Dusky Kob Argyrosomus Japonicus Research Articles

Marine predators segregate interspecifically by space and time in a sheltered coastal bay.

Marine predators are vital to the healthy functioning of coastal ecosystems, but to understand their roles, it is necessary to elucidate their movement ecology, particularly in relation to one another. A decade's worth of acoustic telemetry data (2011-2020) from Algoa Bay, South Africa, was investigated to determine how two mesopredatory species (teleosts: dusky kob Argyrosomus japonicus, n = 11, and leervis Lichia amia, n = 16) and two top predatory species (sharks: ragged-tooth sharks Carcharias taurus, n = 45, and white sharks Carcharodon carcharias, n = 31) used and shared this bay ecosystem. Multi-annual seasonal fidelity to the bay was exhibited by all species, but differences in residency were observed among species. Similarly, species used space in the bay differently-the teleosts moved less and had movements restricted to the central and western inshore regions of the bay. Conversely, the sharks roamed more, but detections were concentrated in the western part of the bay for C. taurus and in the eastern part of the bay for C. carcharias. Social network analysis showed that species segregated in space and time on a fine scale. However, there was some interaction observed between C. taurus, L. amia, and A. japonicus, but to varying degrees. This is likely because of strong habitat preferences exhibited by each species and predator-prey relationships between these predatory guilds. Results highlight that the sheltered marine Algoa Bay is a resource-rich environment, supporting multiple predators with different hunting strategies albeit similar prey preferences. Finally, these species are likely afforded some protection by the current Greater Addo Elephant National Park Marine Protected Area in the bay but are vulnerable to fishing pressure when they leave this ecosystem.

Incorporating estuarine-angler behaviour and delayed blood sampling into the rapid assessment of catch-and release angling on the iconic dusky kob Argyrosomus japonicus

Argyrosomus japonicus is arguably South Africa’s most important estuarine recreational and small-scale fishery species. Although juvenile A. japonicus predominate in estuarine environments, where catch-and-release angling is common, limited C&R studies have taken place. The aim of this study was to use angler-behaviour to robustly examine the physiological stress response, reflex impairment and short-term (12–36-hour) survival of A. japonicus following C&R angling. Observations of estuarine recreational angling informed three air exposure treatments based on minimum (10 s), mean (75 s) and maximum (240 s) observed times, for use in a controlled angling experiment. Based on a prior laboratory study, blood sampling was delayed 30–40 min post-capture to allow for peak accumulations of lactate and glucose. Long air exposure (240 s) predicted significantly higher blood glucose concentrations (ANOVA, p = 0.03) than short (10 s) exposure. Similarly, both long (p = 0.01) and moderate (75 s; p = 0.01) air exposure significantly predicted elevated blood lactate concentrations, when compared with short exposure. In terms of physical impairment, long air exposure (240 s) had a significant negative influence on the reflex response (cumulative link model, p = 0.01) of A. japonicus. An observed short-term mortality of 7.7% was primarily attributed to hooking injury. To reduce significant physiological and physical stress, it is proposed that anglers should aim to reduce air exposure times to below the observed mean of 75 s, and ideally to 10 s. For relevant and meaningful future C&R studies, we propound the incorporation of angler behavioural assessments, and the investigation of physiological stress responses, prior to designing field studies.

Bacterial microbiome of dusky kob Argyrosomus japonicus eggs and rearing water and the bacteriostatic effect of selected disinfectants

The dusky kob Argyrosomus japonicus is an emerging aquaculture species in South Africa. To support industry growth, existing hatcheries frequently sell or donate excess eggs to neighbouring farms, raising concern over the potential transfer of infectious disease agents. Chemical disinfectants are frequently used to prevent transmission of diseases, but their application has shown large variations between finfish species and very little is known about the efficacy and impact of disinfectants on dusky kob eggs or larvae. A thorough understanding of the bacterial communities associated with specific fish species can inform the need for specific chemical agents and guide a more targeted approach for disease control and treatment. Therefore, this study used next generation sequencing (NGS, 16S rDNA gene) and downstream bioinformatics analysis to describe the bacterial microbiome associated with the eggs and rearing water of dusky kob. Four chemical disinfectants [chloramine-T (C-T), povo‑iodine (PI), hydrogen peroxide (H2O2) & formaldehyde (CH2O)], applied at different concentrations and times, were then tested for their ability to disinfect eggs and seawater. The impact of each disinfectant on early larval development was also assessed. A total of 11,605 individual bacterial operational transcriptional units (OTUs) were identified from the dusky kob egg (7352 OTUs) and hatchery water (4253 OTUs) samples. Non-metric multidimensional scaling (NMDS) analysis revealed distinct differences in the composition of the microbiota associated with the egg and seawater samples. A moderate level of bacterial diversity was observed in all samples, with the seawater samples dominated by Rhodobacteraceae (65%) and Flavobacteriaceae (14%); whereas egg samples were dominated by Pseudoalteromonadaceae (49%), Vibrionaceae (27%) and Colwelliaceae (8%). A classical univariate statistical comparison identified 6 differentially abundant OTUs at phylum level, 12 at family level and 19 at genus level; with members of the family Vibrionaceae found to be significantly (P = 0.0011) more abundant on dusky kob eggs samples. All tested chemical disinfectants were capable of reducing the bacterial load in seawater in a dose dependent manner, with the number of culturable bacteria on eggs decreasing significantly following exposure to PI and CH2O for five minutes (43% and 88% reduction in CFUs, respectively), compared with C-T and H2O2 (3% & 10%). Similarly, PI and CH2O significantly decreased the culturability of Vibrio species on eggs (79% & 75%), compared with C-T and H2O2 (12% & 14%). None of the disinfectants appeared to have any adverse impact on larval growth and morphology.

The effect of fishmeal replacement with terrestrial protein sources on growth, body condition and intestinal microbiota of juvenile dusky kob Argyrosomus japonicus

A balanced combination of protein sources to partially replace fishmeal in the diets of cultured carnivorous fish can promote optimal fish health and production performance. In the present study, the growth, body condition and gut microbiota of juvenile dusky kob Argyrosomus japonicus fed fishmeal-substituted diets comprising different blends of animal and plant protein sources, with crystalline amino acid supplementation, were compared. The fish (15.7 [SE 0.12] g fish−1) were fed one of three isonitrogenous (41% protein) and isoenergetic (13 kJ g−1) diets for 60 days. Fish fed a diet with no fishmeal (D0; blood meal and soybean protein sources only) displayed the lowest specific growth rate (p < 0.001). Longer body lengths were achieved in fish fed a 50% fishmeal diet (D50; supplemented with poultry meal, bloodmeal, canola meal and soybean) (13.2 [SE 0.4] cm) compared with those fed either a 100% fishmeal diet (D100) (12.8 [SE 0.4] cm) or a diet with no fishmeal (D0) (12.1 [SE 0.08] cm) (p < 0.001). There were no differences in condition factor, hepatosomatic index (HSI) and feed conversion ratio (FCR) between fish fed diets D50 and D100; whereas fish fed diet D0 had a lower HSI and a higher FCR when compared with the other treatments (p < 0.01). The dietary protein source did not alter the gut microbiota of these fish. In conclusion, 50% replacement of dietary fishmeal with a favourable balance of meals processed from poultry, blood, canola and soybean improved length gain in juvenile dusky kob compared with those fed a diet with no fishmeal replacement.

Physiological stress response and recovery of an important estuarine fishery species, dusky kob Argyrosomus japonicus, after a simulated catch-and-release event

Catch and release (C&R) angling is a popular pastime the world over; however, studies have found that fish can experience considerable stress during a C&R event. To determine the effect of a C&R event on angled fishes, the physiological stress response (PSR) is often assessed. However, the peak PSR only occurs after a delay and is species-specific. The dusky kob Argyrosomus japonicus is a threatened species that is heavily exploited in South African estuarine fisheries. Given its collapsed-stock status, it is critical to understand the effects of C&R angling on the PSR of this species. The aim of this study was to plot the time-course of the PSR of A. japonicus to determine when the peak PSR occurs. Time had a significant effect on blood lactate (p < 0.05) and glucose (p < 0.05) levels, with both peaking 30–40 minutes after a stressor, but time did not have a significant effect on plasma cortisol levels (p > 0.05). Blood lactate and glucose levels reached recovery by 150 minutes. The results suggest that blood sampling should be done 30–40 minutes after a stressor, and although A. japonicus is able to recover quickly from a C&R event, its delayed PSR may render it vulnerable to post-release predation.

Dietary seaweed (Ulva sp.) does not alter fatty acid profiles and concentration in South African juvenile dusky kob (Argyrosomus japonicus, Sciaenidae) fillet

ABSTRACT Seaweeds are potential feed additives for aquaculture but their effect on fish fatty acid content and profile is largely unknown. This study was, therefore, designed to assess the effect of incorporating green macroalgae seaweed (Ulva sp.) into diets of juvenile dusky kob (Argyrosomus japonicus) on fillet fatty acid profile and concentration. Five experimental diets were formulated to contain seaweed at the following inclusion levels: 0 (Ulva0), 50 (Ulva50), 100 (Ulva100), 150 (Ulva150) and 200 (Ulva200) g/kg commercial kob feed on a dry matter basis. Seventy-seven fingerlings (9.14 ± 0.30 g) were distributed into each of 20 experimental tanks and diets were randomly allocated to tanks such that each diet had 4 replicate tanks holding 77 fingerlings. There was no dietary effect on fillet fatty acids (FA) concentrations (P > .05). The concentration of C18:3n3 (α-linolenic acid) tended to increase with Ulva inclusion levels (P > .05). However, the opposite was observed with the fillet concentrations of both C20:4n6 (arachidonic acid methyl ester) and C20:5n3 (eicosapentaenoic acid (EPA)), but the decrease was also not significant. In conclusion, while Ulva-containing diets did not enhance the FA concentration in dusky kob fillet, the values were similar to those observed in fish fed on the relatively expensive commercial kob diet.

Can long-term content analysis of print media be used to examine species composition, population demography and changes in distributional range of recreational fishery species?

Although marine recreational fisheries are socially and economically important, there is often limited funding for their monitoring and assessment. With South African anglers reporting catch declines for almost all targeted species and little long-term monitoring data available, novel methods need to be explored to provide managers with additional information. This study aimed to evaluate the efficacy of content analysis of print-media records for detecting long-term trends in species composition, size composition and distribution of selected recreational fishery species. Information for eight fishes captured in the marine shore-based fishery was collected from South Africa’s most popular recreational fishery magazine, between 1960 and 2009. During the five decades, there were shifts in the catch composition, from being dominated by slow-growing, large predators to faster-growing, large predators and lower-trophic-level species. There were no significant trends in the mean and maximum sizes of the dominant species, except for dusky kob Argyrosomus japonicus, which increased significantly in the maximum size reported during the study period. The distributional range of A. japonicus and leervis Lichia amia significantly increased, and that of galjoen Dichistius capensis significantly decreased. However, it was uncertain whether these trends could be attributed to population changes, changes in angler technology and/or behaviour, or climate signals. Overall, this type of content analysis may provide a cost-effective method to examine changes in species composition and fish distributions over time, but, without detailed knowledge of shifts in angler behavioural patterns, it may not be as useful for understanding changes in the population dynamics of recreational fishery species. It is suggested that fishery-independent monitoring programmes are most suited for this type of complex fishery.

Effect of graded levels of dietary carbohydrate on growth, feed utilisation and intestinal microbial community structure in dusky kob Argyrosomus japonicus fed a pelleted diet

The ability to utilise carbohydrates is limited for many predatory marine fishes. Graded levels of dietary carbohydrate (4.1–24.6%) were formulated using pregelatinised maize starch, to determine optimal levels for dusky kob Argyrosomus japonicus, an emerging mariculture finfish for which pelleted feeds are being developed. Specific growth rate increased with an increase in the carbohydrate level up to 16.72%, after which it declined. Feed utilisation followed a similar trend, with the best feed conversion ratio (1.28) and protein efficiency ratio (1.76) recorded at 16.4% carbohydrate. Lipid vacuolisation of the hepatocytes was evident in all livers examined, with melano-macrophage aggregates in those of fish fed 24.6% carbohydrate suggesting starvation. Gut bacterial community profiles were variable but were not influenced by dietary carbohydrate level and differed mostly between fish fed trout feed prior to the trial and those fed experimental diets containing starch. The dusky kob were able to clear glucose from their blood when fed up to 16.4% carbohydrate, but glucose removal was not achieved at 24.6% carbohydrate. In conclusion, dusky kob has a limited ability to utilise cooked starch as a carbohydrate source, which may be included in pelleted feed at 16.4% without adverse effects. For this species, levels of dietary carbohydrate above this may result in symptoms consistent with physiological breakdown, including reduced growth, reduced feed intake and feed conversion efficiency, prolonged hyperglycaemia, liver pathology and altered microbial communities in the foregut.

Metabolic activity throughout early development of dusky kob Argyrosomus japonicus (Sciaenidae)

The physiology of fishes in the early stages of development remains poorly assessed despite the importance of identifying energy bottlenecks in organisms faced with changing environmental conditions. This study describes the metabolic activity of dusky kob Argyrosomus japonicus throughout its early development, from hatchling to settlement stage. Standard, routine and active metabolic rates (SMR, RMR and AMR, respectively) were assessed to determine the species’ metabolic scope and identify how metabolism changes with growth and development. Distinct metabolic changes occurred in association with developmental changes during the early life stages, with flexion-stage larvae showing significantly reduced metabolic scope (approx. 0.30 µmol O2 ind.–1 h–1), representing an energy bottleneck. Based on these findings, it is likely that larvae of A. japonicus are most susceptible to environmental perturbations during flexion. The variability of metabolic rates during the diel cycle was also assessed and revealed that the early-stage larvae showed no preference for daylight, although settlement-stage juveniles were more active during daylight hours (RMR = 12.78 µmol O2 ind.–1 h–1) than at night (RMR = 5.87 µmol O2 ind.–1 h–1). These results suggest that metabolic measurements of the SMR of A. japonicus larvae can be taken at any time of the diel cycle until the settlement phase, when readings should take place at night.

Use of dried blood spots (DBS) to diagnose hyper- and hypoglycaemia in farmed dusky kob Argyrosomus japonicus in South Africa

Biomarkers require sensitivity and rapid turnover for health assessment of individuals and populations. Blood glucose in fish shows sensitive and measurable changes in response to environmental, physiological or nutritional stressors. Argyrosomus japonicus is currently the only commercially produced marine aquaculture finfish species in South Africa, yet knowledge about its responses to biological and environmental stressors is limited. Herein we confirm the utility of the Accu-Chek® Active diabetic glucometer under field conditions to provide accurate blood glucose data for A. japonicus, in comparison with laboratory-based enzymatic analyses using plasma, whole blood and dried blood spots (DBS). Glucometer, whole blood and DBS glucose was correlated with plasma glucose (R2=0.973, R2=0.955, R2=0.898 respectively). Whole blood glucose was consistently and significantly lower than plasma glucose, thus indicating that more complex sample preparation than storage on ice and freezing at −80°C are required to inhibit glycolysis in whole blood samples and generate accurate results. Diabetic glucometers offer a means to measure on-farm blood glucose with sufficient accuracy and rapidity. We include an analysis of a subsequent sampling from the farm, and demonstrate that through routine glucose measurements we were able to identify hypoglycaemia at the farm level, and that this corresponds to decreased condition of fish. The ease of DBS storage and stability of metabolites offer the opportunity of expanding fish health and condition monitoring by measuring multiple indicators in DBS.

Testing and validating a modified CTAB DNA extraction method to enable molecular parentage analysis of fertilized eggs and larvae of an emerging South African aquaculture species, the dusky kob Argyrosomus japonicus.

This study describes the successful implementation of a modified cetyltrimethyl ammonium bromide (CTAB) protocol to isolate genomic DNA and amplify 14 microsatellite markers from fertilized eggs and larvae of an emerging South African farmed marine fish species, the dusky kob Argyrosomus japonicus. To test and validate the efficiency of this method, genetic data were utilized to resolve parentage and kinship of first-generation (F1) offspring produced in mass-spawning events of wild broodstock fish in a commercial hatchery.

Potential impacts of alternative regulatory interventions on the recreational value of angling on the Breede River estuary, South Africa

This study investigated the support for, and potential impacts of, alternative management measures to address declining fish stocks in the Breede River estuary. A survey of residents and visitors was carried out during 2011–2012. Angling was the most important attraction of the estuary, and accounted for at least R 25 million in annual expenditure in the area. The majority of recreational anglers were concerned about the status of fish stocks and felt that additional regulations were necessary. There was greatest support for banning the trolling of artificial lures, and majority support for a closed area and an upper size limit for one of the main species targeted – the dusky kob Argyrosomus japonicus. The options of a night ban on fishing and a closed season received little support. Ratings of alternative scenarios suggested that anglers were more sensitive to fish abundance than to regulations. Modelled results showed that regulations resulting in increased abundance of fish, and large A. japonicus in particular, would have a positive impact on angling value of the estuary, whereas further declines in fish stocks would result in a decrease in expenditure in the area.

A review of the Oceanographic Research Institute's Cooperative Fish Tagging Project: 27 years down the line

The Oceanographic Research Institute's (ORI) Cooperative Fish Tagging Project was initiated in 1984 and is one of the most enduring and successful environmental projects of its kind in South Africa. By the end of 2011, 5 130 anglers had joined the project, of whom 3 457 had tagged one or more fish. In the project's 27-year history (1984–2011), over 251 900 fish from 368 different species have been tagged and released, of which approximately 13 192 (5.2%) have been recaptured. The five most commonly tagged species to date are galjoen Dichistius capensis, dusky kob Argyrosomus japonicus, dusky shark Carcharhinus obscurus, garrick/leervis Lichia amia and spotted grunter Pomadasys commersonnii. Information from the Tagging Project has been extensively used in numerous research projects, scientific publications, popular articles and postgraduate theses. The results generated by this project have also contributed to policy development and decision-making on linefish management in South Africa. Furthermore, the project has had a positive impact on improving angler awareness and knowledge about linefish resources. The concept of tag and release has partly been responsible for changing the ethics of the recreational fishing community, many of whom now release their catch, thereby contributing to a more sustainable fishing future. This paper critically assesses the contribution that the ORI Tagging Project has made as a conservation achievement in general and to linefish management in particular, and reflects on future directions for the project.

The trouble with estuarine fisheries in temperate South Africa, illustrated by a case study on the Sundays Estuary

This study provides empirical data on the exploitation of fishery resources on the Sundays Estuary on the south-east coast of South Africa. Total annual effort was estimated at 63 785 angler-hours, which accounted for an annual yield of 16 214 fish (8.0 t). Effort and mean catch per unit effort exhibited seasonal trends and were highest during summer. Dusky kob Argyrosomus japonicus and spotted grunter Pomadasys commersonnii were the most commonly targeted species and accounted for the bulk of the catch: 3.4 t (43% gravimetrically, 22% numerically) and 1.9 t (24% gravimetrically and numerically) respectively. The smaller-bodied Cape stumpnose Rhabdosargus holubi contributed only 0.2 t (0.03%); however, it accounted for 30.1% of the catch numerically and was important to the minority subsistence sector. Overall, the sustainability of the Sundays estuarine fishery is questionable due to high proportions of juvenile fish in angler catches, high retention rates of juveniles, and high targeting effort towards vulnerable species, such as A. japonicus, although success rates of the heavily targeted species were low. This is exacerbated by limited law enforcement, no fishery monitoring, ill-informed users and no public awareness campaigns. These threats to fishery resource sustainability are not unique to this system and a comparison of findings from similar studies suggests that the management of estuarine fisheries in South Africa is currently inadequate.

The effect of crowding density on growth, food conversion ratio and survival of juvenile dusky kob Argyrosomus japonicus (Teleostei: Sciaenidae)

Dusky kob or mulloway Argyrosomus japonicus is a potential aquaculture candidate, but data on growth of this species in South Africa are missing. To determine the effect of crowding density (kg m−3) on growth, survival and food conversion ratio, juvenile dusky kob were reared at constant densities of 10, 30 and 50 kg m−3. Both flow index and fish density within a treatment were kept constant at 33 kg fish (m3 water flow h−1)−1. There were no significant differences in water quality, food conversion ratio, growth or survival between treatments, suggesting that this species can be reared using the range of conditions tested. During the 40-day growth trial fish grew at 1.3% of body mass per day, with a length gain of 0.65 mm day−1. Average percentage survival was above 98.5% across all treatments. Thus, the highest density tested (50 kg m−3 at 33 kg (m3 water flow h−1)−1) did not have a negative effect on growth or food conversion ratio.

The ichthyofauna in the Port of Ngqura, South Africa

This study provides one of the first quantitative assessments of a fish community within a pre-operational deep-water port system. Between September 2006 and September 2007 a total of 4 559 fish, representing 47 species and 27 different families, were caught within the Port of Ngqura, Africa's deepest port, in 1 713 h of shore-angling (2.3 fish angler−1 h−1) and 239 h of boat-based angling (2.8 fish angler−1 h−1). Catches included species representative of both estuarine and shore fisheries. The majority (83.0%) of species caught were marine and most were less than size-at-50% maturity (71.4%). The most abundantly caught species were dusky kob Argyrosomus japonicus (25.5%), elf Pomatomus saltatrix (24.9%), garrick Lichia amia (17.7%) and dusky sharks Carcharhinus obscurus (10.7%). Fish assemblages differed significantly (p < 0.05) between three habitats within the port: Dolosse, Quay Wall and Sandy Shore. The Dolosse habitat supported the greatest abundance (3.1 fish angler−1 h−1) and diversity (43) of fish species. The Port of Ngqura is thus functioning as an important habitat for both juvenile and adult fish. The diversity and abundance of fish suggests the potential use of ports and other artificial structures for enhancing local fisheries in South Africa.

Do juveniles of the estuarine-dependent dusky kob, Argyrosomus japonicus, exhibit optimum growth indices at reduced salinities?

The natural distribution of early juvenile (<150 mm TL) dusky kob Argyrosomus japonicus (Sciaenidae) along the east coast of South Africa appears to be restricted to the reduced salinity upper reaches of estuaries. This study experimentally tested the effect of salinity on growth indices in an attempt to explain the estuarine distribution of early juvenile A. japonicus. Growth under laboratory conditions was used as an indicator of relative energy expenditure for osmoregulation at salinities 5 (hypoosmotic), 12 (isosmotic) and 35 (hyperosmotic), as energy used for osmoregulation becomes unavailable for growth. Juveniles (81 ± 1.5 mm SL and 9.6 ± 0.5 g) survived and grew at all three salinities over a nine-week period. Contrary to what was hypothesised, growth of juveniles increased with increasing salinity (average weight 35.57 ± 1.50 g, 42.31 ± 2.70 g and 48.86 ± 1.30 g for the 5, 12 and 35 treatments respectively), with specific growth rate of juveniles at the hypoosmotic salinity (2.05 ± 0.15) significantly lower than at isosmotic (2.52 ± 0.09) and hyperosmotic salinities (2.59 ± 0.09). Food conversion ratio was significantly higher at the hypoosmotic (1.30 ± 0.28) than the hyperosmotic salinity (0.94 ± 0.05), but there was no difference between the isosmotic (1.01 ± 0.08) and hyperosmotic salinity. Salinity did not affect condition factor (range: 2.67 ± 0.03–2.75 ± 0.10) or feed intake (range: 2.43 ± 0.17–2.75 ± 0.53). The results, in part, indicated that early juvenile A. japonicus expend more energy at salinities below isosmotic levels and that salinity alone does not explain their natural distribution within estuaries. It is possible that high conductivity in the upper reaches of certain South African estuaries may mitigate the negative effect of reduced salinity on growth and hence the estuarine distribution of early juveniles.

Estuarine habitat use by juvenile dusky kob Argyrosomus japonicus (Sciaenidae), with implications for management

The spatial and temporal area-use patterns of estuarine-dependent juvenile dusky kob Argyrosomus japonicus in the Great Fish Estuary, South Africa, were examined using acoustic telemetry. In all, 29 individuals (307–400 mm total length) were surgically equipped with individually coded transmitters and monitored for a period of up to 195 days using an array of 11 moored data-logging acoustic receivers. Tagged dusky kob spent most time in the estuary (84%), compared with neighbouring marine (13%) and riverine (4%) environments. Tagged fish made extensive use of the estuary from the mouth to between 6.4 km and 10.3 km upstream, and spent approximately equal proportions of time at each of the seven receivers located between 0.5 km and 6.5 km from the mouth. A total of 18 fish undertook sea trips of a mean duration of 3.5 days, whereas three individuals made riverine excursions (mean duration = 7.3 days). A total of 12 (41%) tagged fish was captured in the estuary prior to, during or after the monitoring period. These findings highlight the importance of estuaries as critical nursery areas, and that the use of these habitats by dusky kob should be better reflected in the management arrangements for this fishery species.

Determination of the optimal water temperature for the culture of juvenile dusky kob Argyrosomus japonicus Temminck and Schlegel 1843

The effects of temperature on growth, food-conversion ratio (FCR) and feeding intensity of juvenile dusky kob Argyrosomus japonicus (23.7±2.6 g fish−1) were assessed over the temperature range 17.5–28.5 °C in a 42-day growth trial. Growth increased with increasing temperature up to an optimum after which it declined. Specific growth rates were 2.05% and 1.2% day−1 for the fastest (25.3 °C) and the slowest (17.5 °C) treatments respectively. Food-conversion ratio peaked at a lower temperature than growth. Optimal (0.72 kg kg gain−1) and least efficient (1.40 kg kg gain−1) FCR were found at 21.7 and 17.5 °C respectively. Feeding intensity was linearly related to temperature within the range of 17.5–28.5 °C. These results corresponded to the thermal preference (25–26.4 °C) and natural temperature distribution (12–28 °C) of South African dusky kob. Determination of the temperature range that does not limit growth is a prerequisite to assess the relationship between growth and environmental variables such as light intensity, feeding regime and stocking density. Consequently, experiments to determine the effects of these environmental variables on growth and aquaculture potential of dusky kob should be conducted at 24–26 °C.

The effect of light intensity on growth of juvenile dusky kob Argyrosomus japonicus (Temminck &amp; Schlegel 1843)

South African juvenile dusky kob Argyrosomus japonicus are more abundant in turbid estuaries than in clear marine-dominated estuaries. Turbidity can reduce light penetration into the water and create an environment different from that experienced by fish under culture conditions in mechanically filtered clear water. In order to optimize rearing conditions of this species, the effects of light intensity (23–315 lx) and feeding method (restricted ration vs. feeding to apparent satiation) on growth and food conversion ratio (FCR) of juvenile A. japonicus were assessed in a 56-day growth trial. Fish weight increased from 7.2±1.6 to 41.9±10.2 g fish−1 at a growth rate of 3.25% body weight day−1. Light intensity did not significantly affect growth or FCR. Feeding method did not significantly affect growth rate, but average FCR was significantly better in treatments fed a ration of 3.6% body weight day−1 than in treatments fed to apparent satiation. Therefore, a light intensity range of 23–315 lx can be used to culture dusky kob juveniles. The better FCR in fish fed a restricted ration suggests that a ration of 3.6% body mass day−1 allowed good growth of juvenile dusky kob.