Population drivers of a Thalia democratica swarm: insights from population modelling

The small salp, Thalia democratica, is known globally to form dense swarms, yet the physiological and oceanographic factors that influence the magnitude and occurrence of these swarms are poorly understood. In this study, two numerical models were used to explore T. democratica population dynamics. A Lefkovitch matrix model identified that the survival of juvenile oozoids was the most important population metric promoting population growth and a size-structured population model was developed to further explore the dynamics of T. democratica. The size-structured model tracks cohorts of four life stages and incorporates size-dependent reproduction and mortality. Model outputs of average generation time and mean abundances of each life stage corresponded well to previously reported values. Factors that promote juvenile oozoid abundance were the most sensitive parameters influencing salp abundance. A 10-year time-series simulation using this population model identified that salp abundances in the Tasman Sea are proportionally higher in winter and spring, consistent with previous studies. This model shows that temperature and phytoplankton concentration are sufficient drivers of large-scale patterns of salp abundance across season and latitude. These results demonstrate the importance of future research focusing on identifying the environmental conditions that influence the size at which females reproduce.