Anthony J. Booth

On the Life History of a Cyprinid Fish, Labeo cylindricus

Aspects of the life history of Labeo cylindricus in Lake Chicamba, a man made hydroelectric dam in central Mozambique were investigated. L. cylindricus was found to be a potadrometic, synchronous spawner concentrating in river mouths prior to migrating up flowing rivers in January to spawn. Length-at-(50%)-maturity was attained at a fork length (FL) of 96 mm for males and 98 mm for females, both within their first year of life. The adult sex ratio was found to be female dominated at 1 male:1.63 females. Scales were used to age the species, with two growth checks deposited annually. Growth was rapid and was best described by the von Bertalanffy growth model as Lt=224.2(1−e-0.66(r+0.03)) mm FL. A maximum age of 4 years was observed for L. cylindricus with the largest male and female measuring 210 mm FL and 235 mm FL, respectively. The rate of natural mortality was estimated to be extremely high at 1.93 year-1 using length-converted catch curve analysis and is possibly a response to heavy predation by the introduced piscivore Micropterus salmoides.

The growth ofOreochromis andersonii (Pisces: Cichlidae) from the Okavango Delta, Botswana, and a comparison of the scale and otolith methods of ageing

SynopsisOtoliths and scales were used for age and growth determination ofOreochromis andersonii from the Okavango Delta, Botswana. Marginal increment analysis showed that an annulus was formed in both the scales and otoliths during the dry summer period. Using scales, the growth ofO. andersonii was described by Lt = 285.27(1-e-0.26(t+2.02)) mm SL and using otoliths by the equation Lt = 267.48(1-e-0.25(t+2.18)) mm SL. Maximum age estimates of 10 years using scales and 13 years using otoliths were obtained and the growth curves were significantly different (p < 0.01). Age estimation using scales tended to over-emphasise growth inO. andersonii resulting in larger predicted lengths-at-age. For this reason, otoliths are considered to be more reliable and suitable than scales in determining the age and growth of this species.

Spatial and seasonal distribution patterns of the ragged-tooth shark Carcharias taurus along the coast of South Africa

Catches from competitive shore-anglers, inshore boat-based anglers and sightings by spearfishers and divers were used to infer the spatial and seasonal movement patterns of young-of-the-year (<1.2m TL), juvenile (1.2–1.8m TL), sub-adult (1.8–2.4m TL) and adult (>2.4m TL) ragged-tooth sharks Carcharias taurus along the coast of South Africa. Adult sharks inhabited the entire coast between Maputaland in the east and St Helena Bay on the West Coast. The geographical range of sharks at earlier life-history stages decreased with size. The vast majority (93.8%) of young-of-the-year sharks recorded from competitive shore-angling club records were between East London and St Francis Bay on the East Coast, suggesting this region to be the primary nursery area for C. taurus. Estuarine systems, although utilised by young-of-the-year and juvenile C. taurus, do not form an important component of their nursery area in South Africa. Catches of pregnant and post partum females taken during the same time of year and in different areas indicated a biennial reproductive cycle. C. taurus appears to display a high degree of affinity for particular reefs. The reason some reefs are chosen over others, despite having similar physical characteristics, remains unclear. A significant increase in the number of C. taurus caught in competitions held by the Border Rock and Surf Angling Association between 1984 and 2004 suggests an increase in the abundance of C. taurus.

Integrating stomach content and stable isotope analyses to elucidate the feeding habits of non-native sharptooth catfish Clarias gariepinus

Sharptooth catfish Clarias gariepinus was introduced into the Eastern Cape Province, South Africa, in 1976 and there are concerns about its possible negative impacts on native biota. This study investigated its trophic impact by examining its feeding habits. Stomach content and stable isotope analyses were compared from three localities—the Great Fish River, Sundays River and Glen Melville Dam. Stomach content analysis indicated a catholic diet dominated by fish particularly in all localities. Spatially, however, the diets revealed differences based on the dominance of macrophytes that were only present in the rivers, and aquatic invertebrates that appeared more diverse within the Great Fish River compared to other localities. By contrast, stable isotopes revealed a more generalised feeding pattern with no clear dominance of particular prey. Stable isotopes further showed that the catfish was a complex predator, with large catfish being top predators whereas smaller size groups appeared to feed lower in the food chain. An ontogenetic shift in diet was evident, with small fish predominantly consuming aquatic invertebrates and shifting towards fish with increasing size. High dietary overlap suggests the potential risk associated catfish feeding, especially the potential of piscivory by small catfish that are more likely to persist in shallow and marginal where endangered indigenous minnows occur. The alteration of environmental conditions, especially flow by inter-basin water transfer (IBWT) schemes, was inferred to have had a probable influence its invasion success. Occurrence of other invaders, which was facilitated by the IBWT together with the catfish, posits the risk of invasion meltdown within the study systems.

Spatial and seasonal distribution patterns of juvenile and adult raggedtooth sharks (Carcharias taurus) tagged off the east coast of South Africa

Understanding the movement patterns of raggedtooth sharks (Carcharias taurus) is crucial in defining habitat use and evaluating the effects of exploitation and anthropogenic activities. Between 1984 and 2004, 1107 C. taurus juveniles ( 1.8-m TL) were tagged and released along the east coast of South Africa. In total, 125 C. taurus juveniles and 178 C. taurus maturing subadults and adults were recaptured, representing recapture rates of 11.2% and 7.5% respectively. The average distance travelled by juvenile sharks was 18.7 km (95% CI = 10.8–26.6 km). Juvenile sharks displayed site fidelity to summer nursery areas. The average distance travelled by maturing and adult sharks was 342 km (95% CI = 275–409 km). One female shark, however, was recaptured 1897 km from its original release site. The average rate at which pregnant sharks moved south from their gestation to pupping grounds was 2.6 km day–1 (95% CI = 2.04–3.16 km day–1). This study highlights the differences in movement patterns between C. taurus juveniles and adults and suggests philopatric behaviour in both life-history stages.

The age and growth of the greenhead tilapia Oreochromis macrochir (Pisces:Cichlidae) from the Okavango Delta, Botswana

Sectioned otoliths were used for age and growth determination of Oreochromis macrochir, a common cichlid species from the Okavango Delta, Botswana. The Okavango Delta is a vast inland wetland ecosystem which receives annual floodwaters from the highlands of southern Angola. Floodwaters reach the northern areas of the delta between January and March and the southern areas between June and September each year. Samples were collected from sites which receive flood waters between May and August. Marginal zone analysis showed that an annulus was formed between October and December during the dry, summer period. Maximum age estimates of eleven years were obtained and growth was described by the 3 parameter von Bertalanffy model as lt = 215.24 (1−e −0.42(t+1.08)) mm SL.

Ocean warming, a rapid distributional shift, and the hybridization of a coastal fish species

Despite increasing awareness of large-scale climate-driven distribution shifts in the marine environment, no study has linked rapid ocean warming to a shift in distribution and consequent hybridization of a marine fish species. This study describes rapid warming (0.8 °C per decade) in the coastal waters of the Angola-Benguela Frontal Zone over the last three decades and a concomitant shift by a temperature sensitive coastal fish species (Argyrosomus coronus) southward from Angola into Namibia. In this context, rapid shifts in distribution across Economic Exclusive Zones will complicate the management of fishes, particularly when there is a lack of congruence in the fisheries policy between nations. Evidence for recent hybridization between A. coronus and a congener, A. inodorus, indicate that the rapid shift in distribution of A. coronus has placed adults of the two species in contact during their spawning events. Ocean warming may therefore revert established species isolation mechanisms and alter the evolutionary history of fishes. While the consequences of the hybridization on the production of the resource remain unclear, this will most likely introduce additional layers of complexity to their management.

The biology of the panga, Pterogymnus laniarius (Teleostei: Sparidae), on the Agulhas Bank, South Africa

Aspects of the biology of the panga, Pterogymnus laniarius, a commercially important endemic southern African, demersal sparid fish species, are described from material collected monthly between February 1994 and July 1995 on the Agulhas Bank, South Africa. Growth studies based on sectioned sagittal otoliths revealed that the panga is relatively slow growing with ages of 16 years being recorded. Growth was best described by the von Bertalanffy growth model as Lt = 379.4 (1-e−0.13(t+1.78)). Estimates of total mortality, natural and fishing mortality were estimated at 0.36 year−1 0.28 year−1 and 0.08 year−1, respectively. Detailed histological examination revealed that panga are late gonochorists, males and females maturing after a non-functional intersexual interval. Females mature at approximately 200 mm fork length or 4 years of age. Reproductive activity occurs throughout the year, peaking slightly in winter when small pelagic eggs are spawned. Gametogenesis was found to be similar to that of other sparid fishes and marine teleosts in general. The panga feeds predominantly on crustaceans with a distinct ontogenetic shift in feeding habits. Juvenile fish feed predominantly in the water column on mysids after which they move to the benthos with subadult fish feeding on ophiuroids and amphipods. Adult fish remain on or near the benthos, feeding predominantly on crabs with polychaetes, ophiuroids and fishes also present in the diet in smaller quantities. Several aspects of the pangas biology such as its late gonochoristic reproductive style, protracted spawning season, maturation before recruitment and the ability to utilise large areas of the Agulhas Bank by feeding on soft substratum prey are thought to enable this species to sustain a higher fishing pressure than other sympatric sparid species. The pangas longevity, slow growth and high natural mortality rate mitigates against these factors and needs to be considered in the development of a management strategy.

Validating and corroborating the deposition of two annual growth zones in asteriscus otoliths of common carp Cyprinus carpio from South Africa's largest impoundment

A total of 816 common carp Cyprinus carpio asteriscus otolith pairs were collected from Lake Gariep, South Africa. Otoliths were interpreted whole, submerged in methyl salicylate and viewed under transmitted light. The precision of growth zone counts of the primary reader was estimated at 5·54 and 7·03% using the average per cent error method and the coefficient of variation, respectively. Age-bias plots indicated no systematic bias between the primary reader and the three secondary readers for up to nine growth zones (95% of the sample). Growth zone deposition rate was validated using a mark-recapture experiment of chemically tagged C. carpio (n = 21) conducted in a large earthen pond under ambient conditions in the vicinity of Lake Gariep. The validation results were corroborated for the wild population by edge analysis and a length-based age-structured model. All three methods suggest that growth zone formation occurred biannually, exemplifying the importance of age validation as a prerequisite for understanding the life history of C. carpio.

Establishment of Translocated Populations of Smallmouth Yellowfish, Labeobarbus aeneus (Pisces: Cyprinidae), in Lentic and Lotic Habitats in the Great Fish River System, South Africa

This study presents an assessment of the establishment success of smallmouth yellowfish, Labeobarbus aeneus (Burchell 1822), in the Great Fish River (GFRI), where they were introduced by the Orange-Fish River interbasin water transfer scheme and in the Glen Melville Reservoir (GMR), where the species was subsequently introduced from the GFRI. Edge Analysis of asteriscii validated an annual deposition rate of one growth increment per year and sampled fish were aged at between 0 and 10 years old. Combined sex length-at-age (Lt ) was described by Lt = 650(l - e -066(t + 4.22)) mm FL for GMR and Lt = 498(l - e -0.23(t + -0.373)) mm FL for GFRI. Natural mortality rate was estimated at 0.56/yr in GFRI and 0.96/yr in GMR. Condition factor was significantly higher in the GFRI than in GMR. In comparison with other populations, the GFRI population had reproductive and growth traits that were similar to those in its natural riverine range. Reproductive assessment showed ripe male and female fish in October to December in the GFRI, while in the GMR, ripe male fish but few ripe female fish were recorded. This was attributed to the lack of spawning cues in the reservoir. As a result, L. aeneus were considered established in the GFRI, but were not established and likely dependent on introductions in the GMR.