APPENDIX 12 Preliminary assessment of the long-term effects of a change in mesh size in the octopus fishery

by

J.A. Pereiro
Spanish Institute for Oceanography
Madrid, Spain

To assess the effects of a change in mesh size from 40 to 60 mm and to estimate the sustainable yield corresponding to different assumptions regarding the exploitation rate, a yield per recruit computation was carried out. Por that purpose the following parametric values were used in a linear Ricker model:

Growth: The two growth curves presented by Messrs Guerra and Hatanaka to the Working Party (section 3.1) were successively used:

1. total length = 130.19 [1 - e ^{-.423 (t +.435)}] presented by Hatanaka for the growth of the Cape Garnett spring recruits:

2. mantle length = 40 [1 - e ^{-.72 (t-.34)}] presented by Guerra for the growth curve of Cape Garnett stock.

mantle length = 14 x (total length)^1.08

W =.00497 (total length)^2.9259
W =.976 (mantle length)^2.691

Natural mortality: A value of M =.2 on a yearly basis was used. This assumption was based on the observations made in section 3.2.1 regarding the likely rate of octopus exploitation phase and according to which every female would die after her first spawning season, and would remain low during the short exploited phase.

It was assumed as well, from sex-ratio studies (Hatanaka, section 3.2.1) that every female dies when reaching a mantle length of 18.5 cm. Hales, on the contrary, were supposed to suffer a natural mortality of M -.2 along all their life span.

Exploitation rate: Production models seem to indicate that the exploitation rate is very high. Similar computations made by H. Hatanaka (pers. communication) lead to the same conclusion. With M =.2, the corresponding figures for F are.5, 1.0 and 2.0 respectively.

The final yields per recruit are given in tables-1 and 2; they are expressed in percentages of the lower result obtained with each growth curve. Estimates given in table 1 correspond to computations made with Guerra's growth curve; those in table 2 correspond to the Hatanaka's curve. In both cases highest yields per recruit are obtained for an exploitation rate of 8 and a 60 mm mesh size. However, for all combinations of input parametric values, a gain is to be expected on the long-term by increasing the mesh size from 40 to 60 mm.

Considering the lack of accuracy of the various parameters used in the computation, these results should however be considered as provisional.

CAPE GARNETT OCTOPUS YIELD PER RECRUIT (LINEAR RICKER MODEL)

Table 1 Guerra's growth equation

E	.7	.8	.9
Mesh size (mm)
40	117	123	100
60	123	138	111

Table 2 Hatanaka's growth equation

E	.7	.8	.9
Mesh size (mm)
40	155	150	100
60	178	198	162