Predicting foraging success of juvenile salmonids is especially important due to their population decline caused by habitat loss. Juvenile salmonids attack drifting preys (e.g., chironomids) that pass within their vicinity. Drift feeding models of juvenile salmonids have assumed that prey density and capture success rate have a linear relationship. However, during higher prey densities, the capture success rate may decrease as a fish may encounter more prey than it can process. Moreover, water velocity and water depth may impact the prey delivery rate and prey detection distance, respectively. Without thorough hypothesis testing, this may lead to either underestimating and overestimation of prey capture success in drift feeding models. In this study, we hypothesized that the prey capture success does not increase linearly in relation to prey density.
To test the hypotheses, we developed a mechanistic model that simulates juvenile steelhead Oncorhynchus mykiss irideus foraging behavior from a study that conducted a lab experiment to test the effects of water velocity on capture success. We acquired precise 3D coordinates of both prey and fish locations (focal, attack, and return point) for successful prey captures. We used this information to predict the movement of the fish and determine capture success using GLMMs.
Results/Conclusions
This is one of the few studies to test the relationship between prey densities and capture success. Accurate data points helped create a 3D prey detection range of which our model predictions were consistent (3.1% lower) with the results from Piccolo et al. 2008. Later, we tested the effects of density and water velocity on capture success. For the preliminary study, we ran 500 simulations of prey capture experiments with six times the prey density (M6PD) than that of the Piccolo et al. (2008) (M1PD) which is within the range of prey encounter rate in Sacramento River, California. The capture success was 35.9% lower than that of the M1PD model (60.7%) in 29cm/s water velocity. In 61cm/s water velocity, the M6PD capture success of our model was 41.3% lower than that of the M1PD (12.1%). This indicates that the capture success is not linear with prey density at multiple velocities. We will further include precise depth data to increase the accuracy of the detection range and test the effects of various prey densities coupled with various water velocities and depth on capture success. This study will generate useful parameters and functions to advance many juvenile salmonid foraging models.
