Estructura y dinámica poblacional del Pargo Blanco Umbrina Canosai (Sciaenidae, Pisces) del litoral de Rio Grande do Sul, Brasil

Institución otorgante:

Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales

Fecha:

1982

Tipo de documento: 

info:eu-repo/semantics/doctoralThesis
 

Formato:

application/pdf

Idioma:

spa

Descripción:

A study of the population structure, dynamics and fishing of the pargo blanco Umbrina canosai was carried out during almost five years in Rio Grande, Rio Grande do Sul, Brazil. The species represents over 27% of the total demersal landings in Rio Grande and yielded up to 18,726 metric tons in 1976. The fishery in southern Brazil is described and the landing statistics for Argentina, Uruguay and Brazil are presented and discussed. The sampling strategy for size frequency distributions in the landings is described and the representativity and presicion of the mean size estimates is discussed. Scales and otoliths were used for ageing. Annual periodicity in the ring and zone formation was demostrated. Aged fish ranged from 0 to 21 years. Growth in length and weigth was described. Females grew faster and slightly larger than males and on the average lived longer. No difference in lengh-weight relation between sexes was observed. Seasonal changes of growth and of stomach content proportions showed that growth and feeding intensity were higher during summer and autumn. Size, sex and age group distributions were studied by means of landing samplings and 3 cruises of the R/V Atlântico Sul. Mean size become increasingly smaller with depth. Fishes under age II largly concentrate at depth over 60 m and older fishes in shallower waters. Yearlings were found in early summer near the coastline. The proportion of females was higher during the spawning season in winter and spring, when the species is more abundant insouthern Brazil platform. Males proportion was higher in the same region in summer and autumn. U. canosai is a serial spawner, larger females start spawning in august and the smaller ones in october and november. Larger females finish spawning and start southward migration first. Smaller females and males remain longer off Rio Grande. Mean size at maturity was 214±5 mm for females a 184±4 mm for males. At age 3, 97,6% of males and 80,1% of females were sexually mature. Fecundity ranged from 44,6 thousand eggs at 187 mm to 1450,6 thousand eggs at 377 mm and correlated positively with size, age and condition factor. Discard of inmature undersized fishes aged between 0 and 2 years was estimated to be 40-50% of the total number caught by trawlers. Most discard occured during the fishing season in areas where fishable socks were present. Age at first capture (tc) is 1,5 years for the cod end mesh sizes in use. Mean age in the landings dropped from 5.5 years during the 1976 fishing season to 3.8 years during 1980 season. Over se same period fishes above 8 years dropped in landings from 11.5% to 3%. Catch per unit effort of a standard fishing boat fell from 5.3 ton/day in 1975 to 2.5 ton/day in 1981. Total mortality (Z) over the study period was estimated to be between 0.6 and 0.8. Small mesh sizes and high fishing preassure are causing growth overfishing and reduction in the reproductive potential of the population which might endanger future recruitments. "Dynamic pool" models showed that 90% maximum yield per recruit for tc=3 years could be obtained at F= 0,2 to 0,38, depending on the natural mortality assumed. Economic yield per recruit for the present fishing mortality (F= 0,5) could increase by 50% if fish under 3 years were allowed to escape. Without management the stock is likely to stabilize at low production levels. If most of the fishing continues to occur in southern Brazil, management should consider a minimum mesh size of 90-100 mm, a minimum legal size of 230 mm and the discoragment in increase of number of fishing boats. Seasonal or regional closures would be difficult to enforce and of doubtful results. If fishing intensity on the species in Uruguay and Argentina increase international management should be considered.

Identificador:

https://hdl.handle.net/20.500.12110/tesis_n1739_Haimovici