Climate Information and prediction services for Fisheries

The case of tuna fisheries

A contribution to the CLIMAR 99

WMO Workshop on advances in Marine Climatology

8-15 September 1999 Vancouver , Canada

Jean-Luc Le Blanc and Francis Marsac

Abstract : Due to the high average market price of tuna, and because the processing and commercialization of tuna have significant economic implications worldwide, tuna fisheries are of high social and economic importance. Market pressure, increasing human populations, improved standard of living, led to additional demand, and thus to additional pressures on fish populations. Thus tuna catches by fisheries throughout the world have been constantly increasing since the beginning of the 1950’s. Therefore, overexploitation is threatening tuna if their fisheries are not soundly managed. Since both the fisherman and the fish are under the influence of weather and climate, it is important to study the relationships between climate and fish behavior in order to better manage and assess their stocks.

Impacts of ocean variability on fish distribution have been known to fishermen for centuries. Only recently has science uncovered the functional relationships existing between the fish and its environment on different spatial and temporal scales. This knowledge is useful for fish stock assessment – a very difficult task - necessary to enable responsible and sustainable fisheries.

The impacts of climate on fish stocks are two-fold :

• Climate variations induce anomalies in the upper layer of the ocean (currents, waves, upwelling, advection, convergence, divergence, turbulence) which influence the feeding and spawning success of fishes ;

• Climate variations increase or decrease the catchability of fish to the gears (purse-seiners and long-liners).

When a fish stock has suffered from bad feeding and/or spawning conditions, and that catchability is increased, dramatic consequences may occur as was the case for the Peruvian anchoveta fishery during the 1972-73 El Niño.

Another typical example is the decrease in yield of purse-seiners in the Atlantic during 1984. The low-catches were first believed to be due to over-fishing, but it was later recognized to be due to a deepening of the thermocline and hence, to a decrease in catchability (since tuna basically follow the thermocline movements).

It is therefore important for accurate stock assessment to be able to determine whether an increase in yields is due to an increase in stocks or an increase in catchability and similarly if a decrease in yields is due to poor environment conditions or to over-fishing.

Physical atmospheric and oceanic parameters useful for that purpose are those that are ecologically relevant to tunas, i.e. :

• SST (sources : COADS) ;

• Winds (source : FSU pseudo-wind stress data set; COADS; Servain) ;

• Upwelling/Thermocline depth (deduced from winds and/or numerical models;  TOGA data set) ;

• Turbulence (deduced from winds and/or numerical modeling; COADS).

In this document, the relationships between these parameters and tuna catches in the Indian Ocean will be summarized considering their spatial and temporal scales. The methodology and processing of data will be presented and the limits of the information will be discussed. Finally, improvements to be made in the quality, the processing and the distribution of data for better application to sustainable fisheries will be proposed.