DEMOGRAPHIC-MODELING IN THE ANALYSIS OF POPULATION-DYNAMICS OF DEEP-SEA MACROBENTHOS

A flexible graphics simulation approach, combining graphics simulation with nonlinear parameter optimisation, is described for estimating likely growth and survivorship from size frequencies measured in samples of deep-sea benthic invertebrates. Relationships between patterns in size structure, such as bimodal size distributions, are shown to result from limited combinations in values of growth and mortality parameters. Although it is possible to fit demographic models to such size data by optimising goodness-of-fit of model frequencies to those observed, the outcome may be dependent on assumptions of a stable age structure from year to year. Simulation studies show that populations with relatively low values of the von Bertalanffy growth-rate constant K and of survivorship, which show year-to-year variability in level of recruitment, will display strongly varying adult size structure at an inter-annual timescale. Those with high K and high adult surivorship, and which typically show a bimodal distribution of size frequencies, will be much more resilient to such changes. However, high adult survivorship in a relatively long-lived species will result in relatively stable adult age structure even when K, and the probability for any recruitment at all, is low. The implications of these findings are discussed in terms of contrasting models developed for populations of the deep-sea ophiuroids Ophiura ljungmani and Ophiomusium lymani, and of the deep-sea echiniod Echinus affinis, in samples from permanent stations in the Rockall Trough, northeastern Atlantic.