Oil-to-meal ratios of pelagic fish taken from the northern and the southern Benguela systems: seasonal patterns and temporal trends, 1951–1993

Abstract

The oil content of pelagic fish can be regarded as an environmental index which may help to predict recruitment. The present study provides a descriptive and statistical evaluation of monthly oil-to-meal ratios of pelagic fish landings processed at different ports of both the northern and southern Benguela systems, i.e. two in Namibia, four on South Africa's west coast, and one on the South Coast. These time-series, and an aggregate of four Western Cape ports, are described and analysed statistically for seasonally, temporal trends and coherence between ports, covering the period since 1950/51. The seasonal signal was important at all ports, explaining about 25% of the variance and having an amplitude of about 45% in relation to average. Both percentages were higher in the earlier part of the time-series. A significant negative trend was also apparent at all ports over the whole time-series, but it was manifested most strongly from 1980 to 1993. A model producing year-factors identified outliers in the time-series and these often coincided with known environmental anomalies. Also, cross-correlation showed highly significant coherence in the data from the northern and the southern Benguela but less coherence between the southern Benguela and the adjacent South Coast. The extent to which the oil data reflect environmental factors as opposed to fishing and technological factors is discussed, and some associations with recruitment patterns are noted.