Using traditional age and growth techniques in endangered species management: eastern freshwater cod, Maccullochella ikei
Butler, G.L. and Rowland, S.J., 2008. Using traditional age and growth techniques in endangered species management: eastern freshwater cod, Maccullochella ikei. Marine & Freshwater Research, 59: 684–693.
The eastern freshwater cod is a large, long-lived, slow-growing freshwater species that is found only in the Clarence and Richmond rivers on the north coast of NSW. It suffered a dramatic decline in numbers during the first half of last century and is now listed as endangered under the NSW Fisheries Management Act and the Commonwealth Environment Protection & Biodiversity Conservation Act. Over the past several years, scientific information about the species has been collected from remnant populations in the Clarence catchment to allow a better understanding of the ecology of eastern freshwater cod and hence to assist with the long-term recovery of the species.
It is well known that a distinctive growth band may be laid down each year in the earbones (otoliths) and other hard parts of a fish’s skeleton (such as spines, opercular bones or vertebrae), providing accurate data on age and growth. Otoliths and other hard parts were collected from archival specimens of eastern freshwater and collected since 1979, from fish hatcheries and from some wild caught fish, but only the otoliths contained consistent rings that could be clearly seen under a microscope. The analysis of these otoliths showed that the largest fish sampled (those over approximately 75 cm long) had an estimated age of only 14 – 16 years. Most fish were much smaller and younger than this, indicating that the population is still in the early stages of recovery. The maximum size of eastern freshwater cod from historical records is approximately 110 cm, and fish of this size (based on data from the closely related Murray cod) might be expected to be several decades old. Growth is rapid during the first 4 years of a fish’s life, progressively slows between 6 and 10 years and is minimal from then on. Variation in food availability is hypothesised to be the likely reason for the large variability observed in the length-at-age relationship.
These data will be used as the basis for an ongoing monitoring program to assess the recovery of this species into the future.