Evaluation of anchor tags used in sea-ranching experiments with Atlantic cod (Gadus morhua L.)

We carried out several field and experimental investigations in order to study the effects of external anchor tags on Atlantic cod in the range 20-41 cm. Tag loss rates after nearly one year in the sea were estimated to be around 10% from releases of double-tagged fish. However, tag loss after three months was negligible in a holding experiment, which also revealed similar growth rates in tagged and untagged fish. The colour of the tag is probably of little importance for the fate of the tagged fish. Blue, red or yellow tags resulted in similar reporting rates from fishermen in a series of release experiments. However, blue tags, followed by red were most frequently found on a roosting site for cormorants and shags, but this may have been a sampling artefact. Cod tagged with anchor tags placed externally, as usual, or internally were found equally frequently at the roosting site. This indicates that externally tagged fish are not more vulnerable to predation from these birds than untagged (internally tagged) fish. Similar results were obtained in a tank experiment, in which tagged and untagged cod were exposed to predation by larger cod. (C) 1998 Elsevier Science B.V. All rights reserved.