ALBERT

Notes: Abstract The spatial, diel and tidal variability in the abundance of piscivorous fishes and their teleost prey, and the dietary composition of predatory fishes were investigated in beds of Heterozostera tasmanica within Port Phillip Bay, Australia, from September 1997 to February 1998. Predatory and prey fish assemblages were sampled from beds of H. tasmanica at three locations during each combination of diel (day and night) and tidal (high and low) cycles. Pelagic and benthic crustaceans represented 〉60% by abundance of the diets of all predatory fishes. Seven species, 54% of all predatory fishes, were piscivorous. These piscivores consumed individuals from seven families, 36.8% of the fish families being associated with seagrass. Western Australian salmon, Arripis truttacea (Arripidae) (n = 174) and yank flathead, Platycephalus speculator (Platycephalidae) (n = 46) were the most abundant piscivores. A. truttacea consumed larval/post-larval atherinids, gobiids and sillaginids. P. speculator consumed late-juvenile/adult atherinids, clinids and gobiids. While the abundances of piscivores varied between locations (P 〈 0.001) and diel periods (P = 0.028), the relative differences in piscivore abundance between sites and diel periods were not consistent between tides. The abundances of A. truttacea varied in a complex way amongst sites, diel period and tidal cycle, as shown by a three-way interaction between these factors (P = 0.026). Only during diurnal periods at St. Leonards was the abundance of A. truttacea significantly higher during high than low tides (P 〈 0.001). During the other diel periods at each site, the abundance of A. truttacea did not vary. P. speculator was significantly more abundant nocturnally (P = 0.017). The abundance of small (prey) fishes varied significantly amongst sites (P 〈 0.001). During the day, the abundance of small fishes did not vary between high and low tides (P = 0.185), but their nocturnal abundance was greater during low tide (P 〈 0.001). Atherinids (n = 1732) and sillaginids (n = 1623) were the most abundant families of small fishes. Atherinids were significantly more abundant nocturnally (P = 0.005) and during low tides (P = 0.029), and varied significantly amongst sites (P 〈 0.001). Sillaginids varied significantly only amongst sites (P 〈 0.001). Seagrass beds provide a foraging habitat for a diverse assemblage of predatory fishes, many of which are piscivorous. Anti-predator behaviour and amongst-location variability in abundances of piscivorous fishes may explain some of the diel and tidal, and broad-scale spatial patterns in small-fish abundances.

Notes: Abstract Daily growth increments on otoliths were used to age larvae of the pleuronectid fluonders Rhombosolea tapirina Günther and Ammotretis rostratus Günther, collected from Port Phillip Bay, Victoria, Australia, in winter 1984. Daily formation of growth increments was confirmed for R. tapirina by examining the growth of the marginal increment on otoliths of larvae collected over two 24h periods in winter 1985. The first distinctive growth increment was laid down approximately 5 d after hatching, at the onset of external feeding. Growth of flounder larvae was exponential from an early feeding stage to notochord flexion at approximately 30 d after hatching. The specific growth rate was very similar for the two species, at slightly over 4% of standard length per day. Predicted absolute growth rate of R. tapirina larvae increased from approximately 0.10 mm d-1 in early feeding larvae to approximately 0.23 mm d-1 in flexion-stage larvae, compared with 0.12 to 0.28 mm d-1 for A. rostratus larvae of equivalent ages. Exponential models did not adequately describe growth of first-feeding larvae, which was slower than predicted. Growth in the field was faster than that recorded for the same species in the laboratory at higher water temperatures and prey abundances. Otolith growth accelerated markedly in relation to growth in length at the beginning of metamorphosis, causing a significant alteration in the morphology of growth increments, and eventually leading to the cessation of production of visible increments.

Notes: Stomach contents and stable isotope analyses were used to determine if secondary planktonic dispersal of King George whiting Sillaginodes punctata post-larvae from shallow inshore habitats in a large embayment in south-eastern Australia was initiated by wave disturbance. Benthic harpacticoid copepods, which live in and amongst seagrass shoots, were found in the stomachs of S. punctata caught offshore in the plankton only during rough weather. Stable isotope analyses showed that the base of nutritional support, estimated from values of δ13C, of S. punctata collected in the plankton changed significantly during rough (waves 〉 0·25 m) compared to calm (waves 〈 0·25 m) weather conditions. Values of δ13C collected from S. punctata in the plankton during rough weather were more consistent with S. punctata values found in the seagrass. Sillaginodes punctata collected in the plankton and seagrass during rough and calm weather failed to show differences in δ15N values. Dietary and isotope analyses support a model whereby newly arrived S. punctata larvae can be resuspended from seagrass beds and dispersed offshore by wave action during onshore winds. Secondary planktonic dispersal in S. punctata would provide a mechanism by which seagrass beds further inside Port Phillip Bay are colonized.

Notes: Sixty-four post-larvae of the King George whiting Sillaginodes punctata were tested in swimming chambers, against one of five flow-through velocities (2, 4, 6, 8 or 10 cm s −1) for up to a maximum of 120 min. Fish were determined by regression to have an FV50 (50% fatigue velocity) of 6.0 cm s−1. No fish survived the full 120 min at 10 cm s−1. Sixteen individuals were tested in a swimming chamber against a flow-through velocity of 6 cm s −1 and allowed to swim to exhaustion. Fish swam between 25 and 538 min with a peak at c. 6–8 h. Total swimming time was not correlated with standard length of fish although the size range examined was narrow. Relative to recent studies on the swimming abilities of late-stage larvae of reef fishes, this study indicates that post-larval King George whiting are weak swimmers. The weak swimming ability of post-larval King George whiting is consistent with studies showing passive dispersal and recruitment of this species.

Notes: In experiments manipulating temperature and food levels, rates of short-term otolith growth and somatic growth of juvenile King George whiting Sillaginodes punctata became decoupled. Food levels were starvation, 100 and 1000 μg per fish per day and temperatures were 12 and 18° C. Short-term somatic growth was influenced predominantly by food, with negligible growth at starvation and low ration, and significant growth at high food ration at both temperatures. In contrast, short-term otolith growth was influenced predominantly by temperature, with significant otolith growth occurring for all food treatments, and elevated otolith growth occurring at the higher temperature across food treatments. The identification of such differential effects of food and temperature leading to decoupling is an important result that has significant implications for using otoliths to estimate short-term growth.

Notes: Otolith check marks of both tetracycline-treated and control juvenile greenback flounders, Rhombosolea tapirina, were attributed to the stress of transfer between aquarium tanks. Subsequent counts showed that otolith increments were deposited daily.