Ondersoek na lengte, massa en ouderdom van hengelvisse in die Hardapdam, S. W. A

Abstract

Abstract taken from "Opsommings", RAU 1975, pp. 79-80:

4. Tydens die ondersoek van fisiese faktore is veral aandag gegee aan temperatuur en suurstof. Deurlopende temperatuur-waardes is op sowel oppervlak- as 4-meter-diepte verkry, terwyl sowel temperatuur- as suurstofstratifikasiemetinge uitgevoer is

5. In die ondersoek na lengte en massa is gevind dat- 1, die waardes van log c en n baie kan varieer en slegs as konstantes in die vergelyking M - cL n beskou moet word; 2. geslagte massavoordele t. o. v. mekaar by sekere lengtegroepe asook tydens verskillende seisoene vertoon; 3. empiriese en berekende waardes van massa vir opeenvolgende lengtegroepe baie goed met mekaar ooreenstem; 4. die verband tussen lengte en massa sterk by die verskillende ouderdomsgroepe varieer

6. Waardes van die relatiewe kondisiefaktor varieer seisonaal baie en kan moontlik aan gonadetoestand en broei gekoppel word

7. 'n Direkte verband tussen totale lengte en skubstraal word by beide S. mossambicus en L. capensis gevind. Seisoenale variasie kom ook hier voor

8. Skubbe word gebruik in ouderdomsanalises. Sterk oorvleueling in lengtegroepe word by die verskillende ouderdomsgroepe aangetref. Die sukses van ouderdomsanalises m. b. v. skubbe word aangedui by die gebruik van die Van Oosten-terugwerkformule, waar jaarlikse toename in lengte en massa vir jaargroepe bepaal is

9. Verwerking van die groot hoeveelheid data is deur 'n rekenaar behartig, wat nie alleen tydbesparend is nie, maar ook berekeningsfoute tot 'n minimum beperk

Synopsis: 1. This project is the first phase of an intensive survey to facilitate the proper management of the fish population of Hardap Dam. Aspects such as commercial exploitation of fish and angling facilities can therefore be controlled scientifically. 2. Passive gill netting was proved to be the most practical and successful fishing gear in Hardap Dam. 3. A total of 6747 of the six angling species was caught in the period October 1971 -September 1972. L. capensis comprised the largest percentage of the total (49, 4 percent). whilst S. mossambicus comprised 19, 58 percent, L. umbratus 16. 43 percent. B. holubi 10, 25 percent, C. gariepinus 3, 25 percent and C. carpio 1. 07 percent of the total. 4. Special attention was given to the physical factors temperature and oxygen. Continuous recordings were made of temperatures at the surface and a depth of 4 metres. Monthly measurements of temperature and oxygen stratification were also recorded. 5. Studies on the length-mass relationship produced the following: (1). The values of log c and n can vary considerably and should only be considered as constants in the expression M = cLn (2). The sexes often displayed differences in mass at certain length groups, as well as during the different seasons. (3). The calculated mass values for the successive length groups compared well to the empirical values. (4). The length-mass relationship of the different age groups varied considerably. 6. The relative condition factor displayed a seasonal variation which could possibly be connected to condition of the gonads and the breeding cycles. 7. Both S. mossambicus and L. capensis showed a direct relationship between total length and scale radius. This relationship also showed seasonal variation. 8. Scales were used for age determination. The different age groups overlapped strongly at certain length groups. The annual growth for both length and mass was calculated by the back calculation method of Van Oosten. 9. A detailed analysis of the large number of data was done by a computer, which did not only save precious time, but also reduced human error to a minimum