Variable Effects of Droughts on the Density of a Sea-Trout Salmo trutta Population Over 30 Years

Abstract

This study shows how a stock-recruitment model can be used to assess the effects of drought on fish survival, using data from a long-term study of a sea-trout population. Droughts were severest in the summers of 1976, 1983, 1984 (also severe spring drought) and 1995, and less severe in summer, but followed by autumn droughts in 1969, 1989 and 1993. The Ricker curvilinear model significantly fit (P < 0.01) the relationship between initial egg density and survivor density for: parr aged 0 + years sampled in late May or early June, and late August or early September; parr aged 1 + years sampled in late May or early June, and late August or early September; spawning females and their total egg production. These analyses provided strong evidence for density-dependent population regulation and also identified extreme outliers, all these being for year-classes affected by the summer droughts. The variable effects of the seven summer droughts were quantified by expressing the difference between each pair of observed and expected values as a percentage (with 95% CL) of the expected value predicted from the Ricker model. The 1969 drought continued with an autumn drought and reduced the density of 1 + parr (1968 year-class), but had no long-term effects on female or egg densities. The 1983, 1984 (plus spring drought) and 1989 droughts reduced 0+ (1983, 1984 and 1989 year-classes) and 1+ (1982. 1983 and 1988 year-classes) parr densities, and led to low densities of returning females and their eggs from the 1982, 1983, 1988 and 1989 year-classes. The 1976 summer drought, and 1993 summer and autumn droughts had little effect on parr densities, but in the long term produced low densities of returning females and their eggs from the 1975 and 1992 year-classes. The 1995 drought reduced 0+ (1995 year-class) and 1 + (1994 year-class) parr densities, but its long-term effects cannot be evaluated until more data are available. Reduced densities of returning females and their eggs were associated with 1+ parr being subjected to droughts; possible reasons for this are discussed, including growth retardation.