Fisheries in the Pacific

Valérie Allain et al.
Status of stocks
In the WCPFC, the status of tuna stocks is based on single-species stock assessments,
which are performed separately for each of the tuna stocks discussed in the section
“Tuna and other species caught in the pelagic fisheries”. The main assessment
approach is MULTIFAN-CL, a computer program that implements a statistical,
length-based, age and spatially-structured 19 model for fisheries stock assessment
(Hampton and Fournier 2001). The main data sets to which the model is fitted are
time series of catch and effort data (i.e., the level of removals from the stock, and the
amount of fishing effort required to achieve these removals), tuna length frequencies
(which supplies information on, for example, the way different fishing gears impact
the stock, e.g., catching larger or smaller individuals), and tagging information
(which provides an idea of movement between areas of the WCPO). Biological
information such as estimates of the rate at which tuna grow, the age at which individuals
become sexually mature, the rate at which young are produced, and the rate
at which tuna die from natural causes (natural mortality, by age), are also used. The
model estimates tuna abundance per age class, quarter and area. 20
The assessments are reviewed by the WCPFC Scientific Committee (and occasionally
by independent experts) and are used to provide advice on the status of tuna
stocks to regional organisations and national fisheries authorities in charge of the
management of these stocks. This advice is based upon outputs from the assessment
model, including estimates of the quantity of fish of each species in the WCPO and
the level of fishing mortality (the rate at which fish are removed from the stock
through fishing). These are compared to ‘reference points’ 21 which indicate specific
levels of these values that are of interest to managers. Related to reference points,
the concept of maximum sustainable yield (MSY) is central to fisheries management.
MSY is the largest average catch (yield) that can in theory continuously be
taken from a stock under average environmental conditions without affecting stock
health. Related indices frequently used as reference points within the WCPFC are:
• F/F MSY , the level of current fishing mortality (F) compared to the fishing mortality which
will support the maximum sustainable yield (F MSY ); 22
19. Five to nine regions defined in the WCPO area according to the tuna species.
20. The assessment model is fitted to the data to best mimic the patterns and values observed in terms
of relative abundance indices (catch per unit of effort), tagging information, and tuna size frequencies.
21. There are two key reference point types. Limit reference points (LRPs) represent stock or fishing
levels that are to be avoided as they result in a biological danger to stock sustainability. Target reference
points (TRPs) represent stock or fishing levels that would achieve the objectives of fishery managers
(e.g., sustainability, good fishery catch rates, profitability). For example, if the stock assessment indicates
that the stock is beyond the limit reference point, or the fishing mortality is too high, management
action would be needed to reduce fishing levels to allow the stock to recover.
22. When F is greater than F MSY or F/F MSY is greater than 1, it is considered that overfishing of the
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