Characteristics of the Burrishoole Sea Trout Population: Census, Marine Survival, Enhancement and Stock-Recruitment Relationship, 1971-2003

This chapter examines the characteristics of the Burrishoole sea trout, Salmo trutta L., population, which has been monitored through fish trapping since 1958. These are the only available data that allow a fully quantitative examination of a sea trout population in a single Irish catchment. The Burrishoole system is located in the mid-west where a severe sea trout population collapse was evident between 1988 and 1990. The percentage of smolts that returned as finnock in the same year ranged from 11.4% to 32.4% over the period 1971 to 1987 with a mean of 21%. In 1988, this return rate decreased to 8.5% and in 1989 to 1.5%. This was followed by finnock return rates fluctuating around a mean of 6.8% until 1999, when it rose to 16.7% - the highest rate since 1986. Returns of older searun fish followed the same pattern. The total migratory trout stock included silvered and unsilvered migrants, from which the estimated number of ova deposited annually ranged between 27.5 thousand to 1.61 million. Recruitment to the sea of downstream migrants from these ova was determined by trapping 0+ and 1+ autumn migrating juveniles and 2+ and 3+ spring migrating smolts. Total recruitment (four year classes) per annual spawning cohort ranged from 1157 to 8457 and smolt output from 632 to 5813. The 1989 spawning stock collapse significantly reduced both the total number of ova deposited and subsequent levels of recruitment. These observed changes in the structure of the sea trout population and the reduction in survival suggested that this problem was related to marine conditions. Monitoring of these events facilitated the testing of the stock-recruitment relationship for the first time in a lacustrine sea trout population undergoing wide fluctuations in abundance, and provides some insight into the relationship between anadromous and resident brown trout populations.