The Distribution and Abundance of Alien Fish Species within the Wilderness Lakes
By
Alexis Olds & Kyle Smith
It has been stated that on a global scale one of the greatest threats to fish biodiversity conservation is
the accidental or, in some instances, deliberate introduction of alien species into a water system.
The impact of alien fish species on native species depends on a number of factors most notably, the
behavioral relationship between the native and alien species. This interaction can be either direct
such as predation or competition (both spatially for living space and through the sharing of food
resources) or indirect which involves more than two species and how one species alters the effect that
a second species has upon a third species. Furthermore if introduced species are able to successfully
reproduce they can have a cumulative effect over time that has the potential to impact entire
ecological communities and has been known to contribute to species extinction.
Within the Wilderness Lakes System four of the worlds 100 worst invasive fish species have been
reported, namely, an extralimital population of the Mozambique Tilapia Oreochromis mossambicus,
which occurs naturally in estuaries down to the Bushmans River in the Eastern Cape, the Central
American mosquitofish Gambusia affinis, the Eurasian common carp Cyprinus carpio and the North
American largemouth bass Micropterus salmoides.
Although various studies have looked at the impact of these species on freshwater systems and have
documented changes such as the alteration of invertebrate communities, predation, habitat alteration
and cascade effects in aquatic communities, the impact on estuarine fish communities is still
unknown. As a biodiversity conservation body and the management authority for the Wilderness
Lakes System, SANParks is legally and morally obliged to monitor alien species and implement
appropriate management actions. Although the most effective management is in prevention,
management of existing populations of alien species is equally necessary. However, until more is
known about their temporal and spatial distribution, abundance, population structure and the type of
pressure exerted on system biodiversity, a management strategy for these alien fish species cannot be
effectively implemented.
In order to provide some of this information and to contribute to an effective alien invasive
management strategy a recent project has been initiated in collaboration with Rhodes University and
the South African Institute for Aquatic Biodiversity. To begin with, the project aims to assess the
spatial and temporal distribution and relative abundance of alien fish species throughout the system.
To do this a number of fish sampling gear types will be used of which 10 and 30 meter seine nets are
a part. The two seine nets used within this study effectively sample the shallower zones with the 30m
net being set approximately 30m offshore and the 10m being used in the very shallow (<1m) littoral
zone and in dense aquatic vegetation.
Within Island Lake the landing of these nets is restricted due to the dense reed beds surrounding most
of the lakes edge. As such we have chosen two sites, one at the SANParks picnic area and the other
in front of the GLYC within which three 30m seine net pulls are to be completed each season and a
total of ten 10m seines. We have just completed the Autumn sampling and the preliminary results
are presented below.
The range of species caught in the 30 and 10 meter seine nets were very similar with some larger
species (white steenbras, Lithognathus lithognathus and leervis, Lichia amia) being caught in the
slightly deeper 30m pulls (Table 1). As one would expect with the larger net and hence larger sample
area substantially more fish were caught with a total of 36 570 fish being caught (the majority 76%
being cape silverside, Atherina breviceps) as opposed to the ten meter seine in which 2 359 fish were
caught. There was very little difference in the average size of fish caught between the two net types
(Figure 1) with the exception of pipefish in which far smaller individuals were caught in the 10m net.
Table 1: The total number of fish caught in the 30m and 10m seine net pulls.
30m
Seine
Net
10m
Seine
Net
Cape
Silverside
Estuarine
Round
Herring
27960
8010
1972
182
Cape
Halfbeak
Mozambique
Tilapia
White
Steenbras
106
481
2
7
137
50
4
0
Goby
Leervis
Pipefish
Mosquitofish
3
1
0
0
13
1
Figure 1: Average size of fish caught within the 30 and ten meter seine nets.
The target species of seine nets are smaller, shoaling species but occasionally larger species that
normally avoid the net are caught and the catches of white steenbras and Leervis were particularly
exciting. Sampling in the densely vegetated littoral zone is important as these areas provide a safe
nursery and feeding area for smaller species and juveniles. For example pipefish reside in the aquatic
vegetation as it provides an ideal camouflage and as a result, higher numbers of pipefish were caught
in the 10m seine nets. On the other hand the Cape halfbeak (Hyporhamphus capensis) is an
epipelagic fish which feeds at the surface on algae, zooplankton, small fish and eggs and are primarily
caught within the 30m seine nets while feeding on the water surface.
The investigation into the extent of alien invasion within the Wilderness Lakes system is a long term
process but this study will provide valuable insight into the state of biological invasion. The data
presented above is only a fraction of what will be collected over the next two years.
Landing the Seine Net
Cape Half Beak