(4.3)
The mortality parameters and the exploitation pattern of the Iberic Stock of hake, Merluccius merluccius, from Divisions VIIIc and IXa of ICES were estimated by the Assessment Working Group of ICES (ICES, 1998b) as:
Exploitation pattern in 1996
age |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
s_{I} |
0.00 |
0.09 |
0.29 |
1.31 |
1.25 |
1.12 |
1.32 |
1.55 |
1.55 |
The growth parameters were estimated as being:
Growth parameters of von Bertalanffy |
Weight length relation, |
L_{∞} = 100 cm |
a = 0.004 |
At the beginning of 1996 the considered stock had the following age structure, with i representing the age and N_{i} the number of survivors at the beginning of the age i, expressed in millions of individuals:
age |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
N_{i} |
83 |
27 |
41 |
30 |
22 |
11 |
6 |
3 |
5 |
GROUP I
1. Estimate the individual weights at the beginning of each age, the total number of individuals and the total biomass of the stock at the beginning of the year.
2. Estimate the individual mean weight, the mean number of survivors, the mean biomass, the total catch in number, the total catch in weight and the mean weight in the catch and in the stock, during 1996.
3. Supposing there was a recruitment in 1997 equal to 100 million individuals, calculate the following, for the beginning of 1997:
a) The age structure of the stock, in number
b) The age structure of the stock, in biomass
c) The total number of individuals of the stock
d) The total biomass of the stock
(notice that the stock at the beginning of 1997 is equivalent to the stock at the end of 1996, except for the 1997 recruitment).
GROUP II
The scientists drew attention to the fact that the fishing level in 1996 was very high and should, therefore, be reduced. They suggested a reduction of about 40 percent so that one could get adequate catches in weight and biomasses in the future.
As an alternative to improve the exploitation of this stock, they also suggested increasing the mesh size of the fishing nets.
1. Fishing managers asked the scientists to evaluate the mean biomass of the stock, the catch in number, the catch in weight and the mean weight in the catch during 1997 in case of:
a) Maintaining the 1996 fishing pattern (status quo situation)
b) Reducing the 1996 fishing level by 40 percent.
Making the necessary calculations in order to estimate a) and b), present the results and comment on the changes in the catch in weight and mean biomass, relative to 1996.
2. Fishing managers also asked the scientists to evaluate the catch in weight and the mean biomass that would result from maintaining the fishing level of 1996 in 1997, but using a new mesh size of the fishing net. To solve this question, the scientists proposed the following new exploitation pattern:
age |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
s_{I} |
0.00 |
0.00 |
0.03 |
1.50 |
1.65 |
1.75 |
1.80 |
1.80 |
1.80 |
Make the calculations, present the results and make your comments on the changes in the catch in weight and the mean biomass relative to 1996.
3. Evaluate the effects on the catch in weight and on the mean biomass, resulting from the simultaneous adoption (in 1997) of the reduction by 40 percent of the fishing level in 1996 and the introduction of the new exploitation pattern. Present the results and make your comments.