Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod?

Daily formation of fish otolith micro-increments is frequently assumed, however applying inferences about timing of life history events and formation of otolith macro-structures requires further validation of the periodicity of micro-increment formation. We analysed micro-increments from Western Baltic cod (WBC, Gadus morhua) otoliths marked with tetracycline-hydrochloride as part of an age validation study to test the assumption of daily formation of micro-increments. We found that the number of counted micro-increments consistently underestimated the age of cod aged 1 and older. Time at liberty was also underestimated, especially for fish at liberty during winter. In contrast, micro-increment counts of otoliths from wild-caught young-of-the-year (YOY) cod could be used to realistically estimate timing of hatch and translucent zone formation. Under ambient conditions, settlement did not correspond to any visible pattern within the otoliths, but could be inferred from the prey switch observed from stomach content analyses. We therefore conclude that micro-increments can be assumed to form on a daily basis until the first winter, and can therefore be used to investigate early life history of YOY WBC. However, the periodicity of micro-increment formation appears to vary seasonally in older individuals, with the number of micro-increments formed during the winter period being particularly low.

Early development of eel-tailed catfish, Tandanus tandanus (Mitchell) (Teleostei : Plotosidae), with validation of daily otolith increment formation

Eel-tailed catfish, Tandanus tandanus, have recently experienced widespread population declines in eastern Australia; with some southern populations facing the risk of extinction, the management and conservation of Queensland populations should be considered a priority. There is a lack of sound, scientific knowledge surrounding the species’ reproductive patterns and early life-history requirements. To assist in clarifying the influence of changes to the natural environment on the reproductive ecology of T. tandanus, we investigated their early life history using naturally fertilised eggs and larvae from a wild population. Nest depth ranged from 0.20 m to 1.35 m, with in situ temperatures between 15.1°C and 29.9°C, and in situ velocities from 0.00 to 0.52 ms–1. Eggs ranged in diameter from 2.6 to 4.0 mm throughout development, and preservation of eggs in ethanol caused significant shrinkage (mean 18.9%). Hatching took 4–7 days (mean 5.29 days; 15.7–28.0°C). Larvae collected from nests ranged in length from 5.1 mm to 15.3 mm SL, and in age from one day old to 16 days old. Larvae are likely to actively disperse from their nest at ~16 days old, as they approach juvenile metamorphosis. Daily otolith increment formation was validated up to 28 days after hatching; and sagittal otoliths were used to develop an age–length relationship for larvae. These results can assist researchers in estimating the timing of critical recruitment events, and investigating how the species’ early life history is influenced by environmental conditions.

Bias, precision and validation of ageing 0+ European barbel Barbus barbus (L.) from their otoliths

The European barbel Barbus barbus L. is considered a ‘flag’ species for river conservation and sport fishing, but it is increasingly threatened in its native range of distribution. To provide accurate age estimates during early life for appropriate management and conservation measures, the bias and precision of otolith (daily) micro-increment counts were evaluated and age determinations validated on laboratory-reared embryos and larvae. Out of the three pairs of otoliths, the lapillus and sagitta provided reliable age estimates for free embryos and larvae up to 17 days of (known) age post-fertilisation, with first micro-increment formation occurring five days postfertilisation. On the other hand, micro-increments on asterisci formed only 16–17 days post-fertilisation. There was agreement in microincrement counts based on lapilli and sagittae, but not between interpreters, indicating that despite consistency between the two pairs of otoliths extensive training and experience are required for reliable age interpretation. The ability to estimate the ages of 0+ B. barbus from their otoliths will contribute to a better understanding of growth rates from both hatchery-stocked and native/introduced cohorts.