Spotted Grunter Pomadasys Commersonnii Research Articles

Acoustic telemetry reveals the drivers behind estuary–sea connectivity of an important estuarine-dependent fishery species, Pomadasys commersonnii, in the Breede Estuary, South Africa

The spotted grunter Pomadasys commersonnii is an important estuarine-dependent fishery species known to spawn along the east coast (KwaZulu-Natal Province) of South Africa. However, recent evidence suggests that this species also spawns along the southwest coast in the Western Cape Province (WC), which has necessitated the investigation of estuary–sea movements of adult P. commersonnii in the WC, and their link with spawning season and changing environmental conditions. Seasonal gonad maturation of adult P. commersonnii (n = 112) was investigated using histology and the gonadosomatic index. Seven adult P. commersonnii were acoustically tagged and monitored in the Breede Estuary (WC) between November 2016 and March 2020. Tagged individuals spent more time in the estuary (83.5%) than in the adjacent marine environment. Sea trips (n = 90) lasted on average 3.23 (SD 4.55) days (range 1–37 days), with most sea trips (82.6%) having a duration of less than 5 days. The majority of sea trips (>50%) occurred in austral summer, which coincided with the peak spawning season. Increases in estuary water temperatures in summer and river inflow in winter also influenced the presence of tagged fish in the sea. The high degree of residency of P. commersonnii in the Breede Estuary and short sea trips, along with increased frequency of reproductively ripe fish during summer months, supports the conclusion that regional spawning is occurring along the southwest coast. We suggest that management strategies should be put in place to protect the P. commersonnii population along the South African southwest coast, especially during summer, for the recovery of this population.

Estuarine and coastal connectivity of an estuarine-dependent fishery species, Pomadasys commersonnii (Haemulidae)

Understanding the level of connectivity between estuarine and coastal waters is essential for appropriate management of estuarine-associated taxa. Most studies have focused on the role of a single estuary, while limited research exists on the importance of multiple estuaries to individuals of estuarine-associated species. This study used acoustic telemetry to assess the usage of multiple estuaries and coastal waters by the estuarine-dependent spotted grunter Pomadasys commersonnii. Twenty-six adult fish were tagged with acoustic transmitters in the Kariega and Bushmans estuaries, South Africa, and their movements along a 300-km stretch of Indian Ocean coastline were monitored for up to 17 months. Tagged individuals spent most of their time in the estuary where they were tagged (55% and 85% for fish tagged in the two estuaries, respectively), followed by time in the sea (30% and 15%) and in other estuaries (15% and <1%). The mean durations of sea trips for fish tagged in the Kariega Estuary or Bushmans Estuary, respectively, were 25 days (range 3–55) and 12 days (range 2–22). Of the fish that went to sea, 93% from the Kariega Estuary and 60% from the Bushmans Estuary visited other estuaries. Most visits were undertaken to the Swartkops, Bushmans and Kowie estuaries, although the longest durations were spent in the Sundays Estuary. Individuals moved to estuaries up to 130 km away. The total distance travelled between estuaries by an individual during the study was ∼529 km, with means of 201 and 184 km, respectively, for fish tagged in the Kariega and Bushmans estuaries. Despite covering large distances between estuaries, individuals often returned to their tagging estuary. Residency in their tagging estuary, combined with frequent visits to a neighbouring estuary, highlights the importance of estuarine habitats for this popular fish species, even after reaching maturity.

Seasonally and spatially referenced estimates of recreational shore-angling effort, catch composition, catch rates and total catch in the Goukamma Marine Protected Area, South Africa

A roving creel survey of the recreational shore fishery along the 16.4-km coastline in the Goukamma Marine Protected Area on the south coast of South Africa was conducted from 2009 to 2011. Some 838 patrols were stratified equally among months, areas and years, but intentionally biased towards weekends. Angler densities at Buffalo Bay and Groenvlei were 0.59 and 0.28 anglers km−1, respectively. Weekend densities were double to quadruple weekday densities and fishing during winter was more popular than during summer. Area, habitat and distance to access points explained variation in angler densities. Shannon–Wiener diversity in catches declined from 2.18 in an earlier (1993–2002) survey to 1.79. Although the order of species abundance in the catches remained largely unchanged, blacktail Diplodus capensis dominance increased to 57.3% by number, at the expense of galjoen Dichistius capensis. Habitat explained 27% of the variance in catch composition. The catch per unit effort (CPUE) for the top nine species ranged from 0.19 to 6.35 fish 100-h−1. The CPUE of all species, except spotted grunter Pomadasys commersonnii, declined. Blacktail and galjoen CPUE declined by 17% and 77%, respectively. The total catch estimate was 2 986 fish y−1. Transgressions of size limits were common. The results suggest that the fishery is overexploited and that catch rates are declining.

Influence of prey abundance and abiotic factors on the long-term home-range and movement dynamics of spotted grunter Pomadasys commersonnii in an intermittently open estuary

Long-term home-range and movement dynamics of spotted grunter Pomadasys commersonnii and the distribution of their dominant prey, the sandprawn Callichirus kraussi, were investigated in the intermittently open East Kleinemonde Estuary, South Africa. In addition, the effects of fish length, diel cycle, season and other abiotic factors on home range and area use were examined. Nine adult spotted grunter (326–489 mm total length) were tagged with acoustic transmitters, and their movements were studied by manual tracking on six consecutive days and nights during each of five tracking sessions from March to November 2004. In addition, movement patterns were recorded by stationary data-logging receivers. Kernel home ranges (95% utilised distribution) varied in size (mean: 76 797 m2 , range: 26 296–165 321 m2), but were all located in the lower one-third of the 3.6 km-long estuary. There was no effect of season, the diel cycle or fish length on the home-range estimates. However, the distribution of prey corresponded with the home-range distribution of tagged fish. Although sandprawn distribution was highly variable within and among transects and depth classes, sandprawns were most abundant in the lower region of the estuary, which corresponded to the home-range distribution of spotted grunter. Spatial distribution of prey appears to be a dominant factor influencing home-range parameters of this species within an intermittently open estuary.

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.

Value and management of the subsistence fishery at Knysna Estuary, South Africa

Knysna Estuary supports an estimated 30 full-time and 200 part-time subsistence fishers involved in bait collection, mud crab harvesting, and fishing. The mud prawn Upogebia africana dominates the bait-fishery, with estimated catches amounting to about 3% of the standing stock, suggesting sustainable use. Harvest of Marphysa spp. and Gorgonorhynchus dayi is conducted in a destructive manner. Most of the value of fishing lies in the setline catches of spotted grunter Pomadasys commersonnii and white steenbras Lithognathus lithognathus, whereas small species and individuals, particularly Cape stumpnose Rhabdosargus holubi, are also caught with handlines. Indications are that the linefishery is also sustainable at present. The subsistence fishery is worth an estimated R0.7-R1.1 million per annum, with full-time fishers earning at least R11 000-R17 000 per annum from the estuary. Currently operating under recreational regulations, the fishery is poorly controlled and fails to reach its full potential. The main threat to the estuary from damaging harvests of bait species, such as Marphysa spp. and G. dayi, should be discouraged through heavy penalisation of buyers and traders. Sales of mud prawn could probably be legalised. If off-take rates are found to be sustainable, sales of certain fish species could be legalised if access to the fishery could be effectively controlled under a co-management arrangement.

Estuarine use by spotted grunter Pomadasys commersonnii in a South African estuary, as determined by acoustic telemetry

Factors influencing estuarine use and marine excursions by spotted grunter Pomadasys commersonnii in the Great Fish Estuary, South Africa, were studied using manual and automated telemetry methods. In all, 20 individuals, ranging from 362 mm to 698 mm total length (TL), were caught and tagged with acoustic-coded transmitters in the estuary. Tagged individuals spent most time in the estuary (67%), interspersed with a number of marine excursions — 15 in all. The number (6–53) and mean duration (0.8–9.5 days) of sea excursions varied among individuals, and were correlated significantly with tide, time of day, fish size and various environmental conditions. On average, fish departed the estuary on the outgoing tide at night and returned to the estuary on the incoming tide during the day. Spotted grunter <400 mm TL spent more time in the estuary than larger fish (403–698 mm TL). The number of fish recorded in the estuary was significantly correlated with barometric pressure, wind direction and sea temperature. The spotted grunter under study exhibited a high degree of estuarine use, highlighting the importance of this habitat to this overexploited, estuarine-dependent fishery species.

Do environmental factors influence the movement of estuarine fish? A case study using acoustic telemetry

Telemetry methods were used to investigate the influence of selected environmental variables on the position and movement of an estuarine-dependent haemulid, the spotted grunter Pomadasys commersonnii (Lacepède 1801), in the Great Fish Estuary, South Africa. Forty individuals (263–698 mm TL) were surgically implanted with acoustic coded transmitters and manually tracked during two periods (7 February to 24 March 2003; n = 20 and 29 September to 15 November 2003; n = 20). Real-time data revealed that spotted grunter are euryhaline (0–37) and are able to tolerate large variations in turbidity (4–356 FTU) and temperature (16–30 °C). However, the fish altered their position in response to large fluctuations in salinity, temperature and turbidity, which are characteristic of tidal estuarine environments. Furthermore, tidal phase had a strong influence on the position of spotted grunter in the estuary.

A telemetry experiment on spotted grunter Pomadasys commersonnii in an African estuary

The feasibility of using telemetry equipment to study the movements of estuarine-associated fish in South Africa was investigated by conducting a tracking experiment on spotted grunter Pomadasys commersonnii in the East Kleinemonde Estuary. The telemetry equipment comprised two VEMCO V8 transmitters and a VEMCO VR60 receiver linked to a directional hydrophone. Field experiments demonstrated that the equipment's maximum detection range was 400m. The transmitters were tuned to different frequencies, and interference between these was found to be negligible. The accuracy of locating the transmitter equalled the previously determined Global Positioning System (Garmin GPS 12) accuracy of approximately 5m. Two fish were tracked over a seven-day period. The fish preferred the lower reaches of the estuary where they made repeated and prolonged use of some areas. A tank experiment was conducted to examine the effects of the transmitter implantation. Fish recovered quickly after the surgical procedure, and no differences in swimming behaviour and growth rates were found between these fish and the controls.

Distribution patterns of fishes in a freshwater deprived Eastern Cape estuary, with particular emphasis on the geographical headwater region

Small seine net fish assemblages in the headwater region of the Kariega Estuary are described and compared to the catch composition in other parts of the system. The geographical headwaters of the freshwater 'deprived' Kariega Estuary were utilised by a range of fish species but, in contrast to the nearby freshwater 'rich' Great Fish Estuary, few important angling species (e.g. spotted grunter Pomadasys commersonnii and dusky kob Argyrosomus japonicus) were recorded. The reduced riverine flow into the Kariega Estuary resulted in an extremely restricted river-estuary interface (REI) zone being made available to resident and migrant fish species. Despite the low freshwater input recorded, the catadromous freshwater mullet Myxus capensis was abundant within the REI zone and headwater reach, probably due to reduced access to riverine habitat above the estuary. Although salinity has been shown to be an important structuring force influencing ichthyofaunal assemblages in many South African estuaries, it is not the primary factor governing the distribution of fish in a freshwater-deprived, marine-dominated system such as the Kariega. Water SA Vol.29(1) 2003: 61-67

Effect of Protein Level and Varying Protein – Lipid Concentrations on Growth Characteristics of Juvenile Spotted Grunter Pomadasys Commersonnii (Haemulidae)

A 60-day feeding trial was conducted to evaluate the effects of dietary crude protein level on growth, feed conversion and protein efficiency ratio of juvenile (3–6 g) spotted grunter Pomadasys commersonnii. In the experiment, six semi-purified diets containing casein and fishmeal as protein sources, and with crude protein levels ranging from 35 to 60%, were fed to three replicate groups of fish per treatment in a recirculating system at the optimum temperature for growth. The relationships between dietary crude protein level and growth parameters were analysed by broken-line regression models. Results suggest that 48 – 50% dietary protein is needed for optimum growth and feed conversion for P. commersonnii. In a second experiment, three protein levels (35, 45 and 55%) at three different lipid concentrations (6, 8 and 12%) were used to formulate nine semi-purified diets that were fed for 60 days to triplicate groups of fish per treatment. Results suggest that a diet of at least 45% protein with a 12% lipid inclusion level is required for the best specific growth rate (5.96% per day) and feed conversion ratio (1.72) at this specific stage in the growth phase of P. commersonnii.

Assessment of the recreational linefishery in selected Eastern Cape estuaries: trends in catches and effort

Boat-based creel surveys were used to collect catch and effort data on the shore- and boat-based linefisheries of the Kromme, Gamtoos, Swartkops, Sundays, Bushmans, Kariega, Kowie and Great Fish estuaries in the Eastern Cape, South Africa, from January 1996 to April 1997. The surveys, which were performed primarily during routine fisheries law enforcement patrols by a regional conservation agency, recorded the catch and effort of 2 468 individual angler outings (12 840 angling hours) during 337 angler-count patrols. Although 26 species were recorded, a large proportion of the catch (85%) comprised only four species. Spotted grunter Pomadasys commersonnii (43%) was most commonly caught, followed by Cape stumpnose Rhabdosargus holubi (16%), dusky kob Argyrosomus japonicus (14%) and white seacatfish Galeichthys feliceps (12%). In terms of mass, the overall catch was dominated by A. japonicus (50%) and P. commersonnii (33%). Overall catch per unit effort (cpue) by number was highest during summer and lowest d...

An assessment of the recreational fishery in the St Lucia estuarine system, KwaZulu-Natal, South Africa

St Lucia is one of the largest estuarine systems in Africa, and attracts thousands of anglers each year. Catchcard data from the National Marine Linefish System for the years 1986–1999 were analysed to determine catch composition, catch per unit effort (cpue) and seasonality of catches by recreational anglers. Because not all anglers completed catch cards, estimates of total catch were made using additional data on the number of private boat outings, the number of boat trailers at boat slipways and the number of boats recorded entering the campsite gates during 1992 and 1993. In all, 27 fish families, constituting 55 species, were recorded by recreational anglers. Dusky kob Argyrosomus japonicus, spotted grunter Pomadasys commersonnii, perch Acanthopagrus berda, Natal stumpnose Rhabdosargus sarba, springer Elops machnata and mini-kob Johnius dorsalis were the most prominent species caught in terms of numbers and mass. Catch rates expressed numerically (fish angler−1 h−1) peaked during the summer and early winter. However, in terms of mass, catch rates peaked during late winter and spring, when there were increased landings of large dusky kob. Fluctuations in cpue were linked to salinity and estuary mouth conditions (i.e. mouth closure). Despite annual fluctuations in cpue, regression analysis revealed an overall downward trend for the dominant species (dusky kob and spotted grunter) and a gradual increase for stumpnose, perch and springer, but with the exception of stumpnose, these trends were not significant. Socio-economic aspects of the fishery were also investigated by conducting an independent boat-angler survey. The value of the recreational fishery, in terms of accommodation and direct angler expenditure, was estimated to be in the region of R9 million during 1992. Angler attitudes towards fishing regulations were positive and anglers generally had a good knowledge of the regulations for target species. Based on this assessment, a number of suggestions are made regarding the future management of the recreational fishery at St Lucia.

Exploitation and conservation of angling fish in two South African estuaries

1. Recreational and subsistence angling in South African estuaries is a small, but relevant, sector of South Africa's diverse coastal fisheries. This paper reviews these fisheries and their effects in two estuaries. 2. The composition of angler's catches in two large Eastern Cape Province estuaries has hardly changed over a period of nearly 20 years. 3. Gill-net catches (in terms of CPUE) indicate a decline in the abundance of the most popular angling fish species, especially the spotted grunter Pomadasys commersonnii. 4. Regular freshwater inputs into estuaries are required to ensure a salinity gradient which is crucial for the maintenance of an abundant and diverse estuarine ichthyofaunal assemblage. 5. Present management and conservation policies are poorly defined, and the philosophy and principles of ecosystem preservation, rather than that of species preservation, should be seriously considered. © 1996 by John Wiley & Sons, Ltd.

The effect of temperature and photoperiod on the growth of juvenile spotted grunterpomadasys commersonnii(Pisces: Haemulidae)

All measured growth parameters of juvenile spotted grunter Pomadasys commersonnii were significantly related to temperatures in the range 20,5 to 28,5°C. Optimum growth, condition and food conversion of juvenile grunter occurred at 24,5°C. Photoperiod had a more subtle influence on growth. Condition factor of fish held under a 12L:12D photoperiod was significantly better than at protracted (l6L:8D) or shortened (8L:16D) light regimes. Growth and food conversion were not significantly influenced by photoperiod.

Sea fishes spawning pelagic eggs in the St Lucia estuary

During September 1993, after the St Lucia estuary had been closed to the sea for about nine months, two samples of live plankton were collected at the mouth inside the closed estuary. Fish eggs from these samples were hatched and identified by the characteristics of the eggs and early larvae. Pomadasys commersonnii and Crenidens crenidens were reared to confirm identification. Eggs of nine species were found and all but one were identified to at least genus level. The unidentified egg was probably that of a clupeiform, and the most common egg was the spotted grunter Pomadasys commersonnii. Subsequent sampling in the Charters Creek area in 1994 confirmed that five of the species also spawn in the main lake, while a further species was added to the list of marine fishes that will spawn in St Lucia given favourable conditions.

Assessment of the Linefishery in Two Urban Estuarine Systems In Kwazulu-natal, South Africa

The recreational linefisheries in Durban Harbour and the Mgeni Estuary were surveyed using roving creel and access-point surveys during the period January–December 2000. In total, 3 351 shore-anglers and 652 boat-anglers were checked for catch-and-effort information, and 432 shore-anglers were interviewed using a demographic and socio-economic questionnaire. Durban Harbour had much higher angling effort than the Mgeni Estuary. Angling activity was higher on weekends (121 and 23 anglers per count for the harbour and Mgeni Estuary respectively) than during the week (34 and 9 anglers per count respectively). Total effort expended in Durban Harbour and the Mgeni Estuary shore-fisheries during 2000 was estimated to be 54 024 and 11 977 angler-outings respectively. Annual effort for the Durban Harbour boat-fishery for the same period was estimated to be 9 991 angler-outings. The flathead mullet Mugil cephalus was the most commonly harvested species in both Durban Harbour and the Mgeni Estuary shore-fisheries (25.2 and 68.1% respectively), and spotted grunter Pomadasys commersonnii (41.5%) was the most commonly harvested species in the Durban Harbour boat-fishery. Because of their small size, a large proportion (>63%) of the total fish catch at both localities was released. Mean catch per unit effort for the Durban Harbour boat-based fishery (0.11 fish angler−1 h−1 or 0.15 kg angler−1 h−1) was higher than that of the shore-based fishery (0.071 fish angler−1 h−1 or 0.034 kg angler−1 h−1). The questionnaire survey indicated that anglers had high site fidelity, considerable years of fishing experience (mean of 17.2 years for the harbour, 13.6 years for Mgeni) and high compliance with possession of fishing permits (86% harbour, 84% Mgeni). Although anglers generally supported the regulations currently applicable to the linefishery (>71% harbour, >77% Mgeni), the questionnaire results showed that specific knowledge for target species was poor (45–70% harbour, 29–60% Mgeni). The level of fisheries law enforcement (6.4% of harbour outings inspected, 7% Mgeni outings inspected) was poorer than in the previously studied KwaZulu-Natal marine shore-fishery. Economic investment by participants in terms of angling equipment used in the shore-fisheries of the two systems was calculated to be >R10 million, and expenditure in terms of bait, travel and tackle costs was approximately R9 million per year.