A-R Childs

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.

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.

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.

Using baseline biological and ecological information to design a Traffic Light Precautionary Management Framework for leerfish Lichia amia (Linnaeus 1758) in southern Angola

The coastline of southern Angola is sparsely populated and, largely because of a protracted civil war, has very lightly exploited inshore fish stocks. This has provided the almost unprecedented opportunity to determine prefishing-state reference points and to implement management strategies at the early stages of exploitation. The leerfish Lichia amia is distributed from the Mediterranean Sea, along the west coast of Africa to the southern and eastern coasts of South Africa. Despite its importance in recreational catches in South Africa and Angola, there is little information on the biology of this species. The leerfish is one of the three most dominant recreational shore-fishery species in southern Angola, and a biological study on the species was conducted in that region between May 2005 and December 2006. The mean length of captured fish was 767 mm fork length (FL) and 6.8 kg, and the catch per unit effort was 0.13 fish angler−1 h−1 and 0.79 kg angler−1 h−1. The growth of the leerfish population (in mm) was described by: L t = 1 137(1 − e0.22(t + 1.58)), and its total mortality was estimated to be 0.41 (±0.05). Female mortality (0.40 ± 0.06) was lower than that for males (0.44 ± 0.14), and the length and age-at-50% maturity was 623 mm FL and 2.4 years respectively. Female fish with ripe ovaries were found between June and November. The male:female ratio was 1:1.9. Leerfish fed exclusively on fish, mainly sardinella Sardinella aurita (62% frequency of occurrence). Based on this biological information, a theoretical Traffic Light Precautionary Management Framework is constructed for the species. Appropriate management regulations and potential future threats to the species are discussed.

Long-term effects of different-sized surgically implanted acoustic transmitters on the sciaenid Arygyrosomus japonicus: breaking the 2% tag-to-body mass rule

Biotelemetry is an increasingly popular and effective tool to study fish movements. Interpretation of the results relies on the assumption that fish behaviour and physiology is not affected by the presence of the tag. Although the long-term effects of tagging should be evaluated before field telemetry studies, this has rarely been quantified. As a precursor to a long-term telemetry study on Argyrosomus japonicus (Termminck and Schlegel 1843) in South Africa, the effects of surgical implantation of two different-sized tags were assessed in a 256-day experiment. We hypothesised that the larger 13-mm-diameter tags, which exceeded the general 2% tag-to-body mass rule, would have adverse effects on growth, survival and tag retention compared with the smaller 9-mm-diameter tags and an untagged control group. Because no adverse effects in growth were observed among the three groups, nor any long-term tag-related mortality, tag expulsion or internal damage, we suggest that the general 2% tag-to-body mass rule should be regarded only as a broad guide. Further, we conclude that juvenile A. japonicus individuals are well suited for surgical implantation of commercially available 9- and 13-mm-diameter tags and are good candidates for long-term telemetry studies.

First Assessment of Estuarine Space use and Home Range of Juvenile White Steenbras, Lithognathus lithognathus

The spatial dynamics and home range sizes of early juvenile white steenbras (Lithognathus lithognathus Cuvier, 1829) in the permanently open Great Fish Estuary, South Africa, were assessed using acoustic telemetry. Seven individuals (154–184 mm fork length) tagged with acoustic transmitters spent the majority of their time in the mesohaline to oligohaline regions of the estuary, between five and seven kilometres from the mouth. Mean home range size was small (97 399 ± 22 557 m2, mean ± S.D.) relative to the size of the estuary (approx. 1 360 000m2). Mean length of the estuary used by an individual was 1707m(±S.D.=1663 m). The fish were highly resident within the estuary, and showed fidelity towards their site of capture.

Area-use patterns and diel movements of white steenbras Lithognathus lithognathus in a temporarily open/closed South African estuary, inferred from acoustic telemetry and long-term seine-netting data

Acoustic telemetry and long-term seine-netting data were used to assess distribution, area use, home range dynamics and movement patterns of estuarine-dependent juvenile white steenbras Lithognathus lithognathus in the temporarily open/closed East Kleinemonde Estuary. Using 15 years of biannual seine-netting data, it was shown that white steenbras abundance was highest in the mouth region and decreased with distance from the mouth. Fifteen fish equipped with acoustic transmitters were monitored for eight months using an array of stationary acoustic receivers, while five of these fish were also manually tracked for two months. Results from both telemetry methods indicated that activity was greatest in the mouth region and lower reaches of the estuary. All individuals exhibited site fidelity and remained resident near their sites of capture. The mean length of estuary used by the 15 individuals was 1 888 m (SD 666). There was a distinct diel activity pattern, with certain individuals utilising deeper parts of the estuary during the day and shallow regions at night. The results indicate that the shallow littoral zone is a critical habitat for juvenile white steenbras.

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.

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.

Response of demersal fish assemblages to an extreme flood event in a freshwater-deprived estuary in South Africa

The Kariega Estuary in South Africa, is an example of a freshwater-deprived estuary. Large-scale water abstraction and droughts have resulted in this estuary either being uniformly marine or hypersaline. A major episodic flood event in October 2012, the largest ever recorded in this system, provided us with an opportunity to investigate the prolonged effect of an episodic flood on the demersal fish assemblage of a freshwater-deprived estuary. A beam trawl net was used for sampling the fish assemblages from December 2013 to November 2014. The flood event before the sampling period ‘reset’ the system, such that there was a longitudinal salinity gradient from the head to the mouth, which resulted in ‘normal’ estuarine conditions, typical of most estuaries. There was an increase in the abundance of benthopelagic marine migrant species, particularly early juveniles of important fishery species, in the middle and upper reaches during the present study relative to previous studies conducted in this freshwater-deprived system. This suggests increased recruitment of these species into their estuarine nursery habitat. The present study highlights the importance of understanding the effect of freshwater inflow on the demersal estuarine fish assemblage and the nursery role of this and other estuarine systems and their management.