Spawning run of Atlantic Salmon (Salmo salar) in the River Tornionjoki monitored by horizontal split-beam echosounding

Fixed location split-beam horizontal echosounding was used to assess the si ze and timing of the Atlantic salmon (Salmo salar) spawning run in the Rive r Tornionjoki. Four transducers, two on each river bank, were mounted acros s the river at the study site 4 km upstream from the river mouth. Net weirs were used on both shores to direct the passage of fish through the acousti c beams. Hydroacoustic monitoring covered 40-50% of the liver cross-section al area. Also test fishing and yearly catch statistics of salmon were used as an indication of the size of the spawning run in the river. Altogether, 7 700, 5 300 and 4 300 salmon-sized targets (target strength, TS greater th an or equal to -29 dB) moving upstream were detected in 1997, 1998, and 199 9, respectively. The fish migration began in all the years by early June an d peaked during the second half of the month; the migration period of large salmon lasted until mid-July. The observations made by the echosounding an d catch statistics were similar in this respect. In 1998 and 1999, however, more targets of TS greater than or equal to -29 dB were detected during la te summer than could be expected by the river catches of salmon. It may be that the large targets in late summer were, in fact, whitefish whose run oc curred during the same time. Hydroacoustic estimation of the total salmon r un at the study site was found difficult. The numbers of salmon-sized targe ts detected were almost the same as the numbers of salmon caught each year by fishermen. Therefore, only an index of the run timing and the size of th e stock can he produced from the data. it was clear that a considerable amo unt of fsh escaped the acoustic monitoring by using areas uncovered by the brain, such as gaps in the bottom and the surface layers of the water colum n near the shores. Moreover, it was found that species recognition based on TS only is not adequate in multispecies environments. Assessment of spawni ng runs remains, however, a key issue in the management of Baltic salmon, a nd with further development, the hydroacoustic monitoring may be the most v iable means of doing it. (C) 2000 Ifremer/CNRS/INRA/IRD/Cemagref/Editions s cientifiques ct medicales Elsevier SAS.