Variability in abundances of fishes associated with seagrass habitats in relation to diets of predatory fishes

The spatial, diel and tidal variability in the abundance of piscivorous fis hes and their teleost prey, and the dietary composition of predatory fishes were investigated in beds of Heterozostera tasmanica within Port Phillip B ay, Australia, from September 1997 to February 1998. Predatory and prey fis h assemblages were sampled from beds of H. tasmanica at three locations dur ing each combination of diel (day and night) and tidal (high and low) cycle s. Pelagic and benthic crustaceans represented > 60% by abundance of the di ets of all predatory fishes. Seven species, 54% of all predatory fishes, we re piscivorous. These piscivores consumed individuals from seven families, 36.8% of the fish families being associated with seagrass. Western Australi an salmon, Arripis truttacea (Arripidae) (n = 174) and yank flathead, Platy cephalus speculator (Platycephalidae) (n = 46) were the most abundant pisci vores. A. truttacea consumed larval/post-larval atherinids, gobiids and sil laginids. P. speculator consumed late-juvenile/adult atherinids, clinids an d 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. truttarcea varied in a complex way amongst sites, di el period and tidal cycle, as shown by a three-way interaction between thes e 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 noct urnally (P = 0.017). The abundance of small (prey) fishes varied significan tly amongst sites (P < 0.001). During the day, the abundance of small fishe s 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) an d sillaginids (n = 1623) were the most abundant families of small fishes. A therinids were significantly more abundant nocturnally (P = 0.005) and duri ng low tides (P = 0.029), and varied significantly amongst sites (P < 0.001 ). Sillaginids varied significantly only amongst sites (P < 0.001). Seagras s beds provide a foraging habitat for a diverse assemblage of predatory fis hes, many of which are piscivorous. Anti-predator behaviour and amongst-loc ation variability in abundances of piscivorous fishes may explain some of t he diel and tidal, and broad-scale spatial patterns in small-fish abundance s.