(Penaeus Indicus H. Milne Edwards, 1837) and Juvenile White Banana

Journal of Experimental Marine Biology and Ecology 309 (2004) 79–108 www.elsevier.com/locate/jembe

Allopatric distribution of juvenile red-legged banana prawns (Penaeus indicus H. Milne Edwards, 1837) and juvenile white banana prawns (Penaeus merguiensis De Man, 1888), and inferred extensive migration, in the Joseph Bonaparte Gulf, northwest Australia

R.A. Kenyona,*, N.R. Loneragana, F.J. Mansona, D.J. Vancea, W.N. Venablesb

a CSIRO Marine Research, P.O. Box 120, Cleveland QLD 4163, Australia b CSIRO Mathematical and Information Sciences, P.O. Box 120, Cleveland QLD 4163, Australia Received 4 August 2003; received in revised form 9 March 2004; accepted 11 March 2004

Abstract

During October to December 1997, we trawled estuarine habitats in the Joseph Bonaparte Gulf (JBG) to determine the distribution of juvenile red-legged banana prawns, Penaeus indicus (H. Milne Edwards, 1837) and white banana prawns, Penaeus merguiensis (de Man, 1888). We made 229 beam-trawls at 185 sites, mostly over a 100-m path (3-min duration). A Global Positioning System (GPS) receiver was used to verify our location. During October to December 1998, we intensively resampled three of the rivers that were sampled in 1997 to confirm the gulf-wide distribution of P. indicus and P. merguiensis and to investigate the microhabitat use of P. indicus. We chose previously sampled and new sites in Forsyth Creek (eastern JBG), the Lyne River (Cambridge Gulf), and the Berkeley River (western JBG). We made 249 trawls at 21 sites, mostly over 100 m. Juvenile banana prawns were abundant in eastern JBG, Cambridge Gulf and western JBG. They were not abundant in southern JBG, although fewer trawls were made there, due to its inaccessibility. In eastern JBG and Cambridge Gulf, over 96% and 73% (respectively) of juvenile banana prawns were P. indicus and they were more abundant there than in the western JBG. Conversely, in the western JBG over 93% of the juvenile banana prawns were P. merguiensis and they were more abundant than in the eastern JBG and Cambridge Gulf. The Lyne River in the northwestern

* Corresponding author. Tel.: +61-7-3826-7274; fax: +61-7-3826-7222. E-mail address: [email protected] (R.A. Kenyon).

Cambridge Gulf seems to be the transition zone; both P. indicus and P. merguiensis are equally abundant. P. indicus are most abundant on the mangrove-lined muddy banks of waterways within mangrove forests, similar habitats to P. merguiensis. Within these habitats, they were most abundant in gutters and small creeks, rather than rivers and large creeks. Few P. indicus or P. merguiensis were caught in 100 m2 trawls undertaken midriver (on the channel bottom and on emergent banks), although these habitats may be only 100 m from the mangrove-lined habitats. In all creek and river habitats, both species are most catchable at low tide (irrespective of daylight or darkness) when they move out of the mangrove forests and accumulate in the remnant water bodies. The offshore fishery for P. indicus is in northwestern JBG in waters 50–80 m deep, about 300 and 200 km, respectively, from where juveniles are abundant in their extensive inshore habitats in east JBG and in Cambridge Gulf, demonstrating a geographical separation of the juvenile and adult phases. Postlarval P. indicus, spawned offshore, must use tides and currents to travel south and east to reach nursery habitats. Emigrant subadults must migrate north and west, across relatively shallow inshore sand substrates (30–40 m deep) to reach their offshore habitats. D 2004 Elsevier B.V. All rights reserved.

Keywords: Hydrodynamic processes; Mangrove habitat; Microhabitat; Penaeus indicus; Penaeus merguiensis

1. Introduction

Throughout the coastal regions bounding the Indian and western Pacific Oceans, the banana prawns (shrimp) Penaeus indicus and Penaeus merguiensis exist in sympatric inshore and offshore habitats (Devi, 1988; Rao et al., 1993; Barus and Mahiswara, 1994; Sambandam, 1994; Mohan et al., 1995; Primavera, 1996). Both species have a similar life cycle; the adults spawn offshore, pelagic larvae migrate inshore, juveniles spend several months in mangrove-lined estuaries and then they migrate offshore (Dall et al., 1990). They support significant coastal and estuarine fisheries adjacent to estuarine mangrove nursery areas (Rao et al., 1993; Mohan and Siddeek, 1995, 1996). Sometimes, juvenile P. indicus and P. merguiensis occupy similar microhabitats among soft-sediment mangrove communities (Mohan et al., 1995). In Australia, the population biology of P. merguiensis has been studied intensively for over 20 years (e.g., Crocos and Kerr, 1983; Vance et al., 1985; Staples and Vance, 1986; Vance et al., 1998). The juveniles are abundant in mangrove-lined estuaries. Their abundance is greatest in the upper reaches of small creeks (Vance et al., 1998), especially among mangrove forests with many small streams probably where the physical complexity of the habitat is greatest (Webb and Kneib, 2002). In contrast, the habitat use and distribution of juvenile P. indicus in Australia is relatively unknown (Somers, 1994). In other countries within their range, juvenile P. indicus occur on muddy sediments in creeks and rivers among estuarine mangrove forests (Subramanian, 1985; Mohan and Siddeek, 1996), where they are tolerant of euryhaline conditions (Subramanian, 1985; Kumlu and Jones, 1995). Often, subadult and adult P. indicus are found over a range of habitats, from coastal bays to inshore and offshore waters where they are commercially exploited at different stages of their life R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 81 history (George et al., 1963; Rao et al., 1993; Mohan and Siddeek, 1995). Historically, in India, P. indicus were fished in 10–40 m depths, while more recently larger vessels allow exploitation in depths up to 100 m (George et al., 1963; Suseelan and Pillai, 1993).In Madagascar, the adults are found close to shore in 15–25 m water depth (Brinca and Mascarenhas, 1985). In Australia, only the offshore adults are fished in waters about 50– 80 m deep. Compared to other tropical penaeids in Australia, P. indicus are found in a very restricted geographic range—the Kimberley and Arnhem Land coasts, and the east coast of Cape York (Grey et al., 1983). They do not extend beyond 20jS on the west or the east Australian coasts, and within this range they are often uncommon. Their limited extent in northern Australia reflects their existence on the southeastern limits of their global zoogeographic distribution. The Timor, Arafura and Coral Seas are the eastern extent of the range of the species, which flourishes in the coastal seas along the western and northern Indian Ocean (Dall et al., 1990). As well, their distribution in northern Australia may be reduced by competition with P. merguiensis, a similar species which dominates mangrove habitats in the western Pacific Ocean and flourishes in the eastern and northern Indian Ocean (Dall et al., 1990). P. indicus are fished in the western Joseph Bonaparte Gulf (JBG) (Fig. 1) and north of Melville Island (11.5jS, 131jE)/Coburg Peninsula, the only Australian locations where they are commercially abundant (Somers, 1994). In contrast to P. indicus, the white banana prawn, P. merguiensis, is found over a wide area of northern Australia, from south of Moreton Bay (27jS, 153.25jE) on the

Fig. 1. Study sites trawled for postlarval and juvenile banana prawns during three surveys undertaken in the JBG during October and December 1997. 82 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 east coast, to Shark Bay (25.5jS, 113.5jE) on the west coast (Grey et al., 1983).In Australia, P. merguiensis are common and fished commercially throughout their range (Somers, 1994) in locations offshore to their estuarine mangrove nursery habitats (Staples et al., 1985). In the JBG, the geographic distributions of the dominant P. indicus fishery and the P. merguiensis fishery do not overlap (Somers, 1994). P. merguiensis are fished from the northwestern extremity of the JBG, westward to Napier Broome Bay (14jS, 126.5jE). The location of the P. indicus fishery in the north western JBG and extensive areas of estuarine habitats (potential nursery habitats) in the southeast JBG, distant from the fishery, raises questions as to the coastal distribution of the juvenile P. indicus that recruit to the fishery. Usually, juvenile banana prawn habitats are adjacent to the fishery, in inshore estuarine habitats (Staples et al., 1985). If the juveniles of both banana prawn species were distributed evenly throughout coastal JBG, why were both species not common in the fishery catch? Was only one species common in the nursery habitats of the JBG? Until recently, juvenile P. indicus were very difficult to separate from P. merguiensis using morphological characters making their study difficult in areas where their distributions overlapped (Pendrey et al., 1999). In this study, our aim was to investigate the broad geographic distribution and abundance of juvenile P. in dic us and P. merguiensis in the coastal and estuarine habitats along 500 km of the JBG. We intended to determine if both species were common throughout all regions, or if few P. merguiensis were found in the JBG. In addition, we studied the habitat use of both species in three regions of the JBG to test for difference in microhabitat use between P. indicus and P. merguiensis. To complete these aims, features of their mangrove habitats were quantified and used to explain the distribution of the juveniles as found. As well, the effects of other environmental factors on the distribution patterns were investigated using a quasilikelihood generalised linear model. P. indicus and P. merguiensis have recently been renamed Fenneropenaeus indicus and F. merguiensis, respectively, by Pe´rez Farfante and Kensley (1997). We do not agree with this nomenclature and have retained the old names for this paper.

2. Materials and methods

2.1. Study sites

2.1.1. Broad geographic distribution During October, November and December 1997, we trawled estuarine habitats between Cape Londonderry (13.746jS, 126.960jE), Western Australia, and Pearce Point (14.423jS, 129.355jE), Northern Territory, in the JBG region of northwest Australia to determine the broad-scale distribution of P. indicus (Fig. 1). We used topographic maps to select sites in the estuaries, upriver and in open-coast locations. As distances were so large, we calculated an approximate latitude and longitude of each target site to assist with our field navigation. Many of the sites were geographically distant from each other, spread over 500 km of coastline, while some were grouped in close proximity at a geographic R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 83 feature such as the confluence of a creek and a river. The sites within each river were usually located over a geographic range of about 10–20 km. However, in some cases (e.g., the Victoria River), sites in the outer estuary were about 80 km from those upriver. In the field, a Global Positioning System (GPS) receiver was used to locate the sites. Tidal influences are a dominant feature of the marine environment in the JBG (with maximum tidal amplitudes of up to 7 m) and sampling was only possible during the neap tides (3–5 m range). As the juvenile habitats of P. indicus in Australia were unknown, we sampled habitats where we would have expected to find juvenile banana prawns; especially habitats similar to those where juvenile P. merguiensis occur in the Gulf of Carpentaria (e.g., Staples et al., 1985; Vance et al., 1990), and similar to where P. indicus occur elsewhere (Mohan et al., 1995; Mohan and Siddeek, 1996). We sampled mangrove-lined banks in rivers, creeks, subcreeks and gutters. Where the water bodies were navigable, we sampled the lower-, mid- and upper-reaches of rivers, creeks and their tributaries, including the subcreeks and gutters at the head of the stream where it bifurcates and where previous work suggested small juvenile P. merguiensis were abundant (Vance et al., 1998). To test whether juvenile P. indicus used different microhabitats to P. merguiensis in Australia, we also sampled habitats where juvenile banana prawns are not usually abundant: midriver sandbanks and mudbanks, mangrove-lined beaches and sand beaches. Each site was classified using descriptions of the water body, position within the river/creek system, type of bank, adjacent vegetation type, sediment type and terrestrial landform surrounding the habitat (Table 1). Trawls were undertaken in October, November and December as the timing of recruitment to the offshore fishery suggested that juvenile P. indicus would be abundant then, and these months correspond to annual periods of high catches of juvenile P. merguiensis in the Embley and Norman Rivers of the Gulf of Carpentaria (Staples, 1979; Vance et al., 1998).

Table 1 Numbers of trawls undertaken in different classes of water body during three cruises undertaken in 1997 and two cruises undertaken in 1998 Water body MS CH MB CK SC GT BB Total ‘‘Distribution’’ trawls October 1997 29 0 0 11 18 6 0 64 November 1997 32 1 0 8 14 1 1 57 December 1997 20 0 2 17 9 5 28 81 October 1998 24 0 0 4 6 9 0 43 November 1998 24 0 0 4 6 8 0 42

‘‘Habitat’’ trawls November 1997 6 5 4 3 3 6 0 27 October 1998 24 12 12 8 12 12 0 80 November 1998 24 12 12 8 12 16 0 84 The trawls undertaken to determine the distribution (‘‘distribution’’ trawls) of Penaeus indicus and those undertaken to determine their habitat preference (‘‘habitat’’ trawls) are listed separately. Mainstream (MS), midriver channel (CH), midriver bank (MB), creek (CK), subcreek (SC), gutter (GT), beach or bay (BB). 84 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

2.1.2. Fine scale distribution In October to December 1998, we resampled three of the rivers that were sampled in 1997 to investigate the fine scale use of different microhabitats by P. indicus. The sampling also allowed comparisons of species distribution and abundance to be made between years for these three rivers. The rivers were chosen from regions in the JBG that had different proportions of the two species of juvenile banana prawns in the catch in 1997: Forsyth Creek in the eastern JBG, where P. indicus was the main species; the Lyne River in the Cambridge Gulf where both P. indicus and P. merguiensis were found in about equal abundance; and the Berkeley River in the western JBG, where P. merguiensis was the main species (Fig. 1). The microhabitats are listed in Table 1 and include riverine mangrove-lined near-bank habitats as described previously, as well as midriver banks and channels without mangrove associations. As many microhabitats as possible were sampled in the rivers, although not all habitats were found in each river. Each site was classified using the qualitative indices given above. Each river was sampled in October/November and again in November/December of 1998.

2.2. Prawn sampling

We sampled postlarval and juvenile P. indicus using a 1.0 0.5 m beam trawl with 2- mm body mesh and a 1-mm mesh codend. We trawled during the last third of the ebb tide, as near as practical to low tide given the time needed to complete all trawls. Our trawls were 100 m long (3-min duration). However, in some cases, we carried out short trawls ( f 10 m long) in side creeks and gutters that were almost empty of water at low tide. At each site, we recorded latitude, longitude, time, duration and length of trawl, salinity, temperature, tide-phase, moon-phase and cloud cover. The degree of turbidity of the water was measured using two methods: measuring the light penetration of the water using a secchi disc, and a qualitative categorisation of the water turbidity in four categories—clear, slightly muddy, moderately muddy and very muddy. The trawl catch was immediately frozen and later taken to the laboratory for sorting. Juvenile prawns were identified as either P. indicus or P. merguiensis following Pendrey et al. (1999).At each site where a trawl was made, we took a sediment sample which was analysed in the laboratory for its grain size composition using mechanical and pipette analysis described in Folk (1968). Most of the trawls (202 out of 229) completed in 1997 were made at 185 different sites to determine the broad-scale distribution of P. indicus among nursery habitats in the JBG (Table 1). Trawls were undertaken mainly during the day, on the last third of the ebb tide, when P. merguiensis is most catchable in beam trawls (Vance and Staples, 1992) and to control for variation in catch due to tide height. To confirm that P. indicus is also more catchable at low tide, and to investigate differences in catch between microhabitats, we trawled in a range of contrasting mud/mangrove habitat types on the last of the ebb tide (low tide) and on the last of the flood tide (high tide) at four selected locations in Cambridge Gulf (27 trawls; Table 1). We trawled midriver sandbank and channel habitats at the same time as the mud/mangrove river and creek habitats; these habitats were within about 400 m of each other. Some of the trawls made to determine distribution were also used to investigate catchability and habitat use. R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 85

In 1998, differences in catches between microhabitats were examined in each of the three selected rivers by beam-trawling (164 trawls, 21 sites) around the time of low and high tides during the day, and during the night (Table 1). To look at differences in broad- scale distribution between years, we trawled the same sites in each river at the same tide- phase and light regime as those trawled during 1997 (85 trawls, 14 sites; Table 1). Some of the trawls made to investigate fine scale habitat use were also used to compare the distribution of P. indicus in 1998 with that in 1997.

2.3. Distribution of prawn habitats

We used ESRI ArcView 3.1 GIS software to estimate the extent of coastal habitat types from the AUSLIG Topo250k data for the JBG region (AUSLIG, 1995). AUSLIG data are current to 1990 and accurate to 300 m, with 90% of the data being accurate to 100 m. Our aims were to estimate:

(i) the total area of each habitat type (as categorised by the AUSLIG data) in the JBG region and in individual river systems, and (ii) the linear extent of each habitat type along the banks of each river system.

Table 2 The area and linear extent of dominant coastal habitats in each of four regions and 12 subregions (river systems) in the JBG, northern Australia Habitat region and Mangrove Saline coastal flat Land subject to inundation river system 2 2 2 Area (km ) Linear (km) Area (km ) Linear (km) Area (km ) Linear (km) Eastern JBG Moon 118 234 380 89 149 0 Fitzmaurice 76 211 367 115 65 1 Victoria 66 125 501 120 133 13 Keep 87 165 587 271 505 12 Total Eastern JBG 347 735 1835 595 852 26

Southern JBG 19 44 829 198 53 0

Cambridge Gulf Delta 83 212 401 421 14 0 Ord River 41 101 540 189 260 38 West 32 143 779 327 26 0 Lyne 5 28 42 66 0 0 Total Cambridge Gulf 161 484 1762 1003 300 38

Western JBG Berkeley 1 5 3 9 0 0 West JBG 16 53 26 30 2 0 King George 2 16 0 0 0 0 Total Western JBG 19 74 29 39 2 0 Total JBG 546 1337 4455 1835 1207 64 86 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

The JBG was divided into four major regions and 12 subregions containing groups of rivers and creeks (Table 2). The major regions were Eastern JBG [the coast from Pearce Point (14.423jS, 129.355jE) to the Keep River (14.885jS, 129.025jE) inclusive]; Southern JBG [the coast from the Keep River to Cape Domett (14.823jS, 128.380jE)]; Cambridge Gulf [the coast south of a line joining Cape Dommett and Cape Dussejour (14.755jS, 128.218jE)]; and Western JBG [west of Cape Dussejour to Cape Londonderry (13.746jS, 126.960jE)]. Three geographic habitat types (as defined by AUSLIG) that may contribute directly to prawn habitat are found in the JBG: intertidal tropical mangrove forest, saline coastal flat, and land subject to inundation (Table 2). We calculated the area and linear extent of these three potential prawn habitat types along the banks of river systems in each region or subregion of the JBG (Table 2). Each habitat polygon in the coverage was defined as being within a region or subregion and some were allocated to individual river systems. Where one polygon lay between two rivers, that polygon was split, and each section allotted to the nearest river. The habitats were confirmed qualitatively and by ground-truthing at locations in two estuaries (Manson et al., 2001). In addition, estimates of the accuracy of the AUSLIG data were made by comparing them with remotely sensed data from other techniques (see Manson et al., 2001).

2.4. Analyses of prawn distribution and microhabitat preference

We calculated the mean catches (prawns 100 m 2) of juvenile P. indicus and P. merguiensis by region, subregion and habitat types. We used ANOVA in SAS (SAS Institute, 1999–2000) to analyse the data. The data were log-transformed (x + 1.5; due to many zero data points) to normalise the data and the variance. We used a two-way ANOVA to test for differences in catches of both P. indicus and P. merguiensis, and postlarval prawns, between subregions (11 levels) and sampling cruises (two levels) in the JBG in 1997. The data were from trawls made on the last third of the ebb tide, because previous work had shown that at high tide the catch of P. merguiensis in mangrove creeks does not represent the population of juvenile prawns in the mangrove habitat, as they move into the forest (Vance and Staples, 1992; Vance et al., 2002). The data from trawls on sand beaches where P. indicus were uncommon were excluded from this analysis. We used a three-way ANOVA to test for difference in catch among microhabitat (six types), tide phase (high/low) and light (day/night) in the JBG, in 1998. However, the effect of light was not significant for postlarval or juvenile banana prawns, so we eliminated light from the analysis and then used a two-way ANOVA to test for differences in catches between microhabitat and tide phase. Other environmental variables, such as moon phase, were relatively constant over the 48 h taken to sample one river. The relationships between the catches of P. indicus and P. merguiensis and the area and linear extent of the different habitats in the four regions and the 12 subregions (i.e., among river systems) were investigated using regression techniques (PROC REG in SAS, SAS Institute, 1999–2000). R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 87

2.5. Relationship between juvenile prawn catches and environment

We investigated how the proportion and the numbers of P. indicus and P. merguiensis in the catch were influenced by environmental variables using quasilikelihood generalised linear models. For the numbers of prawns in the catch, we also used tree-based models that seek to find the major drivers of prawn density using recursive partitioning of the data. S- Plus software was used for these analyses (Venables and Ripley, 2002).

2.5.1. The proportion composition of juvenile banana prawns in the JBG The proportion of P. indicus juveniles, P, in combined P. indicus/P. merguiensis trawl samples was modelled as a function of the following variables—location, temperature, salinity, sediment type (and clay content), cloud cover, secchi depth, water body type, presence-of-mangroves and distance of trawl. The location variables were a coordinate system tilted at 45j so one axis, NWSE, runs from southeast to northwest and the other, SWNE, runs from southwest to northeast. Because the orientation of the major coastal systems is along the NWSE axis, this choice of system minimises the necessity for inclusion of interacting terms. Number of P: indicus juveniles N P ¼ ¼ Total number of banana prawn juveniles in the trawl T

For this purpose, we conditioned on the total number of juveniles, T. The response is a proportion and it is likely to have a variance depending on the true (mean) proportion and this variance will be zero if the true proportion is zero or one, and a maximum if the true proportion is 1/2. According to statistical convention, we denote the true (or expected) proportion by m: E[ P]=m, and assume that the distribution of P is determined by its mean, m, and a scale parameter, f. Our analysis was based on a quasilikelihood generalised linear model (see McCullagh and Nelder, 1989) with the following properties:

The logistic transform of the mean, l, is a linear function of smooth flexible terms, one in each of the predictors, x1,x2,...,xp. In generic model terms l log ¼ b þ s ðx Þþs ðx Þþ:::þ s ðx Þ 1 l o 1 1 2 2 p p

For the smooth flexible functions s1,s2,...,sp we chose natural splines with suitably placed knots. Var[ P]=[m(1m)]/[T/f], where T is the total number of juveniles and f is the scale parameter. This guarantees that if the true proportion is near 0 or 1, then the variance of the sample proportion is low. In addition, the larger the total number of juveniles in the sample the more accurate is the sample proportion as an estimate of the true proportion.

The component functions, si, shows the effect of the determining variable, xi, on the true proportion (through its logistic transform), assuming no interactions. Care is needed to ensure the determining variables are chosen so that significant interactions between them 88 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 are unlikely. Our generalised linear model differed from a generalised additive model only in that the component functions were explicit spline functions, rather than smoothing splines, which have to be estimated using penalised maximum likelihood. This simplified the inference without losing too much flexibility.

2.5.2. Numbers of juvenile P. indicus and P. merguiensis in the JBG Because of the high number of zero catches (67 out of 229), and some high catches (>200 prawns, maximum catch = 1280), we adopted a two-phase approach to the modelling. First, we estimated the probability of finding pawns in the trawl, (using presence/absence as the response), and then we estimated the numbers of prawns found, given that some prawns were caught in the trawl. We then repeated the above analysis checking for the presence of P. indicus only, and of P. merguiensis only.

2.5.2.1. Logistic regression models for presence of any prawns or individuals of any species of prawn. With the data reduced to presence/absence in the first phase, the probability of the presence of prawns was modelled as a function of external variables using logistic regression. Let p be the probability of finding any prawns in a trawl. We use logistic regression models to relate the probability of finding any prawns, p, to the external variables. The model has the form:

p log ¼ b þ s ðx ÞþL þ s ðx Þ 1 p o 1 1 k k where the sj(x) terms are called the partial contributions to the logistic transform. The initial model used natural splines with four degrees of freedom as the smooth functions for each of the continuous variables (except for cloud cover, which used a spline term with 3 degrees of freedom) and for the categorical variables we used separate constants for each level. Using natural splines instead of polynomial terms allows greater modelling flexibility for the same expenditure of degrees of freedom (Venables and Ripley, 2002, Ch. 8). Salinity was modified for modelling purposes. Two stations had salinities that were very low (0.4 and 1.6). In the fitted model, if the salinity was less than 29 it was replaced by 29. Values that are too low act as outliers and unduly influence the modelling process, as well as distorting the graphical representation of the component. At low salinities, the proportion of P. indicus was near zero and this modification does not materially affect the model in any other way.

2.5.2.2. Numbers of P. indicus and P. merguiensis using a parametric model (generalised linear model). We used a generalised linear model with a quasilikelihood error structure, log link and variance function that was proportional to the mean to model the catches of each species. The response variable was: No: of P: indicus Y ¼ Distance travelled in the trawl Under some optimistic but not misleading assumptions, this variable could be approximately proportional to a Poisson random variable, suggesting a variance propor- R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 89 tional to the mean. Let l be the true mean of Y. In symbolic terms, the precise model may be stated as ::: logl ¼ bo þ s1ðx1Þþs2ðx2Þþ þ spðxpÞ where the right-hand side is the now familiar additive function of flexible smooth terms in the predictors. The variance of the response may then be specified as

Var½Y¼l=/ where / is a constant of proportionality. In addition, we postulate that the distribution of Y is in the generalised linear modelling family, in the sense of quasilikelihood.

2.5.2.3. Density of P. indicus and P. merguiensis using a nonparametric (tree-based) model. A simpler approach to predicting density is recursively to partition the data and produce a binary tree predictive device. This modelling strategy can give added insights into the same data set, as it automatically uncovers interactions in the predictors. The data are partitioned into increasingly homogeneous groups with respect to density by a binary splitting on the most effective determining variable, at the most effective place, at each recursive stage (Venables and Ripley, 2002, Ch. 9). After an initial tree is formed, cross- validation techniques are used to see if its degree of complexity is warranted. The tree is then optimally pruned as suggested by the cross validation, so that it is neither too simple (which would result in seriously biased predictions) or too complex (which would result in the errors in the training set being reproduced in the predictions).

3. Results

3.1. The physical environment of the JBG

The water temperatures in the coastal areas of the JBG were similar throughout the regions and ranged from 26.8 to 34.2 jC depending on time of day (Table 3). The salinity in the coastal areas and rivers ranged from 30 to 36, except for the Ord River, where salinity was often < 10 (Table 3). The secchi depths were more variable between regions. In most of the eastern JBG and Cambridge Gulf, secchi depths were very shallow ( < 0.2 m) (Table 3). The secchi depths were deeper at a few sites with an oceanic influence. In contrast, secchi depths in the western JBG were deeper, ranging from 0.2 to 4.5 m. Secchi depths in the Lyne River (0.2–1.6 m) were intermediate between those of the eastern and western JBG, and were deeper than those at most sites in Cambridge Gulf where they were about 0.1 m.

3.2. Littoral habitats in coastal JBG

The largest area and linear extent of mangrove forests in the JBG were found in the large river systems of the eastern JBG (347 km2 and 735 km, respectively; Table 2). The Cambridge Gulf also had extensive mangrove forests (161 km2 in area and 484 90 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

Table 3 The temperature, salinity and turbidity of coastal waters in each of four regions and 12 subregions (river system) in the JBG in 1997 Environmental variable Temperature Salinity Turbidity j Region and river system ( C) Secchi Qualitative depth (m) estimation Eastern JBG Moon 29.5–31.0 30.5–35.3 0.1–0.5 1, 2 Fitzmaurice 29.5–29.9 33.0–34.1 0.0–0.2 1 and 2 Victoria (no delta) 26.8–31.8 31.0–35.1 0.1–0.3 1, 2, 3 (1, 2) Keep 28.4–33.4 30.0–35.5 0.0–0.2 1, 2 Total Eastern JBG 26.8–33.4 30.0–35.5 0.0–0.5 1, 2, 3 Total Southern JBG 31.5–33.5 32.2–33.5 0.1–0.7 2,3

Cambridge Gulf Delta 29.6–31.8 32.0–36.0 0.1–0.5 2, 3 Ord River 30.5–31.2 0.4–23.5 0.0–0.1 1 only West 28.5–34.2 30.0–34.3 0.0–0.3 1, 2, 3 Lyne 28.0–31.5 31.4–34.7 0.1–1.6 2 and 3 Total Cambridge Gulf 28.0–34.2 0.4–36.0 0.0–1.6 1–3

Western JBG Berkeley 29.5–32.9 30.2–33.4 0.3–2.2 2, 3, 4 West JBG 27.5–33.9 32.8–35.1 0.3–3.0 3 and 4 King George 29.7–33.9 33.1–35.0 0.2–4.5 3 and 4 Total Western JBG 27.5–33.9 30.2–35.0 0.2–4.5 2–4 Turbidity is measured by secchi depth readings (m) during the day. Qualitative units of turbidity are—1, very muddy; 2, moderately muddy; 3, slightly muddy; 4, clear. km in linear extent), with a particularly large area found in the northeast (Table 2). The area and linear extent of mangroves in the western and southern JBG were much lower than either of the above regions (combined area = 38 km2 and linear extent = 118 km, Table 2). The highest area and linear extent of saline coastal flats behind the fringing mangroves were estimated for the eastern JBG, Cambridge Gulf and the southern JBG (Table 2). These values were much higher than in the western JBG (28 km2 and 39 km, respectively). Among the four regions, the area and linear extent of ‘‘land subject to inundation’’ followed a similar pattern (Table 2).

3.3. Distribution of P. indicus and P. merguiensis in the JBG

In 1997, we caught a total of 43,918 postlarval banana prawns, 6650 juvenile P. indicus and 7189 juvenile P. merguiensis in 202 trawls made at 185 sites in the Joseph Bonaparte and Cambridge Gulfs. The sites ranged from a creek east of Pearce Point (14.462jS, 129.475jE) to a bay south east of Cape Londonderry (13.955jS, 127.118jE) (Fig. 1). Catches of postlarval banana prawns were highest in the Cambridge Gulf (up to 1500 postlarvae 100 m 2) and the rivers of the western JBG (Fig. 2a). Catches were lower in the eastern and southern JBG (except for the Keep River). Note however, that only three R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 91

Fig. 2. Mean catch ( F 1 S.E.) of postlarval and juvenile banana prawns (P. indicus and P. merguiensis) in beam trawl catches from 12 subregions in the JBG in 1997. trawls were completed in the southern JBG, as it was difficult to access due to extensive tidal sandbanks. Catches of juvenile banana prawns were high in the eastern JBG, Cambridge Gulf and the western JBG (Fig. 2b). Few juvenile prawns were caught in the creeks of the southern JBG. In the eastern JBG and Cambridge Gulf, over 96% and 73% (respectively) of juvenile banana prawns were P. indicus (Fig. 3). P. indicus was the main species in the ‘‘Cambridge 92 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

Fig. 3. Proportion of P. indicus and P. merguiensis in the juvenile prawn catch from 12 subregions in the JBG in 1997.

Delta’’ near Cape Domett (95%) and the rivers of the southern and western Cambridge Gulf (90%). In contrast, in the western JBG only 7% of juvenile banana prawns were P. indicus (Fig. 3). The Lyne River of the northwestern Cambridge Gulf was the only location where the catches of both species were about equal (49 % P. indicus and 51% P. merguiensis). The proportions of P. indicus and P. merguiensis in the three rivers that were sampled in both 1997 and 1998 were similar. For example, P. indicus comprised >98% of the catch in Forsyth Creek in the eastern JBG and < 10% of the catch in the Berkeley River in the western JBG, in both years. The Lyne River had a mixture of both species; P. indicus comprised 49% of the catch in 1997 and 30% of the catch in 1998. The catches of banana prawn postlarvae were high (>220 prawns 100 m2) among all the rivers of the JBG, except the eastern most rivers systems, the Victoria, Fitzmaurice and New Moon Inlet ( < 20 prawns 100 m2), and the Ord River ( < 36 prawns 100 m2). Catches were very high elsewhere in Cambridge Gulf; e.g., the southern and western rivers (1596.2 F 659.9 prawns 100 m2) and the Lyne River (1041.3 F 648.4 prawns 100 m2). The catches of juvenile P. indicus were higher in the Fitzmaurice, Victoria and Keep Rivers of the eastern JBG (>58.1 F 17.3 prawns 100 m2), than in the rivers and creeks of the western JBG ( < 34.8 F 27.4 prawns 100 m2) (Fig. 2b). In the Cambridge Gulf, the catches of P. indicus were higher in the ‘‘Cambridge Delta’’ near Cape Domett (43.3 F 18.4 prawns 100 m2) and the rivers of the southern and western Cambridge Gulf (146.3 F 64.2 prawns 100 m2) than in the western JBG. R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 93

Table 4a Mean squares and significance levels for two-way analyses of variance of the abundance of P. indicus [log(x + 1.5)], P. merguiensis [log(x + 1.5)] and unidentified postlarval banana prawns [log(x + 1.5)] in 11 river systems in inshore habitats in the JBG, northern Australia Source df P. indicus P. merguiensis Postlarvae River system 10 18.1*** 24.7*** 29.4*** Cruise 2 1.6 44.3*** 40.6*** River Cruise 5 3.4 5.9** 9.7* Error 179 3.1 1.8 4.1 Total 197 *0.01 < p < 0.05. **0.001 < p < 0.01. ***p < 0.001.

The catches of juvenile P. merguiensis in the western JBG were greater (e.g., 357.6 F 91.4 prawns 100 m2 in the Berkeley River; 332.7 F 180.9 prawns 100 m2 in western JBG creeks) than those in the eastern JBG and Cambridge Gulf (e.g., < 16.6 F 13.0 prawns 100 m2 in the Keep River and the rivers of southern Cambridge Gulf) (Fig. 2b). The Lyne River of the northwestern Cambridge Gulf was a location where the catches of both species were high (P. indicus, 314.1 F 272.0 prawns 100 m2 and P. merguiensis, 332.6 F 233.8 prawns 100 m2). Neither species was abundant in the east-arm of Cambridge Gulf and the Ord River (Fig. 2b). No prawns were caught at the upstream sites of the Ord River where the salinity was < 5. ANOVA showed that catches of P. indicus were significantly higher in the Keep (323.4 F 233.4 prawns 100 m2) and Fitzmaurice Rivers (58.1 F 17.3 prawns 100 m2) in the eastern JBG than in the King George River of the western JBG (0.2 F 0.2 prawns 100 m2) (Table 4a). Catches of juvenile P. indicus among the other eight river systems did not differ significantly, and they did not differ from those in the Keep and Fitzmaurice Rivers or the western JBG. Catches of P. merguiensis were significantly higher in the Berkeley River (357.6 F 91.4 prawns 100 m2), creeks of the western JBG

Table 4b Mean squares and significance levels for two-way analyses of variance of the abundance of P. indicus [log(x + 1.5)], P. merguiensis [log(x + 1.5)] and unidentified postlarval banana prawns [log(x + 1.5)] in eight river systems in inshore habitats that were sampled during both October and December in the JBG, northern Australia Source df P. indicus P. merguiensis Postlarvae River system 7 20.8*** 31.4*** 32.4*** Cruise 1 3.2 23.8*** 13.0 River Cruise 6 3.4 5.9* 9.7* Error 105 2.8 2.0 3.5 Total 119 *0.01 < p < 0.05. ***p < 0.001. 94 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

(332.7 F 180.9 prawns 100 m2) and the Lyne River (332.6 F 233.8 prawns 100 m2) than other regions (Table 4a). Catches of P. indicus did not differ significantly between October and December, while those of P. merguiensis were significantly different (greater) in December (160.7 F 68.7 prawns 100 m2) than October (87.0 F 39.4 prawns 100 m2) (Table 4b). In 1998, catches of P. indicus were greater in the Forsyth Creek (1619.3 F 586.9 prawns 100 m2), than in either the Lyne or Berkeley Rivers ( < 20 prawns 100 m2). Catches of P. merguiensis were greater in the Berkeley River (422.6 F 96.1 prawns 100 m2), than in Forsyth Creek or the Lyne River ( < 40 prawns 100 m2). These differences were consistent over October/November and November/December. During both 1997 and 1998, the proportion of the trawl catch and the abundance of P. indicus was highest in the eastern JBG and Cambridge Gulf. The proportion of the trawl catch and the abundance of P. merguiensis was highest in the western JBG, demonstrating that the two species use different regions of the Gulf.

3.4. Correlation of banana prawn abundance and habitat extent

The catch of postlarval prawns was not related to the area or linear extent of mangrove, saltflat or inundated habitat.

Table 5 R2 and probability for simple linear regression of the abundance of Penaeus indicus and the area and linear extent of three habitats, and similarly for Penaeus merguiensis, in 11 subregions in the JBG, northern Australia df Slope Probability R2 Habitat areal extent (km2) P. indicus Mangroves 1,10 0.134 0.9 0.002 Saline coastal flat 1,10 0.056 0.67 0.018 Land subject to inundation 1,10 0.309 0.22 0.146 Penaeus merguiensis Mangroves 1,10 2.312 0.04* 0.359 Saline coastal flat 1,10 .365 0.01** [0.03* ( Ord)] 0.510 Land subject to inundation 1,10 0.412 0.19 0.167

Habitat linear extent (km) Penaeus indicus Mangroves 1,10 0.088 0.85 0.004 Saline coastal flat 1,10 0.247 0.40 0.072 Land subject to inundation 1,10 0.382 0.91 0.001 Penaeus merguiensis Mangroves 1,10 1.14 0.03* 0.382 Saline coastal flat 1,10 0.627 0.07+ [0.13 ( Ord)] 0.298 Land subject to inundation 1,10 3.938 0.35 0.087 ( Ord) = results without the inclusion of data from the Ord River. *0.01 < p < 0.05. **0.001 < p < 0.01. R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 95

Among the 11 river systems (subregions) of the eastern and western JBG and the Cambridge Gulf, the abundance of juvenile P. indicus was not positively correlated with the area or linear extent of any of the estuarine or coastal habitats (Table 5). Not all river systems with large areas of mangroves had abundant populations of P. indicus. However, all rivers in JBG with abundant populations of P. indic us did have large areas of surrounding mangrove forests. Among the 11 subregions, the abundance of P. merguiensis was inversely correlated with the area of mangrove habitat ( p < 0.05) and the area of saline coastal flat ( p < 0.01), and inversely correlated with the linear extent of mangroves ( p < 0.05) (Table 5). The analysis of prawn abundance and broad-scale habitat is inconclusive, suggesting that environment or microhabitat may be important in determining the distribution of the two species of banana prawn.

3.5. Relationship between prawn abundance and habitat variables

Banana prawns were caught at 162 of the 185 sites trawled. The sites at which a high proportion of P. indicus (relative to P. merguiensis) was caught were concentrated in the large rivers in the east JBG and in Cambridge Gulf. The proportion of P. indicus in the beam trawl catches increased from the North–West to the South–East JBG. This ‘surrogate’ location variable was the dominant effect in the model (Table 6). P. indicus may prefer a temperature range of about 30–32 jC, but the effect of temperature on the proportion of P. indicus in the catch was not strong. A higher proportion of P. indicus in the catch was found in water with slightly lower than marine salinity (about 30–32). The proportion of P. indicus tended to increase as secchi depth decreases (Table 6, p=0.06). The NWSE variable may be a surrogate for low secchi depth, as secchi depth was consistently shallow (<0.2 m) in the south east corner of the JBG, and higher but more variable in the western JBG. The high variance in secchi depth may well explain why secchi depth itself was not as good a predictor. Except in the Lyne River, all catches of P. indicus that were greater than 500 prawns 100 m2 were made at locations with secchi depths of 0.0–0.2 m. In the Lyne River, catches>500 were made at sites with secchi depths<0.6 m. The secchi depths in rivers in the western JBG (where P. merguiensis dominated the banana prawn community) were 0.2–4.5 m. In the Lyne River, (in northern Cambridge Gulf) where both species were abundant, secchi depths were 0.2–1.6 m, about the middle of the range found in JBG.

Table 6 Significant contributions to the model predicting the proportion of P. indicus in trawls where some juvenile banana prawns were caught Component Degrees of freedom F statistic P-value NW–SE 4145 36.733 0.000 Temperature 4145 3.590 0.008 Salinity 4145 4.063 0.004 Secchi 4145 2.291 0.062 The estimate of the scaling parameter was ˆj = 18.138. 96 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

Table 7 Significant contributions to the model predicting the presence of (a) P. indicus and (b) P. merguiensis in trawls where some juvenile banana prawns were caught (a) Term df Deviance p-value NW–SE 6 18.1337 0.0059 Temperature 4 13.8309 0.0079 Cloud 3 10.4028 0.0154 Salinity 4 18.2579 0.0011 Secchi 4 14.6427 0.0055 Sediment 2 28.9737 0.0000

(b) Term df Deviance Pr(Chi) NW–SE 6 26.3378 0.0002 Secchi 4 13.8510 0.0078 Sediment 2 10.9978 0.0041 Water type 2 6.4482 0.0398

In the trawls that caught either species of juvenile banana prawns, six variables significantly contributed to the model for the likelihood of catching any P. indicus at all (Table 7a). The NWSE axis location was significant, with P. indicus present more often in trawls made in the southeast. Likewise, catches of P. indicus were related largely to the NWSE location variable; they were higher in the southeast JBG (Table 8a). In addition, the remaining terms were significant, suggesting that they have biological significance. These factors support and enhance the previous analysis of the proportion of P. indicus in the catch. P. indicus were found in greater numbers in turbid waters and on muddy substrates (Table 7a). They were caught where water temperature was >32 jC and their presence

Table 8 Significance of terms in a quasilikelihood General Linear Model for the density (catch) of (a) P. indicus and (b) P. merguiensis in trawls made throughout the JBG Term df Deviance F-value Significance (a) NW–SE 4 154.6478 6.7587 0.0000 Secchi 4 238.3981 10.4189 0.0000 Cloud 3 74.6338 4.3491 0.0053 Water Type 2 351.1422 30.6926 0.0000

(b) Secchi 4 29.4738 4.0210 0.0037 Temperature 4 25.5776 3.4895 0.0088 Sediment 2 13.3564 3.6444 0.0278 Water Type 2 42.5568 11.6118 0.0000 Salinity 4 35.4971 4.8428 0.0009 NW–SE 4 242.0877 33.0273 0.0000 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 97 increased as salinity and cloud cover increased, although the effect of these variables was less certain. The density of P. indicus was higher in small creeks than in either the river or coastal sites (Table 8a). The first partition on the nonparametric (tree model) supported this contention. Furthermore, the same variables were of high importance (five terminal nodes for P. indicus). The location variable was the equal third level split for P. indicus. Greater densities of P. indicus were found in turbid waters (Table 8a) and the tree model suggests that they were found in waters with a secchi depth<0.5 m. The presence of P. merguiensis was also strongly affected by the NWSE location variable; P. merguiensis were more likely to be present, and their catch was greater in trawls made in the northwest JBG than the southeast (Tables 7b and 8b).The nonparametric tree model shows the same variables are important (four terminal nodes for P. merguiensis); the initial split in the tree model for P. merguiensis is made on the NWSE location variable. The quasilikelihood parametric model for P. merguiensis suggests a more complex picture than that for P. indicus; six variables contribute significantly to differences in their catch (Table 8b). Like P. indicus, they were less likely to be found in clear water, although the relationship was less strong. Fewer P. merguiensis were caught in highly turbid waters ( < 0.5 m), and their catch was greatest where secchi depths exceeded 0.5 m to about 1.5 m. Both parametric and nonparametric models suggest that the type of water body was significant for P. merguiensis. They were more likely to be caught in creeks (and their abundance was greater in creeks) than in rivers or coastal locations, and regardless of location, they were more likely to be caught on mud than sandy substrates (Tables 7b and 8b). Although several environmental variables are significant determinants of the distribution and abundance of each species of juvenile banana prawn, most vary within each region of the JBG and Cambridge Gulf. The only significant environmental variable that shows a consistent trend across the JBG is the NWSE axis (probably a proxy for consistent very high turbidity in the southeastern JBG and southern Cambridge Gulf).

3.6. Catches of P. indicus and P. merguiensis in different microhabitats in coastal JBG

In 1997, the mean catches of banana prawn postlarvae in the gutters and subcreeks of the upper Cambridge Gulf were 10–100 times higher than those in the creeks and the main rivers. The catches from all near-bank mud/mangrove habitats were higher than those from midriver channels and banks (2.5 F 1.5 postlarvae 100 m2 at the most). Similarly, the mean catches of P. indicus also were far higher (30–70 times) in gutters (1584.5 F 991.7 prawns 100 m2) and subcreeks (672.4 F 611.3 prawns 100 m2) than in creeks (21.0 F 16.5 prawns 100 m2) or main rivers (1.3 F 0.8 prawns 100 m2). They were also higher than in midriver habitats (0.0 F 0.0 prawns 100 m2). The catches of P. merguiensis were higher in the subcreeks (164.1 F 145.0 prawns 100 m2) than the creeks (3.3 F 0.7 prawns 100 m2). Their low catch in gutters (36.0 F 24.9 prawns 100 m2) and rivers (0.0 F 0.0 prawns 100 m2) is not comparable with other habitats due to sampling bias. This bias occurred because the microhabitat sampling in 1997 was limited. Gutters and rivers were sampled only in regions outside the common distribution of P. merguiensis, where their abundance was always low (i.e., southern 98 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

Table 9 Mean squares and significance levels for two-way analyses of variance (GLM) of the 1998 abundance of P. indicus, P. merguiensis and unidentified postlarval banana prawns in six habitat types and two tide phases in the JBG and Cambridge Gulf, northern Australia Source df P. indicus P. merguiensis Postlarval banana prawns Habitat type 5 36.9*** 24.9*** 44.1*** Tide phase 1 103.7*** 168.5*** 214.7*** Habitat Tide 5 24.0*** 14.5*** 22.2*** Error 152 1.8 1.4 1.9 Total 163 ***p < 0.001.

Cambridge Gulf). No juvenile P. merguiensis were found in or on midriver channels and banks. In 1998, the catches of both P. indicus and P. merguiensis differed significantly between microhabitat and tide phase (Table 9), but not between day and night. Their

Fig. 4. Mean catch ( F 1 S.E.) of postlarval and juvenile banana prawns among five microhabitats in three rivers in the JBG in October to December 1998. Low tide data are presented. Channel and Mudbank are ‘midriver’ habitats. ns = not sampled. R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 99 catches were significantly smaller at high tide ( < 5 prawns 100 m-2 in any microhabitat) than low tide. The mean catch of banana prawn postlarvae was significantly greater (five to 100 times; p < 0.001) in creeks, subcreeks and gutters (from 499.9 F 240.0 to 2537.7 F 963.9 postlarvae 100 m2) than in the main rivers, either in near-bank (40.2 F 13.4 postlarvae 100 m2) or in midriver habitats ( < 3 postlarvae 100 m2) (Fig. 4a). In the Forsyth Creek, the mean catch of P. indicus was significantly higher (eight to 30 times; p < 0.001) in near-bank habitats in smaller water bodies (subcreeks and gutters) than the main river (108.0 F 93.8 prawns 100 m2) (Fig. 4b). As well, in the Lyne River, the mean catch of P. indicus was higher in the subcreeks and gutters than the main river (Fig. 4b). At most, one juvenile P. indicus was caught on midriver habitats in either river. No water bodies that were categorised as creeks were sampled in the Forsyth Creek or the Lyne River. In the Berkeley River, the catches of P. merguiensis showed a different pattern. Catches were significantly greater ( p < 0.001) in near-bank habitats in medium-sized creeks than in the subcreeks or the main river (Fig. 4b). No water bodies that were categorised as gutters were sampled in the Berkeley River. In the Lyne River, the catches of P. merguiensis were similar in the subcreeks and the gutters, which were eight times higher than the main river (3.6 F 1.4 prawns.100 m2) (Fig. 4b). At most in either river, < 1 to about six juvenile P. merguiensis 100 m2 were caught in midriver habitats, far fewer than the number caught in near-bank habitats in the same river. However, more P. merguiensis than P. indicus were found midriver. Throughout the duration of the project, microhabitat sampling has shown that both P. indicus and P. merguiensis are more abundant in near-bank habitats in small water bodies in coastal JBG.

3.7. Size of P. indicus and P. merguiensis in the rivers and between different microhabitats of coastal JBG

In 1997, over 85% of juvenile P. indicus in the rivers of the JBG were V 9 mm CL, except in the Victoria River where 75% were V 9 mm CL. The modal size of P. indicus was larger in the east JBG (5–9 mm CL) than in the Cambridge Gulf and the creeks of Western JBG (3–4 mm CL). The size distribution of juvenile P. merguiensis was more variable than that of P. indicus. Within many of the river systems of the JBG over 90% of juvenile P. merguiensis were V 9 mm CL. However, only about 68% of juvenile P. merguiensis in the Victoria River in the eastern JBG, and about 75% of juvenile P. merguiensis in the Lyne River in Cambridge Gulf were V 9 mm CL. The modal carapace length of P. merguiensis was larger (5–9 mm CL) in the eastern JBG than in the Cambridge Gulf and the western JBG (3–4 mm CL, as well as 5–9 mm CL prawns). The majority of trawls to examine differences in prawn abundance between microhabitats were done in 1998 (164 out of 201 trawls), so the size data for catches from each year were grouped. In 1997 and 1998, most juvenile P. indicus caught in near-bank habitats in gutters and subcreeks were < 9 mm CL, and similar in size to those caught in the near-bank habitats of the large creeks and rivers (Fig. 5a). In contrast, all juvenile P. 100 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108

Fig. 5. Carapace lengths of P. indicus (n = 11,184) and P. merguiensis (n = 10,390) in four microhabitat groups over all rivers sampled for differences in abundance among microhabitats in the JBG in 1997/1998. Low tide data are presented. Channel and bank are ‘midriver’ habitats. indicus caught in midriver habitats were z 9 mm CL. The common sizes of P. merguiensis in the same near-bank habitats were similar to those of P. indicus (Fig. 5b). However, P. merguiensis were smaller than P. indicus in midriver habitats ( < 9 mm CL); similar sizes to what they were in near-bank habitats. R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 101

The short time periods (about 10 days each survey) over which sampling was undertaken in both 1997 and 1998 make it difficult to accurately predict the period of recruitment and emigration of juvenile banana prawns in the JBG. However, the presence of postlarval and small prawns in the estuaries of both east and west JBG and Cambridge Gulf from October to December shows that juvenile banana prawns were actively recruiting to their critical habitats when the surveys were undertaken, facilitating our determination of their distribution.

4. Discussion

4.1. Distribution and habitat

Juvenile banana prawns were abundant throughout the coastal mangrove habitats of the JBG and Cambridge Gulf. The use of near-bank shallow waters along mud/mangrove habitats by both P. indicus and P. merguiensis is similar to the use of these habitats at low tide by P. merguiensis in the Gulf of Carpentaria (Staples, 1980; Vance et al., 1990, 1998), and the habitats identified for P. indicus elsewhere in the Indo-Pacific (Subramanian, 1985; Mohan and Siddeek, 1996). As our trawls were made at low tide, the prawns were found in the remnant gutters and creeks where they congregate after the tide has receded from the mangrove forests (Vance et al., 1996a,b, 2002). The two species of banana prawns inhabit different regions within the JBG. During our study, over 95% of juvenile banana prawns that were caught in the eastern JBG were P. indicus, while over 93% of juvenile banana prawns caught in the western JBG were P. merguiensis. The northwest corner of the Cambridge Gulf seems to be a transition zone, where both species were abundant. Although the mixed proportion of the two species in the Lyne River showed some variation, the distributions of the two species were consistent over both years throughout the JBG. The difference in the distribution patterns of the two species of prawns coincides with major gradients in geomorphology and physical characteristics of the river systems across the JBG. In the eastern JBG and the southern Cambridge Gulf, river systems are long and wide with large catchments, extensive floodplains and high turbidity, and extensive mud/ mangrove habitats in the lower and middle reaches. These rivers contrast with the rivers and creeks in the western JBG that are much smaller, have smaller catchments, are confined by sandstone escarpments and have much clearer water, often with sandy substrates. The transition zone for the two species of banana prawns (northwest Cambridge Gulf) also seems to be a transition zone for the local geomorphology; its hilly landforms are less rugged than the western escarpments, but more elevated than the eastern floodplains. The characteristics of the local landscape and rivers reflect differences in the extent of the banana prawn habitat in both the eastern and western JBG; extensive mangrove forests and saline coastal flats (including the water/habitat interface) in the east and a restricted extent of mangrove and saline flat habitats (narrow bands) in the west. Our modelling suggests that there is no strong direct relationship between the extent of mangrove or saltflat habitat and the catch of P. indicus, while there is a negative 102 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 relationship for P. merguiensis within the 11 river systems. We know that P. merguiensis use mangrove habitats in the Gulf of Carpentaria and elsewhere (Vance et al., 2002). In the Gulf of Carpentaria, the abundance of P. merguiensis is positively correlated with extent of mangroves (Staples et al., 1985). Thus, the negative relationship between their abundance and the extent of mangrove habitat is puzzling. Current patterns due to the hydrology of the JBG may limit the advection of P. merguiensis to the mangrove habitats in the eastern JBG, rendering them unable to access their preferred habitat (see Section 4.4). Despite no correlation between mangrove habitat and the catch of P. indicus, 92% of all P. indicus that were caught in 1997 were caught in the river systems of eastern JBG or Cambridge Gulf with their extensive mangrove and floodplain habitats. It is likely that they are readily delivered to these habitats on strong NW/SE tidal currents in the JBG (see Section 4.4), although patchy recruitment would make their within-habitat distribution nonuniform. These data were supported by our analysis using the northwest/southeast-axis variable, which shows that P. indicus were more likely to be present and that significantly more P. indicus were caught in southeast JBG (with extensive habitat). P. merguiensis were more common and significantly more were caught in northwest JBG (with restricted habitat). This direction variable is probably a surrogate for other biologically relevant variables, with dominant tidal currents and consistently high turbidity the likely candi- dates. Our modelling showed that P. indicus were more abundant in highly turbid waters. Thus, the extremely high turbidity and sediment load in the southeast JBG may favour them. Although not all trawls made at sites with low secchi depths caught large numbers of juvenile prawns (trawls were made in different microhabitats accounting for high variability among the catches), all catches of P. indicus that were greater than 500 prawns 100 m2 were made at locations with secchi depths < 0.6 m (mostly < 0.2 m). It is possible that P. indicus are more tolerant of high sediment loads in the water column than P. merguiensis. Conversely, over 85% of all P. merguiensis were caught in the rivers and creeks of western JBG with their limited mangrove and floodplain habitats. Because juvenile white banana prawns (P. merguiensis) are found in great abundance in extensive mangrove habitats in the Gulf of Carpentaria (Staples, 1980; Vance et al., 1998),we would expect them to be abundant in the mangrove habitats in JBG. Mangrove forest habitat is used by both species during their juvenile phase if they can reach it (Mohan et al., 1995; Mohan and Siddeek, 1996; Vance et al., 1996b, 2002). Thus, the low catches in the extensive mangrove habitats of east JBG may be due to poor larval supply rather than habitat preference (see Section 4.4), or to the extremely high suspended sediment loads in the water column. These hypotheses (for both species) need to be tested. Our modelling showed that temperature and salinity had an effect on the presence or abundance of both species, but the range of these environmental variables (27–34 jC and 30–36, respectively) was mostly within the normal tolerance of juvenile banana prawns [20–35 jC and 20–35, respectively (about 30 jC and a salinity of 30 are optimum for P. merguiensis, while a salinity of 20–30 is optimum for P. indicus)] (Staples and Heales, 1991; Bukhari et al., 1993; Kumlu and Jones, 1995). The Ord River was the only location where an environmental variable was outside the normal tolerance for banana prawns (salinity < 10). Upriver sites had salinities of 1.6 and 0.4, in contrast to other rivers R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 103 flowing into Cambridge Gulf where upriver salinities were 31–32. In the Ord River, no P. indicus or P. merguiensis were caught at the upriver sites despite high catches at several comparable rivers nearby (>250 juvenile P. indicus 100 m2 caught). Low salinity ( < 10) is lethal to large P. indicus postlarvae (>PL45) (Kumlu and Jones, 1995) and larger juveniles. Consequently, the low salinity in the Ord River would cause the death or emigration of banana prawns that initially recruited to the river. The low salinity and absence of banana prawn juveniles in the Ord River is due to freshwater flows from both irrigation runoff and the release of dam water to produce an ‘‘environmental flow’’ in the lower Ord (Water and Rivers Commission, 1999; O’Boy et al., 2001).

4.2. Microhabitat preference

The abundance of prawns varied between different water bodies at the same location, as well as between different locations. In the JBG, both juvenile P. indicus and P. merguiensis were more abundant in shallow near-bank habitats (mangrove-lined creek and riverbanks) in creeks and rivers than in midriver habitats, either shallow habitats on mud or sandbanks or deep channels. This pattern matches that found for P. merguiensis elsewhere in tropical Australia (Staples et al., 1985; Vance et al., 1998), and for P. indicus elsewhere in the Indo-Pacific (Subramanian, 1985; Mohan and Siddeek, 1996). In the near-bank habitats in the JBG, red-legged banana prawns were associated with mud substrates rather than sandy sediments, as they are in other locations (Mohan and Siddeek, 1996). In nonriverine or nonestuarine near-bank locations in large embayments or the JBG itself, all trawls caught < 3 juvenile P. indicus 100 m2 (<1 P. merguiensis and < 10 postlarvae 100 m2). Even in areas where sand beaches made up most of the habitat (such as west JBG), few prawns were caught. The exception was three trawls that were made on mangrove islands at the downstream extremity of the expansive Victoria River estuary. Large prawns (10–18 mm CL) were caught (59.7 F 12.0 P. indicus 100 m2 and 3.3 F 2.8 P. merguiensis 100 m2), probably emigrating from the river (Staples and Vance, 1986). Trawls made on mangrove islands further offshore caught no prawns. We caught no, or very few, juvenile banana prawns when we trawled the gutters and creeks at high tide, either centrally or along the mangrove fringe. As the tide rises, the juvenile P. indicus and P. merguiensis disperse from the creeks into the flooded mangrove forests. They move with the currents to seek food and shelter in the intertidal zone at high tide (Vance et al., 2002). The forests are flooded even during the neap tides (3–4 m range), though to a lesser extent than on the spring tides (up to 7 m). As the tide ebbs, the juvenile banana prawns are forced back into the remnant gutters, creeks and rivers by the retreating water level, where they accumulate at low tide and are catchable in our trawls. Their density probably increases in the creeks on the spring low tides. Within the near bank habitats, our methods of statistical analysis (generalised linear and nonparametric modelling and ANOVA) showed that both P. indicus and P. merguiensis were more abundant in the smaller water bodies (creeks and gutters) than the large rivers of JBG, as has been shown previously for P. merguiensis in mangrove estuaries (Vance et al., 1998). However, each species mostly used different types of remnant water bodies, 104 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 with P. indicus being most abundant in small creeks and gutters, while P. merguiensis were the most abundant in subcreeks and relatively abundant in large creek and main river habitats. These data suggest that in the JBG, although they are still very abundant in the uppermost reaches of water bodies, P. merguiensis may use larger streams to a greater extent that P. indicus. A high abundance of penaeid prawn juveniles in the upper reaches of creeks (where the water bodies are small and convoluted) has also been found in other tropical localities in Australia (Vance et al., 1998) and localities in America (Webb and Kneib, 2002). In the upper reaches of streams in a tidal marsh habitat, the area and linear extent of marsh and the marsh/water ‘edge’ habitats are greatest (Webb and Kneib, 2002). Likewise, on the scale of 100s of metres in the JBG, the high catch of juvenile P. indicus and P. merguiensis in small water bodies at low tide may be because of a relatively greater area and interface of mangrove habitats with the small creeks and gutters.

4.3. Size of juvenile prawns and juvenile recruitment

The occurrence of postlarval (1–2 mm CL) and small juvenile prawns (3–9 mm CL) of both P. indicus and P. merguiensis in the gutters, creeks and rivers of the JBG during October to December confirms that both species recruit to nursery habitats there at similar times to other penaeid prawns in tropical Australia (see Loneragan et al., 1994; Vance et al., 1998). The frequency of our sampling did not allow a rigorous measurement of trends in banana prawn recruitment in the JBG. However, the presence of high numbers of postlarvae and small juveniles of one species in some rivers while not in others, and the opposite pattern two months later, suggests that the recruitment of P. indicus and P. merguiensis does not occur simultaneously over the whole of the JBG. It is patchy, even among the regions where each species is common. In the rivers, the majority of postlarvae and small juveniles were found in near-bank habitats. In midriver habitats, only large juvenile P. indicus (>9 mm CL) were caught (Fig. 5a), suggesting that they were migrating from the river systems of the Gulf. In contrast, most of the P. merguiensis that were caught midriver were < 9 mm CL, supporting our suggestion that, in the JBG, P. merguiensis may be more abundant in larger water bodies than P. indicus, or that they emigrate at a smaller size.

4.4. Proximity of juvenile nursery habitats to the offshore fishery and recruitment to inshore habitats

The commercial fishery for P. indicus takes place in the northwestern offshore waters of the JBG (around 14jS, 128jE, in water depths of 50–80 m) (Kenyon et al., 2000). Thus, juveniles of P. indicus are found in estuarine habitats up to 120 km south and 240 km southeast of the southern and eastern limits of the fishery (Fig. 6). There are estuarine mangrove habitats in the western JBG that are much closer, about 50 km to the west and south west of the P. indicus fishery. However, although close, these habitats are not used by juvenile P. indicus. The separate locations of the nursery areas and the adult population suggest that the larvae of P. indicus that result from spawning in the fishing grounds are advected large R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 105

Fig. 6. Distribution of fishing effort relative to the inshore nursery habitats of postlarval and juvenile P. indicus in the JBG in 1998. The sites at where recently emigrated adult prawns were caught, tagged and released are also shown (refer Kenyon et al., 2000). distances to the south and east to their nursery habitats. They also imply that juveniles and subadults migrate from the mangrove nursery habitats, north and west, across shallower sand substrates (30–40 m deep) to the deeper-water fishery (on mud substrates 50–80 m deep). Nautical charts of the JBG confirm the existence of strong NW/SE currents in the inshore regions of the Gulf, but its hydrodynamics are not well understood. Suggestions that adult P. indicus migrate large distances offshore have been made elsewhere (Manisseri, 1988), yet they have not been rigorously investigated. If postlarval P. indicus advect such large distances (a minimum of 170 km to eastern JBG), the ‘‘advection envelope’’ (Rothlisberg et al., 1996; Condie et al., 1999) for P. indicus in JBG must be large. The determinants of advection depend on the behaviour of the postlarvae and local hydrology (Rothlisberg, 1995; Rothlisberg et al., 1995; Vance and Pendrey, 2001). However, a quick estimate suggests that the advective envelope might be about 17,000 km2; this is perhaps 10 times the size of advective envelopes of P. merguiensis in locations in the Gulf of Carpentaria where effective spawning may occur only 44 km offshore (Rothlisberg et al., 1996). The large tides (7 m) and strong tidal currents (4 km h1) in JBG probably contribute to the large size of the advection envelope for the banana prawn postlarvae (Condie et al., 1999). The juvenile P. merguiensis found in the rivers of the western JBG may recruit to these nursery habitats from outside the JBG, possibly further west along the north Kimberley coast where they are fished (Somers, 1994). As well, the juvenile and subadult P. 106 R.A. Kenyon et al. / J. Exp. Mar. Biol. Ecol. 309 (2004) 79–108 merguiensis must emigrate northwest from their nursery habitats to the Kimberley coast. Movement to the northwest is possible as a few banana prawns (P. indicus) that were tagged in the JBG migrated to the west around Cape Londonderry (Kenyon et al., 2000). The hydrology of the JBG may explain why juvenile P. merguiensis are not abundant in the eastern JBG; is it impossible for them to be transported from the Kimberley coast to eastern JBG? The hydrodynamic processes that transport the larvae of either banana prawn species in the JBG are unclear. Advection in the JBG, relative to the adult distribution of the two banana prawn species, may be a major determining factor in the distribution of the juveniles of each species, rather than habitat preference. The nature of these processes needs further investigation. A hydrodynamic model of the JBG would be an integral step towards determining what contribution advection made to the nonoverlapping distribution of juvenile P. indicus and P. merguiensis that we have identified. Furthermore, our results highlight the risk when attempts are made to link offshore fishery production with the extent of adjacent inshore habitats (see Staples et al., 1985). In the JBG, the most productive juvenile habitats for the fishery are not those closest to it, but those about 200 km distant. A sound knowledge of the hydrodynamics associated with postlarval advection and adult emigration, to and from offshore fisheries, is crucial for adequate fishery management in the JBG.

Acknowledgements

We thank Neil Montgomery and Larni Montgomery and the crew on the MV Makaira II, and Wayne Bishop and his skipper Rod Cole and crew of the FV Ace of Spades, for their hospitality and logistic support in providing ‘motherships’ to undertake the fieldwork. Dave Milton and Yimin Ye gave constructive comments on the manuscript. Vicki Smith processed the prawn samples in the laboratory. This work was funded by the FRDC grant (97/105) to study factors affecting the growth, mortality, movements and nursery habitat of red-legged banana prawns in the JBG. [RW]

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