African Journal of Marine Science

Understanding the effects of climate change on Southern Ocean ecosystems

C.D. McQuaid — Thu, 30 Nov 2023 00:00:00 +0000

Understanding the future of Southern Ocean ecosystems requires approaches at micro to macro scales. The Southern Ocean has experienced both top-down and bottom-up perturbations driven by man. The removal of whales and finfish was a top-down disruption, removing enormous levels of biomass of consumers and driving competitive release for other predators. In contrast, climate change is altering physical conditions in complex, interrelated ways likely to change primary-producer community structure, with bottom-up consequences for the efficiency of energy transfer to top predators, and for the biological pump. The physiological effects of increasing ocean temperatures on animal species are likely to be less important than three key physical changes: loss of sea ice, changes to water-column stability, and patterns of water transport; these will affect krill abundances, primary producer community structure, and prey availability to land-breeding predators, respectively. The Southern Ocean is vast and critically influenced by global teleconnections, with different regions already exhibiting different patterns of physical and biological change. Given the strong physical forcing of these ecosystems, many of the initial consequences of climate change will operate through direct physiological effects on the primary producers and indirect effects on the larger organisms. This will disrupt species interactions and drive new ecological relationships.

Mapping fine-scale demersal trawl effort for application in ecosystem assessment and spatial planning

J.C. Currie, L.R. Harris, L.J. Atkinson, T.P. Fairweather, K.J. Sink — Thu, 30 Nov 2023 00:00:00 +0000

Fine-scale maps of fishing activity are valuable information layers for fisheries management, assessments of biodiversity impacts and marine spatial planning. Our aim was to develop an accurate map of demersal trawling intensity in South Africa and to demonstrate its utility at a national scale. We calculated a swept area ratio, representing demersal trawling effort for the entire study period (2005‒2018) and annually. We then plotted spatial and temporal patterns of trawling activity, identified core fishing areas, and examined spatial overlap between trawling, South Africa’s marine ecosystem types and the national network of marine protected areas. A high proportion of trawling effort (90%) was concentrated in 43% of the area exposed to trawling, with the remaining 10% spread across 57% of the fished areas. The fishery overlaps with 33 of 150 benthic and bentho-pelagic marine ecosystem types. Of those, 11 have more than 50% of their extent, and five have more than 80%, within the trawl ring-fence. Our analyses support a systematic prioritisation of ecosystem types for further management and protection. The new South African trawling-intensity map contributes an improved pressure layer for ecosystem assessments, can help identify priority fishing areas and has application in conservation, marine spatial planning and fisheries management.

Patterns in sightings of ocean sunfishes (Molidae) in the greater Dyer Island ecosystem, Western Cape, South Africa

R.G.A. Watson, E. Frey, S. Horbst — Thu, 30 Nov 2023 00:00:00 +0000

This study aims to describe the distribution of ocean sunfishes (family Molidae) in the greater Dyer Island ecosystem off the southwest coast of South Africa. Between 2014 and 2022, local whale watching vessels operating under Dyer Island Cruises conducted 3 227 trips which resulted in 274 trips with Molidae sightings, with a total of 300 individuals sighted. Molidae sightings showed a significant correlation to sea surface temperature and a strong seasonal pattern. Sightings in the greater Dyer Island area increased significantly after 2016, suggesting a correlation with the decline of white shark Carcharodon carcharias sightings, with further increases in 2021. This suggests that the greater Dyer Island ecosystem has become a more favoured habitat for species of Molidae.

Low hypoxia tolerance in larvae of the sardine Sardinops sagax and anchovy Engraulis encrasicolus may limit their stock recovery in the northern Benguela upwelling system

A. Kunzmann, R.M. Imam, S.J. Geis — Thu, 30 Nov 2023 00:00:00 +0000

Physiological traits of five key fish species (Cape hake Merluccius spp., pelagic goby Sufflogobius bibarbatus, Cape horse mackerel Trachurus capensis, European anchovy Engraulis encrasicolus and sardine Sardinops sagax) from the northern Benguela upwelling system (NBUS) were compared during their larval stage by measurements of enzymatic activities of key metabolic enzymes (citrate synthase [CS] and pyruvate kinase [PK]). Two distinct age classes (early larvae: 8‒14 days old; late larvae: 22‒29 days old) for each species and from two areas were compared: Terrace Bay (20° S), the main spawning ground of Cape horse mackerel and anchovy, and Kunene (17° S), near the Angola–Benguela frontal zone, where warm and hypoxic water masses intrude into the NBUS. The results revealed significantly higher CS activity in both larval age classes in Cape horse mackerel, anchovy and sardine compared with Cape hake and pelagic goby. Pelagic goby and Cape horse mackerel had significantly higher PK activity compared with Cape hake, anchovy and sardine, apparent in both larval age classes and both areas. Results for anaerobic metabolism indicate higher capacity for pelagic goby and Cape horse mackerel to recover from oxygen debt built up in prey capture and predator escape behaviour and a higher potential for hypoxia tolerance when compared with Cape hake, anchovy and sardine. These results suggest higher survival probability for pelagic goby and Cape horse mackerel compared with the other species under conditions of a shoaling oxygen minimum zone and may explain their current dominance in the NBUS.

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

B.A. Ziko, T.S. Murra, T.F. Naesje, J.D. Filmalter, P.D. Cowley — Thu, 30 Nov 2023 00:00:00 +0000

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.

Movement patterns of Natal seacatfish Galeichthys troworum (Ariidae) tagged in the Pondoland Marine Protected Area, Eastern Cape, South Africa

B.Q. Mann, K. Cox, G.L. Jordaan, R. Daly — Thu, 30 Nov 2023 00:00:00 +0000

A tag-recapture study was undertaken on the Natal seacatfish Galeichthys troworum in the Pondoland Marine Protected Area (PMPA) on the east coast of South Africa. A total of 473 fish were tagged over a period of 8 years, and 136 individuals (28.8%) were recaptured over a period of 16 years, some of them on multiple occasions. Most recaptures (82%) were caught within 200 m of their release site, with movements ranging between 0 and 617 m, and time-at-liberty ranging between 0 and 3 407 days (9.3 years). Data analysis showed that all recaptured fish remained in relatively small home ranges (of ~360 m linear distance) and none of them abandoned their home range and moved out of the 2-km² sampling blocks. While the no-take area of the PMPA provided effective protection for these fish, their relative abundance was similar in an adjacent exploited area. This is likely because G. troworum is not a desired linefish species, with most individuals caught being released, coupled with high post-release survival.