How do the models work?

Except as noted, inSTREAM and inSALMO use the following key assumptions .

• Four time steps per day, representing dawn, day, dusk, and night. (Most versions before inSTREAM 7 used a one-day time step).
• A spatial resolution of several square meters. Habitat is modeled as polygonal cells large enough to contain the feeding area of at least one large fish.
• Stream flow, water temperature, and turbidity (and sometimes food availability) as the external variables driving the model over time.
• Food, feeding cover, and hiding cover as the resources that trout compete with each other for.

inSTREAM is object-oriented and simulates four kinds of objects: stream reaches, habitat cells, fish, and redds (nests created by spawning trout).

Each model reach represents a contiguous section of habitat, and is made up of cells. Reaches can be linked to represent how fish move through a stream or watershed.

Habitat cells determine their depth and velocity from the stream flow rate; this calculation uses a lookup table imported from any two-dimension hydraulic model. Habitat cells also track the availability of food, velocity shelters for drift-feeding fish, and spawning gravel.

The model fish conduct four major actions each time step.

• 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.
• Habitat and activity selection: Each fish examines the surrounding area and decides whether to feed or conceal itself, 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., 60 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.

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; an experiment manager for automated execution of multi-run experiments; and complete user documentation.

Complete descriptions of the model and its software are distributed with each version.