Objectives and methods
The objective of the present survey was to study the shrimp distribution and to estimate an index of abundance in order to compare with previous estimates.
The methodology applied was the same as during the last survey (BRINCA et al., 1982).
Design of the survey
The design of the present survey was basically the same as the one in the above mentioned report. However, as it was recommended in that report, the main strata were subdivided into smaller strata, keeping the previous boundaries unchanged. Eighteen strata were defined (see Annex 5, table 18).
The stations were allocated using a proportional scheme. Optimum allocation was not used because there is still some doubts concerning the variance of the catches in each stratum. Figs. 38 and 39 shows the strata and the station pattern obtained.
The first four trawl hauls (three from stratum 1 and one from stratum 2) could not be considered because of technical problems with the net. These problems were not solved until station no. 499.
Due to the lack of time, strata 5.2.a, 5.2.b and 7 were not covered.
Indices of abundance
The tables from 19 to 23 and tables from 27 to 29, included in Annex 5, show the individual catch per tow in each stratum, the mean catch per tow, the biomass and the stock size estimates per stratum. The total catch of shrimp and the catch of the main species, Penaeus indicus and Metapenaeus monoceros was considered.
Fig. 38. The stratification scheme for shallow-water shrimp survey.
The problem of constructing valid confidence limits of the mean catch per tow, was not clearly defined in the previous report (BRINCA et al., 1982). Due to this fact, variance per stratum was not calculated in the present report and the stock size was estimated using only the arithmetic mean.
Fig. 39. Station pattern for the shallow-water shrimp survey.
For the computation of the swept area by tow the following values were used:
mean trawling speed - 2.8 knotsFor total shrimp biomass a stratified mean per tow of 5.34 kg was estimated and a mean biomass of 2,077 tons. For the species P. indicus a stratified mean catch per tow of 1.398 gr and 37 individuals was obtained. The biomass was estimated to 544 tons and the stock size as 14x106 individuals. For the species
horisontal opening of the net - 18 meters
M. monoceros a stratified mean catch per tow of 764 gr and 61 individuals was obtained. The biomass was estimated to 297 tons and the stock size as 23x106 individuals.
Biological characteristics of the main species
Measurements of carapace length were grouped in 2 mm classes. At each station the length composition obtained in the sample was weighted by the total catch of the species, expressed in numbers, Within each stratum the length composition was combined and weighted by the stock size of the stratum.
By combining the length composition of the different strata the overall length composition for the total area covered was obtained. The above mentioned methodology was applied considering the different stages of maturity of females. (The gonad maturity stage just after spawning is referred to as 1 d.)
The total length - total weight relationship was obtained for P. indicus and M. monoceros (females and males separately) applying the functional linear regression to data transformed in logarithms. Due to the scarcity of data these relationship were not calculated per stratum.
Penaeus indicus
a) Females
Carapace length varied between 16 and 52 mm, with roost of the individuals (73%) being between 32 and 48 ram (Fig. 40).
Annex 5, table 24 shows that the smallest size occurred in strata 4.1.b and 5.1.a. Annex 5, table 25 shows the percentage of the gonad maturity stages per stratum. For the total area covered, late maturing and mature females constitute 37% of the female population. The highest percentage were found in strata 5.1.c and 6.a.
Fig. 40. Penaeus indicus - Carapace length distribution.
Fig. 41 shows the length-weight relationship, expressed by the following equation:
|
log |
W = 3.5464 log LT - 6.3237 |
|
|
n = 141 |
|
|
r = 0.9779 |
b) Males
Carapace length varied between 16 and 42 mm with most of the individuals (79%) being between 30 and 36 mm (Fig. 40). The smallest size occurred in strata 4.1.a, 4.1.b and 5.1.a (see Annex 5, table 26). Fig. 41 shows the length-weight relationship, expressed by the following equation:
|
log |
W = 3.2181 log LT - 5.6205 |
|
|
n = 200 |
|
|
r = 0.9459 |
Sex - ratio was computed and males were dominant (60 to 80%) except in strata 3.1.a and 6.a.
Metapenaeus monoceros
a) Females
Carapace length varied between 12 and 52 mm with most of the individuals (68%) being between 22 and 34 mm (Fig. 42). Annex 5, table 30 shows that the smallest individuals were found in strata 3.1.a, 4.1.a, 4.1.b and 5.1.a.
Fig. 42. Metapenaeus monoceros. Carapace length distribution.
Annex 5, table 31 shows the percentage of the different maturity stages per stratum. In general, females seem to be at the early maturity stages, except in the stratum 3.2.b, where the percentage of the late maturing and mature females was higher. For the total area covered, only 4.7% of the female population was formed by late maturing and mature females.
Fig. 43 shows the length-weight relationship, expressed by the following equation:
|
log |
W = 3.1441 log LT - 5.4521 |
|
|
n = 423 |
|
|
r = 0.9853 |
b) Males
Carapace length varied between 12 and 44 mm, with roost of the individuals (84%) being between 20 and 30 mm (Fig. 42). The smallest size occurred in strata 2, 3.1.a, 4.1.a and 5.1.a (see Annex 5, table 32).
Fig. 43 shows the length-weight relationship, expressed by the following equation:
|
log |
W = 3.0285 log LT = 5.2449 |
|
|
n = 441 |
|
|
r = 0.9810 |
Sex - ratio was calculated and the proportion between males and females was 1:1.
Conclusions and recommendations for future surveys related to the survey design.
1. The stratification scheme applied in this survey seems to be better than the one used during the previous survey (BRINCA et al., 1982). This is particularly evident in stratum 4.1, which was subdivided into two new strata and these showed a significant difference in catch per tow.
2. The subdivision of stratum 5 had also given better results. However, it seems that a new geographic subdivision of stratum 5.1.a into two new strata may improve the results.
Therefore, it is recommended that during future surveys, a new boundary at 19°05’S should be considered for stratum 5.1.a.
