Dp Cyrus

Turbidity and salinity in a tropical Northern Australian estuary and their influence on fish distribution

Turbidity and salinity and their influences on fish distribution were studied for two and a half years in the Embley Estuary in tropical northern Australia. Both turbidity and salinity varied significantly during the year but three clearly distinguishable seasonal patterns existed. These are referred to as the Wet, Early Dry and Late Dry Seasons. During each of these seasons distinct gradients of turbidity and salinity were present. The turbidity and salinity gradients were continuous with those in the adjacent marine environment of Albatross Bay. The levels and ranges of both factors were largely determined by the seasonal rainfall patterns in the catchment of the Embley River. The distribution and abundance of the 45 most common species was analysed in relation to turbidity, salinity and temperature patterns in the estuary. These data showed that fish densities within the estuary were related to turbidity and salinity but not temperature. There was a strong inverse relationship between turbidity and salinity. The Catch per Unit Effort (CPUE) of each species was determined in each of three broad ranges of turbidity and salinity. From this, patterns related to these two factors were found for 30 of the 45 species of fish.

Habitat utilization by small flatfishes in a North Carolina estuary

Distribution and abundance of flatfish species (<150 mm standard length) were related to habitat characteristics in the Newport River and Back Sound estuaries in North Carolina. Salinity, turbidity, depth, distance from marsh edge, benthic composition, and grain size were used to describe the different shallow water habitats from April through October 1994. One Scophthalmidae, seven Paralichthyidae, one Achiridae, and one Cynoglossidae species were collected during the study including juvenileParalichthys albigutta (gulf flounder),P. dentatus (summer flounder), andP. lethostigma (southern flounder) along with multiple age classes ofCitharichthys spilopterus (bay whiff),Etropus crossotus (fringed flounder),Symphurus plagiusa (blackcheek tonguefish), andTrinetes maculatus (hogchoker). Incidental catches ofAncylopsetta quadrocellata (ocellated flounder),C. macrops (spotted whiff), andScopthalmus aquosus (windowpane) were also made. Flatfish distributions among habitats varied by species, size within species, and season. Regardless of season, the highest densities of flatfishes were found in the upper estuary. All habitats were used by one or more species and most species occurred at several habitats. Some species were significantly more abundant at specific habitat types. Ontogenetic shifts in habitat utilization were found for several species. High densities of smallP. lethostigma, C. spilopterus, S. plagiusa, andT. maculatus occurred in the upper estuary on muddy substrates. LargeP. dentatus, C. spilopterus, S. plagiusa, andT. maculatus utilized sand flats and channels in the lower estuary.

The influence of zooplankton food resources on the morphology of the estuarine clupeid Gilchristella aestuarius (Gilchrist, 1914)

SynopsisThe body shape of the estuarine clupeidGilchristella aestuarius from the St. Lucia system is different from that of the same species in other estuaries. The morphology ofG. aestuarius is discussed in relation to long term food availability in the St. Lucia system.

Recruitment and origin of penaeid prawn postlarvae in two South-east African estuaries

Abstract Input of postlarval stages into the penaeid prawn stocks of Natal, South Africa, was investigated by monitoring immigration into the St Lucia and Kosi estuaries. Five species, Penaeus japonicus Bate, P. indicus Milne Edwards, P. semisulcatus de Haan, P. monodon Fabricius and Metapenaeus monoceros (Fabricius) were recorded. Recruitment into the more northerly Kosi estuary, where offshore conditions are influenced by the south-flowing Agulhas Current, was totally dominated by P. japonicus . At St Lucia, at the northern end of the Tugela Bank, similar numbers of P. japonicus and P. indicus were recorded as well as smaller numbers of the other three species. The differences imply that the Tugela Bank provides the major postlarval input of commercially important species to St Lucia and suggests a greater degree of isolation of the Natal penaeid prawn populations than previously thought.

Fish assemblages in the Mfolozi-Msunduzi estuarine system, KwaZulu-Natal, South Africa, when not linked to the St Lucia mouth.

The fish community of the Mfolozi–Msunduzi estuarine system was investigated with particular emphasis on its role as a nursery area for marine fish when the adjacent St Lucia mouth is closed. The mouth was open during March 2007 and 2008 with high-turbidity river water flowing into the sea, and was closed during August 2007 and 2008. Fish were sampled biannually with seine-nets during March and August in 2007 and 2008 at five sites throughout the system. A total of 5 886 fish from 59 species were recorded, with a higher number of species and CPUE in the Mfolozi Estuary than in the Msunduzi Estuary. Seine-net catches were dominated by Ambassis gymcocephalus, Ambassis natalensis, Leiognathus equula and Valamugil cunnesius. Juveniles of marine spawning species were present throughout the study period, even when the mouth was closed. The sampled fish assemblage structure was influenced by significant temporal differences between the four biannual sampling seasons, with relatively small spatial differences between the sampling areas. Temperature, salinity and sediment characteristics were most responsible for the structure of the fish assemblage as sampled by seine-nets. The results emphasise the importance of the Mfolozi–Msunduzi estuarine system as an alternate nursery area.

Fish breeding in, and juvenile recruitment to, the St Lucia Estuarine System under conditions of extended mouth closure and low lake levels

Ongoing drought conditions along the northern coast of KwaZulu-Natal resulted in the St Lucia Estuary mouth closing, and to evaporation leading to extremely low lake levels and the development of hypersaline conditions in most parts of the system. The mouth closed in June 2002 and has remained closed for more than three years. The St Lucia Estuarine System is an important nursery ground for juveniles of many marine fish species. Ezemvelo KZN Wildlife initiated a once-off survey to investigate the effects of the drought on the fish fauna and to establish if any fish were breeding in the system under the existing drought conditions. Fish were sampled at six sites throughout the system, using seine and gill-nets. In December 2004, the system had become partitioned into four isolated compartments, and at this time a total of 4 475 individuals comprising 30 species were recorded. An analysis of size class data indicated the presence of juveniles of at least 12 species, including nine estuarineassociated marine species, which had spawned after mouth closure. Although some estuarine species had spawned within the system, most appeared to have recruited into the Narrows and lake sections of the St Lucia system during January 2004 when the adjacent Mfolozi River, which is linked to the open Mfolozi Estuary, flooded and overtopped into St Lucia Estuary. Should the St Lucia mouth remain closed for four or more years, it can be expected that the lack of recruitment of first-time spawners from the St Lucia nursery grounds into the off-shore marine breeding stocks will have major impacts on these estuaryassociated marine species.

Benthic faunal distribution and abundance in the Mfolozi–Msunduzi estuarine system, KwaZulu-Natal, South Africa

The Mfolozi–Msunduzi estuarine system historically shared a common mouth with the St Lucia estuarine system. In 1952, a separate mouth was created, 1.5 km south of St Lucia mouth, to prevent silt carried by the Mfolozi River from entering Lake St Lucia. Despite its proximity to the comparatively well-studied St Lucia Estuary, there is very little information on the Mfolozi– Msunduzi estuarine system in general and no information on its benthos. In the present study, 17 taxa were recorded from biannual quantitative sampling in 2007 and 2008. Results indicated that the system was dominated by the polychaetes Ceratonereis sp., Dendronereis arborifera and Capitella capitata, the crab Paratylodiplax blephariskios and the tanaid Apseudes digitalis. The main factors influencing the distribution of the benthos were oxygen concentration, temperature, the open or closed state of the mouth, and salinity, with particle size and organic content of the substratum also being important in determining community structure. Although the dominant taxa were previously recorded as abundant in St Lucia, numbers present in the Mfolozi–Msunduzi system were generally lower than those recorded in St Lucia, suggesting that periodic flooding and the unstable nature of the sediments in the Mfolozi–Msunduzi prevented its benthos from attaining the densities recorded in the adjacent St Lucia Estuary. While the Mfolozi–Msunduzi system is classified as a river mouth, its benthos was more similar in composition to that of the Mhlathuze and Mlalazi estuaries, which are classified as an estuarine bay and a permanently open estuary, respectively. It is suggested that reclassification of the Mfolozi–Msunduzi system as a permanently open estuary would be more appropriate in terms of its benthos.

The sublethal effects of copper and lead on the haematology and acid-base balance of the groovy mullet, Liza dumerili

Toxicity bioassays were conducted on groovy mullet, Liza dumerili, using copper and lead, in order to assess how these metals affected their blood haematology and acid-base balance. Short-term (96 hours) exposure to lead caused significantly more haematological response [PCO2] than copper, when compared to the control. When copper and lead were applied in combination there were significant increases in bicarbonate content and red blood cell count of the fish. Long-term (28 days) exposure to copper and lead resulted in significant increases in blood glucose, haemoglobin content and the acid-base status of the fish. The results of this study highlight the stress to which estuarine fish are exposed through the uncontrolled discharge of heavy metals and the inherent dangers posed by combinations of metals in the aquatic environment. They will also help emphasise the need to control pollution in order to protect aquatic biodiversity.

Juvenile fish recruitment through wave-overtopping into a closed subtropical estuary

The fish fauna of the subtropical Nhlabane estuary in northern KwaZulu-Natal was studied during a regional drought over the period 1991–1995. The estuary mouth closed in late 1991, due to the drought and to water abstraction from the system, and reopened only at the end of the drought in late 1995. The fish community underwent considerable changes in composition and abundance, from marine-dominated during 1992 to freshwater/estuarine-dominated in 1995. The adverse effect of extended mouth closure on the fish fauna and the lack of contact with the marine environment was clearly illustrated. Due to mouth closure, recruitment of estuarine-dependent marine species almost stopped, although some recruitment by means of wave-overtopping was found to continue, even though the estuary mouth remained closed. Recruitment of twelve estuarine-dependent marine fish species was found to occur in this way, suggesting that recruitment through wave-overtopping is an important aspect of estuarine functioning during drought conditions.

The potential for zoobenthic fauna dispersal into the St Lucia Estuary from the Mfolozi–Msunduzi estuarine system, KwaZulu-Natal, South Africa

The onset of a prolonged drought in the St Lucia catchment in 2002 and subsequent mouth closure raised concern for the biota of the estuary. Ezemvelo KZN Wildlife proposed a scheme whereby the Mfolozi–Msunduzi mouth would be closed, causing fresh water to enter the St Lucia Estuary to prevent excessively hypersaline conditions (>65) from developing in the lower and middle reaches of the St Lucia system. Although this proposal caused concern about the impact of mouth closure on the ecology of the Mfolozi–Msunduzi estuarine system, it raised its potential as a source of benthic fauna dispersal into, and the restocking of, the St Lucia system. Quantitative benthic sampling in the Mfolozi–Msunduzi system in 2007 and 2008 revealed a benthic community generally dominated by polychaetes which, although low in terms of total species richness compared with St Lucia, included elements of a core benthic group present in the St Lucia system throughout the current drought. The presence of the gravimetrically important crab Paratylodiplax blephariskios in the Mfolozi–Msunduzi system was considered important as a source from which the once-abundant population in the St Lucia Narrows could be restocked via dispersal through the channel linking the Mfolozi–Msunduzi Estuary to St Lucia. It was concluded that, while the input of fresh water from the Mfolozi River is necessary to ameliorate hypersaline conditions and conserve the important core benthic group in the Narrows and South Lake, consideration must be given to allowing sufficient tidal exchange in the Mfolozi–Msunduzi system to maintain its stocks of benthos with an obligate marine phase.