(Note: This set of exercises has been constructed to illustrate how the calculations in all the previous sections may be combined to provide assessments and practical advice in the management of a fishery.)
The following data were obtained from the trawl fishery for chafer fish in the Round sea.
The data have been grouped into three periods, on the basis of the different levels of the fishing effort in three periods.
1. Catches. The catches are given in thousand tons, numbers in millions, effort in thousand hours' fishing.
|
Year
|
Catch |
Effort |
||
|
Thousand Ions |
Millions |
Thousand hours |
||
|
Period 1 |
||||
|
|
1932 |
82.2 |
118.5 |
30 |
|
1933 |
109.4 |
143.2 |
40 |
|
|
1934 |
66.7 |
82.0 |
24 |
|
|
1935 |
108.0 |
124.9 |
45 |
|
|
1936 |
45.0 |
51.4 |
25 |
|
|
1937 |
111.3 |
155.5 |
46 |
|
|
1938 |
79.1 |
105.3 |
35 |
|
|
Period 2 |
||||
|
|
1950 |
136.0 |
190.1 |
91 |
|
1951 |
102.7 |
144.6 |
72 |
|
|
1952 |
141.1 |
217.1 |
84 |
|
|
1953 |
122.2 |
179.2 |
75 |
|
|
19S4 |
81.1 |
110.9 |
62 |
|
|
1955 |
126.0 |
191.4 |
81 |
|
|
1956 |
164.0 |
241.7 |
103 |
|
|
Period 3 |
||||
|
|
1961 |
128.8 |
177.2 |
116 |
|
1962 |
170.4 |
270.3 |
142 |
|
|
1963 |
158.4 |
256.0 |
138 |
|
|
1964 |
122.1 |
188.7 |
132 |
|
|
1965 |
155.0 |
267.3 |
125 |
|
|
1966 |
135.9 |
216.3 |
128 |
|
|
1967 |
143.9 |
249.4 |
115 |
|
By plotting the data on total catch and catch per effort, against the appropriate effort, can you obtain a preliminary conclusion on the state of the fishing and the expected effect of a change in effort?
2. The age compositions, obtained from age-length keys based on otolith age readings, and the annual length compositions, are (in percentages):
|
Age |
3 |
4 |
5 |
6 |
7 |
|
Year |
Percentage | ||||
|
1932 |
35.4 |
49.3 |
10.0 |
3.9 |
1.4 |
|
1933 |
34.5 |
24.3 |
32.7 |
6.5 |
2.0 |
|
1934 |
32.4 |
25.2 |
16.2 |
21.9 |
4.3 |
|
1935 |
27.9 |
25.0 |
17.5 |
12.5 |
17.1 |
|
1936 |
23.3 |
25.0 |
22.8 |
17.8 |
11.1 |
|
1937 |
55.8 |
13.2 |
12.4 |
11.2 |
7.4 |
|
1938 |
35.4 |
39.8 |
8.0 |
8.8 |
8.0 |
|
| |||||
|
1950 |
40.5 |
34.1 |
16.0 |
6.4 |
3.0 |
|
1951 |
49.0 |
21.2 |
17.5 |
8.8 |
3.5 |
|
1952 |
60.7 |
20.2 |
8.6 |
7.1 |
3.3 |
|
1953 |
49.1 |
31.0 |
10.7 |
4.9 |
4.3 |
|
1954 |
38.2 |
32.4 |
19.9 |
6.9 |
2.7 |
|
1955 |
61.7 |
14.1 |
13.5 |
8.0 |
2.6 |
|
1956 |
50.9 |
30.0 |
9.4 |
5.8 |
4.1 |
|
| |||||
|
1961 |
31.5 |
48.2 |
9.5 |
7.9 |
2.7 |
|
1962 |
66.9 |
10.8 |
16.4 |
3.2 |
2.6 |
|
1963 |
62.6 |
25.9 |
3.9 |
6.4 |
1.2 |
|
1964 |
54.8 |
28.8 |
11.7 |
1.9 |
2.8 |
|
1965 |
73.7 |
15.0 |
7.7 |
3.1 |
0.5 |
|
1966 |
57.0 |
31.5 |
6.5 |
3.5 |
1.5 |
|
1967 |
74.1 |
15.0 |
8.6 |
1.6 |
0.8 |
(a) Estimate the total mortality coefficient (Z) for each of the three periods. Determine the coefficient of natural mortality (M), using these values of Z and the average effort in each of the three periods.
(b) Estimate also the coefficient of fishing mortality F.
3. The relation between age and length in the chafer fish is the following:
l1 = 10 cm; l2 = 22 cm; l3 = 33 cm; l4 = 38 cm; l5 = 45 cm; l6 = 48 cm; l7 =50 cm; l8 =52 cm.
Fit a von Bertalanffy curve to these observations, and estimate the growth parameters. W= 0.01 l3
4. Selectivity experiments with a covered cod-end and mesh size-of 100 mm gave the following results:
|
Length
|
Number of fish |
|
|
In cod-end |
In cover |
|
|
cm |
||
|
29 |
0 |
510 |
|
30 |
30 |
623 |
|
31 |
82 |
728 |
|
32 |
151 |
855 |
|
33 |
554 |
555 |
|
34 |
631 |
276 |
|
35. |
778 |
85 |
|
36 |
698 |
12 |
|
37 |
500 |
0 |
Determine the mean selection length, and the selection factor. Note that the lengths were recorded to the cm below.
5. In recent years (i.e. in the third period under consideration) the mesh size used in the fishery for chafer fish was 100 mm. Make use of the yield tables for the following questions:
(a) Is it possible to obtain a greater catch using this mesh size and increasing the fishing effort?(b) What effort would give the maximum yield for this mesh size, and what is the yield at this level of effort?
(c) What would be the effect on the yield of an increase in the mesh size from 100 to 110 mm, keeping the effort at the level of the third period?
(d) What mesh size would give the maximum yield for this effort, and what would be this yield?
