Modeling the effects of environmental and ecological processes on the transport, mortality, growth and distribution of early stages of Cape Anchovy (Engraulis Encrasicolus) in the Benguela system

Doctoral Thesis


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University of Cape Town

An individual based modelling approach was used to study environmental factors and processes influencing the early life history of anchovy in the southern Benguela region and on the Agulhas Bank. The intention was to then establish the link between these factors and processes and the recruitment success of anchovy in the southern Benguela region. Specific factors that were explored were transport from the spawning grounds to the nursery area, advection, temperature- dependent growth and mortality, vertical migration behaviour, retention in the nursery area, as well as the possibility of a second nursery area on the Eastern Agulhas Bank. Eight individual based models were coupled to the output of a 3-D hydrodynamic model to study dispersion processes. Particles representing eggs and larvae were released (spawned) over the Agulhas Bank, and their movements were tracked during their transport to the west coast (the recruitment area). Although the eggs and larvae were initially considered to be neutrally-buoyant, passive particles, increasing levels of complexity were progressively incorporated by adding processes such as particle buoyancy, temperature-dependency of growth and mortality of eggs and larvae, vertical behaviour and retention. A series of experiments was run by setting the parameters representing the factors being investigated by the model, and estimating a primary, quantifiable response variable representing the dynamics of the system (e.g. particles successfully reaching the nursery area). An analysis of variance was employed to assess the significance and the sensitivity of each model to changes in the parameter values. A pattern-oriented analysis was then systematically applied to assess the validity of the results of the model. The results indicate that five processes and their interactions strongly influence the transport of spawning products arriving at and subsequently being retained in the nursery area: (1) the location of the spawning, (2) the buoyancy of the eggs, (3) transport by the jet current and its inter-annual variability; (4) the 3-D structure of the current in the nursery area on the west coast, (5) The swimming abilities of the pre-recruits in both the vertical and the horizontal planes. Two factors appear to be of major importance in effecting mortality during the period leading up to recruitment, namely temperature and offshore losses through advective processes. A conceptual model of the life history of anchovy in southern Benguela is presented and discussed in the light of the hypothesis proposed by Hutchings et al. (1998). A number of modifications to this hypothesis are proposed, specifically an extension of the spawning area, and the addition of several key biological processes. A new element that is proposed in this thesis is the possibility of the Eastern Agulhas Bank as an additional nursery and spawning area.

Bibliography: leaves 107-119.