Browsing by Author "Vine, Niall"

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    Thermal preferences of two cohorts of juvenile silver kob, Argyrosomus inodorus (Pisces: Sciaenidae)
    (2007) Dickens, Hazel; Vine, Niall; Kaiser, Horst; Waldron, Howard
    The silver kob, Argyrosomus inodorus, has been identified as a viable species for mariculture in South Africa due to its high demand and market value as well as the necessary biological traits such as high fecundity and fast growth rates. In order for the full scale commercial production to become a reality however, it is important that the environmental conditions required for optimum growth at each phase of the species development are fully understood. Until recently, this sciaenid species was misidentified along with the dusky kob, Argyrosomus japonicus, as Argyrosomus hololepidotus, a fact that resulted in a poor understanding of the life history characteristics of the species and therefore the implementation of a poor management plan. As a result, wild stocks have been heavily over-exploited with spawner biomass-per-recruit ratios estimated at 2.9-12.5% of the pristine level. This study hypothesized that juvenile silver kob would exhibit a preferred temperature when a choice was available to them, and that this preferred temperature might change with age. Two cohorts of juvenile silver kob were acclimatized at 18°C and then subjected to identical temperature preference trials using temperature gradient tanks where the temperature ranged between 16 and 21.5°C as well as to identical isothermal control trials where the temperature throughout the tanks was 18°C. Salinity was kept constant at 35ppt and photoperiod remained 12L: 120 throughout. After a day of acclimation, observations were done over a five day period until the distribution of fish within the tanks had stabilized and that final temperature preferendum could be determined. During this time, dissolved oxygen concentrations were measured continuously to ensure that oxygen did not become limiting but remained above 6.4ppt. Results showed that the juveniles did indeed exhibit a preferred temperature with a range of 16.3 to 17.6°C for the first cohorts, and 16.9 to 18.1°C for the second. This difference between the preferred temperature ranges of the two age classes was found to be significant (p<0.01) by a Mann Whitney U Test. Kolmogorov-Smirnov tests showed no significant difference (p<0.01) between the tanks of the isothermal control trials indicating that there was no shoaling behaviour or individual tank bias affecting the results.