Browsing by Author "Branch, George M"
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- ItemOpen AccessAncient stonewall fish traps on the south coast of South Africa : documentation, current use, ecological effects and management implications(2006) Kemp, Lucy Valeska; Branch, George M; Attwood, ColinAncient intertidal stonewall fish traps are found world-wide and those along the SouthAfrican south coast are the focus of my thesis. These fish traps, known locally as 'vywers', have recently enjoyed much media attention as interest increases in both South Africa's cultural heritage and its diminishing fish stocks. Two pioneering studies, by Goodwin (1946) and Avery (1975), provided the only documented knowledge of these vywers. My study aimed to locate, survey and document the main concentrations of vywers within a 300-km stretch along the south coast. A total of 43 sets of vywers was located by aerial surveys, 30 of which are only accessible through private land. This affords them some measure of protection, together with five located off reserves, but diminishes their educational and tourism value. Four sets of vywers were mapped in detail using aerial photogrammetry, a method that provided a rapid, extensive, accurate survey record in the form of geo-rectified ortho-images of these sites. The vywers are built in both exposed and sheltered environments, constructed from in situ rock material built into walls with either angular or curved shapes. These walls may occur singularly or in complexes of up to 25 traps. Vywers are prone to decimation by wave action and storm damage and so require maintenance to retain their characteristic form and associated cultural information. Tensions have, however, arisen between those who maintain and fish the vywers, and fisheries managers. Data from a questionnaire survey compared with records from the literature showed that species composition has not changed significantly in the last five decades. The fish are caught most frequently during new-moon spring-tides, especially in the winter months. The vywer fishery, currently active at only two sets of vywers, targets primarily mullet species (mostly Liza richardsonii) but infrequently enjoys 'bonanza' catches of over-exploited linefish stocks such as galjoen Dichistius capensis. It is these latter catches that concern managers, in addition to the noncompliance of fishers in terms of catch composition and size, permits and gear.
- ItemOpen AccessApproaches to assessing the status of nearshore and estuarine fish and fisheries and their responses to environmental change(2008) Lamberth, Stephen Justin; Branch, George MIn multispecies fisheries knowledge, personnel and funding limitations often create the dilemma over which species should be the first to receive management and research attention. This thesis uses a Multi-Criteria Decision Analysis approach to prioritize 176 South African linefish species on the basis of a number of criteria indicating conservation and socio-economic importance. Each species importance was assessed from the points of view of (1) conservation, (2) each fishery sector separately and combined and (3) the overall combined conservation and sectorial importance. Relative scores within each of the fishery sectors were dominated by two or three species such that the scores separating the remaining species from each other were small. On the whole, the top species included those most important in each sector, biogeographic region and in terms of conservation. The separate and overall ran kings should assist in the development of broadly acceptable management strategies for different fish. Catch data for 129 of 255 functional estuaries on the South African coastline were reviewed and the relationships between fish catch and estuarine size, type and biogeographical region analysed using simple and multivariate models. Estuary size alone explained over 80% of the variation in catch in the warm temperate region and over 90% of the variation in catch in the cool temperate and subtropical region. Further analysis of the two main estuarine types (permanently open and temporarily open/closed estuaries) revealed higher productivity for the permanently open systems. Both estuarine size (ha) and type (5 types) were used to explain catches using general linear models. The models were able to explain 82-98% of the variance in catches. The total estuarine catch in South Africa was estimated at 2 480 tons per annum with a total value of R748 million, of which 99% could be attributed to recreational angling. Estuarine contribution to the inshore marine fisheries was estimated at approximately R846 million per year with estuarine dependent species comprising 83% of the catch of the recreational shore and commercial seine and gillnet fisheries and only 7% of the catch of the recreational spearfishery and commercial and recreational boat fisheries. Total value of estuarine and estuary-dependent fisheries was estimated to be R1.594 billion per annum in 2008 rands. The cool-temperate Olifants and warm temperate Breede estuaries on the west and south coasts of South Africa have both seen a >35% reduction in mean annual runoff (MAR) from a historical reference condition to the present day with a >60% reduction possible under future development. With almost no exceptions, the Olifants Estuary fish assemblage has seen an overall 20% decrease in abundance from reference to the present day and will gradually decline to 55% of reference with a predicted future 60% reduction in MAR. Consequently, future reductions in flow are likely to see the Olifants Estuary progressing towards a low biomass, low diversity, marine-dominated system. In contrast, with the exception of some key exploited species, fish abundance in the Breede Estuary has increased by 6% from reference to the present day and is likely to increase to 115% of reference with future reductions in flow. In all, the fish assemblage will experience a gradual change from a relatively high-diversity low-abundance freshwater-rich system under historical flow conditions to a high-diversity, highabundance, marine-dominated system with future reductions in flow.
- ItemOpen AccessAspects of marine fouling in western Cape waters(1981) Henschel, Johannes Robert; Buys, M E L; Branch, George MTwo independent investigations into aspects of marine fouling were conducted in Simonsbay and Table Bay during 1979 to 1981. The development of macrofouling communities on six test materials was examined at 10m and 20m depths in Simonsbay for periods ranging from one month to one year. Community development was similar on inert non-reactive materials, aluminium, stainless steel, fibre glass and polyvinylchloride, but was reduced on non-wettable silicon rubber and corrodible mild steel. Macrofouling was characterized by seasonal succession with minimum colonization rates during winter, when adverse weather and low temperature conditions prevailed. The nature of fouling differed with depth. At 10m depth, mussel and barnacle-dominated communities developed rapidly, while at 20m depth, ascidian and barnacle-dominated communities developed more slowly. The role of primary film formation in the colonization of substrata by invertebrates was investigated in short-term studies conducted in Simonsbay and Table Bay. Surface-bound antibiotics, streptomycin and penicillin, were used to inhibit bacterial proliferation, while a herbicide, diuron, was employed to prevent diatom growth. The colonization by invertebrates was monitored on these surfaces and compared to surfaces where primary film development was normal, or where it was advanced by pre-culturing in laboratory seawater. It was found that invertebrates attached soon after panel exposure and that differences in the degree of primary film development were of little consequence to their settlement. The apparent discrepancy of these observations with previous findings is discussed, with special reference to the location of test sites in relation to mature communities.
- ItemOpen AccessAssessment of predation by the West Coast rock lobster (Jasus lalandii) : relationships among growth rate, diet and benthic community composition, with implications for the survival of juvenile abalone (Haliotis midae)(1998) Mayfield, Stephen; Branch, George M; Cockcroft, AndrewThis thesis focused on two aspects of predation by the West Coast rock lobster (Jasus lalandii). These were (1) the problem of slow growth currently observed in adult male rock lobsters and the putative link between growth rate and food availability, and (2) the influence of rock-lobster predation on the benthos, but more specifically on sea urchins and juvenile abalone.
- ItemOpen AccessBaseline surveys and metal binding proteins as metal pollution indicators(1985) Hennig, Helmke Friedrich-Karl Otto; Orren, Michael J; Branch, George M; Brandt, Wolf FThe field of metal determination as a part of pollution studies, has been critically examined and metal pollution may be defined in one simple statement: The presence of metal binding proteins confirms toxic metal pollution. It has been shown that current methods of metal determination in biological systems are of little use. This has been illustrated by both a review of metal concentration in Southern African coastal water, sediments and biotopes, and by a comparative baseline study of organisms from Gough Island and Mar ion Island. These showed that extrapolation of results from one geographical area to another are invalid and that this interpretation is made difficult by factors such as age, sex, size life stage of the organisms. Furthermore, it was shown that many reports on metal pollution do not even mention fundamental information such as the size or the sex of the animals. Metal pollution could be linked to metal binding protein through an independent pollution er i ter ia, for example, the out of season moulting of crayfish. The new definition of metal pollution has then been tested by application to five different organisms (crayfish, Jasus lalandii; hermit crab, Diogenes brevirostris; shrimp, Palaemon pacificus; black mussel, Choromytilus meridionalis and limpet, Patella granularis) kept under identical conditions and it was shown that a much more meaningful interpretation of the results could be made. The new definition was al so tested with two naturally occurring metal accumulating organisms (whelk, Bullia digitalis and "kikuyu" grass) and it was shown that dramatic increases in metal may not necessarily be toxic. It was concluded that less effort and time should be spent on metal analysis in determination of metal pollution and attention should rather be directed to the presence or absence of metal-binding proteins.
- ItemOpen AccessThe benthic invertebrate fauna of subantarctic Marion and Prince Edward Islands(1994) Branch, Margo L; Branch, George MThere have been several expeditions to Marion and Prince Edward Islands (MPE) which have sampled the marine fauna and flora, including those of the British ships Challenger (1873-1876) and Discovery (1935), and subsequent surveys by the French ship Marion-Dufresne (Arnaud and Hureae, 1979). South African research began with land-based surveys that concentrated on the intertidal and shallow-water benthos and included work by Fuller (1967), Van Zinderen Bakker et al 1971 ), De Villiers (1976) and Blankley & Grindley (1985). More recently this research was extended offshore by the University of Cape Town, with dredging being undertaken from the SA Agulhas over the period 1984-1989 (GM Branch, Attwood, Gianakouras and ML Branch, 1993) and a quantitative SCUBA survey at depths of 5, 10 and 15m at Bullards Bay, Transvaal Cove and Trypot Point (Beckley and Branch, 1992). The identification of benthic species posed a major problem during each of the recent surveys, despite the fact that the material from earlier expeditions has been referred to international taxonomic authorities. Reference specimens are not available or are housed in European collections. The number of species from the area has also increased considerably. Previous work in the subantarctic has been undertaken by scientists from a large number of countries and over a protracted period of time. Isolated detailed monographs reviewing particular groups have appeared, but these remain largely inaccessible to the more generalist worker. Furthermore many are now out-of-date in terms of the nomenclature employed. This situation prompted the compilation of the present series of identification guides, which are intended to synthesise information on the benthic invertebrate fauna of Marion and Prince Edward Islands and to present it in a manner that will allow relatively easy identification by the non-specialist.
- ItemOpen AccessBenthic-pelagic coupling : rocky intertidal communities and nearshore oceanographic conditions across multiple scales(2006) Wieters, Evie A; Branch, George MIn Chapter 1, I characterize geographic patterns in rocky intertidal communities across 6° of latitude along the west coast of South Africa and examine the spatial structure of functional group biomass in relation to wave action and upwelling intensity. Despite between-habitat differences in biomass, most functional groups showed similar regional trends in exposed and sheltered habitats, but weaker non-significant between-habitat associations were observed when considering differences due to site. Divese geographic trends were observed, with only specialized kelp-trapping limpets showing a smoothly decreasing trend with latitide. Abrupt changes in the abundance of several species were observed at about 32° or near 34.5° S. Correlations between functional groups were strongest in the low (exposed) and mid (sheltered) shores, and supported the possibility that species interactions, particularly (1) competitive dominance by filter feeders and gardening limpets and (2) habitat facilitation by filter feeders partially account for local differences in functional-group abundances. Nearshore oceanographic conditions were characterized using satellite-measured sea surface temperature (SST), verification by in situ loggers, and analysis of Offshore Ekman Transport (OET) indices. A clear discontinuity at about 32° S partitioned the coast into a northern region characterized by consistenly strong and spatially homogenous upwelling, and a southern region distinguished by significant mesoscale variation in seasonal upwelling intensity among sites, with clear upwelling centers alternating with "downstream" areas only weakly influenced by upwelling. Functional group relationships with SST differed between north and south regions. Local abundances were generally more variable in the south, where greater biomass of ephemeral and corticated macroalgae, as well specialized gardening and kelp-trapping limpets, were associated with upwelling centers. In contrast, high-shore filter-feeder and predator biomasses were significantly greater at downstream sites. These results suggest that oceanographic discontinuities around 32° S may frame community dynamics and that upwelling intensity influences community structure and generates regular spatial differences in interaction webs along the South West Cape. This study represents the critical first step to identifying spatial scales at which processes regulate communities, and provides a series of testable hypotheses that can be evaluated by experimental comparative approaches.
- ItemOpen AccessBiochemical and morphological systematics of the Southern African gastropod genera Burnupena (Buccinidae) and Oxystele (Trochidae)(1995) Dempster, Yvonne Laura; Branch, George MGastropods of the genus Burnupena are very common rocky-shore whelks endemic to the Southern African region. However, despite recent revisions of the genus, difficulties in identification persist, due mainly to the occurrence of phenotypic intermediates between sympatric species. Samples of Burnupena were collected from 17 sites along the coast of South Africa. Variation in shell morphology was examined using multivariate analyses. The results revealed that the degree of variation within species is very high, and that shell form and colour can be influenced by local environmental conditions. Nevertheless, most populations of the individual species clustered together. Morphological differences between species were small, although distinctions could be discerned, with not all species being confused with all other species. However, on the basis of morphology, there will always be a few individuals which will remain difficult to identify. Examination of the radulae from all of the species of Burnupena using both light and electron microscopy showed that the radula is of little or no practical value in distinguishing between species. Whilst some differences could be detected in some characters, these were not consistent, either within or between species. However, the radula is diagnostic at the generic level. The populations of Burnupena were analysed for allozyme variation at 25 loci. Variation within species was high, and in all of the species, significant population differentiation was obseNed. A significant correlation between genetic and geographic distance was found. Nei's genetic distances within species were low (means per species ranging from 0.01 to 0.05), and the populations of each species clustered together. Unlike their morphological differences, the species were mostly well differentiated genetically, with Nei's distances between species ranging from 0.1 to 1.45. Compared to the other species, B. cincta and B. lagenaria were relatively little differentiated from each other. B. catarrhacta was strongly differentiated from all of the other species. Comparisons between sympatric populations suggested complete reproductive isolation between the species.
- ItemOpen AccessBiodiversity survey towards conservation of subtidal reef habitats in KwaZulu Natal : biogeography and depth patterns(2005) Lawrence, Cloverley; Harris, Jean M; Branch, George MSubtidal reef communities in KwaZulu-Natal (KZN), South Africa are poorly known. This lack of knowledge is problematic as the biodiversity of these reefs may be severely impacted and inadequately conserved. This study documents and describes subtidal benthic communities occurring on reefs at four depth categories along the whole length of the coast. A distinct difference between northern reefs from those in the south and central parts of the province emerged with substantiating evidence of a discrete biogeographic separation at Cape St Lucia. Pairwise ANOSIM tests found no significant differences in community composition of reefs along a depth range of 10 m to 30 m at nine localities in KZN. However, differences among localities were significant at both regional (R = 0.607, P = 0.1%) and local (R = (1.792, P = 0.1%) scales. In the north, trends in species assemblages and functional groupings revealed a higher percentage cover of fauna (mainly corals) at shallower depths and a greater coverage of algae on deeper reefs. In the southern localities algae dominated shallower reefs while filter-feeding epifauna were more prevalent at deeper depths. Species richness, evenness and diversity were highest at 10m in the northern coral-dominated region while in the central/south region diversity peaked in the intermediate depth zone (15 - 25 m). Appropriate measures to conserve representative habitats in each biogeographic zone are necessary. Further research to assess biodiversity at a finer scale, as well as the establishment oflong-term monitoring to quantify natural variability and human effects, are required.
- ItemOpen AccessBiogeography and potential factors regulating shallow subtidal reef communities in the western Indian Ocean(2009) Porter, Sean N; Branch, George M; Sink, Kerry J; Kaehler, SvenThe biogeography and ecology of benthic shallow subtidal reef communities in the western Indian Ocean is poorly known, particularly in north-eastern South Africa and southern Mozambique. This thesis uses quantitative information to resolve biogeographic patterns, define reef community types, elucidate potential abiotic determinants of community composition, and evaluate whether subsidies of riverine-derived particulate organic matter (POM) support filter-feeder biomass and drive biogeographic patterns. A large-scale biogeographic analysis was conducted using quantitative biomass data derived from 55 shallow subtidal reefs spanning five countries in the western Indian Ocean. Two statistically distinct marine provinces, Tropical Indo-West Pacific and Subtropical Natal, were recognised by differences in community composition and separated by a biogeographic break in the vicinity of Cape Vidal, South Africa. The biogeographic break took the form of a transitional or overlap area corresponding in location to the Delagoa Bioregion, one of three bioregions also revealed by post-hoc analyses. Significant differences in total average biomass and trophic structure were evident among bioregions, with a number of inter-bioregional trends in trophic groups being apparent. In total, 12 reef community types were recognised, based on similarity profile permutation tests. Most reefs in the Subtropical Natal Bioregion were dominated by a community type characterised by a high biomass of the filter-feeding ascidian Pyura stolonifera and various species of articulated coralline algae. In the Delagoa Overlap Bioregion, a comparatively high diversity of community types was defined, many dominated by algal turf, P. stoloniferaand various Alcyonacea and Scleractinia. Further north, P. stolonifera diminished and the contributions of Scleractinia, especially Porites spp., Pocilloporaspp. and Galaxea spp. increased. Many of these community types are not represented within protected area networks, particularly those in southern Mozambique. When the biomass data were correlated with nine abiotic variables, likely determinants of community composition emerged at both inter- and intra-regional scales. Sea surface temperature, significant wave height, chlorophyll-a and suspended inorganic sediment were the variables highly correlated with community composition and therefore most likely to drive biogeographic differences. Within each bioregion, different sets of abiotic variables were found to be important in driving community differences among sites, including turbidity, chlorophyll-a, reef susceptibility to sand inundation, reef heterogeneity and sea surface temperature. Striking differences in the oceanographic conditions of bioregions were evident, especially between Subtropical Natal and Delagoa Overlap bioregions. In particular, the strong influence of wave height emerged as a novel and unexpected correlate at a biogeographic scale. These differences initiated a trophic study conducted in the Subtropical Natal Bioregion, aimed at determining the importance of riverine-derived POM subsidies in supporting the high filter-feeder biomass in this bioregion. Using carbon, nitrogen and sulphur isotopes and a three-source Bayesian mixing model to calculate proportional contributions, I determined that marine-derived POM formed the bulk of the diets of four species of filter-feeders, but the assimilation of riverine-derived POM was nevertheless notable, ranging from 8 to 33 %. I concluded that riverine POM is likely to play an important but secondary role to factors such as increased levels of turbidity and productivity in explaining the high filter-feeder biomass in the Subtropical Natal Bioregion. These findings provide the first evidence of riverine-inshore-pelagic coupling in filterfeeder communities in this bioregion, and throw light on the factors linked to large-scale biogeographic patterns.
- ItemOpen AccessBiologically induced alternative states in two rocky subtidal benthic communities(1987) Barkai, Amos; Branch, George MThis thesis is concerned with the contrasting communities that occur in the subtidal zones of two closely situated islands in Saldanha Bay on the west coast of South Africa: Both are guano islands, supporting substantial populations of seabirds. They have a similar bottom topography and experience similar physical conditions. Despite these similarities their subtidal benthic communities are however, radically different. Marcus Island has prolific beds of the black mussel Choromytilus meridionalis, accompanied by a large number of other species, notably the sea urchin Parechinus angulosus, the holothurians Pentacta doliolum, Thyone aurea and Trachythyone insolens, large population of whelks, mostly Burnupena spp. and the barnacles Austromegabalanus cylindricus and Notomegabalanus algicola. In contrast, Malgas Island is dominated by a large population of the rock lobster Jasus lalandii together with a dense seaweed flora, and has small numbers of the ribbed mussel Aulacomya ater and the whelks Argobuccinum pustulosum and Burnupena papyracea.
- ItemOpen AccessBiology and ecology of the deep-water rock lobsters Palinurus gilchristi and Palinurus delagoae in relation to their fisheries(2000) Groeneveld, Johan Conrad; Branch, George M; Cockcroft, AndrewRecent declines in the biomass of the closely-related deep-water rock lobsters Palinurus gilchristi and P.delagoae off South Africa suggest that both these resources are currently overfished, and that the existing management procedures are inadequate. A lack of detailed fisheries, biological and ecological knowledge is partly to blame, as this deficiency limits the scope and utility of resource assessments, and impairs the capacity of managers to forumulate species-specific management strategies. This thesis aimed to expand the biological and ecological bases on which effective resource assessment rests, and to recommend methods to improve the management of these deep-water rock lobster populations. The steps taken were: (1) determination and interspecies comparison of biological parameters, including spatial and temporal size composition trends, somatic growth rates, size at sexua maturity, female fecundity, mortality rates, moulting, recruitment and migration; (2) assessment of the effect of fishing with different gear types or arrays on abundance and its indices; and (3) incorporation of new information into resource assessments, with recommendations aiming to improve the management of the fisheries.
- ItemOpen AccessCarbon and nitrogen flux through the Cape rock lobster jasus lalandii (H. Milne Edwards), with particular reference to the nearshore Benguela system(1989) Zoutendyk, Peter; Field, John G; Branch, George MObservations and experiments on Jasus lalandii were undertaken over the period 1977 to 1986 to quantify the passage of carbon and nitrogen through populations at Oudekraal and Robben Island on the west coast of South Africa. The terms of the energy flow equation, C=P+R+G+UE+Ux+Uo+F (where C=consumption, P=production, R=respiration, G=reproduction, UE=Exuvial loss, Ux=excretion of exogenous nitrogen, Up =excretion of endogenous nitrogen and F=faeces), were quantified by combining field data and laboratory results. With this information the role played by J. lalandii in the kelp-bed ecosystem under the influence of the Benguela current was explored and quantified.
- ItemOpen AccessA comparison between macrofaunal communities on mixed shores and rocky and sandy shores in False Bay(1999) Smith, Cameron Ewart; Brown, Alec C; Branch, George MThe community structures of three shore types namely: "mixed shores" (those where rocky and sandy-shore habitats are intermixed), pure rocky shores and pure sandy beaches in False Bay, South Africa are compared in this study. Four habitats were identified - pure rock (unaffected by sand), mixed rock (rock affected by sand), mixed sand (sand between emergent rocks) and pure sand (beaches with no emergent rock) - representing a gradation from pure rock to pure sandy beaches. The specific aims of this study were to: (1) Sample quantitatively and describe macrofaunal communities on mixed shores in False Bay; (2) make direct comparisons among both the four types of habitats and three types of shores; and (3) test the hypothesis that sand inundation increases diversity at both habitat (a-diversity) and shore (diversity) level. The biological communities of mixed shores are described in terms of species composition, trophic organisation and zonation. Mixed-shore zonation patterns are different from those previously described for pure rocky shores in the region. The ability of Charomytilus meridiana/is and inability of patellid limpets and various algae, to withstand sand inundation are largely responsible for these differences.
- ItemOpen AccessContributions to the ecology of the Benthic macrofauna of the Bot River Estuary(1986) De Decker, Hendrik Paul; Branch, George MThis thesis forms part of a multi-disciplinary study of the Bot River estuary, situated between Kleinmond and Hawston on the southwest coast of the Cape Province, and falls within the framework of the investigation of all Cape estuaries, initiated by SANCOR and co-ordinated by the Estuarine and Coastal Research Unit of the CSIR. The motivation for the research program on the Bot River estuary, in particular, was the need to obtain detailed knowledge of its dynamics in order to be able to address its unique management problems.
- ItemOpen AccessDiet of the Tristan rock lobster Jasus tristani following the 2011 soya spill at Nightingale Island(2017) Jones, Luke; Blamey, Laura K; Branch, George M; De Lecea, Ander MThe spiny lobster Jasus tristani inhabits the Tristan da Cunha Island archipelago in the South Atlantic Ocean 2,400km from the West coast of South Africa. It is commercially exploited across the archipelago and is the main economic source of income for the local government as it accounts for nearly 80% of the local gross domestic product. The commercial rock lobster industry was established in 1949, and the fishery is currently recognised as sustainable, and as a result was awarded a Marine Stewardship Council Certification in 2011. That same year however, the sinking of the OLIVA at Nightingale Island spilt 60,000 tonnes of soya beans (Glycine max), greatly affecting the local benthic environment, and with probable consequences for the local food web, including the diet of J. tristani. It is still unclear whether the soya beans are still on the seafloor at Nightingale Island as there have been no scientific surveys conducted since the spill. Using samples from 2015, I assessed whether the diet of the lobsters from Nightingale Island differs from that of lobsters from the unaffected Tristan and Inaccessible Islands, and whether there is any evidence of soya in the diet of the lobsters from Nightingale Island. In addition, I examined whether diet differed between lobsters of small or large size, and between shallow and deep depths. In total, 540 lobsters were sampled across the three islands, and a combination of gut content and stable isotope (SI) analysis was conducted to assess the dietary components of the lobsters. Gut fullness was significantly less at Nightingale Island suggesting there may be less food available on the reef. Statistical analyses showed that diet differed between all islands, depths and sizes, although this was difficult to ascertain from visual multidimensional scaling plots as diet showed considerable overlap and variability both within and between islands, depths and sizes. The stable isotope analysis showed significant differences in nitrogen levels among the three
- ItemOpen AccessDoes the implementation of a closed fishing season during the breeding season benefit a species? : a per-recruit-based approach using Cymbula granatina as an illustration(2004) Govender, Anesh ;Arendse, Clement; Branch, George MClosed seasons are generally implemented on the presumption that they increase reproductive output of fished populations. This is based on the assumption that the imposition of a closed season during the breeding season allows more individuals to reproduce, as they are not being harvested. I evaluated the validity of imposing closed seasons during the breeding season by creating a simulation model using the limpet Cymbula granatina as a test case and then compared the fol lowing four scenarios: 1) an unharvested population; 2) no closed season imposed; 3) a closed season imposed during the breeding season; and 4) a closed season imposed outside the breeding season, to determine the effects of the latter three situations on the reproductive output and yield. It was determined that closed seasons do not significantly affect the reproductive output of the population and that the timing of closed seasons made no difference to the reproductive output of the population, as the output was the same for populations with closed seasons during the breeding season or outside the breeding season. Survivors, catch in numbers and yield in biomass were affected by the timing of closed seasons, with higher outputs for each obtained when the closed season was closer to the month when individuals become of harvestable size. It was thus concluded that the imposition of a closed season at any time of the year is an effective management measure if imposing the closed season can reduce annual fishing mortality, but the imposition of a closed season specifically during the breeding season with a view to increase reproductive output brings no benefits relative to closure at any other time of the year. Moreover, any closure will be ineffective if it does not also bring about a reduction of annual fishing mortality. The reasons for advocating closure of a fishery during the breeding season are thus based on false grounds in the case of the limpet C. granatina.
- ItemOpen AccessThe echinoid Parechinus angulosus and its association with other invertebrates(2000) Robertson-Andersson, Deborah; Branch, George MEchinoids are recognized as important components of temperate kelp forests because of the impact that they have by grazing. The provision of shelter by some echinoid species for both juvenile conspecifics and for other species of echinoids has been widely documented. The behaviour is thought to be important in the survival of these species. The role of echinoids as protective "nurseries" for several types of marine animals has also been suggested. Adult echinoids are supported 1 - 2 cm above the substrate by the oral spines, this leaves a gap under the echinoid where other organisms can shelter. This paper examines firstly what types of organisms associate with the echinoid Parechinus angulosus and the degree to which they display tight relationships vs. chance associations. Secondly, it explores how much selectivity of the echinoids was involved when compared to other physical shelters and open substrate. This was done by comparing differences in the species associated with level, flat rocks covered with crustose corallines; the holdfasts of Ecklonia maxima; sand, and under echinoids. The study was done at four sites in False Bay, Cape Peninsula, South Africa. Samples were collected on each habitat type by placing a bottle over the substrate and scraping the sample into the bottle. The organisms in each sample were identified under a microscope and counted. The available habitat at each site was survey by divers swimming 6 am transects and noting the cover of a particular habitat falling under the tape measure. The total number of species and organisms for each site were compared using one-way and two-way ANOVA's on untransformed data. The availability of different habitat types at each site was compared, using one-way fixed-effects ANOVA. Results showed that there was no significant difference between habitat availability at the different sites. Results of the two-way ANOVA's comparing site and habitat for individual species showed that the amphipod Paramoera capensis only occurred in significant numbers at one echinoid site. The fact that 65 % of this species had a colour variation that matched the echinoid, implied that the amphipod gains protection from predators by being associated with subtidal echinoids. This relationship is not obligate as this amphipod was found in other habitats. The echinoid Parechinus angulosus appears to have a dual habitat preference with juveniles less than 1 O mm in diameter sheltering beneath adult conspecifics and juveniles greater than 1 o mm in diameter sheltering in kelp holdfasts. The amphipod Hyale grandicomis occurred in significant numbers associated with echinoids and had a habitat preference for echinoids, suggesting that it has an important relationship with subtidal echinoids. The amphipod is a known grazer of macroalgae and thus it is possible that the amphipod gets access to a food resource in the form of drift kelp, which is trapped by the echinoid. If this is the case then the amphipod would also be gaining protection from predators by being associated with the echinoid as the amount of time it is exposed while foraging is minimal.
- ItemOpen AccessThe ecological effects of bioturbation on the eelgras Zostera capensis : community interactions and the impacts on the biota of an intertidal sandflat(2003) De Vos Siebert, Timony-Lee; Branch, George MThalassinidean prawns in the genus Callianassa have been singled out as important bioturbators because of their size and activity, and because they often occur at high densities and burrow deep into the sediments. The ecological effects of bioturbation by Callianassa kraussi on the eelgrass Zostera capensis, and its indirect effects on the sedentary mudprawn Upogebia africana and the biota of an intertidal sandflat were assessed at Langebaan Lagoon, South Africa, through comparative surveys, observations and field experiments. I hypothesised that C. kraussi and Z. capensis have mutually detrimental effects on each other, with bioturbation by C. kraussi leading to smothering of Z. capensis, and stabilising of sediments by Z. capensis impeding burrowing of C. kraussi. I also hypothesised that C. kraussi would be negatively correlated with U. africana because the latter relies on semi-permanent U-tubes to filter-feed, and bioturbation is likely to disrupt these.
- ItemOpen AccessEcological interactions between abalone (Haliotis midae) juveniles and sea urchins (Parechinus angulosus), off the south-west coast of South Africa(1998) Day, Elizabeth; Branch, George MField surveys were carried out in the kelp beds of the shallow subtidal off the south western Cape of South Africa, which confirmed the existence of a strong positive relationship between the urchins Parechinus angulosus and juveniles of the abalone Haliotis midae. Both species occupied primarily hard substrates, showing preferences for encrusting corallines. Of the juvenile abalone sampled, > 98% were found beneath sea urchins. All small (3-10 mm shell length) and medium sized (11-20 mm shell length) abalone juveniles occurred under urchins, whether on flat or vertical reef, or in crevices. A small proportion (approximately 10%) of large juveniles (21-35 mm shell length) were not found under urchins, and in these instances all occupied crevices instead. Subsequent surveys also revealed a positive, but weaker, association between abalone recruits (<3 mm shell length) and urchins - an unexpected result, given that abalone recruits are cryptic against the encrusting coralline substrate that they occupy preferentially. Thus camouflaged, they were assumed to gain little additional benefit from sheltering beneath urchins. Selectivity indices showed that, amongst different morphological categories of encrusting corallines, recruits showed preferences for strongly-textured corallines, such as the so-called "knobbly" and "paint" textures. Lower densities of recruits per unit substrate area were recorded on the smoother "velvet" corallines. However, almost all (80%) of recruits were found on velvet corallines occurred under urchins, as opposed to 28 and % of recruits on paint and knobbly corallines respectively.
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