African Journal of Marine Science

Further insights into killer whales Orcinus orca preying on white sharks Carcharodon carcharias in South Africa

A Towner — 2024-04-17

Photography and video footage, captured by researchers and tourists on board two vessels, provided insights into the predation techniques employed by an adult male killer whale Orcinus orca in Mossel Bay, South Africa. The incapacitation of its prey, a juvenile white shark Carcharodon carcharias (~2.5 m TL), followed by consumption of the liver, occurred within less than 2 minutes, highlighting the predator proficiency of the killer whale. Notably, another adult male killer whale, and constant traveling companion with the first, was observed approximately 100 m away and was not involved during the predation. The following day a second carcass of a different eviscerated white shark (3.55 m TL) washed ashore in the vicinity, indicating that at least two white sharks may have been killed during the interaction.

Distribution and site fidelity of four endemic catshark species in Walker Bay, South Africa

TL Johnson — 2024-04-17

Catsharks (family Scyliorhinidae) and the recently reclassified deepwater catsharks (family Pentanchidae) are two of the largest families of elasmobranchs and include species that function as important mesopredators in almost all marine ecosystems. This study focuses on four species endemic to the coast of southern Africa: the puffadder shyshark Haploblepharus edwardsii, dark shyshark H. pictus, leopard catshark Poroderma pantherinum and pyjama catshark P. africanum. Similar to most catsharks, these four species are underrepresented in chondrichthyan research. Our investigation aimed to gain insight into the distribution and site fidelity of the focal species through mark-recapture efforts in Walker Bay on the southwest coast. The use of 95% minimum convex polygons indicated
large overlaps in distribution among the species as well as between sexes, except for H. edwardsii. Site fidelity was universally low (0.005) at three key sample sites, although travel distances between sites averaged 3–5.5 km across all species. The results suggest that sexual segregation is not present for the studied catshark species, with the possible exception of H. edwardsii, which had a low capture rate. The low levels of site fidelity and movement also indicated significant levels of site interconnectivity between the three commonly sampled sites as they fell within the same 5-km2 region of the bay. From the present findings, Walker Bay could be considered an area of interest for conservation with respect to the four species, allowing for further study of their population dynamics and the influence of the local marine protected area.

Acoustic assessment of the biomass of aggregated chokka squid Loligo reynaudii on its inshore spawning grounds on the southeast coast of South Africa in the season closed for fishing

I Hampton — 2024-04-17

Recent attempts to estimate the biomass of chokka squid Loligo reynaudii on the southeast coast of South Africa by acoustic surveys of the inshore jig-fishing area are described. Pilot surveys of aggregations, covering part of the fishing grounds, were conducted in 2019 and 2020, followed by a multiphase survey of the entire grounds within the 50-m depth contour by two vessels in 2021. Biomass estimates were made using previous in situ and ex situ estimates of the target strength of L. reynaudii at 38 kHz, and our knowledge of the physical characteristics of spawning aggregations from acoustic studies over the past two decades. Biomass estimates from the 2021 survey ranged from 343 tonnes (coefficient of variation [CV] 47%) for targets conclusively identified as squid, to 2 365 tonnes (CV 9%) if all targets that could possibly have been squid were included. An estimate of 635 tonnes (CV 20%) is proposed as the most reasonable from the survey, although its worth as an absolute estimate is questionable mainly because of identification and target-strength uncertainties. We conclude that a series of wide-area acoustic estimates could provide an additional index of relative abundance for fitting in the current squid assessment model. This would be useful in managing the jig fishery and for improving our understanding of changes in chokka squid abundance and distribution in response to changes in the environment.

Thermal tolerances of littorinid snails from temperate and subtropical South Africa

CD McQuaid — 2024-04-17

Environmental temperature affects ectotherm performance and fitness because physiological performance increases up to a sublethal optimum temperature; this is not fixed, but depends on the species and individual history. We explored the influence of species identity, size and thermal history on thermal tolerance in four species of littorinid snails in South Africa. As expected, heat coma temperature (HCT) was lower than the lethal temperatures (LT₅₀), and was greater for juveniles, while LT₅₀ was greater for adults. The ranking by both metrics was Echinolittorina natalensis > Littoraria glabrata > Afrolittorina africana > A. knysnaensis, reflecting their biogeographical distributions. No species showed an effect of laboratory acclimation for 14 days at temperatures from 20 to 35 °C, though HCT was lower for field fresh individuals than for all other treatments. Nor was there acclimatisation of LT₅₀ between seasons. Bioregion is a proxy for lifetime acclimatisation yet the Afrolittorina species showed nonsignificant differences between conspecifics from different regions. The effect of size differed between the two metrics, and field fresh individuals showed lower HCT values than laboratory-treated individuals— which taken together indicates the need for care when interpreting the results of lab work. The thermal tolerances of these species reflect their biogeography and appear fixed with little or no plasticity, rendering these littorinids vulnerable to future climate warming.

Site- and habitat-dependent variations in the diversity of polychaetes associated with golden kelp Ecklonia radiata holdfasts along the southeast coast of South Africa

N Nkohla — 2024-04-17

Polychaetes are important components of the macrofaunal communities associated with golden kelp Ecklonia radiata holdfasts across different spatial scales. However, the polychaete component varies in different habitat types in response to varying environmental conditions. Here, we compared patterns of variation in Ecklonia radiata holdfast-associated polychaetes between rock pools and gullies along South Africa’s southeast coast. Eighteen species were found in gullies and 28 species in rock pools, with 13 and 16 families represented, respectively. The study sites at Cwebe, Dwesa and Nqabarha had 12, 11 and 20 species, respectively. The rock pools were more species-rich than the gullies, while Nqabarha was the most species-rich site. The composition of species varied both among habitat types and between sites, with a separation in polychaete composition observed between Cwebe and Dwesa. The species that contributed >60% to the distinction between habitat types and among sites were Lepidonotus semitectus, Cirriformia capensis, Eunice aphroditotis, Syllis sp., Chaetopterus variopedatus, L. durbanensis, Arabella iricolor, Lysidice natalensis and Gunnarea gaimardi. The polychaete distribution on kelp holdfasts was influenced by the sediment and geomorphological characteristics of the sampling sites. Our findings highlight the importance of sediment in creating habitat heterogeneity and how this facilitates high species richness in rock pools. However, physical factors explained a small proportion of the variance in the polychaete assemblages. Therefore, biological factors could be more important drivers than abiotic elements, particularly between habitat types.

Impact of interactions between common bottlenose dolphins and purse-seiners in the Moroccan Mediterranean region: case study in the Al Hoceima fishing grounds

M Keznine — 2024-04-17

This study examines the effect of common bottlenose dolphins Tursiops truncatus on the purse-seine fishery for small pelagic fishes in the Mediterranean Sea and the economic consequences thereof. The investigation focused on the fleet registered at the port of Al Hoceima, Morocco, and used information collected from on-board observations and a semi-structured questionnaire with fishermen and ship-owners. A total of 121 dolphins were captured as bycatch during 48 fishing trips, with a mortality rate of 0.23 dolphins per fishing trip. In terms of damage to the fishing gear, the number of observed holes varied between 28 and 230 per net per incident. Though some tears were large, most were <35 cm in height. The cost of repairing the holes in the nets caused by these interactions was estimated at US$179.52 per mending event. The level of interaction between common bottlenose dolphins and the purse-seine fishery targeting small pelagic fishes is a challenge for both fishery management and dolphin conservation.

Biology and ecology of the African blackspot shark Carcharhinus humani on the east coast of South Africa

G Cliff — 2024-04-17

The African blackspot shark Carcharhinus humani (until now commonly known as Human’s whaler shark) is a small-sized requiem shark (family Carcharhinidae) found in tropical coastal waters of the western Indian Ocean as far south as Port St Johns on the east coast of South Africa. It was only recently recognised as being distinct from the blackspot shark C. sealei, which occurs elsewhere in the Indo-Pacific. This study utilised four datasets to investigate the movements, temporal and geographic distributions, life history and population status of this species in South African waters. The recapture of eight individuals of 294 that were tagged by shore anglers revealed an average distance moved of 33 km (range 1–192 km). Competitive shore anglers caught an average of 39 C. humani per annum along the entire KwaZulu-Natal (KZN) coast, with no marked trend in catch rate over time. Catches in the KZN bather protection programme were extremely low but included 30 pregnant females which displayed a highly seasonal pattern of embryo development. Sightings of C. humani on stereo baited remote underwater video systems (stereo-BRUVS) in the iSimangaliso Marine Protected Area were dominated by females (7.7:1) and were mostly in deeper water of 26–35 m. In all datasets most of the individuals were mature and were present year-round, but with a clear peak in summer and autumn, especially in the catches taken by shore anglers. Competitive shore-angling catch data were used to conduct a risk assessment for the South African population, and, based on values of probability of encounter, the sampled population satisfies the IUCN Red List criteria for Least Concern.