The Commercial Fisheries in Three Southwestern Australian Estuaries Exposed to Different Degrees of Eutrophication

(This paper has been first published as R. A. Steckis, I. C. Potter, and R. C. J. Lenanton, The Commercial Fisheries in Three Southwestern Australian Estuaries Exposed to Different Degrees of Eutrophication, in Eutrophic Shallow Estuaries and Lagoons, McComb, A.J., Ed., CRC Press, Boca Raton, FL, 1995, chap. 12)
 
 

I. INTRODUCTION

Estuaries in many parts of the world receive nutrient runoff from urban settlement, agricultural land and forest clearing and thus have become eutrophic. ^1-5 This form of eutrophication is referred to as "cultural" eutrophication. The ways in which estuaries respond to nutrient enrichment are determined by their physical, chemical and geological characteristics. ⁶ These include the depth of the water column, the flushing rate of the system, the amount of chemical conversion (oxidation or reduction) of the nutrients and the sedimentary characteristics of the benthos. Thus, for example, estuaries which experience considerable nutrient enrichment, but have a deep entrance and large tidal exchange, e.g., San Francisco Bay, do not exhibit the extreme manifestations of eutrophication, such as the production of large algal blooms and macroalgal growths. In other words, the high flushing rates in those systems prevent the nutrients from accumulating, and thus increasing primary productivity. ^2,7 On the other hand, those estuaries that are shallower and have a much smaller tidal interchange with the open sea, such as Chesapeake Bay on the east coast of North America, and even more particularly, the Peel-Harvey Estuary in southwestern Australia, which has a very narrow entrance channel (a large artificial entrance was opened in 1994), exhibit a far greater response to nutrient enrichment. These are reflected by the development of large blooms of phytoplankton and changes in the abundance and species composition of the macrophytes. ^1,4,8

Large increases in macrophyte growth can lead to corresponding increases in fish production, either by providing more food and thus an increase in growth rate, and/or more shelter and thereby higher survival rates.^1,9-11 Fish abundance and biomass, and sometimes species richness, are often correlated with macrophyte biomass and therefore tend to be higher in vegetated than in unvegetated areas. ^10,12

An increase in phytoplankton, such as nanoplankton and diatoms, usually leads to increases in zooplankton biomass and thus provides more food near the base of the food chain. ¹³ Artificial enrichment of certain Canadian streams with nitrogen and phosphorus has been shown to result in an increased standing crop of both periphyton and benthic invertebrates and, as a consequence, a faster growth rate among the resident salmonid fish. ¹⁴ Keller et al.¹⁵ have demonstrated that an increased production of diatoms following nutrient enrichment leads to increases in the zooplankton that graze on those diatoms and thus more food for the planktivorous juvenile menhaden (Brevoortia tyrannus) which therefore grow faster. The fish communities that tend to be advantaged by increases in phytoplankton production brought about by nutrient input are those that are represented by "smaller more rapid reproducers" which occupy lower trophic levels, i.e., planktivores.¹⁶  Examples of fisheries exploiting such species are those for herring and sprat in the Baltic Sea and for Atlantic menhaden (Brevoortia tyrannus) in Chesapeake Bay. ¹⁸

Excessive blooms of phytoplankton and growth of epiphytic diatoms in eutrophic systems reduce the amount of light available for photosynthesis and can thus lead to declines in seagrass meadows and other macrophyte communities and thereby to declines in the production of some species of fish through the á destruction of spawning habitats. ^1,19 This type of situation, along with other effects of pollution such as anoxia, have contributed to a decline in spawning and nursery areas and thus, the numbers of larval and juvenile striped bass (Morone saxatilis) in Chesapeake Bay. ²⁰

The food web can be dramatically altered by eutrophication. For example, nutrient enrichment of Cockburn Sound in Western Australia (Figure 1 ) has resulted in the food web changing from one based on the detritus produced by macrophytes to one based on phytoplankton. This has led to a significant change in fish community structure. ²¹ The diets of many commercially important species rely on the animals and plants associated with a detritus-based food web. These species include King George whiting (Sillaginodes punctata), cobbler (Cnidoglanis macrocephalus) and yelloweye mullet (Aldrichetta forsteri). However, the current success of the Cockbum Sound bait fishery is related to the abundance of the planktivorous Western Australian pilchard (Sardinops sagax neopilchardus) and scaly mackerel (Sardinella lemuru). These species belong to the category which Nichols et al.¹⁶ regard as the type most likely to dominate in a eutrophic system.

Marked increases in nutrient inputs to aquatic systems sometimes lead to the production of large blooms of blue-green algae such as Nodularia spumigena ^4,22 These can have detrimental effects on fish stocks through depleting the oxygen levels in the benthos and/or by producing toxins. ^23-25 An example of a commercial species that tends to be disadvantaged by eutrophication is the Norwegian lobster (Nephrops norvegicus) in the Baltic Sea, which can suffer severe declines as a result of the anoxic conditions produced by blooms of Nodularia spumigena. ²⁶

From the above account of the way in which macroalgal growths and phytoplankton blooms can influence fish communities, it is evident that eutrophication can have both detrimental and beneficial effects on commercial fisheries, depending on the circumstances ^9,27

This chapter describes the commercial fishery in the now highly eutrophic Peel-Harvey Estuary (Figure 1). This fishery operates predominantly in the two main basins of the estuary. Particular emphasis has been placed on determining the extent to which the catch, effort and catch per unit effort (CPUE), changed in this system following both the development of massive macroalgal growths in the early 1970s and the production of large seasonal blooms of diatoms and the blue-green alga Nodularia spumigena since the late 1970s. Comparisons are made between the CPUEs for the fishery in the Peel-Harvey with those of the Swan Estuary, 50 km to the north, and with those of Leschenault Inlet, an estuary 85 km to the south. Although the Swan Estuary is nutrient enriched, ²⁸the effects of eutrophication are not evident in the large basins where the fishery is mainly based. In other words, the macroalgal growths and the phytoplankton blooms are not nearly as pronounced in this region of the Swan Estuary as in the basins of the Peel-Harvey. The Leschenault Inlet is eutrophic ²⁹ and, like the Peel-Harvey Estuary, now contains large growths of macrophytes but, unlike that system, does not support conspicuous blooms of blue-green algae. Comparisons within and between the fisheries in these three systems help clarify some of the ways in which macroalgal growths and blue-green algal blooms can affect estuarine fisheries.