Abundance of sea lice larvae in plankton samples: determination of optimal sample sizes

Sea lice infestations have been a major problem for the global salmon farming industry for several decades. To date, few studies have addressed the measurement of lice abundance in plankton samples and a standardized method to quantify sea lice larvae in water samples is still lacking. This study aims to: (1) evaluate the methods used to detect sea lice larvae based on published data and (2) to determine experimentally the volume of filtered sea water needed to obtain precise estimates of sea lice larvae abundance at different lice densities. Twenty-eight publications were reviewed with particular attention to sampling method and depth, total filtrated volume, analysed volume and nauplii and maximum copepodite densities. Moreover, plankton samples were obtained in and around salmon farms to evaluate the optimal water volumes required to estimate sea lice larvae abundance. This study provides a sampling and analysis strategy for quantifying larval sea lice in plankton samples from a cost/benefit point of view. Quantification of sea lice larvae in the plankton communities would be more precise than indirect methods used today (i.e adult sea lice attached on salmonids), and suitable for validation of modelling tools predicting the spatiotemporal dispersal of lice and, hence, the risk of infestation of salmon farms.