African Journal of Marine Science

Mesopelagic fish in the Benguela Current ecosystem and Indian Ocean: environmental influences on communities, trophic interactions and emerging fisheries

2026-10-28T17:13:44+00:00

We introduce here the results of seven studies focused on mesopelagic resources in the southeastern Atlantic and
Indian Ocean, which collectively serve to improve our understanding of fish biomass, distributions and ecology
in the ‘twilight’ zone of the region’s oceans. Most of the studies are products of support by the EAF-Nansen
Programme, South Africa’s National Research Foundation, and the Research Council of Norway—showcasing the
benefits of collaboration.

Regional mesopelagic fish assemblages in the Indian Ocean and Bay of Bengal, and their relationships to hydrographic characteristics

2026-10-28T17:23:05+00:00

Mesopelagic fish often dominate the micronekton in the mesopelagic zone. Despite their ecological importance, information about their diversity, distribution and abundance in some areas of the Indian Ocean remains patchy and incomplete. Using data collected from three acoustic/trawl surveys spanning the western (WIO), central (CIO) and eastern (EIO) Indian Ocean regions in 2018, we investigated the composition and structure of the fish assemblages that were sampled using different pelagic trawl types, namely Macrozooplankton and Multpelt, which targeted the deep scattering layer (DSL) and shallow scattering layer (SSL). We assessed the structure of mesopelagic assemblages across the region at the level of genus and family, and analysed relationships between the samples with concurrently collected hydrographic data using multivariate statistics. Mesopelagic fish taxonomic richness varied across the region, as follows: WIO – 68 genera (13 unique), 34 families (4); CIO – 48 genera (6), 35 families (5); and EIO – 86 genera (26), 44 families (9). Although there was a pronounced difference in the structure of the
assemblages by survey region, gear type and time of day (and many interactions between these), the assemblages were significantly impacted by water temperature, dissolved oxygen and aspects of the chlorophyll a environment—which varied across the sampled areas. Intriguingly, the spatial patterns (and diversity) observed when the mesopelagic fish assemblages were examined at the genus level were likewise reflected when the data were analysed at the family level. The implications of these results are discussed.

Infection by Cardiodectes bellottii (Copepoda: Pennellidae) on Lampanyctodes hectoris (Actinopterygii: Myctophidae) in the southern Benguela Current upwelling ecosystem

2026-10-28T17:29:43+00:00

Hector’s lanternfish Lampanyctodes hectoris is an abundant mesopelagic fish found in the Benguela Current marine ecosystem and is frequently parasitised by the copepod Cardiodectes bellottii. This parasite is relatively large compared with its fish host, and negative impacts of its infection on other lanternfishes have been reported. We examined 1 800 L. hectoris sampled over a wide spatio-temporal range in the southern Benguela to document C. bellottii infection indices in this lanternfish species, and we compare selected biological characteristics of infected and uninfected fish. Infection prevalence showed spatio-temporal variability, with an overall mean of 20.7% (95% confidence limits: 18.9–22.6%; 373/1 800 fish). This is the highest reported infection prevalence by C. bellottii among five lanternfish species for which large sample sizes (of >500 fish) were processed. We found that infection (i) significantly reduced female host weight-at-length but not that of male hosts; (ii) significantly reduced female but not male gonadosomatic index; and (iii) had no significant effect on length frequency distributions. Cardiodectes bellottii infection in L. hectoris does not appear to increase host mortality but can reduce both somatic and/or gonadal growth, and hence reproductive success. These findings suggest that the productivity and population dynamics of L. hectoris in the southern Benguela may be partly driven by infections by this parasitic copepod.

Identification and abundance estimation of mesopelagic fish in the Namibian Benguela using multifrequency acoustics

2026-10-28T17:36:02+00:00

Mesopelagic species are one of the least studied components of the ocean ecosystem, with major uncertainties about their biomass. High abundances of fish, jellyfish, krill and other plankton often occur in mixed aggregations in the Benguela Current region, which challenges conventional acoustic target identification. Multifrequency acoustic techniques were developed to distinguish mesopelagic fishes from other major co-occurring sound-scattering organisms, such as jellyfish, krill and micronekton (here ‘plankton’). Frequency responses were extracted from acoustic reference data collected by the RV Dr. Fridtjof Nansen in the Namibian Benguela in April–May 2019. Acoustic targets were sampled using pelagic sampling trawls and verified with an in-trawl camera system. The acoustic classification algorithm was applied to data collected during surveys in the northern Benguela region in 2017 and 2019 to estimate mesopelagic fish biomass, and its performance was evaluated by visual inspection of echograms, comparing trawl catches with acoustic estimates, and comparing estimates derived from daytime and night-time acoustic data, with and without the assistance of trawl verification. Biomass estimates of the target category mesopelagic fish from the different approaches ranged from 8.0 × 10⁵ to 1.4 × 10⁶ tonnes in 2017, and from 1.0 × 10⁶ to 1.3 × 10⁶ tonnes in 2019. This study demonstrates that the use of frequency responses for mesopelagic fish, krill and ‘plankton’ may lead to improved acoustic biomass estimates in this region.

Trophic coupling and environmental constraints shape the distribution of mesopelagic fishes in the northern Benguela Upwelling System

2026-10-28T17:43:25+00:00

We investigated the environmental drivers of mesopelagic fish density and distribution in the northern Benguela region using hydroacoustic data collected by the RV Dr. Fridtjof Nansen in 2017 and 2019. Multifrequency acoustic data (18, 38 and 70 kHz) were analysed using an acoustic classification algorithm to partition backscatter into three functional groups representing mesopelagic fishes, krill and other ‘plankton’. Mesopelagic fish accounted for 48% and 33% of total backscatter in 2017 and 2019, respectively, while krill contributed 41% and 39%, and ‘plankton’ 11% and 28%. Generalised additive models revealed tight spatial coupling between mesopelagic fish and their primary prey (krill), although fish density also increased with increasing fluorescence and ‘plankton’ density, reflecting an association with productive waters. Mesopelagic fish showed a preference for low-oxygen environments and
exhibited a clear diel relationship with the oxygen minimum zone, occupying its upper boundary during daytime, likely as a refuge from visually oriented predators. Overall, mesopelagic fish density and distribution in the northern Benguela is primarily structured by prey availability in combination with environmental constraints, including oxygen concentration, temperature, light intensity and bottom depth, which together define key habitat zones.

Mesopelagic fishes in the southern Benguela ecosystem: acoustic target strength and biomass

2026-10-28T17:47:00+00:00

Mesopelagic fishes such as Hector’s lanternfish Lampanyctodes hectoris and the Walvis pearlside Maurolicus walvisensis are ecologically important and potentially valuable marine resources in the southern Benguela ecosystem. Reliable biomass estimates are, however, required as inputs into ecosystem and stock assessment models to ensure their potential future sustainable exploitation. This study presents species-specific target strength–fish length relationships derived from in situ data, and acoustic biomass estimates from fisheryindependent surveys conducted between 2006 and 2022. The estimated combined biomass of these two species averaged ~1 million tonnes, but the annual estimates were highly variable, with M. walvisensis generally dominant. Despite the region’s high productivity, the density estimates were lower than those reported elsewhere. Key uncertainties included limited offshore spatial coverage and potential depth-related resonance at 38 kHz; however, diel and multifrequency analyses indicated minimal resonance-related bias within the surveyed depth range.

Seasonal windows of opportunity: exploratory trawling for mesopelagic fishes off South Africa’s west coast

2026-10-29T06:17:25+00:00

Shortages of fishmeal and fish oil and the resultant high prices for these products have intensified commercial interest for potentially harvesting mesopelagic fishes. Data from an exploratory pelagic trawl programme off South Africa’s west coast in 2010/2011 and 2017–2019, largely targeting Hector’s lanternfish Lampanyctodes hectoris, were analysed to investigate drivers of catch per unit effort (CPUE) and to explore operational constraints of a dedicated mesopelagic trawl fishery. Across trips, the mean catch was ~64 (SE = 5.3) tonnes (t) d–1 and CPUE was 2.7 (SE = 0.2) t h–1, but with high variability. Month was the dominant driver of CPUE, with an elevated CPUE in only 3–4 months of the year. Diel differences (day, night, dawn or dusk), towing speed, fishing depth and bottom depth showed no consistent effects. Catches of mesopelagic fishes were concentrated near the 200-m isobath between Lambert’s Bay and Cape Town, close to the existing fishmeal factories at St Helena Bay. A simple benchmark against local trips made by anchovy purse-seiners indicated comparatively higher trip-level catch rates for anchovy
Engraulis encrasicolus (mean ~157 t d–1) and highlighted throughput and cost disadvantages for a dedicated mesopelagic trawl fishery. The fishmeal and fish oil yield was seasonal, peaking in late summer or autumn and not aligned with the peak in CPUE, negatively impacting the economic viability of this prospective fishery.

The trophic ecology of 12 mesopelagic fish species in the northern Benguela Current ecosystem contributes to species coexistence

2026-10-29T06:20:00+00:00

Mesopelagic fishes are abundant and play an important role, even if incompletely understood, in the ecology and functioning of the global ocean. Mesopelagic fishes are predominantly carnivorous, and assemblages may comprise species that feed on similar zooplankton resources. Yet, their species diversity remains generally high. This study examined the trophic ecology of 12 of the most common mesopelagic fish species found in the northern Benguela Current ecosystem in an effort to understand interactions between species. Integrating morphometric traits, stomach content analysis, and stable isotope values revealed complex, multidimensional niche partitioning within the mesopelagic assemblage. Though morphological similarities suggested potential competition, systematic gape-size analysis demonstrated a clear scaling relationship with prey-size spectra, providing a mechanical basis for resource partitioning. Stomach content analysis confirmed this partitioning, with species-specific preferences for distinct zooplankton groups. These dietary differences were reflected in stable isotope biplots of carbon and nitrogen, which delineated a clear trophic hierarchy. By combining these three methodologies, our study indicates that coexistence is achieved through a synergy of morphological specialisation and trophic positioning, rather than spatial segregation alone.