Incomplete or lack of age-specific catch and survey indices
often limits the application of a full age-structured assessment (e.g.
Virtual Population Analysis and many forward projecting age-structured
models). Stock assessments will often rely on the simpler
size/age aggregated models (e.g. surplus production models) when
age-specific information is lacking. However the simpler
size/age aggregated models may not utilize all of the available
information for a stock assessment. Knowledge of a species
growth and lifespan, along with total catch data, size composition of
the removals, recruitment indices and indices on numbers and size
composition of the large fish in a survey can provide insights on
population status using a simple model framework.
The Statistical Catch At LEngth (SCALE) model, previously
known as the
Length Tuned Model (LTM), is a forward projecting age-structured model
tuned with total catch (mt), catch at length or proportional catch at
length, recruitment at a specified age (usually estimated from first
length mode in the survey), survey indices of abundance of the
larger/older fish (usually adult fish) and the survey length frequency
distributions. The SCALE model was developed in the AD model
builder framework. The model parameter estimates are fishing
mortality and recruitment in each year, fishing mortality to produce
the initial population (Fstart), logistic selectivity parameters for
each year or blocks of years and Qs for each survey index.
The SCALE model was developed as an age-structured model that
rely on age-specific information on a yearly basis. The model
is designed to fit length information, abundance indices, and
recruitment at age which can be estimated by using survey length
slicing. However the model does require an accurate
representation of the average overall growth of the population which is
input to the model as mean lengths at age. Growth can be
modeled as sex-specific growth and natural mortality or growth and
natural mortality can be model with the sexes combined. The
SCALE model will allow for missing data.
The SCALE model can be used in two different ways.
model can be used as a stock assessment tool for a species which has
overall growth information, abundance indices at length, total
removals, and size information of the catch but may not possess
production ageing on a yearly basis. Second, the model can be
used as a diagnostic tool for comparison to other fully age-structured
models that are tuned with age-specific data. Failure of the
SCALE model in fitting observed length information over time can
provide insight on inaccuracies in the basic life history understanding
of a population. For example, if a SCALE model consistently
predicts larger fish then were observed during a time period when
fishing mortality if thought to be low then these results can suggest a
possible overestimation in growth of the larger/older fish which intern
may help explain the fully age-specific model results.
SCALE/LTM has been used as an exploratory assessment
tool for Black Sea Bass (NEFSC), Spiny Dogfish (NEFSC), and Thorny
skate (NEFSC). SCALE was used for status determination in the
Monkfish stock assessment.
The calculation engine was built using AD Model Builder by
Nitschke and Dr. Christopher M. Legault (currently at the NMFS
Northeast Fisheries Science Center in Woods Hole, MA). The graphic
interface was also developed at the NMFS Northeast Fisheries Science
Center in Woods Hole, MA.