In a context of declining capture fisheries and public pressure for greater access to marine resources, marine aquaculture is receiving increasing interest from the South African government as a means to increase the diversity of economic activities in coastal regions, thereby providing employment and reducing poverty. The east coast rock lobster Panulirus homarus rubellus is currently harvested by subsistence fisherman along the former Transkei coastline of south-east South Africa and presents a possible opportunity for ongrowing wild juvenile lobsters in culture facilities. Lack of compliance coupled with poor enforcement of the minimum size limit (65 mm carapace length) has resulted in the ongoing harvest of undersize size lobsters by subsistence fishers. Generally, fishers either consume these undersize lobsters or sell them to tourists for low prices. In line with international trends in rock lobster aquaculture, interest has subsequently arisen in the possibilities of ongrowing these undersize lobsters as a means of adding value to the P. h rubellus resource for subsistence fishers. The aim of this physiological study was to assess the biological feasibility of harvesting, transporting and culturing wild caught juvenile lobsters, thereby provide empirical data to inform the development of suitable transport and culture protocols. The experimental objectives were to asses the effect of temperature on growth and survival of P. h. rubellus, as well as the effects of a suite of extrinsic and intrinsic factors on ammonia excretion and oxygen consumption. Juvenile lobsters were collected by hand from near-shore reefs (2-15 m depth) off Mdumbi in the former Transkei, Eastern Cape Province and transported by road (7 hours) to the Port Alfred Marine Research Laboratory where they were held in a recirculating culture system. The effect of temperature over a range of 9.7 °C (18.9±0.7 to 28.6±1.5 °C) on the growth and survival of juvenile P. h. rubellus fed a diet of fresh mussel flesh was investigated. Specific growth rate (SGR) was significantly different between temperatures (p = 0.01), with the highest values recorded for the 24 °C and 28 °C treatments. There was no significant difference in moult increment (MI) between temperatures, however, intermoult period (IMP) differed significantly between temperatures (p = 0.0015) with mean IMP lowest at 24 °C, although not significantly different from the means of the 26 °C and 28 °C treatments. IMP was highest at 19 °C and 21 °C. Apparent feed intake was significantly different between treatments (p < 0.0001) and exhibited a strong positive correlation with increasing temperature. Food conversion ratio (FCR) differed significantly between temperatures (p = 0.02) with 24 °C exhibiting the most efficient FCR. The results for growth rate and food conversion efficiency suggested that 24 °C is optimal for the growout of juvenile P. h. rubellus. In the second study, the effect of body weight, emersion, daily rhythm, feeding and ambient ammonia on the total ammonia nitrogen (TAN) excretion rate was investigated. Body weight (n = 16, range of 16.8 – 322 g) was positively correlated to daytime TAN excretion rate (mg h⁻¹). Re-immersion after one hour emersion in a moist environment was characterized by a significant increase in TAN excretion rate for the first hour compared to pre-immersion levels. The amount of TAN excreted during this period was as expected if basal TAN excretion rates were maintained during emersion. TAN excretion rates returned to pre-emersion levels by the end of the second hour. There was no evidence of a daily rhythm in TAN excretion rate for P. h. rubellus. TAN excretion rates were elevated following feeding. An initial peak in TAN excretion rate after seven hours (7.58 times pre-feeding rate) was followed by a smaller peak after 13 hours (6.69 times pre-feeding rate). TAN excretion rate dropped to levels not significantly different from pre-feeding levels after 23 hours and consistently returned to pre-feeding levels after 42 hours. The TAN excretion rates of lobster exposed for two hours to an ambient TAN concentration of 1.02±0.10 mg l⁻¹ and 2.3± 0.2 mg l⁻¹ were not significantly different from TAN excretion rates recorded at low ambient water TAN prior to exposure. Exposure to an ambient TAN concentration of 4.45±0.78 mg l⁻¹ had a significant effect on the TAN excretion rate, with pronounced ammonia uptake occurring for all animals at this concentration. The third study investigated the effects of body weight, diurnal rhythm, feeding and emersion on lobster oxygen consumption. Body weight was positively correlated to both standard and active oxygen consumption (mg O2 h⁻¹) while body weight was negatively correlated to mass-specific standard oxygen uptake (mg O2 g⁻¹ h⁻¹). Diurnal rhythm exhibited a strong effect on the lobsters oxygen consumption, with average night time values 67% greater than those recorded during the day. This was related to activity driven by intrinsic nocturnal foraging behaviour. Feeding resulted in a classic specific dynamic action (SDA) response, with postprandial oxygen consumption increasing to a peak before decreasing gradually to preprandial levels. Emersion resulted in a significant increase in oxygen consumption, with lobsters rapidly recovering to pre-emersion levels after four hours. Results from these studies suggest that the capture, transport and culture of juvenile P. h. rubellus is biologically feasible. Empirical data generated were used to provide recommendations regarding species optimised transport and culture protocols. A purge time of 48 hours before transport is suggested to ensure that ammonia excretion and oxygen consumption are at basal levels. Furthermore, emersed transport for a period of one hour is characterised by rapid recovery upon re-immersion. In order to prevent the accumulation of stressors, it is suggested that consecutive periods of emersion are interjected with recovery periods (five hours) in water to allow the removal of accumulated ammonia and repayment of the oxygen debt incurred. The recorded ammonia rates indicate that a biological filter size of 4.8 m³ is recommended for 1000 kg of fed lobsters in a culture situation, although this can be reduced considerably if lobsters are being held without feeding (0.72 m³). A flow rate of 112 l kg⁻¹ h⁻¹ is required to meet the metabolic requirements of lobsters. Bottlenecks to the viable commercial culture of P. h. rubellus, and the ability of this practice to provide the socio-economic benefits that were envisioned, are discussed.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:5299 |
| Date | January 2009 |
| Creators | Kemp, J O G |
| Publisher | Rhodes University, Faculty of Science, Ichthyology and Fisheries Science |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Masters, MSc |
| Format | 115 leaves, pdf |
| Rights | Kemp, J O G |
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