Fish larvae were collected weekly from the Breakwater Marina, Townsville between August and November 1987. Sampling was conducted at night using three fluorescent lamps sealed within a clear perspex tube and a 1 m x 250 µm mesh size plankton net. The lamps were switched on and the tube lowered into the water from a jetty to a depth of 1.5 m. The plankton net was then lowered approximately 3 m below the tube. After 15 minutes the plankton net was hauled rapidly up over the perspex tube to the surface. Sampling was repeated 4 times per night, at hourly intervals, commencing at 20:00h. Larvae were immediately preserved in 98% ethanol for later identification and analysis. Two species of clupeid larvae (Herklotsichthys castelnaui and Escualosa thoracata) were identified from samples. Herklotsichthys castelnaui was chosen for this study.The standard length of larvae was measured under a stereo dissecting microscope with an ocular micrometer. Otoliths were teased out of the larvae with tungsten needles and air dried. A drop of 98% ethanol was added to the otoliths and allowed to evaporate to ensure dehydration and the otoliths were mounted in immersion oil for microscopic examination.The sagittae were the only otoliths found to be deposited during the first days of larval life and were used in the analysis. Growth increments were viewed at 1000 x magnification under a compound microscope using polarized transmitted light. A high resolution video camera was mounted on the microscope, which was connected in turn to a video screen on which increments were counted. The system was interfaced with a computer for measurement of otolith radius and growth increment widths.To determine if the increments observed were deposited daily, larvae were collected from the marina in October 1987, transported to the laboratory at the Australian Institute of Marine Science (AIMS) and treated with tetracycline. The fish were acclimated for two days in ambient photoperiod and temperature regimes and were fed twice daily on wild zooplankton captured from Chunda Bay, adjacent to AIMS. Fish were kept overnight in a 4 L tank treated with a 0.25 g/L tetracycline hydrochloride solution and then returned to a 120 L tank and fed as before for 10 nights and 11 days before being killed. Sagittae were viewed under fluorescent UV light with a compound microscope and an ocular marker was aligned with the fluorescent band in the otolith. The otolith was then examined under natural light, and the number of increments between the marker and the otolith margin counted. Both sagittae for each fish were analyzed, and three counts were made of each otolith.For a study of ontogenetic development, larvae were assigned to one of 5 cohorts if birth dates (date of initial increment) were within 2 days either side of a first or last quarter moon phase, to take into account the semilunar spawning periodocity of this species. Larval development was described. Length at age was calculated and compared between cohorts. Increment widths were used to estimate individual growth histories. Standard length at capture and radius of the sagittae were used to estimate the mean daily growth rate.Daily water temperatures were estimated for the sampling period from air temperature records collected at 6 hour intervals at Cape Cleveland, in order to investigate the relationship between water temperature and increment width.
