Although mesopelagic fishes are an important component of marine food webs, the adaptive features used to facilitate niche partitioning among co-existing and presumably competing mesopelagic species is unclear. This study examined the trophic ecology of the two principal mesopelagic fishes off the west coast of South Africa (lanternfish Lampanyctodes hectoris and lightfish Maurolicus walvisensis) sampled during the spring 2014 and autumn 2015 cruises, using stable isotope and stomach content analyses. Stable isotope values were extracted from the white muscle tissue of fishes, but due to the high lipid content of both species, samples were processed in duplicate: δ13C was measured from lipid-extracted samples and δ15N from non-extracted samples. To validate the stable isotope results, stomach contents were examined and the relative importance of prey items was assessed using three measures: frequency occurrence (%F), numerical abundance (%N), and dietary carbon (%C). Both mesopelagic species occupied different isotopic niches that were separated by their δ15N values across a similar δ13C range. Furthermore, the relationship found between trophic position and standard length emphasizes the structuring effect of size within the assemblage, with the larger species (L. hectoris) occupying a higher trophic position than the smaller species (M. walvisensis). Although copepods dominated the diet of L. hectoris in terms of numerical abundance (42%), macro-zooplankton was by far the most important dietary component, with euphausiids contributing 53% of dietary carbon. Conversely, copepods - particularly Calanus sp. - were the most important component of the diet for M. walvisensis in terms of their occurrence (84%), numerical abundance (64%), and dietary carbon (67%). Though some dietary overlap exists between L. hectoris and M. walvisensis, the results of this study suggest resource partitioning within the mesopelagic assemblage, likely facilitated by differences in alimentary morphology (i.e. trophodynamically mediated), and possibly by differences in their respective foraging strategies. Similarly, ontogenetic shifts in trophic position were detected, which suggests that these adaptive features may also be used to mitigate intra-specific competition within populations. Furthermore, the trophic positions of both L. hectoris and M. walvisensis inferred from dietary and isotopic data signify that mesopelagic fishes (in the context of this study) are secondary and tertiary consumers in the marine ecosystem of the southern Benguela. Nevertheless, samples covering a larger area of the southern Benguela and multiple years would be needed for a more complete understanding of the trophic ecology of these two species.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/21001 |
| Date | January 2016 |
| Creators | Tyler, Tamsyn |
| Contributors | Jarre, Astrid |
| Publisher | University of Cape Town, Faculty of Science, Department of Biological Sciences |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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