An age-structured simulation model to investigate species replacement between pilchard and anchovy populations in the southern Benguela

Abstract

Clupeoids form the basis of important commercial fisheries off Namibia and South Africa's Western Cape. Studies on the abundance of pilchard (sardine) and anchovy populations subject to heavy exploitation have revealed the possible succession of sardine by anchovy as the dominant species. In investigating the evidence for species replacement, a simple, age-structured computei simulation model was developed, which assumes that the system is initially in a steady state. The model's sensitivity to starting biomass, natural mortality, harvesting, the level at which density-dependence is initiated and environmental perturbation is consideJ:Cd. Important factors determining the results are the present estimates of natural mortality and the level at which density-dependence is initiated. A lower natural mortality in pilchard results in dominance by anchovy. High threshold values of densitydependence produce dynamic oscillation in the model populations. Harvesting affects the natural balance between populations in a system at steady-state, causing the dominant stock to decline, resulting in species replacement. This situation is particularly evident in scenarios employing harvesting numbers-at-age. The intensity and the duration of harvesting determines whether replacement is temporary or permanent. The model populations show resistance toward environmental perturbation; indeed, in scenarios including it, species replacement is delayed. Klupeo'ides II!tielangrike handelsvisserye te~no?r Nami~ie en Suid-A,frika "': Wes-Kaap len gron?slag. Sru~ies oor die talrykheid van sardyn (pelser)-en ansJovlsbevolkings wat hewlg oRlgm word, het moontlike opvolgmg van sardyn deur ansjovis as die oorheersende spesie aan die lig gebring. Met die ondersoek van die bewyse vir spesievervanging is 'n eenvoudige, ouderdomsgegronde rekenaarsimulasiemodel ontwikkel waarin die aanname gemaak word dat die stelsel aanvanklik in 'n ewewigstoestand verkeer. Die model se gevoeligheid vir aanvanklike biomassa, natuurlike sterftesyfer, ontginning, die vlak waarby digtheidsafuanklikheid inlree en omgewingsversteuring word in ag geneem. Belangrike faktore wat die resultate bepaal, is huidige ramings van natuurlike sterftesyfer en die vlak waarby digtheidsafuanklikheid begin word. 'n Laer natuurlike sterftesyfer by sardyn het oorheersing deur ansjovis tot gevolg. Hoe drempelwaardes vir digtheidsafbanklikheid lei tot dinamiese skommelings in die modelbevolkings. Ontginning be'invloed die natuurlike ewewig tussen bevolkings in 'n stelsel wat in ewewig verkeer en laat so die oorheersende stapel agteruitgaan tot spesieverplasing plaasvind. Hierdie situasie is veral opvallend in scenario's wat van ontginning volgens getal per ouderdomsklas gebruik maak. Die hewigheid en duur van ontginning bepaal of verplasing tydelik of blywend is. Die modelbevolkings toon weerstand teen omgewingsversteuring; inderdaad, in scenario's wat dit insluit, word spesieverplasing vertraag.

Oceanographic anomalies coinciding with humpback whale super-group occurrences in the Southern Benguela

“…Nevertheless, given the oceanographic evidence, there is a possibility that super-groups would become a persistent feature, which draws attention to the role of climatic signals driving the observed phenomenon. The region has experienced regime shifts in the past; it went through a major shift from sardines to anchovies in the ‘60 s, which was most likely caused by overexploitation 56 , while other ecosystem changes occurred in the mid ‘90 s, which were hypothesised to be the result of environmental changes 49 , 57 , 58 .…”