It is well documented that biomass (and catches) for many squid species varies considerably, and moreover, that recruitment strength is strongly related to the environment. The ramifications of catch fluctuations are significant and create uncertainty for resource managers and the fishing industry with the net result of increased risk levels of stock collapse, economic instability, long term investment; and for the semi-artisanal fisheries, socioeconomic hardship for the many fishers. Recruitmentâenvironment mechanisms which underpin biomass are region specific, and in the case of the chokka squid Loligo reynaudii, are unknown. This thesis addresses this knowledge gap and is fundamentally based on Bakunâs (1996)1 generalised triad of requirements for successful recruitment â enrichment, concentration and retention in the ecosystem. For chokka squid this implies that recruitment depends on the survival of paralarvae in terms of food availabilityâfeeding success (i.e. copepods biomass, density distribution and patchiness) and retention in the ecosystem. The nature of this investigation demanded a multidisciplinary approach comprising physical oceanography and biology, as well as a variety of scientific techniques. First a synthesis of basic ecosystem components for the domain in which chokka squid live (i.e. South Africaâs west coast and Agulhas Bank) was prepared using published and new data. It included bottom temperature, bottom dissolved oxygen, chlorophyll, and copepod abundance. Alongshore gradients of these indicated that the main spawning grounds on the eastern Agulhas Bank are positioned where bottom temperature and bottom dissolved oxygen are optimal for embryonic development. This location, however, appears suboptimal for hatchlings because the copepod maximum (food for paralarvae) is typically on the central Agulhas Bank some 200 km to the west. Data on currents suggest that this constraint may be overcome by the existence of a net west-flowing shelf current on the eastern Agulhas Bank, improving survivorship of paralarvae by transporting them passively towards the copepod maximum â a concept referred to as the western transport hypothesis or WTH (hypothesis 1). CTD (Conductivity, Temperature, Depth) data and a temporal analysis of AVHRR (Advanced Very High Resolution Radiometry) satellite imagery reveal the copepod maximum to be supported by a ââcold ridgeââ, a mesoscale upwelling filament present during summer when squid spawning peaks. In situ sea surface temperature (SST) data used as a proxy for cold ridge activity demonstrate considerable inter-annual variability of the feature, especially during El NiÑo-Southern Oscillation events. Negative linear correlations between maximum summer SST (monthly average) and squid biomass the following autumn (r2=0.94), and annual catch (r2=0.69), support the link between the ââcold ridgeâcopepod maximumââ and the early life cycle of chokka squid, and holds promise for prediction. Transport of squid paralarvae hatched on the inshore spawning grounds (<60 m) was also investigated using a bottom-mounted Acoustic Doppler Current Profiler deployed at 36 m on the Tsitsikamma coast (in the Tsitsikamma National Park). Analysis of 12 months of data showed that surface flow was mainly eastward (alongshore), with a maximum velocity of 115 cm sâ1 and an average of 24 cm sâ1. Generally, velocity decreased with depth, with a maximum bottom velocity of 65 cm sâ1 and an average of 10 cm sâ1. Data from a nearby thermistor array show that the water column was usually isothermal during winter (Julyâ September), with bottom flow in the same direction as the surface layer. In summer (DecemberâMarch), vertical stratification was most intense and surface and bottom flows differed in velocity and direction. Potential net monthly displacements calculated for three depths (5 m, 23 m and 31 m) indicate that passive, neutrally buoyant biological material (e.g. squid paralarvae) would likely be transported eastwards in the surface layer for eight of the 12 months, and would generally exceed distances of 220 km monthâ1. Displacement in the bottom layer was more evenly distributed between east and west, with net monthly (potential) transport typically 70â100 km, but reaching a maximum of 200 km. Wind-driven coastal upwelling prevalent during the summer, was observed to cause offshore flow for several days the surface layer of the coastal current resulting in potential displacement distances of 40 km from the coast. This mitigates eastward transport and potentially moves squid paralarvae in the Tsitsikamma current offshore and into the westward mid shelf current which flows towards the cold ridge. Realization that currents may not always be westward led to re-examination of the dependency of squid paralarvae on the cold ridge as the only rich feeding area on the Agulhas Bank. A synthesis of existing data and materials found that chlorophyll and copepods also exist at elevated levels on the thermocline in areas other than the cold ridge, and that currents therefore may not necessarily be so critical to connect hatching position with food. Moreover, varied current data suggested that currents may remove squid paralarvae from the Agulhas Bank ecosystem (hypothesis 2) through the leakage of shelf water into the Indian and Atlantic Oceans. Magnitude and timing of such a phenomenon will impair recruitment, cause biomass fluctuations, and ultimately affect catches. Leakage has been cited as the root cause of the sudden drop in annual squid catches experienced in 2001. To investigate the leakage hypothesis, a Lagrangian IBM (Individually-Based Model) coupled to a ROMS (Regional Ocean Model System) model was setup covering the west coast and Agulhas Bank to 24° E (St Francis Bay). Three simulations were performed for 12 model months using neutrally buoyant particles released from the seabed every second day on the mid shelf of the eastern, central and western parts of the Agulhas Bank. Boundary effects and resolution precluded the release of virtual particles on the inshore spawning grounds. Particles were given life spans of 40 days. Results demonstrated large particle losses from the eastern Agulhas Bank (76%) and the western Agulhas Bank (64%). In contrast few particles were lost from the central Agulhas Bank (2%) making this, in terms of the model, the most suitable place on the Agulhas Bank for spawning. Visualization of the ROMS outputs revealed that leakage on the eastern Agulhas Bank was caused by a cyclonic eddy resident in the Agulhas Bight. Similarly leakage from the western Agulhas Bank was caused by deep water cyclonic eddies in the adjacent Atlantic Ocean. A parallel study was also undertaken using four satellite tracked drifters released on the eastern Agulhas Bank to validate the ROMS-IBM experiments. Two drifters with drogues to 8 m were released on the inshore spawning grounds off the Tsitsikamma coast at the ADCP site. The other two drifters were released on the mid shelf where squid eggs had previously been found and âvirtual paralarvaeâ released in the IBM. One of the mid shelf drifters was tethered to a drogue at 70 m to measure advection in the bottom layer. Both inshore drifters were transported 70 km eastwards in the Tsitsikamma coastal current to Tsitsikamma Point. Here one beached while the other moved offshore onto the mid shelf and then southwards to leave the shelf 20 days after release. The surface drifter on the mid shelf was transported westward across the central and western Agulhas Bank (550 km) to leave the shelf after 58 days south of the Cape Peninsula. The deeper drifter also travelled westward, but remained on the shelf in the vicinity of the cold ridge and was recovered after 40 days and 100 km of the release position. Satellite SST and ocean colour images indicated frequent offshore flows of shelf water near the southern tip of the Agulhas Bank, as well as an intrusion of oceanic water onto the western Bank during this experiment. The latter caused an anti-cyclonic circulation which led to further leakage of shelf water from the inner central Agulhas Bank. The combination of drifters and satellite imagery in this experiment demonstrated that retention of chokka squid paralarvae in the Agulhas Bank ecosystem is not certain, even for the inshore spawning grounds, and that the risk would be less if paralarvae were found near the bottom. So far in situ sampling indicates they occupy the surface layer. Overall, this work has identified (1) environmental niches on the Agulhas Bank which the chokka squid life cycle has evolved to use (e.g. spawning grounds), (2) that the cold ridgeâ copepod maxima is a rich feeding ground on the Agulhas Bank and plays a role in chokka squid recruitment strength (biomass), and (3) that there is potential for chokka squid paralarvae to be advected off the shelf and removed from the Agulhas Bank ecosystem on the eastern and central Agulhas Bank possibly resulting in biomass crashes for the following year. Importantly, the cold ridgeâcopepod biomassâsquid biomass relationship has been quantified, and holds promise for prediction. Prediction will be further strengthened if, in the future, advective paralarval loss can be linked to early retroflection of the Agulhas Current.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/6501 |
| Date | January 2009 |
| Creators | Roberts, Michael J |
| Contributors | Shillington, Frank |
| Publisher | University of Cape Town, Faculty of Science, Department of Oceanography |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Doctoral Thesis, Doctoral, PhD |
| Format | application/pdf |
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