Physiological responses of a juvenile marine estuarine-dependent fish (Family Sparidae) to changing salinity.

Estuaries are subject to high environmental variability coupled to tidal salinity shifts. Under restricted freshwater flow and prolonged drought conditions, salinity may exceed natural ranges and thus organisms may experience stressful hypersaline conditions. This study assessed the physiology of a juvenile marine estuarine-dependent species Rhabdosargus holubi (Family: Sparidae) under changing salinity to determine the impact on respiration and survival under shock and acclimatisation exposures. Oxygen consumption was not significantly different in the 2.5 to 45 salinity range and the interaction between temperature and salinity was not significant at the moderate levels tested. This confirmed the strong osmoregulatory capabilities of marine estuarine-dependent R. holubi. However, respiration was impacted at salinities of 55-62.5. The salinity tolerance ranges of R. holubi were expanded beyond those previously observed in the laboratory, to a maximum of 77 when fish were gradually exposed to daily 20% incremental changes. This indicated the ability to adapt to hypersaline conditions that occur gradually in anthropogenically altered estuaries which is an important aspect for management decisions regarding freshwater inputs. The adaptability of Rhabdosargus holubi to hypersaline conditions in estuaries may thereby ensure the maintenance of populations in the short term under certain environmental conditions, such as those currently occurring in a freshwater-scarce South Africa. However, in the long term and at more extreme salinities (> 55), the physiology of R. holubi and similar species may be compromised, thus placing the species at risk.

Comparing the metallic elemental compositions of Kelp Gull Larus dominicanus eggs and eggshells from the Swartkops Estuary, Port Elizabeth, South Africa.

Metals attributed to pollution may increase their concentrations above the geological background and pose toxic challenges towards humans and biota. We analysed sixteen Kelp Gull eggs and eggshells for 30 metallic elements from the Swartkops Estuary (SE), an important recreational, industrial, and ecological asset for Port Elizabeth, the region, and South Africa. Mean concentrations for eggshell and egg content for Hg was 0.02 and 0.4 mg/kg dm, Cr was 4 and 18 mg/kg dm (the highest yet recorded for any gull or tern egg), for Zn 2.1 and 62 mg/kg dm, for Sr 880 and 12 mg/kg dm, for V 170 and 1.3 mg/kg dm, and for Co 1.7 and 0.002 mg/kg dm, respectively. Zinc, Se, and Hg, increased on a dry-mass basis from sediment via small fish to gull egg content, indicating bioaccumulation. No effect on eggshell thickness was seen. We also determined that eggshell concentrations cannot be used as a proxy for egg content concentrations. Mercury, Cr, V, Co, and Zn were elements we identified as potentially problematic that require source identification and mitigation. Further research into other high-trophic animals such as herons, egrets, cormorants, and otters in the SE system is proposed.

Variability in spatial and temporal occurrence of presettlement and settlement-stage fishes associated with shallow reefs.

Presettlement and settlement-stage fishes were studied in a large, log-spiral bay in temperate South Africa. The aim was to describe the assemblage composition, density and distribution associated with four types of habitats common to the bay: high profile reef, low profile reef, reef-associated sand and open sand spatially separated from reef. Samples were collected with both a plankton ring net and a light trap at each habitat type as part of a mixed-method approach. A total of 4084 presettlement and settlement-stage fishes belonging to 31 teleost families and 84 species were captured. Reef-associated sand and open sand habitats yielded higher species richness and diversity than the high and low-profile reef habitats. Engraulidae, Gobiidae, Clupeidae and Cynoglossidae were the dominant fish families captured with the ring net, while Engraulidae, Clupeidae, Carangidae and Clinidae were captured with the light trap. A temporal difference in the abundance of presettlement fishes occurred between the sampling periods with highest values recorded during the summer settlement period. Habitat type together with associated physico-chemical variables played a pivotal role in determining presettlement fish species composition, density and distribution across habitat types.

Development and ossification of the feeding apparatus in the larvae of two co-occurring species of kob (Sciaenidae), Argyrosomus japonicus and Argyrosomus inodorus, in South Africa.

The teeth of the oral jaws of two sympatric species of Argyrosomus, Argyrosomus japonicus and Argyrosomus inodorus, found along the South African coast developed first on the premaxilla and then on the dentary of the lower jaw. Teeth were observed on the premaxilla of A. inodorus [head length (LH) = 1·0 mm; notochord length (LN) = 2·7 mm] at a smaller size than in A. japonicus (LH = 1·2 mm; LN = 4·7 mm). The ventral elements of the gill arches (hypo- and basibranchials) were not ossified by the end of preflexion. The fifth ceratobranchial began ossifying and possessed pharyngeal teeth by 1·2 mm LH (LN = 4·7 mm) in A. japonicus and 1·1 mm LH (LN = 3·2 mm) in A. inodorus. To complement the osteological data, stomach contents were also analysed as a proxy for feeding apparatus functionality. Prey were first present in the stomach of A. japonicus at 1·2 mm LH (LN = 4·7 mm) and only 22% of the stomachs contained no prey suggesting that A. japonicus is already actively foraging by preflexion. In comparison, 83% of the stomachs of A. inodorus contained no prey and a single prey item was present in the largest examined specimen (LH = 1·6 mm; LN = 5·4 mm). Elements of the feeding apparatus begin to ossify early during ontogeny. While the overall pattern of ossification is similar between the two species, A. japonicus may be able to begin feeding at smaller head lengths relative to A. inodorus in their nursery habitats.

Preliminary assessment of contaminants in the sediment and organisms of the Swartkops Estuary, South Africa.

Urban estuaries are susceptible to metal and organic pollution, yet most remain understudied in South Africa with respect to the presence, concentrations and distribution of contaminants. Metal and organic chemical concentrations were assessed in sediment and organisms from different trophic levels in the lower reaches of the Swartkops Estuary. Species sampled included Upogebia africana (Malacostraca: Upogebiidae), Gilchristella aestuaria (Clupeidae), Psammogobius knysnaensis (Gobiidae), Mugil cephalus (Mugilidae), Lichia amia (Carangidae), Argyrosomus japonicus (Sciaenidae), Pomadasys commersonnii (Haemulidae) and Larus dominicanus (Avis: Laridae). This study is one of the most comprehensive studies to date assessing pollution levels in a food web in estuaries in South Africa. Due to biomagnification, higher concentrations of Arsenic, Lead, Mercury and Cadmium were found in the juveniles stages of popular angling fishes. High concentrations of Cadmium and Arsenic were recorded in the liver of L. amia, A. japonicus and P. commersonnii which exceed international quality food guidelines. Eggs from the gull, L. dominicanus, showed detectable concentrations of PCBs.

Larval fish variability in response to oceanographic features in a nearshore nursery area.

The influence of oceanographic features on ichthyoplankton assemblages in the warm temperate nearshore region of Algoa Bay, South Africa, was assessed. The nearshore ichthyoplankton comprised 88 taxa from 34 families. Samples were collected at six stations between August 2010 and July 2012 using a plankton ring net of 750 mm diameter and 500 µm mesh aperture. The majority of larvae collected were in a preflexion stage, indicating the potential importance of the nearshore for newly hatched larvae. Engraulidae dominated the catch (38·4%), followed by Cynoglossidae (28·1%) and Sparidae (8·4%). Larval fish abundance was highest during austral spring and summer (September to February). Unique patterns in responses of each dominant fish species to oceanographic features in the nearshore indicate the sensitivity of the early developmental stage to environmental variables. Using generalized linear models, ichthyoplankton abundance responded positively to upwelling and when warm water plumes originating from an Agulhas Current meander entered Algoa Bay. Highest abundances of Engraulis encrasicolus and Sardinops sagax were observed during Agulhas Plume intrusions into Algoa Bay. When a mixed and stratified water column persisted in the nearshore region of Algoa Bay, larval fish abundance decreased. The nearshore region of Algoa Bay appears to serve as a favourable environment for the accumulation of ichthyoplankton.

The effects of exposure in sandy beach surf zones on larval fishes.

The influence of wind and wave exposure on larval fish assemblages within a large bay system was investigated. Larval fishes were sampled from two areas with vastly different exposure to waves and wind, namely the windward and leeward sectors of Algoa Bay. In total, 5702 larval fishes were collected using a modified larval seine. Of these, 4391 were collected in the leeward and 1311 in the windward sector of the bay, representing a total of 23 families and 57 species. Dominant fish families included Clinidae, Engraulidae, Kyphosidae, Mugilidae, Soleidae and Sparidae, similar to the situation elsewhere, highlighting continuity in the composition of larval fish assemblages and the utilization of surf zones by a specific group of larval fishes. Nineteen estuary-associated marine species occurred within the surf zones of Algoa Bay and dominated catches (86·7%) in terms of abundance. Postflexion larvae comprised > 80% of the catch, indicating the importance of the seemingly inhospitable surf zone environment for the early life stages of many fish species. The greatest species diversity was observed within the windward sector of the bay. Distance-based linear modelling identified wave period as the environmental variable explaining the largest proportion of the significant variation in the larval fish assemblage. The physical disturbance generated by breaking waves could create a suitable environment for fish larvae, sheltered from predators and with an abundance of food resources.