The major annual peak in chlorophyll a in the water column of the Peel-Harvey Estuary has been shown to be due to blooms of Nodularia spumigena. 4 The trends shown by chlorophyll a between January 1984 and December 1989 thus demonstrate that the density of this blue-green alga produced a sharp peak between mid-October and mid-December in all years except 1987 (Figure 4). The trends also demonstrate that, during those years, the density of Nodularia spumigena was greater in the Harvey Estuary than in the southern part of Peel Inlet, which in turn was far greater than in the northern part of Peel Inlet. This parallels the situation in earlier years and reflects the presence in the Harvey Estuary of an optimal combination of salinity, nutrients and temperature for the growth of this blue-green alga. 5
In an attempt to elucidate the behavior of fish in the presence of Nodularia, comparisons were made between catches of fish taken by seine net throughout the Peel-Harvey Estuary between the middle of spring and the end of summer of a year in which there was no Nodularia bloom (1979/80) with those obtained in the corresponding period in 2 years in which the blooms were large and extended into February (1980/81 and 1981/82). 25 Catches at the Nodularia-affected sites were lower in the 2 Nodularia years than in the year when Nodularia blooms did not occur (Figure 5). Conversely, catches at sites in the entrance channel and the rivers, where Nodularia does not proliferate, were higher in the 2 years when Nodularia blooms occurred than in the year when no such blooms were recorded (Figure 5). In other words, there is strong circumstantial evidence that, when fish in the Harvey Estuary and lower southern Peel Inlet are exposed to blue-green algal blooms, they move out into other areas where such blooms are not present.
Analysis of the data on catches in the Nodularia-affected areas showed that the densities of fish caught at sites with chlorophyll a levels exceeding 100 µgL-1 were significantly lower than the numbers caught at sites with chlorophyll a levels less than l00 µgL-1. 25
Commercial fishermen in the Peel-Harvey Estuary employ gill nets when water clarity is low and use haul nets when clarity is high and schools of fish can be located visually. 38 Thus, in the northern Peel Inlet, the fishermen begin switching from gill netting to haul netting in October as the water starts to clear as a result of declining freshwater discharge (Figure 6). A marked change from gill to haul netting occurred approximately 1 month later in the southern Peel Inlet and 2 to 3 months later in the northern and southern Harvey Estuary (Figure 6). The delays in the switch in fishing method reflect the fact that in October to December, the development of Nodularia blooms in the Harvey Estuary, and to a lesser extent in the southern Peel Inlet, result in an extension of the period when water clarity is poor and thus in an inability to locate fish visually.
Netting in the presence of Nodularia leads to the fouling of the nets, which makes them difficult to clean and, under extreme circumstances, causes them to sink. The fishermen who work in the Harvey Estuary thus tend to move up into the Peel Inlet during dense Nodularia blooms, a feature that is reflected by a decline in both gill and haul net effort in the Harvey Estuary at such times, that is, November and December (Figure 6). This movement of fishermen to the Peel Inlet contributes to the marked increase in haul net effort in that part of the Estuary at that time (Figure 6).
The CPUE in Peel Inlet gradually increases during the late spring and mid-summer,
i.e., in the period when pronounced Nodularia blooms typically occur
in the Harvey Estuary and are followed by a decline in oxygen levels as the
Nodularia decomposes. Although the mean CPUE for haul netting in the
northern Harvey Estuary in December was quite high, there was no conspicuous
rise in the preceding months. Furthermore, the high mean CPUE for haul netting
in December was due to exceptionally large catches that were taken by haul netting
in clear patches of water in 1984 and 1985. These catches were taken at a time
when Nodularia was in rapid decline and chlorophyll a levels had
fallen below 100 µg L-1 (Figure 4).
It should also be recognized that, since fish tend to avoid dense Nodularia
blooms and fishermen make every effort to avoid Nodularia, the CPUE for
the limited amount of fishing that is carried out in the Harvey Estuary during
Nodularia blooms do not represent the relative abundance of fish throughout
the whole of that part of the estuary during those periods.