Rod catch trends for early-running MSW salmon in Scottish rivers (1952-1997): Divergence among stock components

Abstract

Temporal and spatial variations in rod catches of Atlantic salmon for the months February-June were examined for evidence of coherence using data for seven Scottish rivers, from 1952-1997. High levels of coherence were detected among rivers and among months. Examination of the combined catches for all Scottish rivers showed that rod catch trends over years differed among months. Marked declining trends over years were evident for February, March and April. The trends weakened progressively throughout the monthly series and rod catches were more stable in May and June. The divergence among months of trends over years is principally a feature of the later part of the time series (since ca. 1990). Marine and estuary net catches were combined with rod catches to estimate abundance prior to all the homewater fisheries. This suggests that abundance has declined for all the months examined. The contraction of marine and estuary fisheries has resulted in proportionately greater escapement to rivers, reducing the response of rod catches to underlying trends in pre-fishery abundance. We suggest that variation among months of rod catch trends over years is attributable to variations in marine mortality rate. Variation among monthly trends may reflect spatial variation in marine mortality rates among sub-catchment populations. Environmental effects acting on freshwater populations of juveniles in a spatially variable manner may subsequently cause parallel variation in marine mortality rates. Alternatively, variations in marine mortality rate may be attributable to population-specific migrations through spatially variable marine environments. Crown Copyright © 2002 Published by Elsevier Science Ltd on behalf of International Council for the Exploration of the Sea. All rights reserved.