The stock identification of the Cape horse mackerel, Trachurus trachurus capensis (Pisces: Carangidae)
- Authors: Naish, Kerry-Ann
- Date: 1990
- Subjects: Carangidae -- Identification , Trachurus trachurus -- Identification , Carangidae -- South Africa , Trachurus trachurus -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5320 , http://hdl.handle.net/10962/d1005165 , Carangidae -- Identification , Trachurus trachurus -- Identification , Carangidae -- South Africa , Trachurus trachurus -- South Africa
- Description: Three stocks of the Cape horse mackerel, Trachurus trachurus capensis, have been recognised in the ICSEAF convention region off southern Africa (Figure 1) on the basis of catch distribution and biological analyses. They were identified as being off Namibia (ICSEAF Divisions 1 . 3 and 1.4), off the West Coast of South Africa (Div. 1.6) and off the South Coast (Div. 2.1 and 2.2). Recently, speculation as to the accuracy of the classification of the South African populations has arisen. The aim of this study was to determine the number of stocks In Divisions 1.6, 2.1 and 2.2, using methodology which assessed the spatial and temporal nature and the phenotypic and genotypic characteristics of the species. The distribution of horse mackerel was evaluated by studying the trends in catch data and length frequency distributions obtained from the demersal industry during 1986 to 1988 and from the demersal biomass cruises in 1987 and 1988. Adult fish, found in Div. 2.1 and 2.2, probably migrated; during the months in which horse mackerel were expected to spawn, CPUE values were higher over the central Agulhas Bank than in the east of the study region. During the quiescent period, catches and numbers were higher in the latter region. Juvenile horse mackerel were found in the nursery areas utilised by pelagic fish and it is likely that they were transported northward from the Agulhas Bank. A decrease in biomass of individuals in Div. 1.5 indicated a separation between a northern and a southern population. Analyses of the phenotype, or epigenetic characters, of horse mackerel were used as a further test of stock integrity. Monthly samples were drawn from Div. 1.6, 2.1 and 2.2 during May 1988 to June, 1989. Comparisons for the values obtained from growth, length-at-50%-maturity and the season of otolith ring formation from each region showed no significant variation in the phenotype. Morphometric analysis proved inconclusive. Monthly gonadosomatic indices show that fish south of the Orange River share the same spawning season. Reports from the literature show that horse mackerel in the latter region differ from those of northern Namibia in spawning season, age-at-50%-maturity and season of otolith ring formation. The genetic structure of the populations of horse mackerel was evaluated by means of a restriction enzyme analysis of the Mitochondrial DNA of 37 fish collected from Divisions 1.4, 1.5, 1.6, 2.1 and 2.2. Two composite genotypes of horse mackerel were found; one belonging to fish in Div. 1.4 and one to fish in Div. 1.6, 2.1 and 2.2. The genetic distance between the two genotypes, 0.07, was the expected distance between two populations at a subspecies level. Fish in Div. 1.5 consisted of both genotypes and may be a region of mixing between the two populations. The interpretation of results was cautioned; prevlous work has shown that the migration of a small number of individuals between two stocks can be expected to maintain a low variation between the populations. In concluding, it was recommended that the horse mackerel be managed as two stocks, one in Div. 1.3 and 1.4 and one in Div. 1.6, 2.1 and 2.2
- Full Text:
- Date Issued: 1990
- Authors: Naish, Kerry-Ann
- Date: 1990
- Subjects: Carangidae -- Identification , Trachurus trachurus -- Identification , Carangidae -- South Africa , Trachurus trachurus -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5320 , http://hdl.handle.net/10962/d1005165 , Carangidae -- Identification , Trachurus trachurus -- Identification , Carangidae -- South Africa , Trachurus trachurus -- South Africa
- Description: Three stocks of the Cape horse mackerel, Trachurus trachurus capensis, have been recognised in the ICSEAF convention region off southern Africa (Figure 1) on the basis of catch distribution and biological analyses. They were identified as being off Namibia (ICSEAF Divisions 1 . 3 and 1.4), off the West Coast of South Africa (Div. 1.6) and off the South Coast (Div. 2.1 and 2.2). Recently, speculation as to the accuracy of the classification of the South African populations has arisen. The aim of this study was to determine the number of stocks In Divisions 1.6, 2.1 and 2.2, using methodology which assessed the spatial and temporal nature and the phenotypic and genotypic characteristics of the species. The distribution of horse mackerel was evaluated by studying the trends in catch data and length frequency distributions obtained from the demersal industry during 1986 to 1988 and from the demersal biomass cruises in 1987 and 1988. Adult fish, found in Div. 2.1 and 2.2, probably migrated; during the months in which horse mackerel were expected to spawn, CPUE values were higher over the central Agulhas Bank than in the east of the study region. During the quiescent period, catches and numbers were higher in the latter region. Juvenile horse mackerel were found in the nursery areas utilised by pelagic fish and it is likely that they were transported northward from the Agulhas Bank. A decrease in biomass of individuals in Div. 1.5 indicated a separation between a northern and a southern population. Analyses of the phenotype, or epigenetic characters, of horse mackerel were used as a further test of stock integrity. Monthly samples were drawn from Div. 1.6, 2.1 and 2.2 during May 1988 to June, 1989. Comparisons for the values obtained from growth, length-at-50%-maturity and the season of otolith ring formation from each region showed no significant variation in the phenotype. Morphometric analysis proved inconclusive. Monthly gonadosomatic indices show that fish south of the Orange River share the same spawning season. Reports from the literature show that horse mackerel in the latter region differ from those of northern Namibia in spawning season, age-at-50%-maturity and season of otolith ring formation. The genetic structure of the populations of horse mackerel was evaluated by means of a restriction enzyme analysis of the Mitochondrial DNA of 37 fish collected from Divisions 1.4, 1.5, 1.6, 2.1 and 2.2. Two composite genotypes of horse mackerel were found; one belonging to fish in Div. 1.4 and one to fish in Div. 1.6, 2.1 and 2.2. The genetic distance between the two genotypes, 0.07, was the expected distance between two populations at a subspecies level. Fish in Div. 1.5 consisted of both genotypes and may be a region of mixing between the two populations. The interpretation of results was cautioned; prevlous work has shown that the migration of a small number of individuals between two stocks can be expected to maintain a low variation between the populations. In concluding, it was recommended that the horse mackerel be managed as two stocks, one in Div. 1.3 and 1.4 and one in Div. 1.6, 2.1 and 2.2
- Full Text:
- Date Issued: 1990
An assessment of the South African longline fishery with emphasis on stock integrity of kingklip, Genypterus capensis (Pisces : ophidiidae)
- Authors: Japp, David William
- Date: 1989
- Subjects: Genypterus capensis , Ophidiidae , Fisheries -- South Africa , Longlining (Fisheries) -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5209 , http://hdl.handle.net/10962/d1004660 , Genypterus capensis , Ophidiidae , Fisheries -- South Africa , Longlining (Fisheries) -- South Africa
- Description: The South African demersal longline experiment is assessed with emphasis on the target species, kingklip Genypterus capensis. The hypothesis that kingklip on the South African coast comprise a unit stock is tested. Recommendations for the management of the fishery are made. Longlining was found to be commercially viable. Techniques have been developed to target on either kingklip or the Cape hakes, Herluccius capensis and H. paradoxus. Longline fishermen exploit the kingklip spawner stock from August to No v ember by taking advantage of prespawning aggregations on the South-East Coast . Effort switches to the West Coast in late summer and early winter where kingklip are less abundant and a larger proportion of hake is caught. Catch rates of kingklip on the South Coast have declined sharply and the resource there has been exploited at a rate greater than that required to retain 50% of the unexploited biomass . There is a 17,6% probability that the spawner biomass on the South Coast has already been depleted below its pristine level. On the West Coast the catch rates of kingklip are lower than on the South Coast and have not changed significantly, although within the 95% confidence limits there is a possibility that the resource there has also been depleted below 50% of its pristine level. Genypterus capensis on the South African coast comprise a unit stock. Comparison of kingklip morphology using multivariate and discriminant function analysis and of otolith morphology using univariate statistics shows that there are no significant differences between the fish on the West and South Coasts. They are a slow growing species and were aged up to 25 years. Kingklip on the West Coast are smaller and have a lower L~ than those on the South Coast, but this is not a characteristic of discrete stocks. Kingklip on the South Coast mature earlier than those on the West Coast and aggregate to spawn on the South - East Coast in spring. There is no obvious spawning period on the West Coast. It is hypothesized that kingklip spawning on the South-East Coast is in response to favourable environmental conditions that enhances the survival of their eggs and larvae. It is recommended that demersal longlining be established as a permanent kingklip-directed fishery but that effort should not be allowed to increase. The West and South Coasts should be managed .separately. A TAC of 5 OOOt for kingklip should be introduced for 1989 of which 2 OOOt should be allocated to the West Coast and 3 OOOt to the South Coast . A closed season for kingklip from 1 August to 30 September on the South-East Coast is recommended. Hake-directed longlining should not be allowed as its effect on the hake spawner stocks are unknown and could be a potential threat to the stability of the demersal trawl fishery.
- Full Text:
- Date Issued: 1989
- Authors: Japp, David William
- Date: 1989
- Subjects: Genypterus capensis , Ophidiidae , Fisheries -- South Africa , Longlining (Fisheries) -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5209 , http://hdl.handle.net/10962/d1004660 , Genypterus capensis , Ophidiidae , Fisheries -- South Africa , Longlining (Fisheries) -- South Africa
- Description: The South African demersal longline experiment is assessed with emphasis on the target species, kingklip Genypterus capensis. The hypothesis that kingklip on the South African coast comprise a unit stock is tested. Recommendations for the management of the fishery are made. Longlining was found to be commercially viable. Techniques have been developed to target on either kingklip or the Cape hakes, Herluccius capensis and H. paradoxus. Longline fishermen exploit the kingklip spawner stock from August to No v ember by taking advantage of prespawning aggregations on the South-East Coast . Effort switches to the West Coast in late summer and early winter where kingklip are less abundant and a larger proportion of hake is caught. Catch rates of kingklip on the South Coast have declined sharply and the resource there has been exploited at a rate greater than that required to retain 50% of the unexploited biomass . There is a 17,6% probability that the spawner biomass on the South Coast has already been depleted below its pristine level. On the West Coast the catch rates of kingklip are lower than on the South Coast and have not changed significantly, although within the 95% confidence limits there is a possibility that the resource there has also been depleted below 50% of its pristine level. Genypterus capensis on the South African coast comprise a unit stock. Comparison of kingklip morphology using multivariate and discriminant function analysis and of otolith morphology using univariate statistics shows that there are no significant differences between the fish on the West and South Coasts. They are a slow growing species and were aged up to 25 years. Kingklip on the West Coast are smaller and have a lower L~ than those on the South Coast, but this is not a characteristic of discrete stocks. Kingklip on the South Coast mature earlier than those on the West Coast and aggregate to spawn on the South - East Coast in spring. There is no obvious spawning period on the West Coast. It is hypothesized that kingklip spawning on the South-East Coast is in response to favourable environmental conditions that enhances the survival of their eggs and larvae. It is recommended that demersal longlining be established as a permanent kingklip-directed fishery but that effort should not be allowed to increase. The West and South Coasts should be managed .separately. A TAC of 5 OOOt for kingklip should be introduced for 1989 of which 2 OOOt should be allocated to the West Coast and 3 OOOt to the South Coast . A closed season for kingklip from 1 August to 30 September on the South-East Coast is recommended. Hake-directed longlining should not be allowed as its effect on the hake spawner stocks are unknown and could be a potential threat to the stability of the demersal trawl fishery.
- Full Text:
- Date Issued: 1989
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