SANCOR Newsletter Issue #169: March 2002
SAIAB FISH RESEARCH EXPANDS INTO TRANSKEI ESTUARIES
by Sekiwe Mbande, SAIAB
Estuaries are among the most productive aquatic ecosystems on earth. Because of their shallow, sheltered waters, they offer juvenile fish good conditions for growth and protection from a wide array of predators. Marine migrant fish species in particular utilize estuaries as nurseries for their juveniles. Generally, these fishes breed at sea and migrate into the estuary as late larvae or early juveniles. They remain in the estuary for about one to three years before migrating back to the sea where they join the adult stock. Some of these species are of recreational or commercial importance.
Mngazi and Mngazana estuaries are situated to the south–west of Port St Johns in the Transkei region of the Eastern Cape province. Because of the marine sand supply and exposure of the mouth to wave action, Mngazi sometimes closes in winter, whereas Mngazana is a permanently open system. Mangroves dominate the Mngazana intertidal vegetation while Mngazi has reeds and marshes in the intertidal zone. One of the reasons for the absence of mangroves in the Mngazi estuary is the periodic closing of the estuary, which would lead to drowning of the aerial roots of these plants.
It is well documented that the fish community structure in estuaries reflects physical, chemical and biological parameters of the estuary. Few studies have, however, investigated the influence of vegetation type on the food chains of fish in estuaries. The stable carbon isotope ratios in the tissue of an organism give a measure of the assimilated diet of an organism. When these isotope ratios are compared to those of the dominant primary producers, the source of the animals diet can be inferred.
The main aim of this study is to compare the fish communities of the above two estuaries in terms of species composition, diversity and identify environmental parameters that are responsible for the structuring of these communities. The stable carbon isotope ratios of a range of fish species from Mngazi and Mngazana estuaries will give a measure of the assimilated diet by each of the species and be related to primary and secondary food sources in the two systems.
Seasonal temperature variations in the aquatic environment affect the occurrence and abundance of different fish species in South African estuaries (e.g. tropical species are likely to extend their distribution southwards in summer and retreat northwards in winter). Similarly, temperate species may extend their distribution northwards in winter and retreat southwards in summer. According to research done in the 1970s the Mngazana estuary contains both tropical and temperate fish species, the proportions of which differed between summer and winter. There was an increase in numbers of tropical species in summer and most of these species disappeared in winter. Conversely, temperate species that were common in winter became scarce in summer.
Contrary to the above findings, preliminary results from January and June 2001 sampling indicate the presence of an overwhelming preponderance of tropical marine species and only a limited number of temperate species (even during winter). The scarcity of temperate species in these estuaries may be a reflection of possible global warming but further research is required before this trend can be confirmed.
ASSESSING FACTORS THAT AFFECT BREEDING
PRODUCTIVITY OF THE AFRICAN PENGUIN ON
ROBBEN ISLAND
by Jenny Griffin
Avian Demography Unit, UCT
The African penguin (Spheniscus demersus) is endemic to southern Africa and its associated offshore islands. It faces a high risk of extinction in the wild in the medium-term future, with a Red Data Book listing under the status "vulnerable", and is listed on Appendix II of the Convention for International Trade in Endangered Species (CITES). Population numbers have decreased dramatically over the past century from an estimated total population of 2 million in the early 1900’s to 170 000 towards the end of the century.
Robben Island is one of the few breeding colonies of the African penguin where the population is actually increasing in size. After being exterminated by sailors in the 1800’s, African penguins returned to Robben Island to breed in 1983 when 9 pairs were observed breeding. The numbers breeding on the island have steadily increased, with 3100 pairs observed breeding in 1996, and 6723 breeding pairs recorded during the 2001 population census. This is now the third largest breeding colony of African penguins, and has the capacity to expand further.
It is thought that the availability of suitable breeding habitat together with abundant food resources in Table Bay has facilitated the rapid growth of this colony. Alien vegetation on the island provides nesting penguins with shade and protection from predators. It is thought that this cover reduces heat stress and associated nest desertions, and diminishes the impact of nest predation by kelp gulls, hereby increasing reproductive success. However, intensive alien clearing programs are currently underway throughout the Cape Peninsula, and could also be planned Robben Island in order to reduce the risk of fire, and to increase grazing habitat for game on the island. It is therefore vitally important that we assess the impact that removal of invasive aliens such as rooikrans, pines and eucalyptus within the breeding colony will have on the productivity of penguins, whose overall numbers are rapidly declining. This will enable us to make recommendations for future vegetation management on Robben Island.
The main focus of my research is to assess to what extent habitat selection (macro-habitat) and choice of nest site (micro-habitat) affect nesting success in the African penguin (Spheniscus demersus) breeding on Robben Island. Other factors that are likely to affect reproductive success, such as distance of nest site from the sea, climatic conditions, nest density (colonial breeding), food availability and human disturbance will also be examined. Where possible, the effect of nest and mate fidelity on productivity will also be assessed, where the length of the relationship between the mating pair, and thus the strength of the pair bond, will be taken into account.
Approximately 25 nests from each habitat (coastal grassland, coastal rooikrans thicket, inland rooikrans thicket, inland eucalyptus forest, and disturbed boardwalk) will be monitored weekly over at least two breeding seasons. Choice of nest types (burrow, scrape, vegetation, or artificial nest) and percentage cover/shading offered will also be noted. Daily temperature and rainfall data will be retrieved from a weather station situated on Robben Island and correlated to peaks in nest desertions or nest failure. I will also be conducting experiments
(Spheniscus demersus) breeding on Robben Island. Other factors that are likely to affect reproductive success, such as distance of nest site from the sea, climatic conditions, nest density (colonial breeding), food availability and human disturbance will also be examined. Where possible, the effect of nest and mate fidelity on productivity will also be assessed, where the length of the relationship between the mating pair, and thus the strength of the pair bond, will be taken into account. Approximately 25 nests from each habitat (coastal grassland, coastal rooikrans thicket, inland rooikrans thicket, inland eucalyptus forest, and disturbed boardwalk) will be monitored weekly over at least two breeding seasons. Choice of nest types (burrow, scrape, vegetation, or artificial nest) and percentage cover/shading offered will also be noted. Daily temperature and rainfall data will be retrieved from a weather station situated on Robben Island and correlated to peaks in nest desertions or nest failure. I will also be conducting experiments using temperature probes attached to data-loggers to compare nest temperatures between different nest types and habitat types to determine to what extent temperature differs between nest sites and habitats. It is hoped that this will also provide some information regarding what temperature birds are most likely to abandon the nest to cool off in the sea, hereby exposing the eggs to predation by kelp gulls.
From a conservation point of view, it is important for environmental managers to take cognisance of factors that positively or negatively affect breeding success in the African penguin so that we may implement measures to enhance their productivity, and ultimately contribute to the survival of this species. It is hoped that this research will heighten our knowledge and understanding of factors that contribute to reproductive success or failure in these birds, hereby enabling us to make informed management decisions based on sound scientific research.
