Short term effects of relative tag size and surgical implantation on feeding behaviour, survival rate, plasma lactate and growth rate in juvenile to adult rainbow trout (Oncorhynchus mykiss)

• We investigated the applicability of the 2% tag ratio rule to fish of various sizes.
• Feeding behaviour was drastically decreased by tags exceeding a 3% ratio.
• Survival rate decreased with increase in tag ratio from juvenile to adult fish.
• A 5.6% tag ratio was expected to give a 90% survival rate eight days post-surgery.

Surgically implanted tag attachments provide biotelemetric and biologging data that can be used to investigate the behaviour and physiology of fish in their natural environments. However, to ensure the validity of the obtained data, it is essential to understand the effects of tagging on the behaviour and physiology of the fish. Consequently, a number of studies have examined the effects of surgical implantation of a tag. These studies indicate that the tag should weigh less than 2% of the fish body mass (the so-called “2% rule”). However, there is little information on whether the 2% rule is appropriate for fish in the same species but of different sizes or developmental stages (e.g. juvenile or adult fish). This study investigated the question of whether the ratio of the tag to fish body mass (termed tag ratio here) in the rainbow trout (Oncorhynchus mykiss) affected feeding behaviour, survival rate, plasma lactate levels and growth rates. Tag ratios over 3% were found to significantly impair the feeding behaviour at both day 1 and day 8 post-surgery. Survival rate was negatively correlated with tag ratio; there was no effect of fish body mass on survival rate. Our analyses showed that a 5.6% tag ratio was expected to have a 90% survival rate at day 8. Plasma lactate levels and growth rates on day 8 were not affected by the tag ratio or surgical implantation. We conclude from our analyses that the 2% rule may be conservative and is likely acceptable for juvenile stages as well as adult rainbow trout.