Estimates the absolute and relative yield and biomass per recruit and the first order derivative.
ypr(param, FM_change, t = NA)a list consisting of following parameters (not all are required):
Linf: infinite length in cm,
Winf: infinite weight,
K: growth coefficent for investigated species per year [1/year],
t0: theoretical time zero, at which individuals of this species hatch,
M: natural mortality,
tr: age of recruitment,
tc: age of first capture;
vector with ascending fishing mortalities,
default NA
A list with the input parameters and dependent on the model type following list objects:
type = 'ypr'
The Thompson and Bell model incorporates an iteration step simulating the
stock by means
of the stock_sim function. In case changes in gear
characteristics -
here measured in terms of Lc or tc, the length or age at first capture,
respectively -
should be explored, a list with selectivity information about the gear has
to be provided and
the prediction models make use of the selectivity select_ogive
function.
Sparre and Venema (1998) recommend to treat the last length class always as plus group.
This model is very sensitive to zero observations in the ultimate length
classes. If unrealistic results are returned,
it is recommended to cut length classes with zero observations, group
them in a plus group or to change the interval between
length classes.
Equations which are used in this function assume isometric growth, an
assumption often not met. Further, the assumption that there is no relationship
between the parental stock size and progeny
over a wide range of fishing mortalities or exploitation values,
respectively, is also said to be untrue. By default, the functions assume
knife-edge recruitment and selection of gears (Sparre and Venema, 1998).
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