Juvenile Chinook salmon (Oncorhynchus tshawytscha) experience predation pressure from marine predators from the time that they enter the ocean to the end of their first year at sea. In this key period, an ecosystem modeling framework is useful framework to explore spatial and temporal variability of predation pressure and testing management strategies that can be implemented during the freshwater period directly preceding ocean entry to enhance marine survival. This project takes an ecosystem-based approach that integrates shipboard survey data, numerical modeling, lower-trophic level modeling, and predator/prey dynamics to investigate the survival and distribution of juvenile Chinook off central California.

Modeling Framework:

Our ecosystem modeling framework incorporates a regional ocean circulation (ROMS) submodel, a Nutrients-Phytoplankton-Zooplankton (NPZ) submodel for generating prey fields, and an individual based model (IBM) for juvenile Chinook salmon. The salmon IBM consists of a series of modules representing growth, mortality, behavioral movement, and predation interactions.  The mortality submodel includes size-based predation by a central-place seabird predator accounting for both salmon size and the probability of salmon-seabird interactions, scaled by the seabird population size and distribution. Temperature and krill fields from the ROMS and NPZ submodels are used in the IBM for calculating juvenile salmon growth and inform behavioral movement.

Incorporation of Survey Data:

Our modeling approach incorporates data from shipboard surveys to quantify the distribution of seabird predators (Common murre, Uria aalge) and to assess the distribution and amount of alternative forage fish, such as rockfish and anchovy. Predator location information is used to quantify the spatial overlap between juvenile Chinook salmon and common murre. Survey data for juvenile rockfish and anchovy is incorporated into the model to investigate how the presence of alternative forage for the seabird predator may mitigate predation pressure on juvenile Chinook salmon.

Management in a Changing Ocean:

We use the IBM to assess potential management strategies that could alleviate some of the pressure on our Central California populations.  Entering the ocean at different sizes and times can alleviate predation pressure under certain scenarios, and we use the IBM to test these scenarios in a virtual environment representing our real-life ocean system. As some of the driving forces of salmon distribution and growth change, so will the need for adaptive management. We can simulate how rises in temperature and changes in krill (salmon food!) populations in the coming century might impact these juvenile Chinook, and therefore examine these growth, survival, and distribution drivers with an eye to the future.