The Echo-integrator Conversion Factor

Selection of areas with fish populations of homogenious size distribution or species composition was described in section 4.1.2. The extraction of echo integrator values or counts for each of these catagories in 4.1.1. In section 4.2 we discussed methods for averaging the echo-integrator values and echo counts. The next step is to estimate the density of targets from the observed echo-integrals. This may be done using the following equation from Foote et al

1987:-(28) p. _I = (K/<o.»E. _{I I}

The subscript i refers to one species or category of target. K is a calibration factor, <OJ> is the mean acoustic cross-section of species i, E_j is the mean echo-integral after partitioning and F_j is the estimated area density of species i. The quantity is the number or weight of species i, depending on whether 0j is the mean cross-section per fish or per unit weight. Cj=(KI<oD is the echo-integrator conversion factor, which may be different for each species. Furthermore, c_j depends upon the size-distribution of the insonified targets, and if this differs over the whole surveyed area, the calculated conversion factors must take the regional variation into account.

K is determined from the physical calibration of the equipment, which has is described in detail in Foote 1987.

It does not depend upon the species or biological parameters. Several calibrations may be performed during a survey. The measured values of K may be different but they should be within 10% of one another. If two successive measurements are very different the cause should be investigated since the equipment may be malfunctioning. Otherwise, K should be taken as the average of the two measurements before and after the relevant part of the survey.

4.3.1 Single species

The mean cross-section <0> may be determined directly from in situ measurements of target-strength made during the survey. In this case care must be taken to ensure that the targets providing the value are representative of the fish stock being surveyed.

Alternatively it may be derived from a function which describes the length-dependence of the target-strength, normally expressed in the

form:-(29)

aj and bj are constants for the i'th species, obtained from experimental evidence and possibly by agreement with other participants in the sUlvey.

The equivalent formula for the cross-section

is:-(30)

The mean cross-section is calculated as the

°

average over the size-distribution of the insonified fish.

Thus:-<Oi> = 4rc"rJijlO((ai+biLog(Lj))/1O) j

(31)

L_j is the mid-point of the j'th size-class and f_ij is the corresponding frequency as deduced from the fishing samples by the method described earlier (Section 4.1.2.1). The echo-integrator conversion factor is Cj= KI<oj>' The calculation may be repeated for any species with a known target-strength function.

Note that it is the cross-section that is averaged, not the strength. The arithmetic average of the target-strengths gives a geometric mean, which is incorrect. The term "mean target-strength" may be encountered in the literature, but this is normally the target-strength equivalent to <OJ>, calculated as 10l0g_lO( <oj>/4rc). Some authors refere to TS as IOlog( 0bs) the definition of

°

is different from 0bs an"d should not be confused.

It is imporartant to note that a number of different methods are in use for measuring fish length for example, fork length, overal length and standard length. It is essencial to standardize on one method for length measurement and to ensure that target strength values obtained from other sources have been obtained using the same measurement method.

4.3.2 Mixed species

Sometimes several species are found in mixed concentrations such that the marks on the echogram due to each species cannot be distinguished. From inspection of the echogram, the echo-integrals can be partitioned to provide data for the mixture as one category, but not for the individual species. However, further partitioning to species level is possible by reference to the composition of the trawl catches (Nakken and Dommasnes, 1975).

Suppose Em is the echo-integral of the mixture, and Wj is the proportion of the i'th species, calculated from fishing data see section 4.1.2.2. It is necessary to know the target-strength or the acoustic cross-section, which may be determined in the same manner as single species (see section 4.3.1). The fish density contributed by each species is proportional to w_{j •} Thus the partitioned fish densities

are:-(32)

The w_i may be expressed as the proportional number or weight of each species, according to the units uscd for <0i> and c_i. Consistent units must be used throughout the analysis, but the principles are the same whether it is the number of individuals or the total weight that is to be estimated.

4.3.3 Weight-length relationships

The abundance is expressed either as the total weight or the number of fish in the stock. When considering the structure of the stock, it is convenient to work with the numbers at each age. However, an assessment of the commercial fishing opportunities would normally be expressed as the weight of stock yield. Consistent units must be used throughout the analysis. Thus if the abundance is required as a weight while the target-strength function is given for individual fish, the latter must be converted to compatible units. This may be done by reference to the weight-length relationship for the species in question.

For a fish of length L, the weight W is variable but the mean relationship is given by an equation of the form:-(33)

Where af and bf are constants for one species. Suppose the target-strength of one fish is given

as:-(34)

The corresponding function TSW' the target-strength of unit weight of fish has the same form with different

constants:-(35)

The number of individuals in a unit weight of fish is (l/W), so the constant coefficients are related by the

formulae:-(36)

(37)

The weight-length relationship is non-linear. This must be taken into account when estimating the total weight from the numbers in discrete size-classes. Suppose there are nj individuals in the j 'th class, Lj is the mean length and ~L is the interval between successive classes. An unbiased estimate of the total weight

is:-(38)

Assuming uniform distribution of lengths in any length class.

In document B9154.pdf (8.036Mb) (sider 56-59)