PSS.f: Predation size spectrum

In duplisea/size: Size based indicators for the northern Gulf of St Lawrence groundfish survey

Description

The predator-prey weight ratio is modelled as a log-normal distribution. The energy demand of a predator is a function of the predator size. This is not actually the amount of predation that has occured but a predation risk or preference, i.e. given the size distribution of predators in the system, the PSS shows where they would most like to take their prey. Since the index is relative, a QWa parameter is not needed as it is just a scaling parameter.

The PPWR CV 0.25 is from from Hahm and Langton (1984). Their PPWR is likely too large for predators in the Northern Gulf which are eating other fish and large invertebrates such as shrimp. Our default value is 30 which means that a predator is 30 times larger its prey (by weight) on average.

The log normal density function will produce NaNs for some size ranges and warnings will be produced. This is generally nothing to worry about as NaNs are converted to 0. The warnings have not be suppressed, however.

Usage

PSS.f(species.group, codeqc, PPWR = 30, PPWR.cv = 0.25, minPPWR = 5,
  QWb = 0.75)

Arguments

species.group	the species group you want to be represented in the data. The PLF is generally computed only for demersals. Choices are "groundfish", "demersal", "pelagic". If "code", then you need to provide the Quebec species numerical code (codeqc).
codeqc	the numerical code used in Quebec region for a species. 792 is unspeciated redfish. Look at the "species" table (codeqc) for other species
PPWR	the mean predator/prey weight ratio. If 10, then the predator on average weighs 10x more than its prey
PPWR.cv	the coefficient of variation of the predator prey weight ratio
minPPWR	the minimum predator/prey weight ratio allowed. e.g. if 3 then the prey is never allowed to be more than 1/3 of the predator weight
QWb	the exponent of the allometric relationship describing food consumption rate as a function of a fish's weight

Value

A list is returned: element 1: the year vector; element 2: the prey length vector; element 3: the prey weight vector; element 4: a matrix of the predation pressure over all prey sizes (rows) by year (columns). You will likely work with element 1 and 4 for plotting or creation of predation indices on particular prey size classes. The values in the matrix are not scaled but this can be done easily by dividing the matrix by the maximum value of the matrix. They can be scaled annually using the apply function and dividing each value in a column (year) by the maximum in that column. The whole PSS is normalised by dividing by the maximum value.

References

Duplisea, D.E. 2005. Running the gauntlet: the predation environment of small fish in the northern Gulf of St Lawrence, Canada. ICES Journal of Marine Science 62: 412-416.

Hahm, W. and Langton, R. 1984. Prey selection based on predator/prey weight ratios for some Northwest Atlantic fish. Marine Ecology Progress Series 19: 1-5.

duplisea/size documentation built on Sept. 11, 2019, 11:05 a.m.