California Department of Fish and Wildlife

Age Validation of Cabezon and Kelp Greenling using Oxytetracycline

Updated June 30, 2011

Background

Otoliths, often called "ear bones", are inner ear sensory organs used in hearing and balance. In fish, these bony structures have proven useful in determining ages and growth rates. Ages are estimated by counting pairs of growth bands formed in the otoliths, comprised of one opaque and one translucent band, similar to counting the rings in a tree trunk.

Many studies assume that growth bands are laid annually. The validation of these ages, or confirmation that each pair of growth bands represents one year of growth, is a step often overlooked or considered unachievable in ageing studies. However, validation is necessary to ensure that banding patterns accurately and consistently reflect growth over time, and that age estimates are correct. This is important because knowledge of age and growth is integral to understanding life history, a major component of essential fishery information (EFI) needed for sustainable management of our fisheries.

More EFI is needed for two nearshore fish species: the cabezon (Scorpaenichthys marmoratus) and the kelp greenling (Hexagrammos decagrammus). A 2006 stock assessment of kelp greenling was not accepted by management due to the lack of EFI regarding age, growth, mortality rates and abundance estimates. The CDFW is currently investigating the life history characteristics of kelp greenling, and age validation will be an important component of the study. Although information is available on cabezon age and growth, age validation is needed for this species as well.

Study Methods

In this study, we will attempt to validate the periodicity of growth band formation in cabezon and kelp greenling using a chemical marker. Live fish will be treated with oxytetracycline (OTC), an antibiotic which is readily incorporated into calcified tissues such as otoliths. The result will be a permanent mark in the otoliths at the time of tagging which is visible under fluorescent light. After one year, otoliths will be removed from marked fish and their growth bands read in relation to the fluorescent OTC band. A single pair of growth bands, formed after the addition of the OTC mark, will validate the method of ageing for each species.

Findings to Date

In April and July 2010, we collected 5 adult cabezon and 7 adult kelp greenling for age validation. Fish were held in tanks at Granite Canyon Marine Laboratories. Each fish was anesthetized then injected with a concentration of OTC based on the fish's weight. Although no cabezon lived past 11 months after treatment due to parasites or seawater issues, their otoliths are still useful in age validation. Kelp greenling that have survived for 1 year were ready for removal in July 2011. Otoliths will be removed and viewed for the OTC mark in the coming months. Stay tuned!

Cabezon that has been tagged and treated with OTC. Photo by Diane Haas.
Cabezon that has been tagged and treated with OTC. Photo by Diane Haas.

Sectioned kelp greenling otolith under reflected light (no OTC mark). Opaque and translucent growth bands are visible.  CDFW photo by Diane Haas.
Sectioned kelp greenling otolith under reflected light (no OTC mark).
Opaque and translucent growth bands are visible. CDFW photo by Diane Haas.