Showing papers by "Alan K. Whitfield published in 1988"

The fish community of the Swartvlei estuary and the influence of food availability on resource utilization

Abstract: Structural and functional characteristics of the Swartvlei estuary fish community are described. The detritivore group of fishes comprised 49% of the total catch biomass, zoobenthivores 25%, herbivores 18%, piscivores 6%, and epifauna/zooplanktivores 2%. The diets of 18 fish species, based on the analysis of 1,648 stomach contents, are presented. Selection by Swartvlei estuary fishes for zoobenthic invertebrates and aquatic macrophytes in different habitats was calculated using the linear index of selection. Results indicated a strong positive selection for epifaunal invertebrates and poor utilization of infauna and plants. Plant consumption by herbivorous fishes in the estuary centered around filamentous algae and diatoms growing onZostera capensis, rather than seagrass leaf material. The detrital base for both eelgrass- and sand-dominated areas in the Swartvlei estuary is emphasized, and most of the fish biomass was supported directly or indirectly by detritus. Large catches (mass per unit effort) in the macrophyte-free upper reaches site were attributed to the tidal input ofZostera leaves and associated algae to the area and its subsequent utilization by fishes through the detrital food chain.

The role of tides in redistributing macrodetrital aggregates within the Swartvlei estuary

Abstract: The Swartvlei estuary possesses a prolific growth of both intertidal and subtidal eelgrass,Zostera capensis. During 1984 less than 12% of the eelgrass beds were located in the upper half of the estuary, yet deposition ofZostera/macroalgal wrack in this region, when the estuary was linked to the sea (open phase), was similar to that in the lower half. Over a period of 20 semidiurnal tidal cycles there was a net gain of 2.5 tonnes dry mass of plant material into the upper reaches. Export of aquatic macrophytes and filamentous algae from the lower reaches toward the sea over 20 tidal cycles amounted to 1.6 tonnes dry mass. The amount of plant material transported during spring tides was 2 to 3 times greater than that carried during neap tides. Shallowing of the estuary mouth due to sand deposition resulted in a decline in the tidal prism and a decrease in macrodetrital flux. Total export ofZostera and associated algae amounted to 0.87 g ash-free dry mass m−2d−1 and represented a monthly export of 18% ofZostera bed biomass. Deposition of plant wrack during the 1984–1985 closed phase amounted to 63 g dry mass per meter of shore per day at the lower reaches site but only 10 g m−1d−1 was recorded at the upper reaches site. The relatively low latter value was attributed to the absence of tidal action which transports macrodetritus from the lower and middle reaches into the upper part of the system. During the 1984 open phase 70 g m−1d−1 was deposited at the lower reaches site and 68 g m−1d−1 at the upper reaches site. The role of tides in the redistribution of aquatic macrophyte primary production in the Swartvlei estuary was therefore clearly underscored.

Larval development of the Cape silverside, Atherina breviceps Cuv. & Val., 1835 (Teleostei, Atherinidae) from southern Africa

Abstract: The egg and larval development of the Cape silverside, Atherina breviceps, are described and illustrated from material collected in the Swartvlei estuary. The eggs are spherical, 1,40-1,65 mm in di...

Larval development of the Cape silverside, Atherina breviceps Cuv. and Val., 1835 (Teleostei, Atherinidae) from southern Africa

Abstract: The egg and larval development of the Cape silverside, Atherina breviceps , are described and illustrated from material collected in the Swartvlei estuary. The eggs are spherical, 1.40-1.65 mm in diameter, with 6-12 uniformly spaced chorionic filaments attached at both ends to the chorion. The latter condition and the small size of the eggs are unique among the Atherina species. The larvae examined (5.1-15.7 mm) are elongate, have a yolk sac at hatching and are characterized by distinct body pigmentation and a prominent gas bladder. Flexion occurs between 7.6-11.2 mm, all fin elements are formed by 15.7 mm and scales develop by 20.0 mm. The authors compare the larval development of A. breviceps with that of other atherinids and discuss the spawning of this species and the seasonal occurrence of larvae along the southern coast of Africa.

Availability on Resource Utilization

Abstract: Structural and functional characteristics of the Swartvlei estuary fish community are described. The detritivore group of fishes comprised 49% of the total catch biomass, zoobenthivores 25%, herbivores 18%, piscivores 6%, and epifauna/zooplanktivores 2%. The diets of 18 fish species, based on the analysis of 1,648 stomach contents, are presented. Selection by Swartvlei estuary fishes for zoobenthic invertebrates and aquatic macrophytes in different habitats was calculated using the linear index of selection. Results indicated a strong positive selection for epifaunal invertebrates and poor utilization of infauna and plants. Plant consumption by herbivorous fishes in the estuary centered around filamentous algae and diatoms growing on Zostera capensis, rather than seagrass leaf material. The detrital base for both eelgrass- and sand-dominated areas in the Swartvlei estuary is emphasized, and most of the fish biomass was supported directly or indirectly by detritus. Large catches (mass per unit effort) in the macrophyte- free upper reaches site were attributed to the tidal input of Zostera leaves and associated algae to the area and its subsequent utilization by fishes through the detrital food chain.