The diet, reproductive biology age and growth of yellowtail, Seriola lalandi, in South Africa

Includes abstract.Includes bibliographical references.Yellowtail, Seriola lalandi, is an important line-caught fish in South African waters, yet little information is available on their life-history. This study aims to add information on the diet and feeding habits, reproductive biology and the age and growth of yellowtail in South Africa. The diet of 62 yellowtail caught in the Western Cape of South Africa between 2011 and 2012 was investigated. Fish sampled by line and speargun ranged from 488 to 916 mm fork length (FL). Prey items were removed from stomachs, sorted, counted and weighed in order to calculate the percentage number (% N), percentage weight (% W), frequency of occurrence (% F) and index of relative importance (% IRI). Of the 62 stomachs examined 11 (17.7 %) were empty, 16 (26.0 %) contained only unidentifiable remains and 47 (82.5 %) contained identifiable remains. Prey items covered 18 species belonging to five classes: bony fishes, crustaceans, cephalopods, polychaetes and bivalves. Small pelagic fish were the dominant prey type, followed by crustaceans. The remaining taxa were of negligible importance. Some dietary differences were observed between sample areas. Most notable was the increased importance of crustaceans at Dassen Island on the West Coast compared to the sites at Robben Island, False Bay and Struisbaai. The reproductive characteristics of yellowtail were documented from fish collected from 1974 to 2012. Samples were collected from Cape Infanta on the South Coast to Lamberts Bay on the West Coast of South Africa. Histological validation of macroscopic staging criteria revealed that active and developing ovaries are commonly staged incorrectly. A protracted spawning season from November to February with peak spawning in December and January was deduced from GSI values. No hydrated eggs were observed. Females matured at 550 mm FL (95 % CI = 532 - 570 mm) and males matured at 585 mm FL (95 % CI = 555 - 619 mm). The age and growth characteristics of yellowtail in South African waters were determined from readings of whole sagittal otoliths collected from 1974 to 2012. Whole otoliths were considerably easier to read than sectioned otoliths. A total of 524 whole otoliths were taken from fish ranging from 430 to 1080 mm FL, of which 141 (27 %) were discarded and 384 (73%) were readable. Agreement between all three readers was 13 % (n = 50) and between any two was 71 % (n = 274). Maximum ages for male and female yellowtail were 7 and 8 years respectively. Age at 50 % maturity (A50) for males it was 2.3 years while females matured (A50) at 3.6 years. Von Bertalanffy growth parameters did not differ between males and females (P > 0.05). A statistical penalty was used to keep the estimated growth parameters within biological limits and produced a von Bertalanffy growth equation with an L∞ and K of1064 mm and 0.17 y-1 respectively. The growth performance index (φ) of yellowtail in South African waters was found to be 3.51. This is high for the family Carangidae but on par with other species in the genus Seriola. The life history characteristics for yellowtail in South African waters closely resemble those of other yellowtail populations. The diet of yellowtail in South African waters represents that of a robust generalist feeder that is not reliant on specific prey for its survival. The age, growth and reproductive characteristics of yellowtail in South African waters indicate that they are a fast growing and relatively early maturing species. These life-history characteristics indicate that the stock is resilient in relation to other line-fish species, but the large proportion (41%) of fish caught below the 50 % size at maturity suggests that a revision of the minimum size limit should be considered