The diet of selected benthic feeding fish in the Mfolozi-Msunduzi system, KwaZulu-Natal
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Date
2011
Journal Title
Journal ISSN
Volume Title
Publisher
University of Zululand
Abstract
The St Lucia estuarine system is considered to be the largest nursery area
for estuarine-associated marine fish in southern Africa. The prolonged mouth
closure due to regional droughts has resulted in large declines in its fish and
prawn communities with deleterious consequences for regional marine
populations, especially those that require estuaries for completion of their life
cycle. Mouth closure of the St Lucia system contributed to a renewed interest
in the St Lucia-Mfolozi System link, particularly the importance of the Mfolozi
system in terms of its role as a refuge and alternative nursery area for
species which cannot recruit into the closed St Lucia system when the
Mfolozi remains open. The Mfolozi-Msunduzi estuarine system is the closest
open estuary to St Lucia but there is a paucity of data on the ecological
functioning of the system. Studies on the benthos of the Mfolozi-Msunduzi
estuarine system have found it to be impoverished but large numbers of
benthic feeding fish are present in the system at times. This raised the
question: if the zoobenthic community is impoverished and high numbers of
benthic feeding fish are nevertheless present in the system, then what are
these fish feeding on?
The stomach contents of 961 fish of 15 different species collected between
August 2008 and March 2010 using seine and gill nets were examined. Of
these 15 species, only six were caught in sufficient numbers for gut analysis.
The six species that were used in the analysis were Leiognathus equula,
Acanthopagrus berda, Glossogobius giurus, Ambassis ambassis, Ambassis
gymnocephalus and Ambassis natalensis. Four methods were used for
stomach content analysis, namely Frequency of occurrence, Numerical
occurrence, Points (Proportional Volumetric) analysis and Index of Relative
Importance (IRI).
It was found that the diet of L. equula was dominated by the copepod, P.
stuhlmanni, with the macrobenthos contributing <0.5% to the overall diet of
the species. Similarly, the diet of A. berda was dominated by P. stuhlmanni,
with the benthic fauna contributing <1.5% to the diet. The dominant prey in
the diet of G. giurus was Glossogobius sp, while the only benthic species that was fed on was the crab P. blephariskios, which contributed not more than
0.3% to the overall diet of the species. A large proportion of the Ambassids
(A. ambassis, A. gymnocephalus and A. natalensis) had empty stomachs,
whilst most of those that contained food had stomachs < 30% full, based on
the Points determination method. The diet of all three Ambassid species was
dominated by P. stuhlmanni, with A. gymnocephalus feeding only on the
copepod. The total contribution of the benthic fauna found in the diet of A.
ambassis was <30% and <1% in A. natalensis.
Copepods dominated the diets of the majority of the species examined,
indicating that in the Mfolozi system, zooplankton rather than benthic
organisms are being fed on. This tends to tie in with the impoverished state of
the benthic fauna. The results of this study indicate that only those species
capable of adapting their diets to whatever is available and abundant to feed
on will be able to survive in the Mfolozi-Msunduzi system.
Description
Submitted to the Faculty of Science and Agriculture in fulfillment of the requirements for the degree of Master of Science in the Department of Zoology at the University of Zululand, South Africa, 2011.
Keywords
Estuaries -- South Africa -- KwaZulu-Natal, Estuaries--South Africa--St Lucia estuary, Benthic feeding fish