Mangrove fish, zooplankton and forest epifauna at Dickson Inlet, Port Douglas, north Queensland

Three areas in Dickson Inlet were sampled intensively in the wet season (16-18 March 1989) and the dry season (24-27 October 1989) during daylight hours. The first area (FRONT) was slightly north of Magazine Island adjacent to the mangrove stands at the mouth of a small creek. Low tide water depth was between 0.5 and 1.0 m at this site and small patches of seagrass were present. The second area (MID) was in the middle reaches of the inlet, and included parts of the mainstream and small mangrove creeks draining into the mainstream. Area 3 (UPPER) contained small creeks and sections of the mainstream at the extreme upper limits of the inlet.

In Dickson Inlet fish were sampled with three net types. A small seine net (6 m x 2 m, 3 mm mesh) was dragged by two people for 10 m at low tide to capture very small post larval and juvenile fish inhibiting shallow creeks. A large seine net (30 m x 1.5 m, 18 mm mesh), was used to sample larger, more mobile fish species, which use shallow accreting mud-banks as a habitat at low tide. Gill nets (15 m x 1.5 m, 20 mm mesh) were set at high tide at the entry points of small creeks into the mainstream, and left to fish for 3 hours on the ebb tide. Three replicate samples were taken for each net type in each of the three areas. All fish were identified and counted in the field. Measurements of water temperature (°C), salinity (ppt), turbidity (secchi disc (m)) and dissolved oxygen concentrations were taken in each area.

Dip Creek, approximately halfway between Port Douglas and Cooya Point, was sampled during the dry season field trip to determine if fish community structure varied in the region. In Dip Creek, only the two types of seine net were used for sampling.

Zooplankton were sampled at 4 sites in the inlet (FRONT, MID1, MID2, UPPER) in the wet season, but only at 3 sites (FRONT, MID1, UPPER) in the dry season, using oblique tows of a zooplankton net (mouth diameter 0.5 m, mesh size 105 µm, with a center-mounted General Oceanics flow meter). Tows were for 2 minutes duration, and there were 3 and 4 replicates per site in the wet and dry seasons respectively. Zooplankton were preserved in the field and returned to the laboratory for analyses.

In addition to the quantitative sampling of fish and zooplankton, many areas of the intertidal forested areas were also surveyed, and a record was kept of the degree of leaf litter accumulation and the relative abundance of crabs (Family Grapsidae, Sub-Family Sesarminae).