power remains the same and the fishing efficiency of the gear will change in pro­portion to changes in the stock. Considering tha t fish­ing efficiency is based on actual catch data over a long period of time, it is also more accurate than other estimates arrived a t theoretically on the basis of vari­ous assumptions concerning fish behaviour. Thus fish­ing power, fishing effort and fishing efficiency have become interrelated. I t is sufficient to know the defini­tion of the fishing power unit adopted for a fishing

Fishing unit measures 57

m ethod to be able to determ ine the units for measuring the fishing effort and fishing efficiency.

T he proposed definitions of fishing power, fishing effort and fishing efficiency are given in Figure 24 a, b, c. These have been taken from another version of the present paper (Treschev, 1971). For a definition offish- ing power, all fishing gear m ay be divided into three groups, (a) those for which fishing power is defined directly by their dimensions and m ethod of use, (b) those for which fishing power is defined by experiment and (c) those for which a comparison of catches is required. T he m easurement of catch for all fishing gears is in metric tons and fishing time is in days (24 hours).

U N IT S O F M EA SU R EM EN T

W ithin the groups referred to above, the size, fishing power and other characteristics of different gear may vary considerably and this m ay make the use of the proposed units inconvenient. For example, the otter trawls used by big freezer trawlers on the one hand and small trawlers on the other, both placed in the same gear group, differ significantly in the scale of their fishing characteristics, and the m easurem ent of these w ithout applying a scale factor would not be convenient. In this connection it would be advisable to use the decimal system of the units for measuring the fishing characteristics, i.e. to use the one-tenth, one-hundredth and one-thousandth fractions of the basic units. T he system of units proposed is given in Table 22. The results of measurements obtained with the help of these units are universal for all classes

(groups) of gear, i.e. are independent of all peculi­arities except swept volume.

T he scale of the un it in this system is chosen so tha t the annual cycle of the most efficient fishing gear is approxim ately equal to one unit of fishing power. For example, in traw l fisheries the power developed during the annual fishing cycle by the trawl used on the big freezer trawlers is taken as the unit of fishing power. W ith this choice of scale, the estimation of the total fishing power of the fleet becomes m uch simpler.

This m ethod allows the determ ination, in com par­able units, of the fishing power and other characte­ristics of fishing in earlier years. O f special interest is the use of this m ethod to estimate the dynamics of fishing efficiency in relation to the dynamics of fishing effort in the most im portant fishing areas. An im ­portan t advantage of the proposed system of units for m easuring fishing lies in the fact tha t it m ay be used to provide the basis for working out future fishery plans for the rational exploitation of a stock and the appropriate fishing technique.

The unification of fishing unit measures a t present is equally im portant both for technical and biological purposes.

REFERENCES

FAO, 1968. Classification, definition and codification of fishing effort statistics. FAO Fish. Circ. 229.

Treschev, A. 1958. Classification of fishing gear. ICES, CM 1958, Doc. No. 62: 5 pp (mimeo).

Treschev, A. 1971. Fishing unit measures. (The second supple­mented and revised report). ICES, C.M. 1971/B:9, 13 pp (mimeo).