Observations on the biology and seasonal variation in feeding of the East Coast redeye round herring (Etrumeus wongratanai) (Clupeiformes), off Scottburgh, KwaZulu-Natal, South Africa
The basic biology and ecology of Etrumeus wongratanai was investigated from samples of fish collected by jigging off Scottburgh, Kwazulu-Natal throughout 2014 and 2015. A total of 516 otoliths, 401 gonads, 140 stomachs and 163 samples of white muscle tissue was examined, and length and weight data of 3 637 fish caught between 2013 and 2016 was also analysed. Counts of annuli deposited on sagittal otoliths indicate that sampled E. wongratanai ranged from 0 to 3 y. (year/s) of age. Length-at-age for males (females) was estimated at 15.5 cm (16.2 cm) for 1 y. olds, 17.7 cm (17.4 cm) for 2 y. olds and 19.6 cm (18.8 cm) for 3 y. olds. The length at 50% maturity was estimated to be 15.6 cm for males, and 16.1 cm for females. Etrumeus wongratanai showed high gonadosomatic index values from June to December indicating that the breeding season lasts for 6 and 8 months for males and females, respectively, and that spawning takes place from the onset of winter to early summer. Condition factor was lowest in May through to August and increased from September. The low condition factor values from May through to August could be due to the physiological strain before and during the spawning season. Fish larvae were the most important food item in samples collected in summer, whereas eucalanids were the most important prey item in autumn and winter. Overall, large copepods were the dominant prey items in terms of frequency and importance. Stable isotope data suggests that there are gradual changes in the trophic level and diet of E. wongratanai as it increases with size, and whilst δ¹⁵N values differed between seasons δ¹³C did not. The results obtained here are compared with other species of Etrumeus, regionally and globally.