Theoretical minimum sizes of perch caught in gill nets of various meshes are
calculated from regressions of log, maximum girth on log, length and log. weight.
128
11. Some Estimates of Theoretical Minimum Expected Sizes of Perch in Gill Nets by C. KIPLING I
Abstract Theoretical minimum sizes of perch caught in gill nets of various meshes are calculated from regressions of log, maximum girth on log, length and log. weight. Very little advantage is gained by using multiple regressions log, girth - log. length - log. weight. Differences in "condition" in different years and seasons can cause appreciable variations in the minimum sizes caught. Examples of length frequency distributions of perch caught in gill nets and by other methods are shown graphically for comparison with the theoretical results. This short paper describes an attempt to calculate theoretically the expected minimum sizes of perch (Perca fluviatilis Linn) meshed by the body in gill nets. The method used is similar to that used by Margetts (1954) in studying mesh selection of trawled haddock and whiting. As opposed to the trawl, the gill net meshes can be assumed to be completely flexible, and, as the gill net is set for at least 24 hours, all undersize fish will have time to escape. The minimum lengths and weights of perch expected to be meshed by the body in gill nets of various mesh sizes were derived from length, weight and girth measurements of angled and trapped fish. Confidence limits were calculated for the estimates and the theoretical minimum lengths and weights were compared with actual catches from gill nets. The data were taken from two samples of female mature fish caught in Windermere, the first (110 fish) angled in October 1948, the second (38 fish) trapped in November and December 1956. The maximum girths were measured by encircling the fish with a piece of string, firmly
but without constricting the flesh, and then measuring the string on a ruler. This is referred to by Margetts (1954) as the natural girth. This method of measurement may result in a slight underestimate of minimum sizes, as the fish which just squeeze through with some distortion of the body are omitted. This factor is probably of less importance for the perch, which has a relatively firm structure, than it would be for some other species (e.g. char and pike) which are more flexible. Multiple regression equations of the logarithms of girth on the logarithms of length and the logarithms of weight, and also the multiple correlation coefficients, were calculated for each sample by the method of least squares. It was apparent that no appreciable advantage would be gained by using multiple regression equations and that the simple logarithmic regression equations were adequate for the present purpose. It was assumed that the minimum size of fish held by a net was one whose maximum girth was equal to the circumference of the meshes. On this basis theoretical minimum lengths and weights of fish were calculated from the log length-log girth and log weight-log girth regression equations, by substituting the logarithm of the mesh circumference in centimetres for girth in each case. The results are shown in Table 1, together with the 0.95 confidence limits calculated from the standard errors of estimate. It should be noted that the samples consisted of ripening female fish all taken at the same time of year. Samples which included spent and ripe fish, or both sexes, or taken throughout several years would certainly have more variation, and hence wider limits; also no allowance has been made for possible slight variations in mesh size within a net.
'Freshwater Biological Association, The Ferry House, Far Sawrey, AmbleSide, Westmorland.
ICNAF SPEC. Pow-. No. 5, 1963
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I3 A
5 5
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129 TABLE 1.
Estimated minimum sizes
Aleslisize (in) knot to knot 1t 1 1 1 11 1 t 1
PERCENTAGE FREQUENCY
Date of sample 1948 1948 1948 1948 1956 1956 1956 1956
0.95 confidence limits (g) 87 119 48 66 22 30 8 10 102 160 59 92 28 44 16 10
Weight (g) 101 56 26 9 128 74 35 13
30
21.5 18.2 14.4 10.6 23.5 19.8 15.8 11.6
20.1 16.9 13.5 9.9 22.0 18.6 14.8 10.9
II
10
TRAPPED 11
,H11111111
i m!Ininn
0 1 1 2 13 14 15
1 6 17 18 19 20 21
erg 1948
30■—•
3/4" GILL NET
20
TRAPPED
10 JIiIIII 10
10
II
I''
13
14
15
i6
17
I
I
18
19
rms.
1 15
116
117
1 20
1111111
1 953
?
3/4" GILL NET =1
118
1I9
210
2'1
22
2'3
214
d g
MIMEO
or o
114
15
1 953 3/4"GILLNET
27
1"GILLNET
L 18 19 20 21
22
23
I 1 1 18 19 1 20 21
25
26
I "GILL NET 1 22
1 23
1 24
22
23
24
1954
OE
1 I/4"GILL NET 20 21
25
26
27
•=si
30 20
?? 1954
10
1 1 /4"GILLNET ANGLED
o-
24
d' d 1 953
5r 0 1—
I
1
26
1 953
117
16
A
NMI
IMI•••■• SOMME= 1111M-11■1 1•EM im
IN im
•■•
41,, e/AP /11.11 /Pil , 4 ,-04
•04' J'ArI'd/AP.••••..
1 9 20 21 22 23 24 25 26 27
LENGTH IN CENTIMETERS Fig. 1. Length frequency distributions of perch.
confidence
limits (cm)
'1 1 1 1 1 1
3/4" GILL NET
•'•',.7 7 7
*.t.•
0.95
cm
? 1948
CV4
20
Length
23.1 19.5 15.5 11.4 25.1 21.2 16.9 12.5
130 It can be seen that there is a considerable difference between the estimates from the two years. Fish of the same girth were longer and heavier in 1956 than in 1948 (i.e. fish of the same length were thinner in 1956). It seems possible that this can be accounted for by the particularly bad summer of 1956. Such differences in condition can affect selection for age by the nets. For example, in general perch in Windermere attain a mean length of about 10 cm in the second year of growth. In 1948 with an estimated minimum length of 10.6 cm for the inch gill net many of this age group would be vulnerable, whereas in 1956 with the estimated minimum 11.6 cm far fewer would be of the required size, even disregarding the likelihood that the mean length attained was probably less in the latter year. If equations of this type were to be used to predict the catching limit of a gill net in a particular year, it is possible that a satisfactory
method of allowing for condition could be worked out by applying a correction. Examples of length frequency distributions of perch caught in gill nets are shown in Figure 1. When strictly comparable samples were taken by other methods these are also shown. Comparison of the minimum lengths taken in the various nets with the calculated values given in Table 1 shows considerable agreement in general with the estimates made from the 1948 data. I am most grateful to Mr. E. D. Le Cren for the use of the data and for helpful advice and criticism. REFERENCE
MARGETTS, A. R., 1954. The length-girth relationships in haddock and whiting and their application to mesh selection. J. Cons. int. Explor. Mer, 20, (1) :56-61.
