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Reliable survival estimates based on tagged fish depend on using small enough tags

Publisert 29.10.2020

Results in this study suggest that 23-mm long PIT tags can be used to tag fish down to 13 cm total length, and 12-mm long PIT tags can be used to tag fish down to 7 cm total length
Results in this study suggest that 23-mm long PIT tags can be used to tag fish down to 13 cm total length, and 12-mm long PIT tags can be used to tag fish down to 7 cm total length

 

USE OF PIT-TAGS TO STUDY MARINE MORTALITY

PIT-tags are electronic tags that are often used to tag juvenile salmon. In studies of marine mortality, the fish are PIT-tagged in the river before they leave for the sea migration. When they return to the river after the long sea migration, antennas installed in the rivers record which of the tagged fish were returning. Unique tag codes enable recognition of individual fish.

 

IMPORTANCE OF ASSESSING TAGGING EFFECTS

PIT-tags are small tags that can be inserted into the body cavity of the fish, and are therefore often regarded as being among the less invasive tagging methods. However, handling and tagging of wild fish will always impact the fish in some way.

 

It is important to know how the fish are affected and to which extent. This is important to be able to evaluate and improve the welfare of the tagged fish. We also need to be aware of how the methods we use can impact the results and conclusions of our studies.

 

Even though the tags are small, juvenile salmonids are small too, and it is important to know how potential tagging effects relates to fish size. On one hand, we do not want to tag fish that are so small that the tags and tagging affects the conclusions of the study. On the other hand, we want results that are representative of the population, and then we do not want to assess sea survival only for the biggest juveniles, because sea survival may differ between the small and large juveniles in a population.

 

REVIEW STUDY

In a new publication, we assessed impacts of PIT-tags on growth and survival in juvenile salmonids, including Atlantic salmon. The study was based on a review of all available laboratory studies on effects of PIT-tags that we could find, and a meta-analysis of the data from those studies.

 

RESULTS

The results showed that juvenile salmonids can be negatively affected by PIT-tagging if the tags are too large compared with the size of the fish. The mortality risk rapidly increased as smaller fish, or larger tags, were used.

 

Juvenile salmonid mortality increased curvilinearly with the tag:fish length ratio. The tag:fish length ratio effect on daily length or mass gain increased linearly. The results provided an estimate of the effects of the tag:fish length ratio on mortality and growth in juvenile salmonids.

 

RECOMMENDATIONS

Based on the results in this study, we suggest that researchers can follow best practices for tagging juvenile salmonids by using tags that are not greater than 17.5% of fish total length. This means that we recommend that 23-mm long PIT tags can be used to tag fish down to 13 cm total length, and 12-mm long PIT tags can be used to tag fish down to 7 cm total length.

 

 

CONTACT

Knut W. Vollset

 

The publication can be downloaded here (free access):

Vollset, K.W., Lennox, R.J., Thorstad, E.B., Auer, S., Bär, K., Larsen, M.H., Mahlum, S., Näslund, J., Stryhn, H.& Dohoo, I. 2020. Systematic review and meta-analysis of PIT tagging effects on mortality and growth of juvenile salmonids. Reviews in Fish Biology and Fisheries doi.org/10.1007/s11160-020-09611-1.