8. PLOTTING CHARTS OF RELATIVE ABUNDANCE

Generally speaking, isoline (from the Greek “isos”, meaning “equal”) is a generic term which describes a type of map¹ widely used by geographers to show the geographical pattern of the survey characteristic(s). The basic data used in preparation of isoline maps are samples of absolute measurements, for particular points, or ratios for certain areas.

¹ There are two fundamental types of isoline maps:

1. The isometric map, in which the lines are drawn through points of equal value or intensity and,

2. The isopleth map, in which the lines connect equal rates or ratios for specific areas.

Some of the more familiar examples of isoline maps include those pertaining to a series of points of equal temperature, which are called “isotherms”; of equal elevation “isohypses” (or more commonly, “contour lines”), etc.

Charts of relative fish abundance are isopleth type maps (see footnote 1). They are exclusively used in acoustic surveys providing a graphic presentation of the geographical distribution (relative) of the target biomass in the survey area. Charts of relative fish abundance are constructed soon after the completion of an acoustic survey and quickly distributed to the users.

For the construction of isopleth maps there are four considerations which must be borne in mind:

1. Because it is impossible to use base areas of the same shape and size, great attention should be paid to the general appearance and comparability of the map.

2. Because the patterns of distribution of the survey characteristics are uneven, the problem is to determine the most typical or representative points in the area.

3. The third consideration is the laying out of the “isopleths” which are drawn with reference to the values indicated by the control points in each statistical area.

4. The fourth consideration is the choice of intervals for the isopleths.

1. Obtained values of integrator readings (mm) on an ESDU basis are plotted on the respective sample tracks of the acoustic survey (see cruise map 1).

2. A choice of the density levels of the target biomass is made by examining the frequency distribution of the obtained integrator readings (mm), in general the following four levels of density are used:

a) very scattered fish
b) scattered fish
c) dense fish
d) very dense fish

3. Control points are located on each sample track.

4. When suitable density levels have been determined, smoothed curves are drawn through points of equal values (see map 2).

Map 2: Chart of relative fish biomass