The recruitment of Palaemon pacificus to the Bushmans River estuary was studied over a 12 month period. The dynamics of a population of shrimps in a small, tidal channel in the lower reaches of the estuary was investigated over a similar time span. Temperature and salinity tolerances of various stages and sizes of shrimps were compared in controlled experiments and are discussed in the light of the distribution and general ecology of the shrimp. Stage 6 zoeae and post-larvae entered the estuary on nocturnal flood tides throughout the year with peak recruitment occurring in March/April. The population of shrimps in the study site was predominantly juvenile for most of the year, with peak sub-adult numbers occurring in midsummer. The sex ratio was female-biased throughout the year except for short bursts of male predominance in February and April. Average growth rate was 0,043 mm carapace length per day. Maximum residence time in the study site was estimated to be four months and in the estuary at least six months. Flooding of the estuary resulted in a considerable reduction in juvenile numbers in the study site and some loss of sub-adults. Shrimps tolerated a temperature range of 4,3 to 30,9⁰C for 144 hours. Their long term tolerance range is probably narrower (10 to 28⁰C) because of the effects of temperature related factors such as starvation and disease. Their distribution in South Africa (Olifants River to Kosi Bay ) is consistent with this tolerance range. Sub-adults tolerated salinities of 1 to 79 ‰ and post-larvae 2 to 60 ‰ at 15⁰C for 144 hours. Low moulting success (low and high salinities ) and starvation (high salinities) would probably reduce these tolerance ranges to 2 to 56 ‰ (sub-adults) and 4 to 56 ‰ (post-larvae). Higher and lower temperatures (10 and 20⁰C) reduced the tolerance of post-larvae to low salinities, but sub-adults were only affected at 30⁰C. Stages 4 and 5 and stage 6 zoeae were intolerant of salinities below 14 ‰. The development of salinity tolerance with age is consistent with the retention of a marine breeding phase. It also explains the high loss of juveniles from the study site following the flood. It is postulated that recovery of the population after a flood would be fairly rapid except in extreme cases when estuarine vegetation is severely affected.

The distribution of Callianassa kraussi in southern Africa extends from Lamberts Bay on the west coast to San Martinho in Mocambique. This represents a northerly extension of the range from the previous known limit of Inhaca Island. In this area 59 localities were visited and records obtained for an additional 16. Three areas were selected for detailed study. These were the west Kleinemonde estuary, which is normally closed off from the sea by a sand bar and the open Swartkops estuary both on the southeastern Cape coast, and the Swartvlei system, which consists of a lake connected to the sea by a channel which closes intermittently, on the southern Cape coast. The nature of the burrows constructed by C. kraussi was investigated in these areas. Burrow complexes may have up to nine entrances but more commonly two to four. Laboratory studies suggest that the number of entrances does not exceed the number of prawns present and consequently burrow opening counts will give an estimate of population density. Water movement can result in the closing off of burrows while strong water and sand movement will exclude C.kraussi from an area. C.kraussi is also excluded from areas which have very coarse substrates. A three year regular sampling program showed that C.kraussi breeds mainly in winter/spring with a smaller breeding peak in summer. Egg development time at 20°C in a salinity of 35° /₀₀ is 30-33 days. There are two larval stages which last 3-5 days. Development time of the eggs is tripled at temperatures of 13-15°C while larval development time is extended to 9-14 days. Development is possibly slightly accelerated at 25 ± 3°C but the number of eggs hatching is markedly reduced. Larval development times were similar to those at 20° and 35°/₀₀. There are no planktonic larval stages. Growth was more rapid and greater size was attained in the open Swartkops estuary than in the closed Kleinemonde estuary. Prawns hatching in winter/spring breed for the first time in the following summer at an age of about 16 months and then again in the following winter/spring at an age of about 2 years. Prawns hatching in summer breed in the following winter at an age of about 18 months and then again in the immediately following summer. Longevity in both sexes is about 2 years. Dispersal is accomplished by migration of the post-larval juvenile phases at an age of 3-5 months. Non-selfmaintaining populations exist in areas where salinities are too low to permit breeding. Population densities were found to vary markedly in different areas. Investigations of osmotic and ionic regulation showed that C.kraussi is a strong hyper-regulator and thus distinct from any other known species in the genus. Volume regulatory ability is well developed and depends on variable rates of urine production. Salt loss in dilutions occurs almost equally via the gills and the urine. The general responses of C.kraussi to dilutions of sea water are discussed. Various suggestions for conservation measures based on the distribution and iife cycle of C.kraussi were made. The factors affecting the distribution of C.kraussi are discussed. The problem of the larval development in C.kraussi was discussed in relation to reviews of larval types of benthic invertebrates. The importance of the burrow, the possible route by which C.kraussi has invaded estuaries and the differences between open and closed estuaries as shown by the effects on C.kraussi were discussed.

In South African estuaries the anomuran mud prawn, Upogebia africana (Ortmann), is the main organism exploited as bait by recreational and subsistence anglers. In the Knysna estuary, three groups of bait collectors were identified on the basis of their source of income: subsistence fishermen who rely on bait-collecting and fishing for their income, supplementary anglers who catch fish to supplement their income and leisure anglers who draw no income from fishing. Two groups were identified based on the methods used: lelsure anglers who collect bait using a prawn pusher or pump and fish using a fishing rod and tackle, and non-leisure anglers who collect mud prawns using tin cans and fish with handlines. The hypothesis was erected that bait-collecting does not affect the U africana populations in the Knysna estuary. The approach adopted was to assess the distribution, density, population structure and reproductive patterns of the bait stock and to estimate the intensity of bait collection, to test the validity of the hypothesis. U africana has an extensive distribution, occupying 62 % of the available intertidal area of the Knysna estuary. Mud prawns have a broad intertidal distribution from the high water level (Spartina zone) to the shallow subtidal. The density, biomass and population structure of the mud prawns vary significantly with distance up the estuary and with tidal height on each shore. The Invertebrate Reserve supports very low densities of U africana (x = 11.7 m⁻²), whereas a relatively inaccessible centre bank in the middle reaches of the estuary appears to be a very effective natural mud prawn reserve as it supports the highest densities (x = 176.5m⁻²). The mud prawn stock of the estuary is estimated to be 2.19 x 108 prawns (82.7 tonnes dry mass). The numbers of bait collectors present per mudbank is highest on public holidays (x = 34) and higher during the summer holidays (x = 16) than during the winter (x = 4). A total of 1.858 x 106 U africana or 700.53 kg (dry mass) are removed by bait collectors annually from the 6 popular bait-collecting sites studied. This represents 8.49 % of the mud prawn stocks at these sites and 0.85 % of the entire estuary stock. 85 % of the U africana taken as bait annually, is removed by the 77.12 % of bait collectors who are non-leisure anglers. Recreational or leisure anglers are responsible for removing 14.2 %. The reproduction of female U africana in the Knysna estuary is seasonal and occurs from late July to April. There is evidence that this breeding season consists of two merging breeding cycles. The largest percentage (63 %) of ovigerous females is found in the middle reaches of the Knysna estuary at the Oyster Bank where the highest numbers of stage 1 larvae (165 m⁻³) are released. Larvae were exported from the estuary on the crepuscular ebb tide with peaks in abundance of nearly 1500 m⁻³ in November 1995 and January 1996. Although numbers caught are not significantly higher, larvae are nearly twice as abundant on crepuscular neap tides following a new moon (waxing quarter) than on those following a full moon (waning quarter). Recruitment of juvenile U africana to populations in the estuary was highest in December (45 m⁻²) and decreased over the summer. The highest numbers of recruits (31 m⁻²) were found at those sites closest to the mouth or on the main channel. Recruitment to the Leisure Isle and Thesens sites which are intensively exploited by bait collectors is high (20 - 32 m⁻²). The number of juveniles recruiting to U africana populations is estimated to be four times as high as the numbers of mud prawns taken by bait collectors. Legal methods of bait collection used in the Knysna estuary by the majority of anglers appear to cause minimal sediment disturbance and are not likely to affect the associated infauna. Illegal bait-collecting methods are however highly destructive and must be prohibited. It is concluded that the null hypothesis is acceptable, as mud prawn stocks of the Knysna estuary are not over-exploited and appear to be naturally regulated. The recruitment rate of juvenile U africana is estimated to be sufficiently high to sustain the present levels of exploitation.

The spatial and temporal patterns in the population demographs and feeding ecology of the caridean shrimp, Palaemon peringueyi, was investigated over a 12 month period in the permanently open Kariega Estuary situated on the south-east coastline of South Africa. Shrimps were sampled monthly from three stations corresponding to the lower, middle and upper reaches of the Kariega Estuary. The shrimps were collected at the different stations using a pull net. Temperature, salinity, dissolved oxygen and % cover of submerged vegetation were measured at each sampling station. Finally, the feeding ecology of the shrimp was investigated employing stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes analysis. Temperature, salinity, dissolved oxygen and vegetation cover showed distinct seasonal patterns. The shrimps attained total abundances and biomass of between 0 and 245.6 individuals per m² (ind.m⁻²) and between 0 and 13.6 grams wet weight per m² (g wwt.m⁻²), respectively. Mean abundance ranged between 2.3 (± 3.0) and 88 (± 27.3) ind.m⁻² in the lower reach, between 0.9 (± 0.4) to 21.5 (± 4.5) ind.m⁻² in the middle reach and 0 to 1.6 (± 0.5) ind.m⁻² in the upper reach. Mean biomass ranged between 0.4 (± 0.5) and 4.4 (± 1.4) g wwt.m⁻² in the lower reach, between 0.1 (± 0) to 4.3 (± 0.9) g wwt.m⁻² in the middle reach and between 0 to 1.6 (± 0.5) g wwt.m⁻² in the upper reach. Greatest abundances and biomasses were recorded in winter following heavy rainfall within the catchment area. Freshwater inflow which is due to rainfall is thought to provide a cue for the recruitment of marine breeding organisms. The spatial patterns in the distribution of the shrimps showed that juveniles occupied the lower and middle reaches and adults the entire length of the estuary. There were no significant relationships between the abundance and biomass of the shrimps and the selected physico-chemical variables (p > 0.05 in all cases). Results of the stable isotope study indicated that the mean δ¹³C of the shrimps in the lower, middle and upper reaches were -12.4 (± 1.3) ‰, -15.6 (± 0.5) ‰ and -17.5 (± 0.5) ‰, respectively. The δ¹⁵N isotopes of the shrimps in the lower reach was 11.6 (± 0.5) ‰ with middle and upper reaches having similar δ¹⁵N values, 13.5 (± 0.4) ‰ and 13.4 (± 0.4) ‰, respectively. There was no significant difference in the δ¹³C and δ¹⁵N isotope (p > 0.05) between the sexes of the shrimps. Two-way ANOVA indicated a significant difference in the δ¹³C isotope of the shrimps between the sites (p < 0.05). The difference in the δ¹³C and δ¹⁵N ratios of the shrimps suggests spatial variability in the diet of P. peringueyi within the Kariega Estuary. The diet of the shrimps in the lower reach is primarily composed of Nanozostera capensis epibionts. In the middle reach the diet of the shrimps is likely comprised of the detritus of benthic algae. In the upper reach of the estuary, P. peringueyi likely derives its carbon from a combination of epibionts, benthic algae and detritus derived from Codium.

The role of the sandprawn, Callichirus (=Callianassa) kraussi (Stebbing), as an ecosystem engineer was assessed in the lower reach of the temporarily open/closed Kasouga Estuary situated along the Eastern Cape coastline of southern Africa over the period April 2010 to June 2011. The study comprised two distinct components, a field study and a caging experiment. The field study assessed the correlation between sand prawn densities and selected physico-chemical (organic content of the sediment and bioturbation) and biological (microphytobenthic algal concentrations and macrobenthic abundance and biomass) variables in 50 quadrants in the lower reach of the estuary. Densities of the sand prawn within the quadrants ranged from 0 to 156 ind m⁻² (mean = 37 ind m⁻²). There were no significant correlations between the densities of the sandprawn and the estimates of the organic content of the sediment and the abundance and biomass of the macrofauna (P > 0.05 in all cases). Numerical analyses failed to identify any effect of the sandprawn density on the macrofaunal community structure. The rate of bioturbation was, however, strongly correlated to the sand prawn density. Similarly, the microphytobenthic alga concentrations were significantly negatively correlated to the sand prawn densities ((P < 0.05). The absence of any distinct impact of the sandprawn on the macrobenthic community structure appeared to be related to their low densities in the lower reach of the estuary during the study. To better understand the role of the sandprawn as an ecosystem engineer, a caging experiment was conducted using inclusion and exclusion treatments (n= 5 for each treatment). Densities of the sandprawn in the inclusion treatments (80 ind m⁻²) were in the range of the natural densities within the estuary. The experiment was conducted over a period of 18 weeks in the lower reach of the estuary during summer. The presence of the sandprawn, C kraussi, contributed to a significant decrease in the microphytobenthic algal concentrations and the abundance and biomass of the macrofauna (P < 0.05 in all cases). The decrease in the microphytobenthic algal concentrations in the presence of the sandprawn appeared to be related to the res-suspension of the sediments (bioturbation) generated by the burrowing and feeding activities of the sandprawn. The observed decrease in macrofaunal abundances and biomass in the inclusion treatments appeared to be mediated by both the decreased food availability (mainly the microphytobenthic algae) and the burial of organisms within the sediments. Numerical analysis indicated that the sandprawn did, however, not contribute to a change in the species composition of the macrofauna. Results of the current study indicate that C.kraussi plays an important role in structuring the invertebrate community and energy flow within temporarily/open closed Kasouga Estuary.