It has been emphasized that the large basins of the Swan Estuary, where the commercial fishery in that system is based, have not exhibited the massive growth of macrophytes that have occurred in the last 20 years in the Peel-Harvey Estuary and 25-35 years in Leschenault Inlet. The values obtained for the CPUEs for the fishery in the Swan Estuary can thus be used as a "control" to elucidate whether any changes in the CPUEs in the other two estuaries are likely to have been related to the more extreme manifestations of eutrophication displayed by those two systems. However, it should be recognized that, during the 1970s, commercial fishermen in all southwestern Australian estuaries have increasingly replaced multifilament nets with monofilament nets and turned to using trailerized dinghies. These changes increased fishing efficiency to some extent and thus help account for rises in CPUE for the total fishery in the Swan Estuary during the 1970s and 1980s.
A comparison between the results of the statistical tests given in Table 2 show that, while the CPUE for the total fishery in the Peel-Harvey and Swan Estuaries in the 1960s did not differ significantly, this was not the case in the 1970s or 1980s when, in both of those latter decades, the CPUEs for the former estuary were significantly higher. It would therefore appear highly relevant that massive growths of macroalgae first appeared in the late 1960s and have extended through the 1970s and 1980s.8 The CPUEs for the total fishery in Leschenault Inlet were greater than those for the Swan Estuary, not only in the 1970s and 1980s, but also in the 1960s. It is therefore noteworthy that the macrophyte growth started to become conspicuous earlier in Leschenault Inlet than in the Peel-Harvey, i.e., in the 1960s rather than the 1970s. Since the macrophyte growths in Leschenault Inlet are due to seagrasses and brown algae, rather than to green macroalgae as in the Peel-Harvey, a range of different plant growths can be accompanied by high CPUEs in southwestern Australian estuaries.
The presence of higher CPUEs in macrophyte-dominated waters could reflect an increased catchability of fish, faster growth rates (and thus a greater biomass of fish) or enhanced protection from piscivorous birds (and thus reduced mortality) or a combination of these effects. Fishermen in the Peel-Harvey felt that the fish became more abundant with the advent of macroalgae in the 1970s, and that the CPUE therefore did not mainly reflect increased catchability (B. Toussaint, personal communication) 59 The growth rates of the main fish species in the Peel-Harvey were not greater than in the Swan Estuary. 9 Thus, increases in the biomass of populations of those species in the Peel-Harvey, resulting from the effects of nutrient enrichment, would not appear to have been due mainly to increases in the growth rate of fish, as is usually the cause when the biomass of fish rises in response to such nutrient enrichment. Although this could imply that the growth rates for the main species in the Peel-Harvey may have been approaching their optima, it seems at least equally likely that the rise in CPUE following eutrophication reflects an increase in the abundance of fish. 9 Since there is a very large piscivorous bird community in the region of the Peel-Harvey, it therefore seems possible that the greatly increased cover provided by macroalgae from avian predators led to a reduction in predation and thus an increase in abundance. 9
The far higher CPUEs recorded for the fisheries in the Peel-Harvey and Leschenault Inlet than in the Swan Estuary reflect in part the greater CPUEs recorded for the yelloweye mullet in the first two systems. The yelloweye mullet is an omnivore, 60 grows quickly, is relatively low in the food chain and has a high reproductive rate and is the type of species that would be expected to do well in a eutrophic environment dominated by macrophytes. 16 However, it is also highly relevant that, while the CPUEs for the sea mullet and cobbler in the Peel-Harvey did not differ significantly from those in the Swan Estuary in the 1960s, they were both significantly greater in the former system in both the 1970s and 1980s. The apparent increases in abundance of sea mullet and of cobbler (at least vis à vis the Swan) in the Peel-Harvey during the 1970s and 1980s would be consistent with the fact that both of these species would be likely to benefit from the decomposition of large amounts of plant material. The former species is a detritivore, while the latter feeds largely on benthic invertebrates. 52,60
Although there is strong circumstantial evidence that macrophytes can be beneficial for estuarine fin fisheries in southwestern Australia, there is also good evidence to suggest that species which tend to coat the substrate with both living and decaying material, e.g., Cladophora montagneana, can have a detrimental effect on the fishery for the western king prawn, presumably through inhibiting burrowing. 41
In contrast to the situation with macroalgae, the effects of Nodularia spumigena are highly seasonal. However, the dense blooms of this blue-green alga, which appeared in the late spring and early summer in all but three of the years between 1977 and 1993, apparently had a considerable impact on the fish fauna and fishery during those blooms. There is for example circumstantial evidence that fish tend to move away from Nodularia-affected sites, especially when the chlorophyll a levels that reflect the presence of this blue-green alga rise above 100 µg L-1. 25 It is also conspicuous that the advent of Nodularia leads to fishermen transferring their activities from the Harvey Estuary into those pacts of the Peel Inlet where the blooms are far less dense. 38 This movement is triggered to a large extent by their desire to avoid having their nets "clogged" by Nodularia and to be able to use their preferred method of fishing, i.e., haul netting, in the clearer waters of Peel Inlet in spring.