Estimates of vital rates and predictions of population dynamics change along a long-term monitoring program

Despite the widespread recognition of the importance of monitoring, only a few studies have explored how estimates of vital rates and predictions of population dynamics change with additional data collected along the monitoring program. We investigate how estimates of survival and individual growth, along with predictions about future population size, change with additional years of monitoring and data collected, using as a model system freshwater populations of marble (Salmo marmoratus), rainbow (Oncorhynchus mykiss), and brown trout (Salmo trutta L.) living in Western Slovenian streams. Fish were sampled twice a year between 2004 and 2015. We found that in 3 out of 4 populations, a few years of data (3 or 4 sampling occasions, between 300 and 500 tagged individuals for survival, 100–200 for growth) provided the same estimates of average survival and growth as those obtained with data from more than 15 sampling occasions, while the estimation of the range of survival (i.e., the difference, over all sampling occasions considered, between maximum and minimum survival estimated in a sampling occasion) required more sampling occasions (up to 22 for marble trout), with little reduction of uncertainty around the point estimates. Predictions of mean density and variation in density over time did not change with more data collected after the first 5 years (i.e., 10 sampling occasions) and overall were within 10% of the observed mean and variation in density over the whole monitoring program.