A spatially disaggregated, length-based, age-structured model for yellowfin
tuna (Thunnus albacares) in the western and central
Pacific Ocean is described. Catch, effort, length-frequency and tagging data
stratified by quarter (for the period 1962–99), seven model regions
and 16 fisheries are used in the analysis. The model structure includes
quarterly recruitment in each region, 20 quarterly age classes, independent
growth patterns for juveniles and adults, structural time-series variation in
catchability for all non-longline fisheries, age-specific natural mortality,
and age-specific movement among the model regions. Acceptable fits to each
component data set comprising the log-likelihood function were obtained. The
model results suggest that declines in recruitment, and as a consequence,
population biomass, have occurred in recent years. Although not obviously
related to over-exploitation, the recruitment decline suggests that the
productivity of the yellowfin tuna stock may currently be lower than it has
been previously. Recent catch levels appear to have been maintained by
increases in fishing mortality, possibly related to increased use of fish
aggregation devices in the purse-seine fishery. A yield analysis indicates
that average catches over the past three years may have slightly exceeded the
maximum sustainable yield. The model results also reveal strong regional
differences in the impact of fishing. Such heterogeneity in the fisheries and
the impacts on them will need to be considered when future management measures
are designed.
Correction: Distinct Yellowfin Tuna (Thunnus albacares) Stocks Detected in Western and Central Pacific Ocean (WCPO) Using DNA Microsatellites
