The macroalgal populations impact on amateur and commercial fisheries
in different ways, which are detailed elsewhere in this volume. The catches
of fish 'per unit effort' have increased somewhat with increased macroalgal
biomass, and because the amount of primary production is so high in the system,
the increase in fish catch has been attributed to other factors, such as additional
shelter for juveniles.31 Prawns
and crab populations have been affected in ways which depend on the species
of macroalgae, and this is explained in detail in Chapter 13.41
However, despite this general increase in catch, fish kills have occasionally
been reported where large amounts of organic material have decomposed rapidly
in a local area and led to oxygen depletion. In addition the presence of large
amounts of macroalgae leads to problems in the use of boat motors and to net
fouling.32
Winter diatom blooms are grazed by large populations off the calanoid copepods Sulcanus and Gladioferans. While these might in turn be grazed and support food webs, they are rapidly eliminated when Nodularia blooms become established; this may be because they are unable to cope with the filaments of the blue-green.20 Fish may leave the estuary during severe blooms.33 While severe oxygen depletion is not a general feature of eutrophication in Harvey Estuary, there are occasional local events of severe oxygen depletion at depth during Nodularia blooms; high mortality of invertebrates have been reported in the Estuary and are attributed to deoxygenation. There is also the possibility of toxicity. Early, classic work linked stock deaths with drinking water containing Nodularia in Lake Alexandrina in South Australia,34 and the strain of Nodularia, which occurs in Harvey Estuary, has been shown to produce the toxin under laboratory conditions.35 While government concern has led to warnings against swimming in the Estuary during the blooms, there have been no reported human responses to the blooms, nor of death of pets and other animals: the occasional deaths of fish and invertebrates referred to above can he attributed to oxygen depletion.
The occurrence of large populations of macroalgae and Nodularia have led to the production of nauseous odours which offend local residents and visitors. While there are no quantitative data. a reasonable case has been made that these odours, the appearance of the estuaries and the need to walk through rotting algae on what were once sandy beaches, must have reduced potential visitor numbers and affected real estate values near to the shore.