Management of the linefish resource in Southern Mozambique : a case study for Marreco (Chrysoblephus puniceus).
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Date
2001
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Abstract
This study provides information on the biology, stock status and the management of C.
puniceus, a key linefish resource in southern Mozambique. This is regionally endemic to
Mozambique and KwaZulu-Natal. Fairly resident species, is found on the continental
shelf ranging north to Zavora and south to KwaZulu-Natal and Transkei, and inhabits
rocky seabeds, ranging between 20 and l00m. In Mozambique is manly exploited by
semi-industrial fleet, and exported to South Africa.
The reproductive biology, assessed through gonad somatic index as well as microscopic
and macroscopic assessment, indicates that spawning extends over the spring months
from August to November, peaking in September. It is a protogynous hermaphrodite,
relatively slow growing and long lived species.
The length-weight relationships for male and female C. puniceus show to be different,
and the overall length frequency distribution shows clear difference in size between male
and female C. puniceus, with male length frequency distribution restricted to the larger
size classes. The monthly length frequency distribution of female C. puniceus is
unimodal and peak at 300 mm FL, male shows unclear year classes.
Age determination was carried out using otoliths and growth was estimated using the
model developed by Punt et al (1993) for protogynous hermaphrodites reproductive
styles. Validation of annuli was done by examining the outer margin of otolith, and also
through mark and recapture information. Validation following the first method indicated
that the opaque band is laid down twice a year, but the mark recapture results were
inconsistent. This contradicts previously published information on C. puniceus, and thus;
both single and double scenarios were modelled. The Von Bertalanffy growth parameters
found for C. puniceus suggest relatively slow-growth, with the number of rings found
from reading the sectioned otolith ranging from 2 to 18. Reproducibility of age estimates
was evaluated using the average percentage error (APE) technique, and was equal to
22%. The age at full recruitment was found to be 2.5 and 5 years for bi-annual and annual banding, respectively. The analysis of the age-at-50% maturity, based on double band
scenario, suggests that C. puniceus mature at 1.5 year-old, which corresponds to a mean
FL of 240mm.
A preliminary yield per recruit assessment revealed that at the current fishing mortality,
C. puniceus fishery is moderately overfished, with the spawning biomass-per-recruit at
35.43% and 36.57%, for one and two bands, respectively, of its unexploited level.
Fishing mortality was equal to 0.2 year-1 and 0.41 year-1, for single and double band,
respectively. YPR analysis shows that the single band scenario is less conservative than
the double band assessment, which has a bearing on the management approach.
It is suggested as the preliminary management strategy a reduction of the number of
boats. Indeed the average number of crew per boat, needs to be evaluated in terms of
overall effort. To complement this management measures, there is a strong and urgent
need to establish marine reserves in order to protect spawning stock, and also, to develop
an overall linefish management plan, which will help in the management of the whole
linefish resource in Mozambique. Furthermore, an age and growth study for C. puniceus
over a larger geographical area needs to be done as a mean to overcome the differences
between previous study and this study, once C. puniceus is being shared between the two
countries (Mozambique and KwaZulu-Natal).
Description
Thesis (M.Sc.)-University of Natal, Durban, 2001.
Keywords
Chrysoblephus Puniceus., Fish populations--Measurement., Sparidae--Mozambique., Fishes--Indian Ocean., Theses--Marine Biology.