A review of the assessment techniques used for population monitoring at different life stages of sturgeons

Sturgeons are a unique group of species that were historically widespread across the northern hemisphere. According to the latest IUCN Red list assessment, more than 80% of the species globally are threatened with extinction, making it essential to identify the life stages at which they are suffering from impacts the most, while at the same time to compare among river systems and populations based on standardized assessment techniques. All sturgeon have similar but not identical, life-history strategies. Therefore, monitoring techniques developed for one sturgeon species would be applicable to most other species. Monitoring can be conducted at each life stage. However, while each life stage will provide different information about the population, not all will necessarily help to assess population trend or status. Life stages that are highly variable prove to be less quantifiable even after expending very high effort. Collectively, these assessments could be very informative on population status, limitations, and trends. However, monitoring at each stage is time-consuming and expensive. Clearly defined objectives are therefore required when embarking on an assessment program. The objective of this study was to review the assessment techniques used for the different life stages including eggs, drifting larvae, age-0, juveniles, subadults, and adults to provide a common basis for population assessments that can be standardized to some extent and thus facilitate comparisons between the results obtained. For this purpose, this review presented the most common assessment techniques for each life stage, assessed the pros and cons of assessing each life stage, and examined if the methodology was qualitative or quantitative to assist in establishing long-term monitoring initiatives.