Nadine A. Strydom

Description and ecology of larvae of Glossogobius callidus and Redigobius dewaali (Gobiidae) from temperate South African estuaries

ABSTRACT This paper describes the morphology and ecology of the larvae and early juveniles of two common gobiids in warm temperate South African estuaries. The early developmental stages of Glossogobius callidus and Redigobius dewaali were collected during plankton surveys in seven permanently open and five intermittently open estuaries along southeastern South Africa. Larval G. callidus have a characteristic ventral row of pigment that extends from the cleithral symphysis along the ventral midline of the tail to the end of the caudal peduncle; notochord flexion takes place at the yolk-sac stage, between 3 and 4 mm BL. Larval R. dewaali are characterized by having two very large stellate melanophores on the ventral surface of the trunk and tail, and internal pigment between the snout and hindgut; notochord flexion takes place between 4 and 6 mm. Body lengths at settlement for G. callidus and R. dewaali are 13 mm and 16 mm, respectively. G. callidus and R. dewaali dominate the larval gobiid catch in warm temperate estuaries. Estuary type and freshwater input played a defining role in the occurrence of larvae of these, often sympatric, species in temperate estuaries. Larval G. callidus occurred in both permanently open and intermittently open estuaries, and were most abundant in mesohaline regions over the spring/summer period. By contrast, larval R. dewaali occurred exclusively in permanently open estuaries and were more prevalent in the fresh and oligohaline regions in autumn. Larvae of both species were more prevalent in freshwater-rich estuaries. Densities of G. callidus and R. dewaali peaked at 101 and 3829 larvae per 100 m3 respectively. The usefulness of using the early stages of these gobiids as indicators of estuarine health based on their specific and often prolific estuarine occurrence is discussed.

Diet of Largemouth Bass, Micropterus salmoides (Centrarchidae), an Invasive Alien in the Lower Reaches of an Eastern Cape River, South Africa

Largemouth bass (Micropterus salmoides) have been introduced to many South African river systems where they become invasive and pose a threat to native biota. The diets of small (32–138 mm TL) and large (192–448 mm TL) sized bass were analysed and compared in a marine fish nursery area in the lower Kowie River on the warm temperate coast of South Africa over a one-year period from March 2009 to February 2010. Dietary differences were detected between the two size groups. Amphipod sp. (% index of relative importance (IRI) = 69.2) and dipterans (Insecta) (%IRI = 21.9) dominated gut contents of small bass while larger bass preyed mostly on odonates (Insecta) (%IRI = 16.3) and the brachyuran Potamonautes sidneyi (%IRI = 80.0). Fish prey was of low importance during this study but comparisons with previous work on the lower Kowie River showed that when the river is flowing, young marine fish recruiting into the freshwater from the estuary become important prey items. These data suggest that in the lower Kowie River bass utilize invertebrate prey at low fish prey abundance and opportunistically feed on migrant fish when these are available.

Utilization of shallow subtidal bays associated with warm temperate rocky shores by the late-stage larvae of some inshore fish species, South Africa

ABSTRACT This work describes the utilization of shallow, subtidal bays, associated with rocky shores, by the late-stage larvae and early juveniles of some common inshore fishes. Larval and early juvenile fishes were collected in seven small bays (30–50 m wide) between Port Alfred (33°36′S, 26°53′E) and Hamburg (33°17′S, 27°29′E), using a modified, beach seine net (500μm mesh). Four bays were sampled weekly during peak fish spawning (September-November 2004) to firstly assess whether larval fishes occur in these previously-unstudied habitats and secondly, to assess the extent of this utilization in terms of species occurrence, abundance and potential nursery function. An additional three replicate bays, situated > 50 km away near Bira were used for comparison. In total, 14 fish families represented by 26 taxa were recorded but the catch was dominated by a small suite of common species. Bays were extensively utilized by Sparidae (Diplodus capensis and Sarpa salpa), and to a lesser extent by Clinidae (Clinus superciliosus), Mugilidae (Liza spp.) and Kyphosidae (Neoscorpis lithophilus). Mean CPUE of larvae was high and exceeded 900 individuals per seine haul. Larval abundance peaked at 4000 larvae per haul and at these times, was dominated only by the two sparid species. Abundance of sparid larvae exceeds that recorded for these species in surf zones, estuaries or the shallow nearshore habitats sampled to date in temperate South Africa by an order of magnitude. Larval abundance was significantly higher during new moon sampling throughout the study period, but was not significantly different between repeated samples collected per site on each fieldtrip. The length-frequency of larvae utilizing the bays was dominated by postflexion stages ranging 7–14 mm SL. Length-frequency distribution of Diplodus, Sarpa, Clinus, Sparodon, Neoscorpis was indicative of grow-out and use of bays as interim nurseries. The nursery function of these bays is supported by gut analysis of a sub-sample of the dominant species, Diplodus capensis, which showed that these larvae were actively feeding in the area, predominantly on copepods. This work prompts further research on shallow habitats associated with rocky shores particularly the shelter and feeding role that these under-studied habitats are playing in the early life history of coastal fishes.

Larval Fish Dynamics in the Nxaxo-Ngqusi Estuary Complex in the Warm Temperate—Subtropical Transition Zone of South Africa

The larval fish assemblage was investigated in the permanently open Nxaxo-Ngqusi Estuary Complex, situated in the subtropical—warm temperate biogeographic boundary zone of South Africa. Larval and early juvenile fishes were collected in summer and winter of 2007 and 2008 using a mixed method approach by boat-based plankton tows in mid-channel waters and seine net hauls along the margins. In total, 20 455 fishes were collected representing 21 families and 40 taxa, of which 13 900 fishes were from mid-channel sites (15 families and 21 taxa). Seine net hauls along the estuary margins produced 6550 fishes representing 18 families and 33 taxa. Gobiidae was the dominant fish family in the mid-channel waters followed by the Clupeidae whereas the opposite was found in marginal waters. Estuary-resident larvae were the dominant group in the estuary complex. The catches typically reflected a more warm-temperate community, although subtropical-associated species were present. Fish density in mid-channel waters was higher in the Ngqusi arm than that of the Nxaxo arm, whereas along the marginal waters, CPUE was higher in the latter. Fish densities in mid-channel waters and CPUE along the margins were typically higher in summer.

Non-reliance of metazoans on stromatolite-forming microbial mats as a food resource

Grazing and burrowing organisms usually homogenise microalgal mats that form on benthic sediments of many aquatic ecosystems. In the absence of this disruption, microalgal mats can accrete laminated deposits (stromatolites). Stromatolites are rare in modern coastal ecosystems, but persist at locations where metazoans are largely excluded. This study aimed to assess the trophic structure at stromatolite locations where metazoans co-occur, to determine the grazing influence exerted by the metazoans on the stromatolite-forming microalgae (cyanobacteria and diatoms). Stable isotope signatures (δ13C and δ15N) were used as food-web tracers and dietary composition of consumers was calculated using source mixing models. Results clearly demonstrate that the dominant macrofaunal grazers do not utilise stromatolite material as a food resource, but rather subsist on autochthonous macroalgae. For instance, the mean (±SD) dietary composition of two of the most abundant grazers, Melita zeylanica (Amphipoda) and Composetia cf. keiskama (Polychaeta), consisted of 80 ± 11% and 91 ± 7% macroalgae, respectively. This suggests that the stromatolite-forming benthic microalgae are not disrupted significantly by grazing pressures, allowing for the layered mineralisation process to perpetuate. Additionally, grazers likely have a restrictive influence on pool macroalgae, maintaining the competitive balance between micro- and macroalgal groups.

Fish Utilization of Surf-Zones. Are They changing? A Case Study of the Sheltered, Warm-Temperate King's Beach

Surf-zone fish communities and their shifts over time are generally poorly understood. The aim of this study was to compare the current surf-zone fish assemblage at Kings Beach, South Africa, to a similar study conducted three decades ago, before the collapse of many exploited shore fishes in the region. Beach seine nets (mesh sizes of 10 and 50 mm) were used to target juvenile and adult fishes bimonthly from February to August 2011 over the high tide around sunset. A total of 14 species were recorded in both the 30 m and 100 m seine nets. The catch in these seine nets was dominated by Pomadasys olivaceus and Liza richardsonii, and this was significantly different to three decades ago, when P. olivaceus, Sarpa salpa and Diplodus capensis dominated the catch. Important linefish species belonging to the Sparidae and Sciaenidae families were significantly smaller and less abundant in this study. Two sparids, S. salpa and Lithognathus mormyrus, which made a large contribution to the surf-zone catch three decades ago were absent during this study. Reasons for the significant shifts in the surf-zone fish community, including overexploitation of the linefish and potential habitat modification, are discussed.

Dynamics of recruitment of larval and juvenile Cape stumpnose Rhabdosargus holubi (Teleostei: Sparidae) into the Swartkops and Sundays estuaries, South Africa

Recruitment of early life stages into estuaries is an integral part of the life cycle of many marine fish species. Although estuaries are naturally environmentally dynamic, they also are subject to anthropogenic disturbances, including land use and climate change, which may affect recruitment. Rhabdosargus holubi is an endemic marine-spawning species predominantly associated with freshwater-rich estuaries which serve as nursery areas for postflexion larvae and juveniles. This study assessed the effect of environmental variables on the dynamics of recruitment of R. holubi larvae and juveniles into the Swartkops and Sundays estuaries, South Africa. Over a period of two years, fyke nets were set at each estuary mouth to monitor movement into the estuaries at each tidal phase over a 24 h cycle during two sampling sessions per season. Rhabdosargus holubi larvae recruited into estuaries primarily in summer and autumn and during the ebb tide at night, while juvenile movements showed no pattern. Salinity, turbidity and temperature were significantly important factors affecting R. holubi recruitment, with pH having no significant effect. Turbidity and salinity are affected by rainfall and freshwater abstraction which may thus influence recruitment. Rhabdosargus holubi is dependent on estuaries, so a combination of future changes in turbidity, salinity and temperature due to predicted climate change may place populations at risk.

Diet and prey selection in late-stage larvae of five species of fish in a temperate estuarine nursery

The plankton assemblage of the permanently open Sundays Estuary in South Africa was sampled seasonally to provide further information on the feeding ecology of the larval stages of fishes in temperate estuaries. Collections were made between winter 2007 and autumn 2009 at eight sites by means of boat-based plankton tows. Physico-chemical variability in the water column was typical for temperate systems. Mean density of larval fishes and zooplankton showed similar seasonal and spatial trends. Gut content analyses were conducted in five dominant larval fish species: Gilchristella aestuaria (Clupeidae), Pomadasys commersonnii (Haemulidae), Monodactylis falciformis (Monodactylidae), Myxus capensis (Mugilidae) and Rhabdosargus holubi (Sparidae). The first record of selective consumption of copepod eggs by G. aestuaria is reported. Insect larvae (chironimids) dominated the diet of some marine fish larvae (M. falciformis and M. capensis). The copepod Pseudodiaptomus hessei formed the largest volume of the diet of P. commersonnii whereas R. holubi selected for the amphipod Corophium triaenonyx, with P. hessei also being a frequently consumed prey item. Seasonal and spatial variability with salinity was evident in the diet. Larvae also showed changes in diet at different stages of development. Pelagic prey species constituted the largest volume of the diet of G. aestuaria whereas all other larvae consumed large volumes of bentho-pelagic prey. All larvae selected P. hessei as a prey item. Myxus capensis and R. holubi also selected this copepod and showed the largest diversity in diet.

Composition, Abundance, Distribution and Seasonality of Larval Fishes in the Sundays Estuary, South Africa

The larval fish assemblage was studied in the permanently open Sundays Estuary on the southeast coast of South Africa. Seasonal samples were collected between winter 2007 and autumn 2009 at eight sites along the estuary by means of boat-based plankton tows using two modified Working Party 2 (WP2) nets. Salinity ranged from 1.5–36.5 and temperature from 11.6–26.8°C during the study period. A total of 8174 larval and early juvenile fishes were caught, representing 12 families and 23 taxa. The Clupeidae, Gobiidae and Blenniidae were the numerically dominant fish families. Common species included Gilchristella aestuarias (Clupeidae), Omobranchus woodi (Blenniidae), Caffrogobius gilchristi, Glossogobius callidus (Gobiidae), Liza dumerilii and Myxus capensis (Mugilidae), largely reflecting the adult assemblage of the estuary. Catches varied significantly between seasons, but not between sites and salinity zones within the estuary. Highest catches were recorded in summer (mean of 464 and 928 larvae per 100 m3 in 2008 and 2009, respectively). Species diversity also varied seasonally, with highest diversity occurring during summer. Highest larval fish density occurred in the euhaline zone while the oligohaline zone supported the highest diversity. Estuarine-resident species (category I) dominated the system (91%). The preflexion stage of development dominated the catches suggesting a high degree of local production in the Sundays Estuary. Larval sizes of the estuarine-resident G. aestuarias and catadromous species M. capensis tended to increase towards the middle and upper reaches of the estuary. Important variables regulating larval fish dynamics in the Sundays Estuary included salinity, temperature and turbidity.

Evidence for Habitat Residency and Isotopic Niche Partitioning in a Marine-Estuarine-Dependent Species Associated with Mangrove Habitats from the East Coast of South Africa

Estuaries are valuable fish nurseries due, primarily, to an abundance of food and the provision of shelter in a spatially heterogeneous area. Habitats within estuaries, however, vary in their specific supply of these two factors with more structurally complex habitats tending to offer more of both food and refuge. In this study, we investigated the site residency of an abundant estuarine-dependent sparid, Rhabdosargus holubi, from two nearby but dissimilar mangrove habitats using two approaches: mark-recapture and stable isotope analysis. Results showed that rates of residency for both short- (in the order of days) and long-term (weeks) were high. Stable isotope ratios (carbon and nitrogen) were also used to compare the isotopic niche of two size groups between the two mangrove habitats. Both small and larger juveniles from a more complex creek associated with red mangroves and seagrass had a significantly broader isotopic niche width when compared with the groups from the more homogenous white mangrove habitat. R. holubi appear to establish strong residency at sites soon after recruitment which may persist until their departure from the estuary despite apparent differences in habitat quality.