Oceanography and skipjack dynamics in the Cook Islands
Background
Over the past five years, the total catch of the four main species of tuna from the Western Central Pacific ocean (WCPO) has increased, with the catch of ~2,468,000 tonnes in 2009 being the highest recorded. Skipjack dominates the total catch of tuna from the WCPO and almost all the catch of this species is taken by purse seining. There is only occasional catch of skipjack by the purse seine fisheries in the Cook Islands Economic Exclusive Zone (EEZ) despite that this species is likely present in abundance. Its distribution is however likely not constant and varies greatly according to ENSO events.
The Ministry of Marine Resources is conducting an exploratory study to assess the potential of development associated to this resource in a sustainable way. In particular, MMR is interested to acquire in a very short term an overview of the oceanography and variability in skipjack population dynamics in the Cook Islands EEZ. Then MMR would like to explore fishing effort scenarios. This report provides a study of oceanographic conditions in the Cook Islands EEZ and presents the results of simulations conducted for the analysis of skipjack dynamics in the Cook Island EEZ.
Regional oceanographic environment
The physical oceanography of the tropical Pacific is strongly dominated by the equatorial current systems. Under the influence of the trade winds blowing from east to west, the surface water is transported along the same direction (North and South Equatorial Currents: NEC and SEC). In the eastern side and the central Pacific, this dynamic creates a divergence at the equator with an upwelling of deep and relatively cold water and a deepening thermocline from east to west. The upwelled water is rich in nutrients and increases the primary (algal) production in the surface layer, creating a productive “cold tongue” of surface water, well visible from satellite, and contrasting with lower productivity of the north and south subtropical gyres and the western equatorial warm pool.
Climate variability
El Niño or La Niña events have direct effects on the movement and vertical distribution of skipjack tuna. Spatial distributions of purse seine catch and tagging data in the central western Pacific allowed to demonstrate a spatial shift in abundance that follows the eastward extension of the warm pool during an El Niño event
SEAPODYM Model
The model SEAPODYM (Spatial Ecosystem and Population dynamics model) is used to describe the spatial dynamics of tuna species at the scale of the Pacific Ocean, under the influence of both fishing and environmental effects. This model is being improved continuously to provide a general framework that allows integration of the biological and ecological knowledge of tuna species, and other large oceanic predators, within a comprehensive description of the pelagic ecosystem.
The main features of the model, which incorporate an optimization approach, are illustrated below (Fig 15) and include:
• Analysis (forcing) of the effects of environmental variables, such as temperature, currents, primary production and dissolved oxygen concentration, on tuna populations;
• Prediction of the temporal and spatial distributions of functional groups of prey, and age-structured predator (tuna) populations;
• Prediction of the total catch and the size-frequency of catch by fleet; and
• Parameter optimization based on fishing data assimilation techniques.
Biomass estimates
The skipjack spawning biomass was estimated around 5 million metric tonnes, in agreement with MFCL prediction for the Western Central Pacific ocean (WCPO), whereas the predicted biomass of young skipjack was still higher than in MFCL.
In average, spawning grounds and larvae are predicted to concentrate in the warm waters of the equatorial (10°N-10°S) western central Pacific Ocean with a prolongation of a relatively favourable habitat between 5°N and 10°N reaching the eastern Pacific coast. Adult fish extend their habitat towards central and eastern Pacific where they can be caught occasionally by longline fishery, and towards medium latitudes.
Skipjack distribution in the EEZ of Cook Islands
A focus in the region of the EEZ of Cook Islands indicates that in average the north of the EEZ (north of 15°S) is favourable for high skipjack production, ie annual recuitment of young fish, while total biomass is shifted further north (north of 10°N).
The total biomass of skipjack for the EEZ is estimated to ~190,000 t in average for the period 2004-2008, close to 3% of the total biomass for the WCPFC. It includes a similar proportion of adult (spawning biomass) and 3.5% of the annual production of the stock. For comparison, similar statistics are provided for the two high seas pockets that were intensively fished during the same period and provided 13.7% of the total WCPFC, resulting in a decrease of 33% in spawning biomass and 26% in total bioamass.

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