A quantitative assessment of the impact of goosander, Mergus merganser, on salmonid populations in two upland rivers in England and Wales

Authors: Wilson, B.R., Feltham, M.J., Davies, J.M., Holden, T., Cowx, I.G., Harvey, J.P. and Britton, J.R.

Pages: 119-136

Abstract:

Fish-eating birds are frequently considered to be deleterious to fish stocks, but quantitative estimates of impact are rare, particularly for goosander, Mergus merganser L., in England and Wales. The diet and distribution of goosanders was assessed during three breeding seasons (1996-1998) on two upland river systems, the River Wye in mid-Wales and the River Hodder in north-west England. AMonte Carlo Simulation model was used to produce estimates, with confidence limits, of salmonid losses from these rivers. These values were related to fisheries biomass estimates to determine the likely proportion of standing crop removed by goosander broods. Goosander productivity showed marked annual variation in the Wye study area, but remained relatively stable on the Hodder. Stomach content analysis of birds shot on the Wye (n = 11 birds in 1996 only) showed duckling diet to be dominated by salmon, Salmo salar L., (63% by mass), with the remainder comprising brown trout, Salmo trutta L., minnow, Phoxinus phoxinus (L.), eel, Anguilla anguilla (L.), bullhead Cottus gobio L., and stoneloach, Barbatula barbatula (L.). This was in marked contrast to the Hodder (n = 21 birds over three years), where salmon accounted for only 14% by mass, depending on area. The total mass of salmonids removed by goosanders on the Wye study site fell from 21.3 to 4.6 kg ha-1 during the course of the study. Estimates for the River Hodder were consistently lower, 3.4-4.0 kg ha-1. The proportion of the juvenile salmonid standing crop removed was as high as 683% on the Wye in a single year. Where estimates were corrected for predation mortality, the proportions of salmonid biomass removed were between 67% and 172% on the Wye and between 29% and 39% on the Hodder. The possible reasons for these high estimates of salmonid losses, and their implications, are discussed.

Source: Scopus