Mulloway Stock Enhancement Project

It is necessary to mark released fish somehow, so they can be identified as of hatchery origin upon recapture. There are several methods of doing this, however we prefer to mark fish using harmless stains, as many fish can be stained at once, and the method is fast and the marks are easy to detect. The stains are deposited into the mulloway’s otolith (or ‘jewel’), and can be visualised using a special microscope that emits UV light.

Olympus microscope used for visualising the fluorescent stains in mulloway otoliths. The bright blue flare you can see is produced when UV light is reflected off the oxytetracycline stain. The UV light is produced by passing a current through a special Mercury filled bulb in the microscope, and is sorted into a series of different wavelengths using sets of filters.

Immersion in oxytetracycline (OTC) produces a yellow stain when visualised under UV light. Another stain used that is much more expensive is alizarin complexone. Alizarin complexone produces a much brighter red stain, which is easier to find in the otolith than OTC. Stains can be used multiple times in each batch of fish to provide a unique series of marks if several different batches of fish are to be stocked.

Oxytetracycline marks in a mulloway otolith. The panel on the left shows part of the otolith visualised under normal light. The panel on the right shows the exact same part of the otolith visualised under UV light, with two OTC marks.

Alizarin complexone (ALC) mark in a mulloway otolith. The panel on the left shows part of the otolith visualised under normal light. The panel on the right shows the exact same part of the otolith visualised under UV light, with a single alizarin complexone mark.

A combination of marks in a mulloway otolith. The panel on the left shows part of the otolith visualised under normal light. The panel on the right shows the exact same part of the otolith visualised under UV light, with an alizarin complexone mark and an OTC mark.

We have also developed methods of identifying hatchery-reared mulloway using by looking at the patterns of growth on the scales of the fish. When in the hatchery fish grow fairly fast compared to their wild counterparts. This can be seen in the growth increments on the scales of the fish, and usually show up as a wider spacing between increments in hatchery reared fish. By developing a mathematical function to describe the pattern of increments, we can differentiate between hatchery-reared and wild fish with a fairly high degree of accuracy.