Temperature and salinity tolerance of juvenile Rhabdosargus holubi [Steindachner (Teleostei: Sparidae)]

Temperature and salinity tolerances of juvenile Rhabdosargus holubi (Steindachner) were determined experimentally. Results indicated that they are tolerant over a wide range of salinities (0.7–70‰) and temperatures (10–30°C) and that little interaction between temperature and salinity occurs. R. holubi is restricted to the warm temperate waters of southeastern Africa. It is absent from the subtropical Indian Ocean and from temperate Atlantic Ocean waters. The juveniles occur mainly in the inshore waters and estuaries. Evidence from tolerance experiments and salinity and temperature data from southern African estuaries suggested that the distribution of R. holubi is controlled by temperature.     

Factors affecting the distribution of juvenile estuarine and inshore fish

The differential distributions of juveniles and adults of 25 species of teleost were investigated and compared from four habitat types in sub-tropical Moreton Bay, Queensland. The aim of the study was to identify factors influencing the distribution of juveniles, particularly the species which enter estuaries. The following habitats were sampled: a shallow, sheltered tidal estuary (Caboolture); a shallow, exposed bay with muddy substrates (Deception Bay); an exposed area of sandy substrates and seagrass (Toorbol Point) and a sheltered oceanic site with sandy substrates and seagrass (Kooringal). Data on diet, spawning seasons and recruitment periods of fry are presented together with measurements of salinity, temperature and turbidity. Species entering estuaries recruited mainly in summer (rainy season). The possible preference of juveniles for calm water, the roles of food and predation pressure, the effects of salinity, temperature and turbidity are discussed in relation to the biology and distribution of the fish. Salinity and temperature were probably not important to most juvenile fish. The effects of calm water, suitable food and predators vary according to species. Although all juveniles studied preferred shallow water, in the case of those entering estuaries, turbidity was the single most important factor. Juveniles of the same species occurred in both the estuary and Deception Bay where abiotic and biotic factors other than turbidity were different. During summer, turbidity gradients extended from east to west in Moreton Bay with highest turbidities in Caboolture estuary and Deception Bay. In winter, turbidities throughout Moreton Bay were low and relatively uniform. At this time many of the ‘clear water’ species occurred in Deception Bay. The influence of high turbidity on fish may be linked to reduced predation pressure and perhaps food supply in shallow water. Turbidity gradients in summer may aid fry in locating estuarine nursery grounds. It is apparent however, that juveniles of many species are probably not attracted to estuaries per se but to shallow turbid areas.     

Ontogenetic plasticity of food habits in the Mexican spiny-tailed iguana, Ctenosaura pectinata

Ontogenetic shifts from insect consumption by juveniles to plant consumption by adults are rare in the herbivorous lizard family Iguanidae. My investigations on diet and digestive tract anatomy of the iguanid lizard Ctenosaura pectinata show that this species has an ontogenetic diet shift. Insects were rare in adult diets but constituted 86.5% (by volume) of the food eaten by the smallest juveniles. All age classes ate some plant parts from a range of plant types, but flowers and leaves of legumes were a primarily food source. Non-adult lizards had the widest food niche breadths. Arthropods in the diet of juveniles and immatures covaried seasonally with the decline of arthropod abundance. Several hypotheses could explain this ontogenetic plasticity in diet. I rejected hypotheses that gut structure constrained juveniles to an arthropod diet and that insect consumption was purely an artifact of plant consumption because (1) size-adjusted gut morphology and capacity was similar among age classes, and (2) no food plants sampled had an excessive density of arthropods. I supported an alternative hypothesis that juveniles can eat plants but do not because insects provide a more nutritious diet. This conclusion was based on the observation that the juvenile hindgut is similar to that of herbivorous adults, and the propensity for juveniles to consume primarily, but not exclusively, insects when they were most abundant. The hindgut represents the site of fermentative plant fiber breakdown in many herbivorous lizards. Insect foods can compensate for size-related nutritional needs (energy and protein) and digestive limitations in juveniles. Opportunistic feeding to maintain a broad diet might help juvenile and immature lizards through high-predation-risk growth periods by reducing searching costs, increasing nutritional and energetic gains due to associative effects, and increasing new food exposure. Received: 20 January 1999 / Accepted: 25 January 2000     

The population structure and growth of juvenile Rhabdosargus holubi (Steindachner) (Teleostei: Sparidae) in a closed estuary

The population structure and growth rate of Rhabdosargus holubi (Steindachner) in the closed West Kleinemond estuary in south east Africa, from 1971–973, are described. Juvenile R. holubi invade the estuary at a length of 1–2 cm but migrate back to the sea at a length of about 14 cm without attaining maturity in the estuary. Adult R. holubi are rarely encountered in estuaries. The annual growth increment of O group R. holubi was 6 cm, almost all occurring in summer. The growth of Lithognathus lithognathus (Cuvier) was also measured in the same closed estuary and O group fish of this species also grew 6 cm per annum although 1 + fish grew 8 cm per annum.     

Habitat quality of estuarine nursery grounds: Integrating non-biological indicators and multilevel biological responses in Solea senegalensis

Estuaries are highly valuable ecosystems that provide various goods and services to society, such as food provision and supporting nursery habitats for various aquatic species. Estuarine habitat quality assessment is thus critical in managing both ecological and economic value. In this work, various biological and non-biological indicators of habitat quality in estuarine nursery areas were determined, encompassing local environmental conditions, chemical contamination, anthropogenic pressures, juvenile Solea senegalensis condition, biomarkers response to contamination and juvenile density. The various indicators provided an integrated view on habitat quality and their responses were broadly concordant. Nursery quality assessment based on anthropogenic pressure indicators and fish biomarker responses were very similar, signaling nursery areas with higher anthropogenic pressure in Tejo and Ria de Aveiro estuaries. Yet, favorable environmental conditions across all sites could have contributed to lessen the potential hazardous biological effects of exposure to anthropogenic stressors, resulting in soles’ fairly good condition and generally high juvenile density. Nevertheless, a mismatch between high juvenile density and high estuarine contribution to adult coastal populations was observed in areas with higher anthropogenic pressures. Although a causal relationship cannot be established, the results emphasize the need to fully understand how the estuarine period spent in estuaries and local processes determine the quantity and quality of juveniles exported to marine adult populations, which is critical to achieve the full potential of the fish production service of estuaries and coastal stock replenishment.     

Recruiting juvenile damselfish: the process of recruiting into adult colonies in the damselfish Stegastes nigricans

Juveniles of Stegastes nigricans occur in adult colonies, solitarily, and occasionally in juvenile colonies. We concentrated on solitary juveniles and those in adult colonies. We examined the costs and benefits of different settlement strategies, quantified the territory requirements of adults, and investigated the process of how juveniles make the transition to adult territorial fish. An adequate adult territory lies next to those of other adults, is proportional in area to the size of the adult, and contains a refuge tunnel whose entrance is sufficiently large. Compared with solitary juveniles, those <4 cm total length inhabiting adult colonies experienced reduced heterospecific competition for algal food and consequently benefited from a greater density of algae. A cost of recruiting into an adult colony, however, was increased attacks by adults. Juveniles that settled in adult colonies avoided attacks by retreating into small holes inaccessible to adults. As juveniles in adult colonies grew, they were chased less often by adults, whereas they themselves chased adults and heterospecific fish more often. Because territory size correlated with fish size in adult colonies, its area had to expand as the young fish grew, and that expansion was done at the expense of neighbors. Obtaining the space needed by an adult may be possible only when the juvenile settles directly into an adult colony. Juveniles that first settle down solitarily, or in juvenile colonies, may later attempt to enter adult colonies. However, because they do so as larger juveniles, they would have difficulty insinuating themselves into small refuges, which is essential for retreat from the adults. Received in revised form: 4 January 2001 Electronic Publication     

Effects of altered estuarine submerged macrophyte bed cover on the omnivorous Cape stumpnose Rhabdosargus holubi

The ecological importance of submerged macrophyte beds to fishes within estuaries was investigated through the example of the ubiquitous Cape stumpnose Rhabdosargus holubi, an omnivorous, vegetation and estuary-dependent species, using stable-isotope techniques and long-term abundance (catch-per-unit-effort) data from the East Kleinemonde Estuary, South Africa. Outputs from a Bayesian mixing model using δ(13) C and δ(15) N signatures indicated that the submerged macrophytes Ruppia cirrhosa and Potamogeton pectinatus were not a primary source of nutrition for R. holubi, confirming previous work that revealed that macrophytes are consumed but not digested. Long-term seine netting data showed reduced abundance of R. holubi during a prolonged period of macrophyte senescence, suggesting that submerged macrophyte habitats provide shelter that reduces mortality (predation risk) and a food-rich foraging area.     

Juvenile sparids (Rhabdosargus holubi) consistently select structurally dense vegetated habitat in nursery seascapes

Seagrass habitats provide structural complexity in coastal estuarine and marine environments, which offer fish optimal foraging grounds and refuge from predation. However, seagrasses are some of the most threatened ecosystems globally, with anthropogenic activities such as population growth and environmental degradation leading to the fragmentation, thinning, and loss of these habitats. Rhabdosargus holubi is one of only a few vegetation-associated marine fish species in South African estuaries. Although field studies have shown a strong association with seagrass over other aquatic vegetation for the juveniles of this species, habitat choice has never been empirically tested. Here, we used artificial vegetation units to test habitat choice (different structural complexities) for this species. We also tested whether habitat choice is influenced by a predatory threat, with fish preferentially selecting dense habitat in the presence of a predator and whether this effect may be more apparent in smaller individuals. We found that R. holubi significantly prefer greater structural complexity over less complex habitats, in both the absence and presence of a predator and for both small and large juveniles, showing that R. holubi actively choose more complex structures and are attracted to the structure per se irrespective of the threat of predation. This study highlights the importance of dense seagrass as nursery areas for this species and demonstrates how the loss of these habitats could impact the nursery function of estuaries.     

Feeding selectivity of four species of sympatric stickleback in brackish-water habitats in eastern Canada

Although sympatric populations of sticklebacks occur commonly along the north-eastern coast of North America, very few studies have considered the interspecific relationships within such populations. This paper investigates prey selectivity, feeding behaviour and potential prey availability within sympatric populations from five brackish-water sites in New Brunswick. Differences between the diets of species from different sites were related to differences in the range of prey available. Fish collected from sites with abundant aquatic vegetation consumed greater numbers of prey than those from vegetation-poor sites. The proportions of prey types eaten by both adults and juveniles reflected prey species abundance at the different sites; thus shifts in diet over time were found to be related to changes in the abundance of potential prey and not to shifts in prey preference on the part of the fish. The size of prey consumed was related to mouth size; adult and juvenile Gasterosteus wheatlandi , having the smallest mouths, consumed the smallest prey. Differences in prey size detected in the four species studied were due to differences in the types of prey consumed. Laboratory experiments on feeding behaviour showed that adult Apeltes quadracus and Pungitius pungitius were more efficient at capturing benthic prey than were Gasterosteus aculeatus and G. wheatlandi. Competition for food is thought not to occur in these populations, because of the abundance of potential prey and the morphological constraints on feeding behaviour.     

Ontogenetic niche shifts in two populations of juvenile threespine stickleback, Gasterosteus aculeatus, that differ in pelvic spine morphology

Most research on ontogenetic niche shifts has focused on changes in habitat or resource use related to food resource distribution and heterospecific size-limited predation. Cannibalism, an intraspecific interaction, can also affect habitat selection or resource use by vulnerable size classes. Morphological defenses, such as spines, increase the effective size of an individual, making it more difficult to consume. The importance of such defense structures in affecting niche shifts in early life history stages is unclear. Using a combination of field observations and experiments in aquaria and wading pools, we examined the relative roles of cannibalism and morphology in determining juvenile habitat use in two populations of threespine stickleback that differ in pelvic spine morphology. Juveniles were categorized into three size classes: small (5–10 mm), medium (11–15 mm), and large (15–25 mm). In experiments assessing the relative vulnerability of juveniles to cannibalism by adults, we documented a significant difference among size classes in the number of juveniles eaten such that more large juveniles were eaten from the population lacking pelvic spines. The natural distribution of small and large juveniles in two distinct littoral microhabitats, open water and vegetation, was determined in each lake. In both populations, small juveniles were more abundant in vegetation. In the population with pelvic spines, a greater proportion of large juveniles was observed in open water than in vegetation. In the population without pelvic spines, the proportion of large juveniles did not differ between the two habitats. Experiments comparing juvenile habitat use in the presence or absence of adult conspecifics suggest that differences in habitat use may not only depend on the size of the individual, or the size of the individual relative to the size of the adult predator, but also on the degree of development or expression of defensive structures.     

PRELIMINARY OBSERVATIONS ON FOOD HAWITS OF GERENUK IN TSAVO NATIONAL PARK, KENYA

In the context of widespread vegetation changes in Tsavo National Park, food habits of gerenuk are being investigated; the present paper gives an account of the results obtained during the first year of the study. Feeding of free-ranging gerenuk was observed from a landrover in two study areas with different vegetation types. Plant species eaten were determined in the field or collected for identification. Gerenuk feed almost exclusively on leaves, shoots, flowers, and a few fruits of trees and shrubs, as well as some climbers and vines, the latter mainly during the rainy seasons. No small herbs or grass were found to be eaten. The composition of the diet differed considerably between the two study areas, largely as a result of differences in the available vegetation. Seasonal variations in the diet are brought about mainly by variations in the availability of different food plants, notably the ratio between deciduous and evergreen species at different seasons. On the basis of a preliminary quantitative survey of the vegetation in the study areas, actual preferences have been evaluated. Some common plant species were found to be rejected altogether, although they are eaten by other browsing animals. A total of 68 plant species has so far been recorded as food plants, indicating that gerenuk are able to utilize a wide range of plants. In view of this adaptability, and of the abundance of preferred food plants in the study areas, the vegetation changes in Tsavo National Park do not, at present, appear to endanger the continued existence of the gerenuk.     

Abundance and species richness as a function of food resources and vegetation structure: juvenile fish assemblages in rivers

Availability of food and habitat complexity are two factors generally invoked to explain the high density of fish in vegetated habitats. The role of food resources (zooplankton) and habitat complexity (expressed by a vegetation structural index) in determining juvenile fish abundance and fish species richness in three morphologically contrasted macrophyte types (Sagittaria, Ceratophyllum and Nuphar) was studied for a large, lowland river.
Our results showed that fish abundance increased with food availability, and was maximal for intermediate values of vegetation complexity. Food resources and vegetation complexity did not, however, explain the higher juvenile fish abundance observed in Sagittaria beds. We suggested that plant growth form, acting on fish foraging success and risk of predation, might influence patterns of juvenile fish distribution.
Species‐abundance relationships were similar among the three macrophyte types, but relationships between number of fish species (fish richness) and number of samples differed. Fish richness in terms of total number of fish species found at each sampling point showed the same pattern as for fish abundance: it increased with food availability and was highest at intermediate vegetation complexities. However, since both fish abundance and fish richness responded in the same manner to food availability and vegetation complexity, we were not able to distinguish statistically any effect for the specific fish richness formulated by the number of fish species per unit fish abundance. The current paradigm that structural complexity of vegetation provides a wider range of niches, increasing species diversity, was thus not verified. This finding indicates a simple species‐abundance relationship (the passive sampling hypothesis), and suggests that no special mechanism acts directly on fish species richness.     

FISH CONSERVATION IN SOUTH AFRICAN ESTUARIES: PRESSURES,PROBLEMS AND PROSPECTS

Summary

A review of the conservation status of fish in the estuarine environment around the South African coastline reveals that some species face serious problems associated either with habitat destruction, and its associated biological, physical and chemical components or exploitation. The 65 species considered fall into three categories; truly estuarine species, which are dependent on estuaries for their entire life; marine species dependent on estuaries during the juvenile phase of their life cycle; and marine species whose juveniles occur mainly in estuaries but are also found at sea. Included in the first two categories are 14 species of fish which are on the South African Red Data list. These comprise one species which is endangered, five which are vulnerable and eight which are rare. All groups are considered in relation to factors in estuaries which are affecting their populations. A conservation strategy is suggested for certain estuarine types or for specific estuaries which could ensure the survival of the entire range of estuarine faunas.     

Microhabitat use by the rainbowfish <Emphasis Type="Italic">Melanotaenia duboulayi</Emphasis> in a subtropical Australian stream

Microhabitat use and feeding behavior of the rainbowfish Melanotaenia duboulayi (Castelnau) were investigated in a slow-flowing stream adjacent to riparian forest in south-eastern Queensland, Australia. Fish were more abundant in vegetated areas, but did not enter dense Vallisneria beds, where predators were observed. In sunny conditions shoals of juveniles occurred near the water surface feeding floating material on the surface, but larger fish tended to occur at the bottom near submerged vegetation, often utilizing the overhanging aquatic plant community as a refuge and food source. In the middle of the day, juveniles and small fish seemed to show behavioral thermoregulation at the surface in the warmest site. Under cloudy conditions, however, fish of all sizes preferred deeper water. The present study suggests that in still and sunny pools thermal change caused by sunlight influences the microhabitat choice of small fish. A field experiment using a kingfisher model implies that fish swimming at the surface could escape from aerial predators in sunlit conditions by responding to moving shadows, but could not do so under cloudy conditions. Electronic Publication     

Comparison of microridges in juvenile and adult sunfish,Lepomis gibbosus

Microridges are F-actin-based surface protrusions of the superficial layer cells of fish epidermis. Microridge patterns progress in complexity during fish embryogenesis, often transitioning from abundant surface microvilli to the classical fingerprint arrangement. This progression suggests pattern changes may also occur during later stages of fish development. Fluorescent labelling of F-actin and morphometric analysis were therefore used to assess changes in epidermal microridge patterns in juvenile and adult sunfish (Lepomis gibbosus). The microridge patterns found in adult pumpkinseed were similar to that described for many fishes, consisting of whorls or complex multi-branched ridges. The microridge patterns of the scales from three different-sized groups of juvenile pumpkinseed were distinctly different from that of adult, however, and were present mainly as unbranched concentric or nearly concentric rings in the two larger juvenile groups. In the smallest juveniles, microridges were often apparent as fragmented ridges with abundant actin puncta. Larger juveniles sometimes displayed mixed patterns, with some microridges similar to that of both adult and juvenile patterns. The results show a transition from simple microridge patterns in juvenile pumpkinseeds to distinctly different, diverse and more complex patterns in adults. The different pattern types may reflect particular microridge functions relevant to fish size and age.     

Endogenous swimming rhythms of juvenile blue crabs, Callinectes sapidus, as related to horizontal transport

The blue crab Callinectes sapidus settles and metamorphoses in areas of aquatic vegetation in estuaries. Crabs in the first-fifth instar stages (J1-5) then emigrate from these areas by walking on the bottom or pelagic dispersal throughout estuaries. The present study was designed to characterize the timing of this migration pattern relative to the light-dark and tidal cycles. Field sampling in Pamlico Sound, NC, USA indicated that J4-5 juveniles were most abundant in the water column during the night. J4-5 juveniles were collected from Pamlico Sound in an area near Oregon Inlet that has semi-diurnal tides, a Mid-Sound area where tides are weak, and on the West side where regular tides do not occur. Crabs from all three sites had a circadian rhythm in which they swam up in the water column during the time of darkness in the field. Peak swimming consistently occurred at about 0300 h, but was not related to the timing of the tidal cycle. Similar results were obtained for juvenile crabs from an adjacent estuary having semi-diurnal tides. Dispersal at night reduces predation by visual predators, and allows early juvenile blue crabs to disperse planktonically from initial settlement sites.     

Social enhancement of foraging on an ephemeral food source in juvenile walleye pollock,Theragra chalcogramma

Synopsis The foraging effectiveness of walleye pollock juveniles, Theragra chalcogramma, was determined experimentally to test the hypothesis that social cues may facilitate the ability of individuals to exploit ephemeral food patches. Fish were tested when isolated, paired with one other fish, and in a group of six fish. Test fish exploited more food patches while in a group of six than when they were isolated. Patch exploitation by paired fish was intermediate to but not statistically different than isolate or grouped treatments. The number of pellets eaten by test fish in a group and a pair was more than 3.5 times that of when they were isolated, although the overall relationship between the amount of food eaten and group size was not statistically significant. Results support the hypothesis that juvenile walleye pollock exploit ephemeral food patches more effectively in the presence of conspecifics. In planktivores such as walleye pollock, social cues may enhance foraging on transient food sources either by facilitating detection of food patches (local enhancement) or by stimulating foraging activity when a food patch is located (social facilitation).     

Connectivity between estuaries and marine environment: Integrating metrics to assess estuarine nursery function

Natural or anthropogenic induced variations in estuaries and the dynamics of marine fish populations potentially promote differences in connectivity between estuaries and marine areas, i.e. in their importance as nursery grounds. Within this context, an integrated assessment of the differential nursery function of the main estuaries along the Portuguese coast for commercial fish species common sole Solea solea, Senegalese sole Solea senegalensis, flounder Platichthys flesus and sea bass Dicentrarchus labrax was performed through several indicators based on available data. Contribution of individual estuaries to marine subpopulations was measured with potential metrics (juvenile density, habitat quantity, juvenile number and habitat quality within estuaries) and effective metrics (estuarine source of young adults in marine environment measured via otolith elemental fingerprints). The relationship between the two types of metrics was also assessed. Estuaries identified as important nursery and/or effective juvenile habitat (EJH) differed with species and no single estuary was best for all, highlighting species-specific regulation of nursery function. Multiple species assessment of nursery and EJH function differed among estuaries. Management and conservation of estuaries should focus on sites with higher contributions to adult subpopulations of multiple species. The importance of defining precise scientific and management objectives was emphasized by the different rankings of estuaries obtained with nursery or EJH criteria. Potential and effective contribution of estuaries were not significantly correlated, but in a quantitative analysis juvenile densities and number of juveniles seem related with effective contribution in some species. An agreement between potential and effective contributions of estuaries is concurrent with the acknowledged minor role of juvenile stage processes in regulation of recruitment to adult subpopulations.     

Trophic interactions among native and introduced fishes in a northern Patagonian oligotrophic lake

In summer the littoral zone of Lake Rivadavia contained almost all the juveniles of the native fish species present in the lake together with juveniles of the introduced salmonid Oncorhynchus mykiss . In particular, a high degree of foraging interaction was found among juveniles of the native species Aplochiton zebra and Percichthys trucha and juveniles O. mykiss. Differences in the juvenile assemblage composition, both in the presence and in the relative proportion of the species were associated with areas with different densities of vegetation. The adults of nearly all species were generalized benthic invertebrate predators. Only adults of A. zebra were absent in the littoral. Juvenile and adult O. mykiss were the only fish that preyed on planktonic organisms. Oncorhynchus mykiss was the only species that performed an ontogenetic diet shift between juveniles and adults, changing main prey categories from benthic to planktonic species.     

Observations on the ontogeny of feeding behavior in mountain gorillas (Gorilla gorilla beringei)

This report describes observations on the ontogeny of food choice in mountain gorillas (Gorilla gorilla beringei), made during a 17-month field study of mountain gorilla feeding ecology in the Parc National des Volcans, Rwanda. Data are presented on the feeding behavior of two infants observed from birth to the age of 8 months and on older infants and juveniles. This information is compared with data on the composition and diversity of the diets of young adults and adults in the same social group. Initial feeding by infants is usually synchronized with the mother's behavior: infants ingest the same food, or a different part of the same plant species, currently being eaten by the mother or just eaten by her. This suggests that observational learning is largely responsible for the transmission of food preferences. Most feeding by young infants, whether or not synchronized with the mother's, is on those foods eaten most frequently by adults. Infants also independently sample potential foods, some of which are apparently not consumed by adults. The frequency of sampling declines with age, although even adults occasionally ingest foods not observed to be eaten by other adults. By the age of 3 years, young mountain gorillas have developed the basic dietary patterns of adults, in terms of the number of foods eaten, the proportions in which specific foods are consumed, and diet diversity and equitability. There is a strong possibility that chemical cues influence food choice, but their role remains unclear.