REGURGITATION RATES OF INTRAGASTRIC TRANSMITTERS BY ADULT ATLANTIC SALMON (SALMO-SALAR L.) DURING RIVERINE MIGRATION

Regurgitation rates of tagged salmon in the wild are difficult to esti mate as few fish are recovered and thus the fate of the majority of ta gs is difficult to assess unambiguously. During a study on the River T weed between 1994 and 1996, 281 returning adult salmon were caught by net and coble, and radio tagged. Salmon recaptured by rod and line wer e used as a sample of the radio-tagged population from which the frequ ency of tag regurgitation was determined. A total of 27 tagged fish we re recaptured by anglers over the 3 year study period, of which 4 (14. 8% +/- 1.2% standard error) did not have radio tags in their stomach. Regurgitation rates were consistent between years; 16.7% in 1994 (2 ou t of 12), 12.5% in 1995 (1 out of 8) and 14.3% in 1996 (1 out of 7). R egurgitation was not associated with the immediate aftermath of taggin g. However, too few salmon were recaptured in the present study to ide ntify other factors which may be important in determining transmitter regurgitation rates in radio tracking studies. No statistically signif icant relationship could be demonstrated between the lengths of the fi sh regurgitating their tags and those of the fish which retained them (Mann Whitney test: N=27, U = 33.5, P = 0.392). Fish which regurgitate d tags were tagged over a wide range of months (between April and Sept ember) and the subsequent positions of regurgitated tags were widely s cattered within the river system.