Climatic and Oceanic Variations Affecting Yield of Icelandic Stocks of Atlantic Salmon (Salmo salar)

Abstract

Regression analysis was used to investigate the effects of variations in climate, weather, and ocean conditions on yields of Atlantic salmon (Salmo salar) to anglers from 15 Icelandic rivers. Catches of grilse from the northern and northeastern group of rivers fluctuated together, as did catches from the southwestern and western group of rivers, but seldom did catches from the two groups of rivers fluctuate together. The dividing point for grouping rivers with similarly fluctuating catches was the northwest peninsula, which separated warm Atlantic water from colder north Icelandic waters of the Iceland Sea. Changes in atmospheric circulation (mid-1950's to mid-1960's) resulted in marked changes in hydrography off north Iceland from 1965 to 1970, which coincided with declines in primary production and standing crops of zooplankton, in reduced abundance and altered distribution of pelagic forage fishes, and in declines in salmon yields from north coast rivers. Few grilse were also caught from most rivers in 1980 after low sea and air temperatures in spring of 1979. Highly significant relationships were found between mean June–July sea temperatures at Hraun on the north coast and yield of grilse the following year from the northern rivers Vatnsdalsá, Vídidalsá, Hrútafjardará, and Midfjardará and its three tributaries. Sea temperatures in April–May and May–June were also closely correlated (P < 0.01 and P < 0.05) with grilse catches from these rivers. Correlations were similar but less close for two-sea-winter salmon. Fewer statistically significant relations were found between sea temperatures and yields for south coast rivers; sea temperatures were higher there and varied less between years. Inasmuch as low sea temperatures and low river temperatures often occur in the same years, salmon yields may be regulated by several adverse climatic and hydrographic factors acting in combination, resulting in delayed smoltification and reduced growth and survival at sea.