Abstract
In this study, we used the X-11 census technique for modelling and forecasting the monthly swordfish (Xiphias gladius) catches in the Greek Seas during 1982-1996 and 1997 respectively, using catches reported by the National Statistical Service of Greece (NSSG). Forecasts built with X-11 were also compared with those derived from ARIMA and Winter's exponential smoothing (WES) models. The X-11 method captured the features of the study series and outperformed the other two methods, in terms of both fitting and forecasting performance, for all the accuracy measures used. Thus, with the exception of October, November and December 1997, when the corresponding absolute percentage error (APE) values were very high (as high as 178.6%) because of the low level of the catches, monthly catches during the remaining months of 1997 were predicted accurately, with a mean APE of 12.5%. In contrast, the mean APE values of the other two methods for the same months were higher (ARIMA: 14.6%; WES: 16.6%). The overall good performance of X-11 and the fact that it provides an insight into the various components (i.e. the seasonal, trend-cycle and irregular components) of the time series of interest justify its use in fisheries research. The basic features of the swordfish catches revealed by the application of the X-11 method, the effect of the length of the forecasting horizon on forecasting accuracy and the accuracy of the catches reported by NSSG are also discussed.
Highlights
The swordfish (Xiphias gladius) is a highly exploited cosmopolitan species in the Atlantic Ocean and the Mediterranean Sea
Fisheries statistics for Greek waters have been recorded since January 1964 by the National Statistical Service of Greece (NSSG, 1965-1999; Stergiou et al, 1997a)
NSSG data suffer from various biases, which are higher for inshore fisheries, and the degree of bias cannot be estimated, they are the best figures available with respect to: (a) length of time, (b) spatial and temporal resolution, (c) consistency and degree of subjectivity in data collection, and (d) statistical design of data collection (Stergiou et al, 1997a)
Summary
The swordfish (Xiphias gladius) is a highly exploited cosmopolitan species in the Atlantic Ocean and the Mediterranean Sea. Fishing pressure on the different swordfish stocks has increased dramatically in the last twenty years because of its high market demand. The annual swordfish catches increased sharply from about 4000 t in 1976 to 20000 t in 1988. Several aspects of the swordfish biology and stock structure have been studied in the Mediterranean Sea (De Metrio and Megalofonou, 1987; De Metrio et al., 1989; Tsimenides and Tserpes, 1989; Tserpes and Tsimenides, 1995), in which it forms a unique stock, distinct from the Atlantic ones (Kotulas et al, 1995)
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