Dr Bruce Hodgson

Following the planned FAO Ecosystem Restoration Programme for estuarine habitats to support estuarine fisheries and early life stages of estuary-dependent marine fish, direct relationships of total seagrass and eelgrass Zostera m. capricorni areas and biomass with fish harvest were derived for a range of slightly to highly urbanized coastal lagoons that are expected to support the larvae and juveniles of estuary-dependent marine fisheries. Fish harvest and seagrass area and biomass in the lagoons increased with moderate catchment total suspended sediment and total phosphorus loads due to lagoon flushing rates directing excess silt and nutrients out to sea via the lagoon entrances. Well managed, sewered catchment management works are shown that could assist estuary managers maintain seagrass for estuarine and offshore estuary-dependent fisheries by maintenance of seagrass and fishery ecological processes. Further research is suggested to investigate estuary-dependent post-juveniles leaving estuaries and lagoons migrating to nearshore, offshore and shelf marine fisheries.
•Coastal lagoon seagrass for larvae and juveniles of estuary-dependent marine fish.•Relationships of Zostera m. capricorni areas and biomass with fish harvest.•Increased seagrass and fish harvest with moderate sediment and phosphorus loads.•Increases aided by flushing rates transport nutrients to sea via lagoon entrances.

A theoretical basis for Ecosystem-based Fisheries Management (EBFM) was derived for pelagic fish by applying marine ecology theory of analytical relationships of predator-prey biological production transfers between trophic levels to FAO guidelines for an ecosystem approach to fisheries. The aim is to describe a simple method for data-limited fisheries to estimate ecosystem-based F MSY and how EBFM modellers could mimic the way natural fish communities function for maintaining ecological processes of biological production, biomass and ecosystem stability. Ecosystem stability (ES) F MSY were estimated by proportion of biological production allocated to predators, giving ES F MSY of 0.23 for small pelagic and 0.27 for pelagic finfish, prioritising ecosystem over economics. To maintain both stability and bio-mass (SB) a full pelagic EBFM SB F MSY of about 0.08 was obtained for both small pelagic and pelagic finfish, having mostly ecosystem considerations. As the F MSY are single-species averages of catchable species targeted in a specific trophic level, multispecies fishing mor-talities were proportioned by the biological production of each species in the trophic level. This way catches for each species are consistent with the average ecosystem F MSY for a trophic level. The theoretical estimates gave similar results to other fisheries for sustainable fish catches that maintain the fishery ecosystem processes. They were also tested using six tropical Ecopath Models and showed the effects of imposing commercial fishing mortalities on predominantly EBFM conditions. The ecosystem stability ES F MSY is suggested to be investigated for sustainable fish catches and the full EBFM SB F MSY for protected areas or recovery of heavily depleted stocks.

The purpose of this study was to review Ecopath model results for six tropical shelves (Christensen and Pauly, 1993, Bulman, 2006) to investigate the role of zooplankton production to pelagic fish production as a fundamental process to inform acceptable levels of fish catch in these fishery areas, at the trophic ecosystem level. The investigation of the relationships between productions of pelagic fish with acceptable levels of catch was prompted by Allen’s 1971 paper on the ”Relation between Production and Biomass”. Zooplankton production transfers to small pelagic fish, and from them to pelagic finfish production, was investigated using trophic transfer efficiencies in the six tropical shelf fishery areas. These relationships were then used at Heron Island to estimate the potential acceptable level of fish catch from measurements of zooplankton biomass and estimates of production near Heron Island, in the southern area of the Great Barrier Reef marine park. The six tropical shelves were reviewed for the following productivity and fish catch relationships: Estimating Trophic Transfer Efficiencies from zooplankton to fish; Determining relationships between fish production and the commercial pelagic fish catches; Considering the role of fish production in the various fishery areas in relation to acceptable levels of fish catches; Estimating pelagic fish production at Heron Island from the trophic transfer efficiencies and potential fish catch from the tropical shelf relationships between fish production and catch. The close-fitting relationships between the tropical shelf zooplankton production, pelagic fish production and fish catch data suggests that the fish catches across all these areas are strongly related to the productivity of the fisheries. This suggests that fish catches are at acceptable levels because they keep pace with the available production inputs from zooplankton production, into the fishery. Subject to further investigation, the potential benefits of this approach for fishery management could include consideration of production based fish catches as acceptable levels in marine parks and surrounding areas, with protection of appropriate areas (eg. spawing areas) that generate the fish production supporting the fish catch.

The differing contribution of the wetland areas of seagrass, mangroves and saltmarsh to the commercial catch in estuaries were unraveled using step-wise regression of each wetland area against catch, and using existing knowledge of their importance. The areas of mangroves, saltmarsh and seagrass all made important contributions to the commercial catches. The areas of each wetland developed within the estuaries are proxies for estuarine production of food that supports the fish catch, suggesting that the fish catch is related to the productivity of the estuaries. The estimated catch was about 15% lower than the reported catch, apparently due to contribution of the microphytobenthos to the estuarine production by "bare' areas. The catch in the Port Stephens Marine Park was lower than predicted, apparently due to increased management relative to the other estuaries. Mangroves are potentially more important to the commercial fish catch than currently understood.

Nutrient fluxes and primary production were examined in Lake Illawarra (New South Wales, Australia), a shallow (Zmean=1.9 m) coastal lagoon with a surface area of 35 km2, by intensive measurement of dissolved nutrients and oxygen profiles over a 22-h period. Rates of primary production and nutrient uptake were calculated for the microphytobenthos, seagrass beds, macroalgae, and pelagic phytoplankton. Although gross nutrient release rates to the water column and sediment pore waters were potentially high, primary production by microphytobenthos rapidly sequesters the re-mineralized nutrients so that net releases, averaged over times longer than a day, were low. Production in the water column was closely coupled with the relatively low sediment net nutrient release rates and detrital decomposition in the water column. Dissolved inorganic nitrogen and silica concentrations in the water column are drawn down at the beginning of the day. The system did not appear to be light limited so photosynthesis occurs as fast as the nutrients become available to the phytoplankton and microphytobenthos. We conjecture that microphytobenthos are the dominant primary producers and, as has been shown previously, that the nutrient uptake occurs in phase with the various stages of the diatom growth.

Investigation of role of zooplankton feeding by coral at Heron Island in the southern Great Barrier Reef. It was estimated that coral obtained 10% of its carbon by consumption of zooplankton and 90% by photosynthesis. The zooplankton consumption was an important part of the coral symbiosis as it provided nitrogen and phosphorus to support zooxanthellae photosynthesis in a low nutrient aquatic environment.

The area occupied by aquatic angiosperms and the biomass of Zostera capricorni have been documented for southern Lake Macquarie and the Tuggerah Lake from 1980 to 1985. The area of seagrasses in Lake Macquarie ranged from 4.81 to 6.75 km2, and the total biomass of Zostera from 543 to 1099 tonnes. Winter biomass was significantly greater than summer biomass. In the Tuggerah Lakes the area was 13.13-19.11 km2 and total biomass of Zostera 840-1888 tonnes. Differences between years were not significant because of the wide variation shown in both parameters. The data show no time-related trends or predictable changes. When vegetation survey is to function as baseline data this inherent viability in seagrass populations must be recognized.