Investigating the trophic ecologies of early life stages of small pelagic fishes in the Benguela upwelling ecosystem
The Benguela upwelling system, located on the west coast of southern Africa, harbours large biomasses of three small pelagic fishes; anchovy Engraulis encrasicolus, sardine Sardinops sagax and west coast round herring Etrumeus whiteheadi. Despite fisheries management interventions, stocks of sardine have been low for the past decade following consecutive years of poor recruitment. This has highlighted that despite low fishing mortality, variance in recruitment may be largely governed by fluctuations in early-stage mortality rates. Therefore, examining early life traits that may explain a fraction of this mortality could be fundamental to understanding the underlying mechanics of recruitment. This study seeks to investigate the ontogenetic development in morphology and trophic ecology of early life stages of these three small pelagic fish species in the Benguela, and how they compare interspecifically and spatially. Increasing our temporal resolution of the ecological ontogeny of a species will enable accurate future analysis of trophic pathways and linkages within ecosystems and their feedback to vital ecosystem services such as fisheries. Results show that there is a high likelihood of trophic partitioning between sardine and anchovy larvae through distinct differences in their ecomorphology. Sardines exhibit traits in their development such as increased relative gut length, that would suggest a higher degree of herbivory compared to anchovy. Contrastingly, anchovy display positive allometric growth patterns in their cranial development at a younger age, suggesting a greater importance of particulate feeding on larger prey. These results provide an anatomical basis of how both the feeding ability and behaviour of these two species may differ which would predispose them to distinct diets. Both species exhibited trophic progression inferred through (a) a 15N enrichment and (b) an increase in carnivory fatty acid trophic markers. Interspecific partitioning is facilitated through distinctions in prey-size composition, with anchovy consuming comparatively larger prey than sardine. Surprisingly, sardine exhibit a higher δ15N, arising from the increased presence of smaller carnivorous copepods in their diet, which highlights the complexity of the food web dynamics in the Benguela. Prey-size selection is an important regulator of both intra- and interspecific competition in these species. Round herring pre-recruits are further trophically disassociated from sardine than anchovy, consuming on average larger particles with very little herbivory compared to sardine and anchovy. This was corroborated by (a) a 15N enrichment from sardine to anchovy to round herring indicative of feeding at higher trophic levels, and (b) 13C depletion in the opposite order, indicative of a reduced dependence on autotrophic components. Stomach content analysis suggested substantial filter-feeding for anchovy and sardine, but relatively little for round herring. Therefore, the compounding effect of the feeding mode with the functional traits provides potential answers for variances seen in their trophic ecology. The Ecopath model that included a finer life history of anchovy, sardine and round herring exhibited substantially more small pelagic fish-centred trophic pathways that were previously grouped into plankton groups than the model that did not. As a consequence, there was an improvement in model fit of 14 out of the 19 functional group time series analysed. In terms of overall model function, there was no change in biomass or system variability. However, there was a significant change in the trophic level estimation of functional groups, that had system-wide ramifications. This study has shown that despite the relative mass insignificance of small pelagic larvae in the system, they still have a considerable effect on the ecosystem dynamics. However, it seems that when disaggregating functional groups into life history stanza, if the same increase in resolution is not reflected in their prey, then vital trophic relationships can be lost. This research has highlighted crucial distinctions between the ecology of anchovy and sardine early life stages that may give rise to differences in condition and starvation and hence mortality in the face of varying prey compositions. The foundational work detailed here will contribute to future studies aimed at understanding the population dynamics of these species, and so contribute to the already multi-faceted toolkit that aims to sustainably manage the small pelagic fisheries of the Benguela.
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