POSSIBLE IMPACTS OF THE GREENHOUSE EFFECT ON COMMERCIAL PRAWN POPULATIONS AND FISHERIES IN NEW SOUTH WALES

INTRODUCTION The Prawn Fisheries Penaeid prawns are currently the second most important fiSheries resource in New South Wales, being worth -$25 million annually at the point of first sale. Four species comprise over 99% of commercial landings (Table 1). Eastern king prawns are exploited offshoce from the Swains Reefs off southern Queensland (21 °S) to the Gippsland Lakes in eastern Victoria (38°S). The fishery operates year round north of South West Rocks, NSW (fig. I) and between summer and winter south of there. School prawns are exploited offshore between Noosa Heads in southern Queensland (26°S) and Gippsland Lakes from spring through to winter. Commercial and recreational estuarine fiSheries for both of these species operate between Noosa Heads and Gippsland Lakes between spring and autumn. Likewise, greasyback prawns are caught between Noosa Heads and Gippsland Lal:es from spring to autumn, but they are exploited only in estuaries. Royal red prawns are fiShed between Tweed Heads in northern NSW (28°S) and Bermagui in southern NSW (36°5) but only offshore. The seasonality of this latter fishery varies between regions but tends to become more year round further to the north.


INTRODUCTION
The Prawn Fisheries Penaeid prawns are currently the second most important fiSheries resource in New South Wales, being worth -$25 million annually at the point of first sale.Four species comprise over 99% of commercial landings (Table 1).
Eastern king prawns are exploited offshoce from the Swains Reefs off southern Queensland (21 °S) to the Gippsland Lakes in eastern Victoria (38°S).The fishery operates year round north of South West Rocks, NSW (fig. I) and between summer and winter south of there.School prawns are exploited offshore between Noosa Heads in southern Queensland (26°S) and Gippsland Lakes from spring through to winter.Commercial and recreational estuarine fiSheries for both of these species operate between Noosa Heads and Gippsland Lakes between spring and autumn.Likewise, greasyback prawns are caught between Noosa Heads and Gippsland Lal:es from spring to autumn, but they are exploited only in estuaries.Royal red prawns are fiShed between Tweed Heads in northern NSW (28°S) and Bermagui in southern NSW (36°5) but only offshore.The seasonality of this latter fishery varies between regions but tends to become more year round further to the north.

Penaeid Prawn Life Cycles
The above four species differ in the variety of habitats they utilise during various life cycle stages.The life cycles of eastern king and school prawns are examples of the most common form found amongst penaeids.These incorporate both estuarine and oceanic dependent stages (fig.2).The planktonic larvae are spawned offshore and, after about a month, postlarvae enter inshore coastal and estuarine nursery grounds.After spending several months (generally through autumn and winter) in these habitats, juveniles gradually move offshore, become sexually mature, spawn, and thereby complete their life cycle.Similar life cycles are pursued by greasyback and royal red prawns; however, these are completed in the estuarine and oceanic environments, respectively.

The Eastern King Pr,awn (Penaeus plebejus)
The eastern king prawn is endemic to waters off the east coast of Australia.It has been recorded from Hayman Island (WOS) to north-eastern Tasmania (42"S) and eastwards to Lord Howe Island (Ruello 1975).The species occurs in waters out to a depth of 240m and constitotes a single stock (Ruello 1975).Its life cycle is characterised by northerly movements over record distances for adult crustaceans (Ruello 1975, Montgomery in press}.Spawning occws offshore between January and June (Dakin 1938) in depths greater than 50 m off northern New South Wales and south-QStem Queensland (Racek 1959).Laboratory studies indicate that salinities greater than 25%.> are required for spawning to take place (Preston 1985).Postlarvae euler estuaries year round but peak abundance occws between autumn and spring (Young & Carpenter 1977, Coles & Greenwood 1983).The estuarine nursery habitat of A. eas1ern king prawns generally comprises shallow areas (less than 2 m depth) with a strong marine influence (Young 1978).Easlem king prawns settle in both seagrass beds and on bare substrata (Young & Carpenter 1977).Juveniles amtolerale salinities as low as 3%o but adults are less tolerant (surviving in salinities as low as 7%o {Dalll980)).Their preferred habitat is therefore well within their tolerance range for salinity.It is also possible for eastern king prawns to complele their whole life cycle in offshore waters (Young & Carpenter 1977, Coles & Greenwood 1983).

Tbe School Prawn (MttiJpenans •acltayi)
The school prawn is also endemic to walers off eastern Australia.It has been reccxded from Tin Can Bay (26'S) to Comer Inlet (39°S) and occun in waters to 55 m depth (Racek 1959).Once offshore, school prawns move only relatively short distances from their eswarine habitat (Ruello 1977, Glaister 1978a).Consequently.several stocks are present over the species' distribution (Ruello 1977).Spawning lakes place between summer and aurumn in offshore waters (Racek 1959).Postlarvae enter estuaries from autumn to winter.Juveniles prefer a fme to medium sand substratum (Ruella 1973a) and settle in both seagrass beds and on bare substrata (Young 1978).The tolerance of school prawns to low salinities has not been srudied, although RueUo (1971) found school prawns in salinities as low as l%o.Juvenile prawns stay in the nursery habitat over winter, then move seaward from spring to summer.Both regional rainfall and river discharge are significantly correlated to annual prawn landings in the Hunter and Clarence River school prawn ftsheries (Ruello 1973b, Glaister 1978b, e.g.fig.3).These fisheries collectively account for more than 41% and 72% (by weight) of the NSW estuarine and offshore school prawn landings, respectively., The Greasyback Prawn (Mttaptnatus btnnttlat) Greasyback prawns have been recorded from Cooktown (!5°S) to eastern Victoria (Kirkegaard & Walker 1970).Spawning can lake place within either the estuarine or inshore marine environments (Preston 1985).This occurs between November and February in New South Wales (Racek 1959) and from October to May off southern Queensland (Dall 1958).Postlarvae are found in estuaries year round but peak abundances occur between March and June (Young &Carpenter 1977).Postlarvae and juveniles prefer a shallow "intermediate to riverine habitat" (Young 1978), but can occur in both freshwater and marine environments (Da1l 1958).
Juveniles move towards the seaward end of the eswary between spring and summer (Dall 1958, kacek 1959, Coles & Greenwood 1983) and will move downstream also during times of flood (Da1l 1958).Dall (1980) concluded that this species was a highly efficient osmoregulator (with little difference between juveniles and adults) and that this characteristic was necessary for its basically estuarine life cycle.

Tbe Royal Red Prawn (Haliporoidts sibogat)
Royal red prawns are recorded across the Indo-West Pacific.Off eastern Australia, they have been recorded in waters from central Queensland to north-eastern Bass Sltait in depths from 275m to 820 m (Graham & Gorman 1985, Poner & Dredge 1985).Little is known of the biology of this species, although two spawning periods have been identified off New South Wales, namely March-April and July-August (Graham & Gorman 1985).

GREENHOUSE EFFECTS
This paper assesses possible environmental changes associated with the Greenhouse effect and potential impacts on each of the four prawn species mentioned above.Possible changes considered are a more consistent southward flow of the East Australian Current below latitude 320S; a temperature rise of 2-40C with a 10 to 20 year lag for ocean temperatures; a sea level rise of between 0.2 and 1.4 m; increased wave action as a result of increased wind speeds south of latitude 36°S; and an increase in the level of rainfall over summer with associated changes in salinity regimes and frequency of flooding.Each effect has been considered separately, ignoring complex ecological processes such as changes in predator/prey relationships.l'osslble fuwre changes in management regimes, fishing methods and mari<:eting strategies that may be necessary because of greenhouse-induced clintale change have 110( been considered in this paper.------------------~------------------~---o 5000 10000 discharge (megalitres) Figwe 3 Relationship between river discharge and annual school prawn landings for the Clarence River region between 1966-67 and 1985-86.The correlation is that delermined by Glaister (1978b) for the years 1966-67 to 1975-76.

POSSmLE IMPACTS OF THE GREENHOUSE SCENARIO Eastern King, School and Greasyhack Prawns
The stimuli which influence prawns to emigrate from estuaries are not well understood.However.if they are temperature relaled.a 2-4"C warming in estuarine water temperatures may stimulate school and eastern king prawns to emigrate from estuaries earlier in the year than at present.Greasyback prawns may also move downsa-eam earlier.Warmer marine water may also stimulate these species to spawn earlier in the year than at present.and eastern king prawns could spawn at higher latitudes.
A more consistently flowing East Ausualian Current may assist in providing less variable recruitment of prawn larvae to eslllaries south of latitude 32°5.This and warmer seawater temperatures may provide for the geographical distributions of all three species to be exlended southwards.
A rise in sea level has the potential to increase the area of nursery habitat available for juvenile prawns.Some flood plains may become permanently covered with saltwater and tidal influence might extend fwtber upstteam.Further, a rise in sea level coupled with an increase in wave action may result in some intermiuently opening estuaries opening to the sea for longer periods, again providing more available nursery habitaL Increased levels of rainfall over summer will cause increased runoff.This in tum has the potential to lower salinity, increase river discharge and increase the incidence of flooding.All three species appear to be capable of tolerating low salinities during the juvenile and adult phases of their life cycles.Lower salinities therefore should have little effect upon the survival of these life history stages, but may force eastern king and school prawns to emigrate from the estuarine habitat earlier and possibly to move further seaward.Greasyback prawns may also move further downstteam and out to sea during periods of flood.
The survival of prawn larVae under conditions of varying salinity and water temperatwe will depend much upon the ambient conditions under which they hatched (Preston 1985).Salinity is the most important of these two environmental factors 10 the survival of larvae and has most effect up to the mysis stage of development (Preston 1985).
The effect of lower salinities upon the survival of prawn larvae wiD depend therefore upon the stage of larval developmenL The period of highest eastern king and school prawn postlarval abundance is at a time when summer rainfall would be expected to have least effect upon salinity.and therefore less impact upon the larvae of these two species.Greasy back larvae are the most likely to be exposed to lower salinities and are the most capable of tolerating them.It is therefore possible that lower salinities will hlwe only minor effects upon the survival of prawn larvae.Increased runoff and river discharge may cause increased siltation.Any change in the structure of the habitat substratum, whether it be within the estuary oc offshore, has the potential to affect the distributions of all three species.
Cllanges to seagrass beds resulting rrom any rise in sea level and increase in turbidity (from increased runoff) may or may not affect the survival of prawns.All three species seule on bare substrata as well as on substrata covered with seagrass, though the relative importance of these two habitat types is unknown.Increased raleS of emigration (caused by high levels of river discharge) have the potential to result in greater aggregations of prawns.As a result, catch rates in the fishery may increase.There is the polential therefore, for annual landings of all species to increase, but particularly those of school and greasyback prawns.

Royal Red Prawa
Wanner seawater temperatures may have the pocentiaiiO affect the geographic disttibution of this species and its time of spawning.A m~R consistently southward flowing East Australian Current, containing warmer water, may extend the southern distribution limit With a completely oceanic life cycle, it is unlikely that the royal red prawn will be affected by sea-level rise or increased wave action.Being disttibuted along the Continental slope, it is also unlikely that this species wiU suffer from pocentially lower coastal salinities.

CONCLUSIONS
The greenhouse scenario as presented here has the pocentiaiiO increase the annual landings of inshore prawn species.Landings of deepwater species sucb as royal red prawns should be unaffected.With increased emigration there is the potential for formation of greater aggregations of the inshore prawn species, panicularly during times of flooding.As such effects may be completely independent of any increase in population abundance, any increase in the catchability of prawns has the potentiaiiO be detrimentaiiO population abundance.
The possible rise in sea level and increase in wave action have the potential10 increase the area of nursery habitat This could result in increases in overaU prawn abundance for aU three inshore species.Prawn abundance will also be affected however, by changes in salinity.This wiU have the grealesl effect during the larval phase of the prawn life cycle.The survival of enough postlarvae 10 take advantage of increased habital area wiU depend upon the ambient conditions a1 hatching, the severity of any changes in the salinity regime, and whethez prawn larvae enter coastal waters at a time when salinity is low.It is impossible 10 assess the interactions of these different processes in terms of prawn survival.

Figure I
Figure I Geographic dislribution of inshore )nwn fiShing grounds off the east coast of mainland Ausualia.

Figure 2
Figure 2 Generalised life cycle of a penaeid prawn.