African Journal of Marine Science

Future of the IUCN Endangered white steenbras Lithognathus lithognathus (Sparidae) – a tale of two estuaries

A.K. Whitfield — 2024-10-29

This review draws on a long history of ichthyological and fisheries research to examine the changing stock status of the estuary- dependent white steenbras Lithognathus lithognathus in the South African Swartkops and Knysna estuaries. The available evidence points to a major decline in the L. lithognathus population of the Swartkops Estuary, from an already exploited status in the late 1800s and early 1900s to a collapsed status by the late 1900s, and the loss of adults which were recorded as prevalent in the estuary in 1915. In recent decades, the spotted grunter Pomadasys commersonnii appears to have occupied the niche previously used by L. lithognathus in the Swartkops Estuary, although climate change may also be playing a role in this trend. The Knysna Estuary had a healthy juvenile L. lithognathus population in the 1970s and 1980s, but increasing anthropogenic pressures on the system, including exploitation by recreational anglers and small-scale fishers, are cause for concern, particularly as fishery regulations for this species are not being adhered to in this and other systems. No-take marine protected areas (MPAs) appear to have been successful in providing some opportunity for stock recovery by adult populations of L. lithognathus in certain coastal regions within the distributional range. In conjunction with adequate enforcement of current fishery regulations, the proclamation of estuarine protected areas (EPAs) for juveniles in important estuarine nursery areas is required to enable the stock to recover and for the status of this fishery resource to be upgraded from collapsed to sustainable in the future. By providing protection for L. lithognathus in selected no-take EPAs, these same areas could also benefit other overexploited marine fish species, such as the dusky kob Argyrosomus japonicus and leervis Lichia amia.

Partitioning the sources of sediment organic carbon in South African seagrass meadows

A. Bossert — 2024-10-29

In the sediments of seagrass meadows, allochthonous carbon sources can account for 50–90% of sediment organic carbon (SOC), in which non-seagrass carbon is derived from external organic matter advected into the meadow’s sediments or trapped by the seagrass canopy. Identifying the SOC origin is essential to accurately estimate the climate change mitigation potential of seagrass ecosystems, yet the ratio of allochthonous to autochthonous SOC in South African seagrass populations has not been investigated. In this study, we measured the SOC and carbon isotope ratio (δ13C) in Zostera capensis seagrass meadows in the Berg River and Breede River estuaries and applied Bayesian mixing models to disentangle the SOC contributory sources. Sediment organic carbon differed not only between estuaries, but also showed variability within each estuary, with autochthonous carbon accounting for between ~35% (SD 0.23) and ~42.3% (SD 0.18) of total meadow SOC. Other sources of SOC included macroalgae, salt marsh and seagrass species. Our work highlights the importance of accounting for allochthonous carbon in disentangling the drivers of high variability of carbon stocks in estuarine environments and contributes to improving the accuracy of South African seagrass carbon stock estimates.

Local knowledge, utilisation and consumption of seaweed in coastal communities of southwestern Madagascar

C. Rodine — 2024-10-29

This study explored the local knowledge, utilisation and consumption of seaweeds (lomotsy in Malagasy) among coastal communities in southwestern Madagascar. Primary data were collected in eight villages, through a survey questionnaire (with a total of 629 respondents) and daily focus group discussions, conducted between May and December 2022. The findings reveal widespread familiarity with seaweed, used predominantly for farming, cleaning fishing equipment, and shading fish during transport. A smaller proportion of respondents (9%) had experimented with seaweed as a food source, notably incorporating species like Kappaphycus alvarezii and Hydroclathrus clathratus into soups and sauces. Despite the coastal inhabitants’ recognition of seaweed, the potential of the seaweed resource has not been fully developed, with utilisation restricted mainly to limited traditional practices. An interest in incorporating seaweed into a person’s future diet or in seaweed cultivation was expressed by 58% and 45% of the respondents, respectively. This study underscores the importance of enhanced and sustainable use of the seaweed resource in Madagascar’s coastal communities, presenting opportunities for economic development and ecological conservation.

Migratory movement of photo-identified humpback whales Megaptera novaeangliae along the southeastern coast of Africa

B. Tree — 2024-10-29

The global conservation of humpback whales Megaptera novaeangliae relies on continuous assessments of the populations’ post- whaling status, which in turn require information on population mixing and movements. One of the seven breeding stocks in the Southern Hemisphere, breeding stock C and its associated four substocks (C1–C4), utilises the western Indian Ocean in several discrete breeding grounds. However, information on the mixing of stocks across breeding grounds is limited. This study utilises photo- identification methods applied to the first multi-regional catalogue of fluke images available for substock C1S (the southern component of substock C1) which uses the southern portion of the breeding ground, developed as part of this study, to investigate the whales’ movements along the southeastern coast of southern Africa. The identification of 1 746 unique individuals, including 11 withinyear matches and 48 between-year matches of 45 individuals, provides new insight into the intra-regional migration patterns, fidelity and structure of substock C1S. The within-year match results confirmed that individual humpback whales are broadly seasonally present for extended periods and visit multiple subregions along this coast, while the between-year matches revealed long-term fidelity to this coastline. Moreover, five of the between-year matches between subregions in South Africa and Mozambique linked the South African migration corridor to the breeding ground in southern and central Mozambique. These findings provide valuable insights into the movement patterns of substock C1S and significantly address the challenges of effective population management, which includes difficulties associated with the whale-watching industry on the east coast of South Africa.

Genetic consequences in the southern African endemic seabream Polysteganus undulosus (Sparidae) after eight decades of overfishing

C.J. Oosthuizen — 2024-10-29

The Critically Endangered seventy-four seabream Polysteganus undulosus, a slow-growing sparid that forms spawning aggregations off South Africa, faced heavy exploitation from 1910 until a fishery moratorium was put in place in 1998. Utilising temporal samples from 1962/1963 (mid-collapse) and 2005/2006 (post-collapse), we assessed genetic diversity at six microsatellite loci. Amplification success for archived samples was low (43%), necessitating a rarefaction approach, revealing a 40% decrease in allelic diversity. Significant genetic differences between recent and archived samples confirmed the impact of overfishing. Simulation studies indicated that missing genotypes did not affect these tests, validating the genetic differences. Using a coalescent-based approach, a 10-fold decrease in effective population size (Ne) was estimated over this 43-year period (Ne = 43.88 to 4.76). Simulations provided corrected Ne of 36.86, accounting for missing genotypes that were responsible for inflated values. Assuming 25% of pristine levels in the 1960s, pristine Ne values ranged from 147 to 176 in 1910, suggesting a 96.8–98.3% genetic decline over 88 years of exploitation. This study emphasises severe genetic consequences of overfishing on P. undulosus diversity and effective population size. The results provide vital genetic baseline data for recovery assessment and conservation efforts, as well as an analytical framework to evaluate the stock decline using partial genotyping data gathered from degraded archived samples.

Contrast in larval sensitivity to low pH in sea urchins from neighbouring seagrass meadows at Inhaca Island, Mozambique

J.A. Duvane — 2024-10-29

Ocean acidification presents a major threat to marine life, and a large body of evidence has documented its negative effects on various marine species and ecosystems. Recent meta-analyses highlight the key role of local adaptation and have linked species’ sensitivity to extremes of the present range of pH variability. More specifically, ‘physiological tipping points’ can be correlated with the lowest experienced pH where the organisms live. We verified this hypothesis by comparing the responses of larvae of the collector urchin Tripneustes gratilla originating from two neighbouring seagrass meadows at Inhaca Island in southern Mozambique. The two seagrass meadows experience different conditions: the site at the Inhaca Marine Biology Research Station (Estação de Biologia Marítima de Inhaca, EBMI) is subtidal, and the site at Banguá is intertidal. Larvae of adults collected from the two sites were cultured in the laboratory under four different target pH treatments (8.04, 7.67, 7.46 and 7.29) for 7 days. The results showed an overall negative effect of lower pH on survival and growth as well as on an index of symmetry. Larvae originating from the intertidal seagrass habitat at Banguá which experiences large pH fluctuations were more sensitive to a lower pH than those from the subtidal seagrass habitat at EBMI. This finding suggests that different mechanisms for pH tolerance and local adaptation may apply in subtidal versus intertidal environments. Consequently, the unifying principle that physiological tipping points are correlated with the extreme low pH in the present range of variability, may reach its limit in increasingly extreme environments under natural conditions.

To move or not? Tourists’ perceptions and management considerations of a beached whale carcass in a South African national park and marine protected area

M.K.S. Smith — 2024-10-29

The handling of beached cetacean carcasses requires social, legal, financial, ecological and logistical considerations. However, limited research on the topic hinders informed decision-making. A large humpback whale carcass that washed ashore at a South African marine protected area (MPA) provided an opportunity to gain insight into tourists’ perspectives on carcass management within a national park and MPA setting. Eighty tourists were interviewed, and a thematic analysis approach was adopted to identify key themes and quantify the responses. Chi-square tests of independence were used to determine (i) whether the respondents’ views on cetacean carcass management were dependent on their knowledge of the site’s MPA designation, and (ii) whether the provision of contextual information would influence the respondents’ perceptions. Tourists had mixed responses to whale carcass management, with half the respondents indicating that the carcass should be left to decompose naturally. Viewpoints were significantly influenced when context and information on the decision-making process were provided, with more respondents stating that no management intervention should be necessary. We propose a simple flow-diagram as a decision-support tool, which, in combination with spatial zonation maps to identify applicable management options, will help guide decision-making for management authorities. Inclusivity, transparency and consultation with experienced role-players from multiple agencies will provide legitimacy to the final management decisions.