Individual-based stream trout research and environmental assessment model
Summary Description
The objectives of inSTREAM derive from its intended application to instream flow assessment of hydropower projects and ecological research. The model is designed to (a) predict the long-term trends in trout populations that result from changes in stream flow, temperature, and turbidity; (b) predict the long-term trends in trout populations that result from changes in such biological factors as inter-species competition and food availability; (c) provide a tool for identifying natural and anthropogenic factors that limit stream trout populations; and (d) provide a framework for monitoring and adaptive management programs for projects that affect stream trout.
We use the following key assumptions .
- A one-day time step for all model processes.
- A spatial resolution of several square meters. Habitat is modeled as rectangular cells with dimensions typically in the range of one half to several meters across the stream and several meters in the longitudinal direction.
- Stream flow, water temperature, turbidity, and food availability as the external variables driving the model over time.
- Food and feeding cover as the resources that stream trout compete with each other for.
inSTREAM is object-oriented and simulates three kinds of objects: habitat cells, fish, and redds (nests created by spawning trout).
Habitat cells determine their depth and velocity from the daily stream flow rate; this calculation uses a lookup table imported from an external hydraulic model. The popular PHABSIM and RHABSIM hydraulic models can be used readily. Habitat cells also track the availability of food, velocity shelters for drift-feeding fish, and spawning gravel.
The fish in our model conduct four major actions each day.
- Spawning: Adult fish spawn if they meet a number of readiness criteria. Upon spawning, fish find appropriate habitat and create a new redd, with the number and size of eggs depending on the spawner's characteristics.
- Movement: Each fish examines the surrounding area each day and moves to the site with the best habitat that is not already occupied by larger fish. Defining the "best" habitat is crucial for the success of the whole model and has been a major focus of our research. We assume fish move to habitat offering the highest probability of surviving and growing to sexual maturity over a specified horizon (e.g., 90 days). Survival and growth to maturity are functions of (a) habitat-related mortality (extreme temperatures or velocities), (b) predation mortality, and (c) food intake (which affects starvation mortality and growth). We do not explicitly impose territorial or non-territorial behavior. Instead, we model how much food is available in each habitat cell and how much food is used up by the larger fish in the cell. The amount of food an additional fish would get is a function of food production in the cell, the fish's feeding ability, and food consumption by larger fish in the cell. This formulation allows a number of complex and realistic movement behaviors to emerge.
- Feeding and growth: We simulate energy intake resulting from two feeding strategies: stationary drift feeding and searching the stream bottom. Intake varies with depth, velocity, fish size, and food availability. Energy consumption due to swimming also depends on the feeding strategy and the availability of velocity shelters. We assume fish use the most profitable of the two strategies, and calculate growth from energy intake and consumption using standard bioenergetics methods.
- Survival: Daily survival is a function of such mortality functions as high temperature, high velocity, spawning stress, starvation, and predation. Our predation formulation includes separate functions for terrestrial and aquatic predators, with survival probabilities a function of depth, velocity, temperature, and fish size.
Redds are modeled from when they are created by spawning until eggs have emerged as new fish, with the development rate a function of temperature. Redds can suffer egg mortality due to dewatering at low flows, scouring at high flows, high or low temperatures, and superimposition of a new redd on an existing one.
A complete description of inSTREAM Version 2 is provided in a Forest Service report “Individual-based Model Formulation for Cutthroat Trout, Little Jones Creek, California”, available on our Products page.
The inSTREAM software is designed to make the model easy to observe, revise, calibrate, and conduct experiments with. Software features include graphical displays of habitat, fish, and redds; a batch mode and Experiment Manager for automated execution of multi-run experiments; and complete user documentation.
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