# 2.8.1. Predation mortality¶

The central assumption in OSMOSE is that predation is an opportunistic process, which depends on:

the overlap between predators and potential prey items in the horizontal dimension

size adequacy between the predators and the potential prey (determined by

**predator/prey size ratios**); and when the information is availablethe accessibility of prey items to predators, which depends on their vertical distribution (this being determined by means of

**accessibility coefficients**). Thus, in OSMOSE, the food web structure emerges from local predation and competition interactions.

Consider a predator school \(S_{pred}\).

## 2.8.1.1. Size predation¶

Size-predation matrix is controlled by two parameters. The predator school \(S_{pred}\) can only feed on prey schools whose length meets:

with \(R_{min}\) and \(R_{max}\) the maximum and minimum predator/prey size ratios. Therefore, the minimum and maximum sizes of a prey that a predator can eat is given by:

predation.predPrey.stage.structure |
Structure to determine thresholds for predator/prey size ratios ( |

predation.predPrey.stage.threshold.sp# |
Array of age or size thresholds |

predation.predPrey.sizeRatio.max.sp# |
Array of \(R_{max}\) values |

predation.predPrey.sizeRatio.min.sp# |
Array of \(R_{min}\) values |

Danger

\(R_{min} \gt R_{max}\)

Since resource groups are defined by a range of sizes, and not by a single sizes, the predator will feed on a given percentage of the resource:

which is the overlapping range of the predator accessible range and of the resource size range.

## 2.8.1.2. Accessibility¶

First, the accessibility of all the preys to school \(S_{pred}\) is determined from an accessibility matrix for every species and stages. This matrix must not be used to define diet preferences but rather to take into account for a difference of positions in the water column (meaning some schools might evolve around the same geographical area but never meet because they do not occur at the same depth).

Prey / Predator |
lesserSpotted < 0.45 |
lesserSpotted |
redMullet < 0.25 |
redMullet |

lesserSpottedDogfish < 0.45 |
0.05 |
0 |
0 |
0.05 |

lesserSpottedDogfish |
0 |
0.8 |
0.4 |
0 |

redMullet < 0.25 |
0 |
0.4 |
0.8 |
0 |

redMullet |
0.8 |
0.4 |
0 |
0.8 |

pouting < 0.25 |
0 |
0.4 |
0.8 |
0 |

pouting |
0 |
0.8 |
0.4 |
0 |

whiting < 0.25 |
0 |
0.4 |
0.8 |
0 |

whiting |
0 |
0.8 |
0.4 |
0 |

Dinoflagellates |
0 |
0.5 |
1 |
0 |

Diatoms |
0 |
0.5 |
1 |
0 |

Microzoo |
0 |
0.5 |
1 |
0 |

Mesozoo |
0 |
0.5 |
1 |
0 |

Macrozoo |
0 |
0.5 |
1 |
0 |

VSBVerySmallBenthos |
1 |
0.5 |
0 |
1 |

SmallBenthos |
1 |
0.5 |
0 |
1 |

MediumBenthos |
1 |
0.5 |
0 |
1 |

LargeBenthos |
1 |
0.5 |
0 |
1 |

VLBVeryLargeBenthos |
1 |
0.5 |
0 |
1 |

backgroundSpecies |
0 |
0 |
0 |
0 |

Each line of the matrix corresponds to a prey (including plankton groups), and each column to a predator. The file must be understood as follow: lesserSpottedDogfish of age class less than 0.45 (line 1) are only accessible to young lesserSpottedDogfish (5%) and old redMullet (5%).

predation.accessibility.stage.structure |
Threshold type. Must be |

predation.accessibility.stage.threshold.sp# |
Array containing the stage thresholds for a given species. |

predation.accessibility.file |
CSV file containing the accessibility matrix |

Warning

**In versions <= 4.2.0, the order of the rows and columns must follow the indexing of species (focal, background and resource) and stages
(e.g., species0; species1; species2 stage0; species2 stage1; species3). The threshold values provided in the CSV file are not used in this version**

Since version 4.3.0, the `predation.accessibility.stage.threshold.sp#`

parameter has been threshold has been deprecated, since the thresholds are read directly from the CSV files by matching
the `<`

character. It is assumed that if there is no match, no threshold is provided. However, when `<`

is matched, it is assumed that what follows is the upper bound of the class.

Furthermore, the column and row order is no more important, since a match of the species name is performed.

Finally, since version 4.3.0, accessibility matrix can vary over time with the following parameters, which follow the parameterization of movements.

predation.accessibility.file.acc# |
CSV file containing the accessibility matrix |

predation.accessibility.initialYear.acc# |
Start year when to use the accessibility matrix |

predation.accessibility.finalYear.acc# |
Start year when to use the accessibility matrix |

predation.accessibility.years.acc# |
List of years when to use the map (instead of setting initial and final years) |

predation.accessibility.steps.acc# |
List of time steps when to use the map |

Danger

If the `predation.accessibility.file`

(with no `.acc`

suffix) is found, Osmose will assume constant
predation accessibility matrix.

## 2.8.1.3. Predation rate¶

Finally, the predation rate is computed as follows. First, the total accessible biomass for the predator school is computed:

The total biomass that a predator can eat is also computed as follow:

with `N_`

the number of sub-step of mortality processes, \(B_{pred}\) the total biomass of predator and \(I_{max}\) the maximum ingestion rate for each species, expressed in grams of food per gram of fish
and per year. It is assumed that predator eat as much as they can.*mort*

The effective biomass that will be eaten by the predator is

Finally, for each prey, the biomass eaten by the predator is given by:

Finally, the success rate is computed as:

predation.ingestion.rate.max.sp# |
\(I_{max}\) (grams of food per gram of fish and per year) |