The Department of Primary Industries and Regional Development has the largest collection of fish ages in Australia after fisheries scientists recently recorded the 350,000th entry (November 2018). The database held at the Western Australian Fisheries and Marine Research Laboratories dates back to 1990 (and earlier for some species) with species ranging from small estuarine to large marine fish.
The database includes the State’s oldest recorded fish, an 84-year old female Bight redfish caught off Albany in 2013 (Figure 1) as well as Australia’s oldest pink snapper, a 41-year old male caught off Bunbury in 2007.
Researchers determine the age of bony fish by studying their otoliths, or ear bones. Fish use otoliths for balance and to hear underwater.as a fish grows, tiny white and clear bands of calcified material are laid down in the otolith, similar to growth rings in a tree.
The otoliths (Figure 2) are removed from fish frames (Figure 3) – skeletons of filleted fish with head and guts intact) and embedded in resin before a very thin section through the core of the otolith is cut. The section (Figure 4) is placed on a microscope slide and growth bands are counted under a microscope to determine the age of the fish (a process called reading).
Figure 2. (Left) Two whole otoliths and (Right) The otoliths are extracted from the brain case of bony fish.
Otoliths of some species can also be read whole under a dissecting microscope using reflected light, when they are immersed in glycerol, which provides greater clarity for reading of the rings.
The information contained in otoliths helps researchers to understand the age structure of a population, in other words, how many fish there are in each age group (Figure 5). Assessing the structure over several years, researchers can see the impact of successful recruitment, and the effect of fishing. This information is vital for fisheries scientists to observe long-term demographic changes for a diverse range of fish stocks thus informing sustainable management strategies.
Researchers can also find out about the environmental conditions in which the fish lived through analysing the chemistry of otoliths.
Commercial fishers, fish processors and recreational fishers all contribute ‘fish frames’ to Fisheries researchers. The frames are used to measure the length of the fish, examine the reproductive organs and extract the paired otoliths.
Collecting fish frames and analysing age structures of fish species is a major initiative of the Department of Primary Industries and Regional Development. Information about the species currently being collected and drop-off locations may be found here.
Jenke, J. 2002. A guide to good otolith cutting. Fisheries Research Report 141. Department of Fisheries, Western Australia, Perth.
Department of Fisheries WA, Recreational Fishing.
http://www.fish.wa.gov.au/Fishing-and-Aquaculture/Recreational-Fishing/Pages/default.aspx [28 November 2018]
Department of Fisheries WA, Research Angler Program (RAP).
http://www.fish.wa.gov.au/About-Us/Pages/Volunteers.aspx [28 November 2018]
The number of fish of different ages within a population.
Similar band in a part of an animal, such as fish ear stone, marking a period of growth.
Calcium carbonate stone located in the brain cavity of bony fish. Fish ear stone.
Animals living in a specified area.
Addition of fish to a stock or population as a result of reproduction, migration or growth to legal size.