This study investigated the ecology of juvenile fish in two mangrove-lined estuaries in subtropical Moreton Bay, eastern Australia. Habitat utilization, recruitment, age, growth, food selectivity and sources of primary fixed carbon were examined for juvenile fish species in Deception Bay and Fisherman Island. The vital question of why juvenile fish select mangroves was also addressed.
A total of 53 species of juvenile fish were caught over a two year study period. Comparing juvenile fish communities among mangrove forests, seagrass beds and mudflats identified significant differences in species richness and abundances of juveniles. Seagrass communities comprised distinct species of resident and non-resident fish of little economic importance. Mangrove forests and mudflats had many shared species but mangrove forests were dominated by smaller or younger juveniles in greater abundances. Only 4 species were exclusive to seagrass whereas 27 species were exclusive to the mangrove/ mudflat habitat.
Salinity, temperature and turbidity were similar in all habitats, so could not account for differences in habitat choice of juvenile fish. Most juvenile fish in mangroves during summer were non-residents, and species richness and abundance were highest in summer and lowest in winter. There were significant differences among sites and years in the numbers of species and individuals, however the trends were similar and demonstrated clearly that mangrove habitats in Moreton Bay play a more important role, and have greater potential, as nursery areas than do adjacent habitats.
Spatial and temporal patterns of recruitment and habitat use by the 14 numerically abundant species of juvenile fish were examined. Peak recruitment occurred during summer, and of the non-resident species, 83% recruited preferentially to mangrove habitats at a small size (<10mm). Growth of fish in the mangrove habitat was rapid, ranging from 0.1-0.5mm/day depending on the species, with growth being greatest in summer recruits and lowest in winter recruits.
Migration through several habitats concomitant with ontogenetic development occurred in at least five species, all of which demonstrated a shift in habitat from mangroves to the mudflats with an increase in size. These five species remained on the mudflats until their emigration to the adult feeding grounds. There were at least six species that emigrated to feeding grounds directly from the mangrove habitat, and one that emigrated directly from seagrass.
Emigration did not follow a distinct seasonal pattern, but most individuals had left by winter. Emigration was size-specific at ca. 50mm. All 12 non-resident species examined emigrated to deeper water at a size which was smaller than adult size. On average, individuals spend ca. one year within the estuary and emigrate as 0+ individuals, but many had emigrated after 8 months.
Three hypotheses were tested in field and laboratory experiments to identify why mangroves habitats are so attractive to juvenile fish relative to nearby mudflat and seagrass areas, viz. (1) juvenile fish are attracted to the physical structure per se of mangrove forests, (2) the risk of predation is lower in mangrove forests than in nearby habitats, and (3) food availability is greater in mangroves than in other areas.
Artificial mangrove forest structure (bare stakes simulating pneumatophores and trunks of saplings) in the field attracted slightly but significantly more juvenile fish than areas devoid of stakes. Artificial structure left to accumulate epiphytic algae attracted four times the total number of juvenile fish than bare areas or areas with clean artificial pneumatophores. The overall composition of assemblages of juvenile fish attracted to simulated mangrove structure with epiphytic algae was different to that of areas without structure or with clean structure. Five fish species were attracted to structure covered with algae but not to clean structure, while two species were associated with artificial mangrove forest regardless of the presence of epiphytic algae. Increased abundance of algae was linked to increased food availability, and it seems that this is an important selection criterion for some species, whereas others are attracted to structure for other reasons.
Predation pressure influenced habitat choice in several fish. In the absence of predators four of the five species tested avoided shelter, but after the addition of predators all species actively sought shelter. In the field, losses of small juvenile fish to predators were minimal in both mangrove and seagrass habitats, but significantly greater in mudflat areas.
Feeding rates of small juvenile fish were significantly greater in the mangrove habitat than in adjacent seagrass and mudflat habitats, indicating increased food availability or foraging efficiency. However, larger juvenile fish foraged most effectively on mudflats compared to adjacent habitats. These larger fish showed decreased vulnerability to predators and they did not seek shelter following addition of predators in tank experiments. These experiments demonstrate clearly that for small juvenile fish, the complex structure of mangrove forests provides greater food availability and minimises the incidence of predation. As fish grow larger they manifest a shift in habitat from mangroves to mudflats at a size when feeding rates are greatest on mudflats and when vulnerability to predators is greatly reduced.
Food selectivity for six of the fourteen numerically dominant species of juvenile fish demonstrated a shift in diet with increased size. The shift is represented by a gradual increase in prey size from copepods, zoea or protists, to polychaetes, fish, gastropods or insects. Larger dietary items such as polychaetes and gastropods were more abundant on mudflats where five of these six species are known to occur at a larger body size.
Small dietary items (e.g. copepods and zoea) were characteristic of mangrove habitats, and in particular were associated with macroalgal epiphytes on pneumatophores. Five distinct feeding groups were evident, viz. algal feeders, mangrove feeders, detritivores, piscivores and mudflat feeders. The first three were associated with feeding in the mangrove environment. Only small size classes and resident species were associated with the mangrove feeder group while medium and large size classes were associated with the mudflat feeder group.
Food consumption was significantly greater in mangrove and mudflat habitats, than in nearby seagrass, indicating higher foraging efficiency in these habitats. In general, juveniles in mangrove and mudflat habitats fed opportunistically, selecting dietary items that were most abundant within their respective habitat Dietary overlap was high (>50%) among species that shared a particular habitat (mangrove or mudflat). Overlap in diets among size classes was low (<10%) in species that shift diet and/ or habitat, but was high among size classes of those species that did not demonstrate shifts in habitat.
Food webs in mangrove forests were traced using stable isotope analysis. Carbon and nitrogen derived from mangrove production is unimportant in the diet of juvenile fish in Moreton Bay. The major source of nutrition for small (<18mm, SL) juveniles of four of the eight species studied was phytoplankton. The remaining four gained nutritional input from seagrass. Enrichment of N15 values of small and medium-sized juveniles of up to 8‰ suggests they may be higher level consumers. Medium to large-sized (>18mm, SL) juveniles had isotopic signatures that reflect a mixture of seagrass and phytoplankton values. Large juveniles were enriched in nitrogen by up to 12‰ and this reflects an increase in trophic level from small and medium-sized juveniles associated with a change in diet Juvenile fish are able to assimilate carbon rapidly, adopting the isotopic value of their diet within 3 weeks. After 2 weeks, isotope values are represented by a mixture of the new and old diet.
In the face of increasing demand for real estate, knowledge on the importance of mangroves to fisheries is of paramount importance. This thesis provides a timely review of the current state of knowledge on the role of estuarine habitats as nurseries for juvenile fish, and to some extent, crustaceans. The overall conclusion is that mangrove habitats are of critical importance to many species of fish, including several species that are harvested commercially in habitats offshore.