Michigan Department of Conservation
Research and Development Report No. 78
Institute for Fisheries Research Report No. 1728, 1966
Marks for Rainbow Trout
Thomas M. Stauffer and Martin J. Hansen
Michigan Department of Conservation
Abstract.-One thousand 2-year-old rainbow trout (Salmo gairdneri) were divided into five differently marked lots of 200 trout (average length, 9.0 inches) and held together in a pond at the Thompson State Fish Hatchery to determine and compare survival, growth, and loss of marks. Trout in four lots were marked as follows: (1) No. 8 ring tag and left maxillary (bone) clip; (2) No. 10 ring tag and right maxillary clip; (3) No. 3 strap tag and adipose clip; and (4) right pectoral and right pelvic fin clips. A fifth lot, the control, was not marked. The maxillary and adipose clips were used to identify trout which had lost tags; these minor clips remained visible for the duration of the study and it was assumed that they did not affect survival and growth. The trout were counted, measured, and checked for lost marks at 6, 14, 18, and 24 months after marking. Trout that died between examinations were recorded.
Condition factors of several groups of marked and unmarked fish from Lake Michigan were available for comparison from another study.
In the hatchery, virtually all of the tags remained in place for at least 14 months and until a length of 14.5 inches was reached. At 18 and 24 months, when the trout had reached lengths of about 17 inches, 94 and 86% of the trout had retained their tags; most of the losses occurred among tagged trout with No. 3 and No. 8 tags. We judge that tag retention of trout released in a natural environment would not be less. In the hatchery, survival of tagged trout was not different from that of unmarked or fin-clipped fish. No inference can be made concerning survival of jaw-tagged fish released in the wild. Growth of trout in the hatchery was inhibited slightly by the tags. On the other hand, the condition factor of tagged trout recovered in Lake Michigan was not significantly different from that of similar groups of unmarked fish, although this observation must be qualified because of small sample size.
Our hatchery experiment showed that pelvic, pectoral, and adipose fin clips can be useful for field experiments since 96, 99 and 100%, respectively, were easily recognized (one-half or less regenerated) at the end of 24 months. In the hatchery, fin-clipped fish survived as well as unmarked fish, but the effect of fin clips on survival in a natural environment is not known. The growth of fin-clipped fish in the hatchery and in Lake Michigan was not different from that of unmarked fish from the same environments.
In the hatchery, 95% of the maxillary bone clips were easily recognizable (one-half or less regenerated) after 24 months. This mark would be useful in field experiments conducted by trained observers.