Occurrence of canthaxanthin in Atlantic salmon, Salmo salar L., fry in Irish rivers as an indicator of escaped farmed salmon

The rapid growth of Atlantic salmon, Salmo salar L., culture in north-weste rn Europe has given rise to concerns regarding the biological consequences of fish farm escapes on wild salmonid populations. Canthaxanthin, a caroten oid pigment additive to farmed salmon feed which is passed from females to their progeny, may be used as an indicator of the numbers of escaped farmed salmon which spawn in the wild. In the present study, thin-layer chromatog raphy and high-performance liquid chromatography (HPLC) were used to screen emergent Atlantic salmon fry sampled from seven river catchments in Irelan d for canthaxanthin. The incidence of fry containing canthaxanthin at great er than trace levels ( < 5% of total carotenoid pigment) was 0-4%, with an average of 1.7%, among the seven rivers sampled, indicating that the progen y of farmed salmon were present at similarly low frequencies. Canthaxanthin was detected at trace levels in an unexpectedly high proportion (35%) of s almon fry. Canthaxanthin was present at levels exceeding trace amounts in 2 4% of 21 non-anadromous brown trout, Salmo trutta L., sampled from six Iris h rivers and present at trace levels in a further 57% of the fish, indicati ng that dietary canthaxanthin is freely available to salmonids in Irish riv ers. The widespread presence of trace levels in salmon fry may be attributa ble, at least in part, to the increased sensitivity of the HPLC methods and to rapid dietary uptake during early post-emergence feeding.