IV. INFLUENCE OF EUTROPHICATION ON THE COMMERCIAL FISHERIES

A. INFLUENCE OF MACROALGAE

The total annual catch in the Peel-Harvey Estuary between 1952 and 1970 lay between 322 and 454 t (Figure 2). It rose sharply to reach a maximum of 995 t in 1976. Large catches were obtained in each subsequent year and were as high as 891 t in 1983. Although a substantial catch was obtained in 1988, the catches have essentially declined since 1983. The decline in 1989-1991 to the levels of the 1960s largely reflects the influence of the fall in the value of bait fish.

Effort remained relatively stable between 1953 and the early 1970s and then, as a result of legislation, declined progressively (Figure 2). The decline in the last 3 years of the study period, i.e., 1989-1991, also reflects a reduction in effort as a consequence of the reduced value of bait fish. CPUE followed similar trends as catch.

The annual values for catch, effort and CPUE between 1952 and 1970 tended to be more variable in Leschenault Inlet than in the Peel-Harvey. However, the subsequent trends shown by these three variables in Leschenault Inlet were similar to those exhibited in the Peel-Harvey.

In contrast to the situation in both the Peel-Harvey and Leschenault Inlet, the catches and CPUE in the Swan Estuary rose less sharply in the 1970s and the catch declined markedly after 1975.

Analysis of variance showed that the CPUEs for the total fin fishery and for the fishery for Aldrichetta forsteri differed significantly among the 1960s, 1970s and 1980s in the Peel-Harvey Estuary, Swan Estuary and Leschenault Inlet (Table 1). Tukey's a posteriori test showed that, in all but the case of A. forsteri in Leschenault Inlet, these CPUEs were significantly higher in both the 1970s and 1980s than in the 1960s. Furthermore, the CPUE for A. forsteri in Leschenault Inlet in the 1980s was significantly higher than in the 1960s, and the values for the 1970s, were markedly higher than the 1960s, but not sufficiently so as to be significant at p < 0.05.

There was a significant difference between the CPUEs for Mugil cephalus in the three decades in the case of the Peel-Harvey and Swan estuaries, but not with Leschenault Inlet (Table 1 ). The CPUEs for this species were significantly higher in both the 1970s and 1980s than in the 1960s in the Peel-Harvey. The CPUEs in the Swan Estuary for the 1980s were significantly higher than those in the 1970s, but not than those in the 1960s. Although the differences between decades were not significant in Leschenault Inlet, it is worth noting that the CPUEs in this system in the 1970s and 1980s were still 34.5 and 75% greater than in the 1960s, respectively.

The CPUEs for C. macrocephalus did not show any significant interdecade differences in either of the Peel-Harvey and Swan estuaries (Table 1). This is almost certainly related to the large 95% confidence limits, which in turn reflect the "boom and bust" cycles that are undergone by the fishery for this valuable species. ⁵⁶ These occur as a result of intense fishing pressure and a relatively slow recovery rate, which is due to a combination of the fishing of individuals before they reach maturity, the minimum time taken to reach maturity (3 years) and the very low fecundity. ^56,57 Extreme fishing pressure almost certainly accounts for the marked decline in the CPUE for C. macrocephalus in Leschenault Inlet between the 1960s and 1980s (Table 1).

It is relevant that the increase in CPUEs for the total fishery and of the fishery for the three main species in the 1970s was much greater in the Peel-Harvey than in the Swan Estuary and that it was only in the former system that massive macroalgal growths developed during the 1970s. ANOVAs showed that the CPUEs for the total fin fishery and for the fishery for A. forsteri, M. cephalus and C. macrocephalus differed significantly among the three systems in the 1970s and 1980s, and the same was true for the total fishery and the fishery for A. forsteri in the 1960s (Table 2). While CPUEs for the total fin fishery and the fisheries for M. cephalus and C. macrocephalus in the Peel-Harvey did not differ significantly from those of the corresponding fisheries in the Swan Estuary in the 1960s, they were significantly greater in the Peel-Harvey in both the 1970s and 1980s (Table 2). These differences should be considered in the context of the fact that the 1970s was the period when massive macroalgal growths appeared in the Peel-Harvey, and that these continued into the 1980s, if not quite so profusely. No such macroalgal growths developed in the Swan Estuary during the corresponding period. The CPUEs for the fishery for A. forsteri were significantly higher in the Peel-Harvey than in the Swan in all 3 decades.

The CPUEs for the total fin fishery and for the fishery for A. forsteri were significantly greater in Leschenault Inlet than in the Swan Estuary in all 3 decades. It would thus appear relevant that the development of extensive growths of seagrasses and brown algae was initiated earlier in Leschenault Inlet than in the Peel-Harvey, i.e., in the late 1950s or in the 1960s rather than the 1970s. The onset of these growths was apparently related to the construction of a new entrance to the Leschenault Inlet in the 1950s which increased tidal exchange with the ocean, thereby making the environment in the inlet more marine and thus probably providing improved conditions for the growth of marine macrophytes. Although the CPUEs for M. cephalus in Leschenault Inlet did not differ significantly from those in the Swan Estuary in any of the 3 decades, it is worth reiterating that the increase in CPUE in the Leschenault during the 30 years of this study was greater than in the Swan Estuary (Table 1 ), in which there was no comparable development of macrophytes.

The annual catches of western king prawns in the Peel-Harvey Estuary rose sharply from 1.2 to 11 t between 1952 and 1962 to 24 t in 1964 (Figure 3). The annual catches subsequently declined markedly from about 10 t in 1966 to 1968 to less than 5 t between 1969 and 1978 (Figure 3). These latter very low catches correspond to a period when Cladophora montagneana was the dominant macroalga in Peel Inlet, i.e., in the region where king prawns are usually abundant. ⁴¹ It is almost certainly relevant that this macroalga, which coated the sandy substrate surface with algal balls and a thick ooze of decomposing algae at that time, inhibited the king prawns from burrowing into the substrate. ^41,58 The conclusion that Cladophora had a detrimental effect on the western king prawn is consistent with the fact that in 1979 and the 1980s the subsequent change to more free-floating types of macroalgae that occupy shallower waters of the estuary, such as Chaetomorpha linum and Enteromorpha intestinalis, ⁵⁸ was accompanied by a marked rise in the catches of king prawns. ⁴¹