Tropical case study
Banana prawns, Fenneropenaeus merguiensis, use tropical estuaries as nursery grounds (Vance et al. 1990, Sheaves et al.
2012), where they rely on saltmarsh vegetation for part of their nutritional requirements (Abrantes and Sheaves 2009).
Banana prawns were chosen as a study species for this case study because they are:
(a) Highly fecund and an annual stock that will recruit rapidly to repaired environments;
(b) A commercially important food species;
(c) Important targets of recreational fishers throughout north Queensland estuaries;
(d) Vital prey of other high profile commercial/recreational species such as barramundi; and
(e) Known to exhibit a random escape response which means that an assumption of minimal under-sampling is valid.
Fish are highly variable in escape-patterns to sampling by most types of sampling gears that can provide accurate estimates
of density. Compared to fish, banana prawns are ideal targets, particularly for cast net sampling, a gear type that is particularly
suitable for small mangrove lined estuaries and this gear type provides for accuracy in sampling through generally high replication
(Johnston & Sheaves 2007). Banana prawns as a highly valued stock for both commercial and recreational catch also makes them
an ideal indicator species to use in any broad estimates of the benefits of seascape repair.
The tropical study and prior work found that estimates of productivity of individual components of the estuary were highly
variable and depended on a number assumptions that are difficult to validate (Minello et al. 2008, Rönnbäck et al. 1999, Rozas
& Minello 2011). In comparison, estimates at the whole-of-estuary level, “seascape” require a relatively low number of assumptions
and produce estimates with relatively low variability (Sheaves 2017, Nagelkerken et al. 2015). For the tropical case study,
Sheaves et al. (in review) found as a conservative estimate a maximum juvenile prawn biomass of 6.5 g m-2 for the 2m wide
bands along the estuary edge where prawns are found. For the estuary studied, with an edge area of 5.6 ha and 11.5 km
total length, the conservative total biomass of juvenile r was 370 kg.
The actual estuary productivity would likely be much higher because this estimate only relates to the maximum juvenile stock
for a sampling occasion and does not take into account continual movements of prawns to offshore adult habitat once they
reach a sufficient size. To more precisely calculate estuary productivity for banana prawns information would be needed
on patterns of recruitment, growth rates, mortality, predation and emigration. Furthermore, estuaries host a variety of
organisms and similar information would also be required for all other crustacean and finfish as well as the net primary
productivity that is exported from the estuary. Suffice it to say an estimate of banana prawn productivity 370 km for an
estuary of 11.5 km total length is probably orders of magnitude below total estuary productivity. While this provides a
baseline estimate that can be used to demonstrate the potential benefits of seascape repair, much more extensive studies
would be required to link production of banana prawns to particular areas of saltmarsh habitat (Sheaves and Johnston 2010,
Sheaves et al. 2012).
Sub-tropical case study
To provide a case study for the east coast subtropical regions, we selected school prawns (Metapenaeus macleayi) because
(1) they are highly reliant on estuarine nursery habitats and primary productivity derived from estuarine habitats for rapid
growth through their early life history stages (Taylor et al in press), (2) the species is important to both commercial
and recreational fisheries (Taylor et al 2017), (3) the species is highly fecund, and given reasonable freshwater inflow
to estuaries it is unlikely to experience stock-related limitations to recruitment, and (4) school prawns are a fast growing
annual species. School prawns are mostly harvested commercially, this commercial harvest provides product for human consumption
and is widely used as bait by recreational fishers. Given the life-history characteristics of the school prawn, benefits
of successful repair of their habitat are likely over a short time frame.
Temperate case study
Compared to East Coast Tropical and Subtropical locations in Australia, there is very little knowledge of prawn and fish
use of temperate saltmarshes. Saltmarshes of the Circular Head region of north-west Tasmania waswere chosen for the East
Coast Temperate case study for a number of reasons. First, the Circular Head region is home to about a fourth of all coastal
saltmarshes in Tasmania and form part of a rich seascape matrix with expansive tidal flats, seagrass beds and buffering Melaleuca
ericifolia swam forests (Mount et al 2010). Second, the area is very important for commercial and recreational fisheries.
Third, the Circular Head region saltmarshes have been subject to most extensive clearing and drainage works, with potential
for habitat repair through tidal restoration works (Prahalad 2014). Fish use data for are likely to determine whether there
is a case for saltmarsh repair to improve the habitat value for fish as part of the broader seascape nursery.
Prahalad et al (in review) found 11 fish species using Circular Head saltmarshes with a high mean density of > 72 fish
per 100 m-2 (sample data from April-May 2017). The family Atherinidae (Silversides) contributed 3 species and 74% of the
total catch numbers. Commercial and recreational species that utilise these saltmarshes in northwest Tasmanian seascapes
include: yellow-eye mullet (Aldrichetta forsteri), australian salmon (Arripis truttaceuos) and greenback flounder (Rhombosolea
tapirina) . These three species contributed close to 20% of the total catch numbers. Of these yellow-eye mullet was most
abundant and common, present in 24 (65%) of the 37 nets that caught fish and made up 19% of the total catch. Extended sampling
throughout the year may reveal further species using saltmarshes.