Patterns of abundance and size structure in the blue groper, Achoerodus viridis (Pisces, Labridae): evidence of links between estuaries and coastal reefs

Linkages between estuarine nursery areas and coastal reefs are thought to be important for sustaining populations of some reef fishes. Patterns of abundance and size structure in the blue groper, Achoerodus viridis (Pisces: Labridae), were documented at sites extending from sheltered reefs and seagrass, Zostera capricorni, habitats, in shallows of estuaries, to adjacent exposed reefs in New South Wales, Australia. Numbers of juvenile fish (< 200 mm SL) decreased from shallow to deep areas of reef within a site and from inner to outer estuarine sites within two estuaries. Increased numbers of large fish (> 400 mm SL) were found on the more exposed coastal reefs. These patterns were consistent over the 21/2 year study (May 1991–December 1993). Recruits were found in both seagrass and rocky reef habitat, and showed similar patterns of abundance to juveniles. Recruitment of A. viridis to seagrass habitat occurred in distinct seasonal pulses each year; peak recruitment occurred in September and October of each year. Patterns of abundance and size structure were consistent with a model of estuarine recruitment and movement to the open coast, but alternatives, such as differential mortality, could not be discounted.     

The effects of light and nutrients on Caulerpa taxifolia and growth

Caulerpa taxifolia, an invasive species elsewhere in the world, is native to Moreton Bay where its distribution has been increasing in recent years. In Australia, dense beds of C. taxifolia are predominantly found in areas of low light and high nutrients (low water quality). Monitoring data from Moreton Bay suggests that native C. taxifolia is not directly replacing seagrass, but that there is a successional trend of seagrass loss and subsequent C. taxifolia colonization. The current study examined responses of C. taxifolia in relation to changes in environmental conditions using ambient water quality and a light/nutrient manipulative experiment. In the ambient water quality experiment we found that C. taxifolia grew significantly faster in areas with higher light (lower turbidity). The manipulative experiment demonstrated that nutrients stimulate C. taxifolia growth, however, light availability and seasonality appear to influence the response of C. taxifolia growth to nutrients in Moreton Bay. These findings suggest that C. taxifolia is unlikely to colonize seagrass beds in areas with high light and low nutrients; however, in areas with moderate light and moderate to high nutrients C. taxifolia and seagrass are likely to coexist.     

Identification, habitats and seasonal occurrence of juvenile whiting (Sillaginidae) in Moreton Bay, Queensland

Three sympatric whiting species-(Sillaginidae)—sand whiting, Sillago ciliata , golden-lined whiting, Sillago analis , and trumpeter whiting, Sillago maculata , are common in Moreton Bay, Queensland. Their identification, habitats and seasonal occurrence of juveniles are studied. A key to identification of these species is provided. Juveniles of the three species inhabit shallow shores including lower sections of creeks and rivers; but 5. ciliata prefer sandy substrates with less than 1 m depth and S. analis favour muddy-sand substrates also with less than 1 m depth, while 5. maculata juveniles stay in muddy-sand to muddy substrates at 1–3 m depth. Sillago analis juveniles appear at the waters' edge from October to May each year.     

The influence of turbidity on juvenile marine fishes in estuaries. Part 2. Laboratory studies,comparisons with field data and conclusions

The turbidity preferences of juveniles of 10 marine species common in estuaries of southeastern Africa were investigated in an experimental turbidity gradient. The tests allowed the elimination of all environmental factors except turbidity.Liza dumerilii (Steindachner), was found to be a clear-water species (< 10 NTU);L. macrolepis (Smith),Rhabdosargus sarba (Forsskal),Gerres filamentosus Cuvier, andValamugil buchanani (Bleeker), preferred “clear to partially turbid” water (< 50 NTU);Monodactylus argenteus (L.) preferred intermediate turbidities (10–80 NTU); and the remaining four,Rhabdosargus holubi (Steindachner),Acanthopagrus berda (Forsskal),Pomadasys commersonni (Lacépède) andTerapon jarbua (Forsskal), were indifferent to turbidity. Statistical analysis of laboratory and field data revealed significant correlations. The results of these are discussed in relation to fish distribution patterns, ecology, and the importance of turbidity to juvenile marine fishes in estuaries.     

Importance of the upper estuary as a nursery ground for fishes in Ariake Bay,Japan

To examine the importance of the upper estuarine areas of Ariake Bay as a nursery ground for fish, assemblages of larvae and juveniles were compared among various aquatic habitats. The upper estuaries of the bay (the Rokkaku and Hayatsue estuaries) are brackish, highly turbid waters with high tidal velocities, and differ substantially from the Isahaya area, which has been separated from the bay by a man-made dike, to the middle estuary (the Kikuchi estuary). Abundances of larvae and juveniles were higher in the estuaries than in the open bay and Isahaya areas. Abundant species in the upper estuaries were similar to each other, but differed from those of the middle estuary. This was primarily due to larvae and juveniles of fishes that occurred almost entirely in the upper estuaries, such as Acanthogobius hasta, Boleophthalmus pectinirostris, Coilia nasus, Cynoglossus abbreviatus, Nibea albiflora, Odontamblyopus lacepedii, Trachidermus fasciatus and Tridentiger barbatus. These results suggest that the upper estuaries play an important role as nursery grounds for fishes.     

Spatial and temporal distribution of whiting (Sillaginidae) in Moreton Bay, Queensland

Five species of whiting (Sillaginidae) inhabit Moreton Bay. The school whiting, Sillago bassensis , and stout whiting, S. robusta , are rare, caught occasionally by trawlers in areas of sandy substrates. The sand whiting, S. ciliata , golden-lined whiting, S. analis , and trumpeter whiting, S. maculata , are very common throughout the year. Sillago maculata is distributed almost throughout Moreton Bay, from 1 to 30 m depth. Sillago ciliata and S. analis are shallow water dwellers, from the shoreline to 3 m and 5 m depth, respectively. In the shallow waters, S. ciliata prefers the sandy substrates and S. analis the mud-sandy substrates. Distributions related to the depth, habitat preference, juvenile-adult grounds, food availability, interspecific competition, and winter season are discussed.     

Identification of seagrasses in the gut of a marine herbivorous fish using DNA barcoding and visual inspection techniques

Traditional visual diet analysis techniques were compared with DNA barcoding in juvenile herbivorous rabbitfish Siganus fuscescens collected in Moreton Bay, Australia, where at least six species of seagrass occur. The intergenic spacer trnH-psbA, suggested as the optimal gene for barcoding angiosperms, was used for the first time to identify the seagrass in fish guts. Four seagrass species and one alga were identified visually from gut contents; however, there was considerable uncertainty in visual identification with 38 of 40 fish having unidentifiable plant fragments in their gut. PCR and single-strand conformational polymorphism (SSCP) were able to discriminate three seagrass families from visually cryptic gut contents. While effective in identifying cryptic gut content to family level, this novel method is likely to be most efficient when paired with visual identification techniques.     

The reproductive biology of Gerres in Natal estuaries

An investigation of the reproductive state and gonad development of Gerreidae in Natal showed that the fry of Gerres rappi, G. acinaces and G. filamentosus entered estuaries at 10 mm (s.l.). They stayed in the estuary until attainment of maturity, which was between 70 and 110 mm according to species. Gonad activity was initiated in estuaries but all females left before their eggs were ripe. Testes ripened while the fish were still in the estuary but no ripe running individuals were found. Since few large and no spent individuals occurred in the estuaries sampled, it appears that most fish do not return from the sea. Sequence of oocyte maturation and testes development were determined histologically. The estuarine area may be of importance to the fry as a safe nursery ground and to maturing individuals as an environment in which gonad maturity is reached for the first time.     

Aspects of the biology of Tilapia mossambica Peters (Pisces: Cichlidae) in a natural freshwater lake (Lake Sibaya, South Africa)

An account is given of some aspects of the biology of Tilapia mossambica Peters in Lake Sibaya, South Africa. Lake Sibaya is a warm shallow coastal lake with extensive shallow terraces in the littoral which shelve abruptly into underwater valleys. The substrate is predominantly sandy. Aquatic macrophytes are generally restricted to water 1–7 m deep. Adult fishes (over 8 cm standard length) are usually absent from water deeper than 12m and shallower than 0–5 m, whereas juveniles may occur at all depths, and fry only in very shallow water.
T. mossambica inhabits the littoral and sublittoral in the warm and transition periods (August–April) but moves into deep water in the cool season (May–July). Exposed and sheltered areas are utilized for different purposes by adult fishes, the former for nesting, and latter for feeding and mouth–brooding. Habitat selection by males is governed by the availability of suitable nest sites. Nests are most common in sparsely–vegetated sheltered areas. Brooding females preferred sheltered littoral areas but ventured onto the terrace to release the fry. Juveniles and fry inhabited exposed shores with a temperature gradient which reversed diurnally. The breeding, shoaling and feeding behaviour is described. The biology of T. mossambica in Lake Sibaya is compared to data on the same, and similar species, in other systems. The utilization of available resources in the lake by T. mossambica is discussed and reference is made to the importance of the retention of generalized characters for the successful habitation of the cyclically–renewed habitat of the littoral.     

A comparison of phytoplankton assemblages in the Chesapeake and Delaware estuaries (USA), with emphasis on diatoms

The Delaware Bay is characterized as having greater nutrient and turbidity levels than the Chesapeake Bay. In reference to these differences, a one year study was conducted to identify any similarities and differences in the phytoplankton populations in these estuaries. The results indicated patterns of similarity in the diatom composition, with the total phytoplankton assemblage forming two site groups along a salinity gradient in each bay. These site groups were associated with stations located in the tidal fresh-oligohaline and meso-polyhaline regions of both estuaries. The seasonal concentrations of diatoms and total phytoplankton in both of these regions were higher in the Chesapeake Bay.Subtle differences between the two estuaries include a more diversified and abundant assemblage of neritic phytoplankters (including dinoflagellates) are present in the lower Chesapeake Bay. In contrast, a diatom dominated community is more characteristic of Delaware Bay. It is suggested the entry of neritic species into lower regions of the estuaries was enhanced by the reduced amount of rainfall and flow rates that occurred during the study period. The greater success of neritic species in the Chesapeake Bay is attributed to the lower turbidity of that estuary compared to Delaware Bay.     

Fish Assemblages Associated with Natural and Anthropogenically-Modified Habitats in a Marine Embayment: Comparison of Baited Videos and Opera-House Traps

Marine embayments and estuaries play an important role in the ecology and life history of many fish species. Cockburn Sound is one of a relative paucity of marine embayments on the west coast of Australia. Its sheltered waters and close proximity to a capital city have resulted in anthropogenic intrusion and extensive seascape modification. This study aimed to compare the sampling efficiencies of baited videos and fish traps in determining the relative abundance and diversity of temperate demersal fish species associated with naturally occurring (seagrass, limestone outcrops and soft sediment) and modified (rockwall and dredge channel) habitats in Cockburn Sound. Baited videos sampled a greater range of species in higher total and mean abundances than fish traps. This larger amount of data collected by baited videos allowed for greater discrimination of fish assemblages between habitats. The markedly higher diversity and abundances of fish associated with seagrass and limestone outcrops, and the fact that these habitats are very limited within Cockburn Sound, suggests they play an important role in the fish ecology of this embayment. Fish assemblages associated with modified habitats comprised a subset of species in lower abundances when compared to natural habitats with similar physical characteristics. This suggests modified habitats may not have provided the necessary resource requirements (e.g. shelter and/or diet) for some species, resulting in alterations to the natural trophic structure and interspecific interactions. Baited videos provided a more efficient and non-extractive method for comparing fish assemblages and habitat associations of smaller bodied species and juveniles in a turbid environment.     

Using new electrofishing technology to amp‐up fish sampling in estuarine habitats

A prototype, boat‐mounted electrofisher capable of operation in estuarine waters (where electrical conductivities often exceed 20 000 µS cm^?1) was assessed. Electrofishing was compared to fyke and mesh netting in four riverine estuaries and to seining in a lagoonal estuary (consisting of a series of brackish coastal lakes separated from the sea by a barrier system of sand dunes). Fish assemblage composition, length distributions and the probability of detecting ecological fish guilds (relating to estuary use, position in the water column and body size) were compared among gears. The assemblage composition of electrofishing samples differed from those of fyke nets in all riverine estuaries and from mesh netting in two. The assemblage composition of electrofishing samples differed from those of seining in structured seagrass habitats of the lagoonal estuary. When all species were pooled, the electrofisher sampled a broader range of lengths than either fyke or mesh netting in riverine estuaries or seining in lagoonal estuaries. The bias of electrofishing and netting towards particular species and size classes affected the probability of detecting some ecological guilds, highlighting the potential implications of gear choice on understanding estuarine ecological function. The detection of guilds varied with gear type and environmental conditions, including stratification, water depth and surface electrical conductivity. Assessments with the aim to characterize the structure of fish assemblages will benefit from the use of multiple gears. Electrofishing shows immense promise for discretely sampling highly structured habitats to test hypotheses about their use.     

Changes in rocky reef fish assemblages throughout an estuary with a restricted inlet

Rocky reef habitat is common in many estuaries, yet its role as a habitat for fishes is poorly understood. There is also limited understanding of how access of coastal species into estuaries and habitat quality can affect the distribution of rocky reef fishes within estuaries. This study used baited remote underwater video stations to determine spatial patterns in fish assemblages associated with rocky reef habitat throughout a barrier estuary with a permanently open but restricted inlet. Estuarine rocky reefs provided habitat for a diverse assemblage of fishes, many of which were large juveniles and subadults. In the absence of a pronounced salinity or temperature gradient, a clear transition in fish assemblages occurred from coastal waters, through the inlet channel, to the central estuary, and into the inner estuary. The inlet channel, notably its narrowness and length, limits tidal input into this estuary, which acts as a significant impediment to the dispersal of many coastal fishes, and insufficient habitat excludes many coastal rocky reef species from the inner estuary. This study highlights the need to recognise estuarine rocky reefs as providing habitat for diverse fish assemblages and the role inlets play in restricting access of coastal species.     

The effects of suspensoids on fish

High suspensoid loads are a common feature of many Southern Hemisphere inland waters. Case studies on a natural lake (Chilwa), a man-made lake (Le Roux) and S.W. Indian Ocean estuaries reveal that the effects of turbidity on fish in these systems differ widely. In Lake Le Roux, high suspensoid loads influence fish by causing a reduced growth rate, a decrease in size at first maturity and maximum size, and a movement inshore by large fishes to feed on phytobenthos. High turbidities in Lake Chilwa sharply reduce food availability in benthic offshore zones, and restrict fishes to pelagic and inshore food resources. The resuspension of sediments by wind action may cause fish mortalities through deoxygenation of the water column. Moderate turbidity levels appear to be beneficial to fish in estuaries by affording protection from predators in shallow, food-rich areas. Turbidity gradients may also provide a navigational aid to fish entering estuaries.While ecological studies on Southern Hemisphere inland waters are fairly advanced, experimental work is restricted. Such research in the Northern Hemisphere has revealed that high suspensoid loads may influence breeding success, egg and larval survival, population structure and size, as well as food availability and feeding efficiency. The effect of suspensoids on the breeding success and feeding efficiency of Southern Hemisphere fish needs further investigation.Moderate suspensoid loads are a natural feature of many inland waters, but sustained high levels reduce the photic zone, blanket the benthos and interfere with the feeding efficiency of fish. The curtailment of soil erosion should therefore be regarded as a priority in the management of inland fisheries, especially in Third World countries where poor land-use practises have resulted in a marked deterioration of water quality. The need to study and manage river catchments holistically is emphasized.     

Recruitment patterns and trophic relationships of fish in Sulaibikhat Bay, Kuwait

The juveniles of nine species of fish constitute over 96% of the fish community captured in Sulaibikhat Bay, a shallow, semi-enclosed bay located at the northern end of the Persian Gulf. These juveniles show four patterns of recruitment to the bay, including recruitment in early spring, in late spring, in summer and all year round. Stomach content analysis allowed five groups of diets to be distinguished. Within these five groups, competition for food resources was probable, particularly in the spring and autumn months.     

Distribution patterns of macrobenthic biodiversity in the intertidal seagrass beds of an estuarine system, and their conservation significance

Intertidal stands of seagrass are important elements in the ecology of many estuaries yet the manner in which their associated macrobenthic biodiversity is distributed throughout any single estuary has never been determined. This has now been attempted for the Knysna estuarine bay in the Garden Route National Park, South Africa, an important site for one vulnerable and declining seagrass, Cape dwarf-eelgrass (Nanozostera capensis), and for several associated animals. Although spanning a salinity range of <5–35, the seagrass beds of this estuary contained a unitary fauna with local variation in frequency and abundance of the various species. Faunal biodiversity was highest along the main estuarine channel in the marine-influenced region and declined both upstream and into the sheltered peripheral zones, overall faunal abundance being inversely correlated with species diversity (though not with species richness). This pattern results in the beds around the 4 km shoreline of a single island near the mouth supporting 91 % of the total macrobenthic invertebrate species present in the system. This situation is discussed in relation to such data as are available from other comparable systems and to the conservation of estuarine seagrass faunal biodiversity. It is concluded that in general and with caveats for some individual species of concern where conservation resources are limited attention would most profitably be focused on the seagrass meadows of downstream estuarine areas.     

Recruitment and growth of juvenile yellowfin bream, Acanthopagrus australis Günther (Sparidae), in an Australian intermittently open estuary

Recruitment and growth of juvenile yellowfin bream, Acanthopagrus australis Günther (Sparidae), an economically important species, were examined in an intermittently open Australian estuary for 13 months. Fish had a mean fork length of 77.9 (± SE 9.5) mm after 1 year. Growth rates were faster than those found in previous studies in permanently open estuaries and appeared to be dependent upon the water temperature, which ranged from 0.04 mm/day in winter to 0.67 mm/day during summer. However, periods of slower growth may also reflect the emigration of larger fish from the cohort when the estuary was open and which coincided with lower water temperatures. Growth and length-at-age of fish using length-frequency data may be estimated more accurately in intermittently open estuaries because the time of recruitment can be determined, especially when the estuary opening period is short.     

The influence of turbidity on juvenile marine fishes in estuaries. part 1. field studies at Lake St. Lucia on the southeastern coast of Africa

Lake St. Lucia, the largest estuarine system in Africa (325 km²), was chosen as the field study area for a 3.5-yr ((1980)–83) investigation into relationships between water turbidity and estuarine fish distribution. The variety of habitats, from clear water, open sandy shores to shallow muddy substrata and turbid waters, together with high species diversity (108 species) rendered the area suitable for this study. The relationships between fish distribution and environmental factors were monitored by monthly seine netting of fishes at seven sites representative of the range of conditions in St. Lucia. Simultaneously, water turbidity, salinity, and temperature were recorded.The possible influences of substratum type and food availability were also investigated by using recently published data on invertebrate benthos and Zooplankton distributions. Published data were also used to determine the diet of the common fish species. The results showed that the distribution of juveniles of the 20 commonest fish species were statistically correlated only with water turbidity, water temperature, and food availability. The correlation with temperature was related to seasonal not spatial temperature patterns.Turbidity and food type influences were difficult to separate but exceptions were the anchovyThryssa vitrirostris (Gilchrist and Thompson) and the soleSolea bleekeri Boulenger which occurred only in turbid water despite the widespread occurrence of their prey, andGenes acinaces Bleeker,G. rappi (Barnard), andG. fllamentosus Cuvier, all of which occurred only in clear water although the greatest densities of their bivalve prey were in turbid waters. Similarly, the sparidsRhabdosargus holubi (Steindachner) andR. sarba (Forsskal) were distributed according to turbidity and not their preferred foods.Principal component analysis with a minimum spanning tree plot and a canonical correlation test showed that the fish fauna could be divided into five groups according to their occurrence in various turbidities. These were: clear water species (e.g. Gerreidae) in < 10 NTU, clear to partially turbid species (e.g.Liza dumerilii (Steindachner) andL. macrolepis (Smith)) in < 50 NTU, intermediate turbidity species (e.g.Valamugil cunnesius (Valenciennes) andLeiognathus equula (Forsskal)) in 10–80 NTU, turbid-water species (e.g.Elops machnata (Forsskal) andThryssa vitrirostris) in > 50 NTU, and species indifferent to turbidity (e.g.Acanthopagrus berda (Forsskal) andTeraponjarbua (Forsskal). It is, therefore, suggested that turbidity plays a significant roˆle, either singly, or in combination with other variables in determining the distribution of juvenile marine fishes in estuaries.     

Rotifer assemblages in the Neva Bay,Russia: principles of formation,present state and perspectives

A comparative study of species composition, abundance and spatial distribution of the rotifers of Neva Bay (Gulf of Finland, Baltic Sea), between 1982–1993, was carried out. The rotifer fauna in the central basin of Neva Bay is determined by and originates in Lake Ladoga and is very similar to that of the Neva River. High turbidity and effect of winds affecting water residence time influence composition and density of rotifers in this shallow estuary. The importance of rotifer assemblages for evaluation of the quality of the estuary is discussed.     

Opportunistic use of estuarine habitat by juvenile bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, from the Elwha River before,during, and after dam removal

Estuaries are used by anadromous fishes, either as the definitive marine habitat or as transition habitat as they move to fully marine waters, and extent of estuary use may vary with habitat conditions and fish attributes. Bull trout (Salvelinus confluentus) are commonly fluvial or adfluvial, though anadromous populations also exist. However, little is known about estuary use, especially by juveniles of this threatened species. We sampled the estuaries of the Elwha River, where a spawning population exists, and the nearby Salt Creek, where none exists, to reveal seasonal timing of estuarine use by juvenile bull trout, size of those using the estuary, and possible use of the non-natal estuary. We captured juvenile bull trout (all ≥100 mm FL, most <300 mm) in the Elwha River estuary in all months except August, but primarily December through May. None was captured in Salt Creek’s estuary despite comparable sampling effort. We also evaluated how dam removal on the Elwha River influenced bull trout estuarine occupancy by sampling before, during, and after dam removal, because this process enlarged the estuary but also increased turbidity and sediment transport in the lower river. Catches were low before dam removal, increased during and immediately after removal, and returned to low levels in recent years, suggesting that juveniles temporarily sought refuge from conditions associated with dam removal. Our findings indicate juvenile bull trout occupy estuarine habitat opportunistically; this information may aid conservation efforts as anadromous populations occur elsewhere in rivers with estuaries altered by human development.