304
J. Robertson et al.
Changes in suitability can also occur as a result of prey switching i.e. the
probability of a predator eating a prey item altering with respect to local and global
prey biomass levels. In the North Sea little is known about such effects. There are also
potential changes in suitabilities caused by non-linearities in the model itself. For
example, when prey switching is included in the MSVPA model, the equations can
exhibit one stable and two unstable solutions (Hildén, 1988). In this analysis prey
switching per se was not included, although the effects were noted in the output of the
ECOPATH model.
Method for Estimating Predator Consumption Rates
Various methods have been used to estimate rations or consumption rates; historically
these have involved gastric evacuation rate models (Daan, 1973; Elliott and Persson
1978; Jobling, 1986; Persson, 1986; dos Santos, 1990; Bromley, 1991). Estimates are
based on the volume of the stomach contents or in-situ gut fullness and gastric
evacuation rates. The ICES standard estimates are derived from the model of Daan
(ibid.)
A more recent study by Hansson et al. (1996) has compared the results
obtained from a selection of these gastric evacuation models with a bioenergetics
model developed by Hewett and Johnson (1992). The model simply examines
consumption (C) as:
where: metabolic loss = respiration + specific dynamic action; waste loss =
egestion + excretion; and growth = somatic + gonadic growth.
The results show that the gastric evacuation models currently used by ICES
for cod consistently produces low rates especially for age 1 fish; the Bromley model
predicts annual consumption 300% higher than the standard ICES model; the dos
Santos model gives consumption rates for age 5 fish that are twice those of the other
models; and the bioenergetics model gives rates intermediate to these. As the authors
point out, the North Sea fish community has changed considerably over the past few
decades, with a nineteen-fold change in herring biomass over the period 1974 - 1989
(Anon., 1991). Such significant changes in prey availability will not necessarily be
reflected linearly in the growth rates of predators, because of the trade-offs in prey type
(e.g. lean or fat) and the non-linear relationship between food consumption and
growth. This presents a significant problem in the MSVPA, because few experimental
data sets exist to be able to establish and validate a bioenergetic model for the main
predator species other than cod.
In this study the bioenergetics results were used for cod; for the remaining
species the daily food consumption rates (g/day) were calculated after Greenstreet
(1996). For haddock, saithe and whiting the mean stomach weight at age in each
quarter (w) was taken from Daan (1989), and Jones' (1974) digestion model applied.
J. Robertson et al.
Changes in suitability can also occur as a result of prey switching i.e. the
probability of a predator eating a prey item altering with respect to local and global
prey biomass levels. In the North Sea little is known about such effects. There are also
potential changes in suitabilities caused by non-linearities in the model itself. For
example, when prey switching is included in the MSVPA model, the equations can
exhibit one stable and two unstable solutions (Hildén, 1988). In this analysis prey
switching per se was not included, although the effects were noted in the output of the
ECOPATH model.
Method for Estimating Predator Consumption Rates
Various methods have been used to estimate rations or consumption rates; historically
these have involved gastric evacuation rate models (Daan, 1973; Elliott and Persson
1978; Jobling, 1986; Persson, 1986; dos Santos, 1990; Bromley, 1991). Estimates are
based on the volume of the stomach contents or in-situ gut fullness and gastric
evacuation rates. The ICES standard estimates are derived from the model of Daan
(ibid.)
A more recent study by Hansson et al. (1996) has compared the results
obtained from a selection of these gastric evacuation models with a bioenergetics
model developed by Hewett and Johnson (1992). The model simply examines
consumption (C) as:
where: metabolic loss = respiration + specific dynamic action; waste loss =
egestion + excretion; and growth = somatic + gonadic growth.
The results show that the gastric evacuation models currently used by ICES
for cod consistently produces low rates especially for age 1 fish; the Bromley model
predicts annual consumption 300% higher than the standard ICES model; the dos
Santos model gives consumption rates for age 5 fish that are twice those of the other
models; and the bioenergetics model gives rates intermediate to these. As the authors
point out, the North Sea fish community has changed considerably over the past few
decades, with a nineteen-fold change in herring biomass over the period 1974 - 1989
(Anon., 1991). Such significant changes in prey availability will not necessarily be
reflected linearly in the growth rates of predators, because of the trade-offs in prey type
(e.g. lean or fat) and the non-linear relationship between food consumption and
growth. This presents a significant problem in the MSVPA, because few experimental
data sets exist to be able to establish and validate a bioenergetic model for the main
predator species other than cod.
In this study the bioenergetics results were used for cod; for the remaining
species the daily food consumption rates (g/day) were calculated after Greenstreet
(1996). For haddock, saithe and whiting the mean stomach weight at age in each
quarter (w) was taken from Daan (1989), and Jones' (1974) digestion model applied.