A spatially disaggregated, length-based,
age-structured population model of yellowfin tuna (Thunnus
albacares) in the
western and central Pacific Ocean
John Hampton and David
A.
Fournier
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[View the document] (4503 Kb)
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Abstract. 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. |
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