Spatial variation in life history characteristics of snapper (Pagrus auratus) within Shark Bay, Western Australia

Abstract

Life history characteristics of snapper (Pagrus auratus) were found to vary at fine spatial scales (less than tens of kilometres) in the sub-tropical waters of Shark Bay, Western Australia, from research undertaken between 1997 and 2004. Differences in the timing and duration of the spawning season, length and age at maturity, and maximum age and growth between snapper from the Eastern Gulf, Denham Sound and Freycinet Estuary were attributed to spatial differences in environmental conditions and density-dependent responses to fishing-induced changes in spawning biomass. Such fine-scale spatial variation in the biological characteristics of snapper is consistent with results of previous stock identification studies and further supports the geographic scale at which local snapper stocks are managed to protect both stock productivity and genetic diversity.

Keywords:

• Sparidae
• stock structure
• World Heritage
• recreational fishery
• fisheries management

Introduction

Individual fish stocks typically demonstrate intrinsic life history characteristics such as length and age at maturity, maximum length and age, and rates of growth and mortality that reflect the underlying population dynamics and the complex interaction of biological, ecological and environmental processes. Differences in biological characteristics have long been used to identify separate fish stocks for management purposes (Begg 2005); however, whether such variation is genetic or environmental in origin can be difficult to determine (Trippel 1995; Law 2000).

Snapper, Pagrus auratus (Bloch & Schneider 1801), are widely distributed throughout warm temperate and sub-tropical waters of the western Indo-Pacific where they typically support important coastal fisheries (Paulin 1990). Around Australia, snapper inhabit waters out to depths of c. 250–300 m from Hinchinbrook Island in Queensland (18°S) to north of Barrow Island in Western Australia (21°S) (Kailola et al. 1993). State-managed commercial and recreational fisheries in Queensland, New South Wales, Victoria, South Australia and Western Australia contribute to a national snapper catch of c. 3000 tonnes/year (Henry & Lyle 2003; Anon. 2006). In Western Australia, c. 75% of the total commercial catch historically came from the oceanic waters adjacent to Shark Bay (26°S, 113°E, Fig. 1) (Moran et al. 2005) while snapper in the inner gulfs support an important recreational boat-based fishery (Stephenson & Jackson 2005).

Fig. 1  Denham Sound, Freycinet Estuary and Eastern Gulf areas of inner Shark Bay, Western Australia. Boundaries represent recreational fishing areas for management purposes and are taken to delineate the approximate distributions of the three Pagrus auratus stocks in this study. Hamelin Pool is a marine nature reserve and closed to all fishing.

Shark Bay has a unique marine environment resulting from the combined physical and biological processes that operate within its waters and the surrounding environs (Wyatt et al. 2005). The bay is a large, semi-enclosed marine embayment and is comprised of open, deeper waters to the north and two shallower inner gulfs to the south (Eastern Gulf, Western Gulf = waters of Denham Sound and Freycinet Estuary combined) (Fig. 1). The climate is arid (average rainfall 200–220 mm/year) with minimal terrestrial runoff (Logan & Cebulski 1970). Limited tidal exchange combined with persistent winds and high evaporation rates (average 2000–2200 mm/year) have resulted in highly elevated salinities and marked environmental gradients (Nahas et al. 2005). Salinities in the inner gulfs consistently exceed the oceanic level (35 PSU) with metahaline conditions maintained in the Eastern Gulf and southern waters of Denham Sound (38–48 PSU) and Freycinet Estuary (45–48 PSU), whereas hypersaline conditions (50–65 + PSU) persist in Hamelin Pool (Logan & Cebulski 1970). Shark Bay was inscribed on the World Heritage List in 1991 for the high conservation value of features of its marine environment including: the distinct biological zones produced by the salinity gradients; the highly restricted marine communities adapted to the hypersaline conditions; and the significant genetic variability in some taxa including P. auratus (Environment Australia 1999).

Shark Bay is near the northern extent of the geographic range of snapper on the west coast of Australia (Kailola et al. 1993). A number of independent genetic (allozymes) studies indicate that snapper in the oceanic waters off Shark Bay, in the Eastern Gulf and Freycinet Estuary, are reproductively isolated from each other, whereas fish in Denham Sound are partially isolated from the other populations (MacDonald 1980; Johnson et al. 1986; Whitaker & Johnson 1998; Baudains 1999). Tagging (Moran et al. 2003; G. Jackson, unpublished data) and otolith chemistry (Edmonds et al. 1999; Bastow et al. 2002; Gaughan et al. 2003) indicate little or no mixing of juvenile, sub-adult and adult snapper between oceanic waters and the inner gulfs, or between the gulfs. Eggs and larvae inside Shark Bay are retained within meso-scale eddies (kilometres in diameter) effectively isolating the main spawning grounds from each other (Nahas et al. 2003). Given the complex snapper population structure in the region, four unit stocks are currently recognised for fisheries management purposes: an oceanic stock found along the continental shelf outside Shark Bay and, in the inner gulfs, separate stocks in the Eastern Gulf, Denham Sound and Freycinet Estuary (Stephenson & Jackson 2005).

Beyond Shark Bay, three other snapper stocks are currently recognised around Australia: an ‘East Coast’ stock off southern Queensland, NSW, and eastern Victoria; a ‘Western Bass Straight’ stock off central and western Victoria; and a ‘Great Australian Bight–South Australian’ stock between the mouth of the River Murray and eastern Western Australia (Donnellan & McGlennon 1996; Meggs et al. 2003; Sumpton et al. 2008). No attempt has previously been made to investigate variation in life history characteristics amongst these Australian snapper stocks.

In this study, we compared some life history characteristics of snapper from closely adjacent areas of inner Shark Bay, which is located at the northern extent of the species range on the Australian west coast. We investigated the hypothesis that the growth and reproductive characteristics of snapper varies at a spatial scale of tens of kilometres given: (1) the complexity of regional population structure; (2) Shark Bay's marked environmental gradients; and (3) historic differences in exploitation amongst the three areas.

Materials and methods

Snapper were collected from the Eastern Gulf, Denham Sound and Freycinet Estuary (Fig. 1) between 1997 and 2004, principally using rod and line fishing. In addition, smaller fish (c. 50–200 mm, fork length, FL) were obtained during research trawl (twin otter trawls, 45-mm cod-end mesh; Moran & Kangas 2003) and trap surveys (Antillean-Z fish traps, 12-mm mesh; Jackson et al. 2007) that were conducted in all three study areas. All fish were measured (FL, nearest 1 mm) and a random sub-sample weighed (whole wet weight, nearest 10 g). Data were logarithmically (natural ln) transformed and used to derive an allometric length:weight relationship for each area; differences between areas were tested using analysis of covariance and where non-significant, the corresponding data were pooled. After adjustment for log-bias (Quinn & Deriso 1999), the resulting relationship was used to estimate the weights of all unweighed fish. Gonads were sexed, weighed (nearest 0.01 g) and staged macroscopically. For females, a six-stage classification system (1 = immature, 2 = resting, 3 = developing, 4 = developed, 5 = spawning, 6 = spent) was used, whereas for males, a five-stage classification system (1 = immature, 2 = resting, 3 = developed, 4 = spawning, 5 = spent) was used (Mackie et al. 2009). A random selection of ovaries (n=1497) that had been fixed and used to prepare histological sections was examined microscopically and staged based on histological characteristics (Mackie et al. 2009). Sagittal otoliths were sectioned and individual ages determined using methods described by Jackson (2007). The periodicity of increment formation was validated using marginal increment analysis and oxytetracycline (or calcein) marking of otoliths (Jackson 2007).

Length and age at maturity

Logistic regression analysis was used to determine the parameters α and β of the logistic relationship between the probability, p, that an individual is mature and the individual's length, L, i.e.

Individual fish were classified as mature if, during the peak spawning period, they possessed gonads categorised as stages 4–6 for females and stages 3–5 for males. Lengths at which 50% and 95% of individuals were expected to be mature were calculated as L ₅₀= − α / β and L ₉₅={log_e(19) − α}/ β, respectively, for both sexes separately within each area. The analysis was repeated using each of 1000 sets of re-sampled data with the 95% confidence limits for values of L ₅₀ and L ₉₅ estimated as the 2.5 and 97.5 percentiles of the resulting parameter estimates. Ages at maturity (A _50, A ₉₅) were estimated directly using a similar approach based on individuals for which both gonad maturity stage and otolith-based estimates of age were available. Length and age at maturity for both sexes were compared amongst the three areas using a likelihood-ratio test (Kimura 1980; Cerrato 1990) with the significance level adjusted for multiple comparisons using the Bonferroni method (Quinn & Keough 2003).

Timing and duration of spawning season

Mean monthly gonadosomatic indices (GSIs) for fish with lengths of the estimated L ₅₀ or greater were determined for females and males within each area using the equation GSI = 100×W_g/(WW − W_g), where WW = whole wet weight of fish and W_g=wet weight of gonads. To determine the seasonal patterns of gonadal development, the monthly percentage frequency for each gonad maturity stage was calculated for females and males with lengths equal to or greater than the estimated L _50, and plotted as frequency histograms.

Growth

Growth curves were fitted to the lengths at age for females and males separately and for all individuals combined, for each area, using non-linear least-squares regression (S-Plus, v. 7.0, Insightful Corp., Seattle, WA, USA) and the von Bertalanffy growth model:

where L _t =mean length of fish (FL, mm) at age t (year); L _∞=mean asymptotic length (FL, mm); k=growth coefficient (per year); t=age of fish (year); and t ₀=hypothetical age (year) at which mean length is zero if fish had always grown at the rate described by the equation.

Juveniles for which sex could not be determined were randomly assigned to either the male or female sets of length-at-age data. Adequacy of fit for all cases was investigated by plotting standardised residuals against observed ages. Growth curves for both sexes were compared, both within and amongst the areas, using a likelihood-ratio test under the assumption that the residual variances differed (Cerrato 1990), with significance level adjusted using the Bonferroni method.

Results

The relationships of natural ln-transformed whole wet weight (WW, g) against FL (mm) for snapper from Denham Sound and Eastern Gulf (insufficient length:weight data were available for Freycinet Estuary to allow statistical comparison) were not significantly different (df = 315, between slopes P=0.608, between intercepts P=0.732). After these data were pooled, the relationship ln WW = 2.6703(ln FL) − 8.817 (r ²=0.99) was derived, which, after adjustment for log bias resulted in the non-linear relationship, WW = 0.000148FL^2.6703.

Length and age at maturity

For females, mean length at 50% maturity decreased from 420 mm FL in the Freycinet Estuary to 401 mm FL in Denham Sound and to 348 mm FL in the Eastern Gulf (Table 1, Fig. 2). Female mean age at 50% maturity was youngest at 3.2 years in the Eastern Gulf and increased to 4.5 years in the Freycinet Estuary and to 5.5 years in Denham Sound. Males were consistently smaller (by c. 90–120 mm) and younger (by 1.6–2.8 years) at 50% maturity compared with females in all three areas. Mean length and age for males at 50% maturity was smallest (243 mm FL) and youngest (1.6 years) in the Eastern Gulf, and was higher in Denham Sound (276 mm FL, 2.7 years) and the Freycinet Estuary (330 mm FL, 2.7 years).

Fig. 2  Proportions of mature female and male Pagrus auratus in each 50-mm length class (•, right y-axis) during the peak spawning period in Denham Sound, Eastern Gulf and Freycinet Estuary, Western Australia, 1997–2004. Relationship between the proportion of mature fish and length fitted using logistic regression analysis. The numbers of samples within each length class are indicated by the vertical bars and left y-axis (note different scale for females and males). Values of mean length at 50% maturity, L ₅₀, ±95% confidence intervals are indicated by horizontal bars on the fitted curves. (Sample sizes are given in Table 1.)

Table 1  Maturity and growth characteristics for Pagrus auratus and key environmental descriptors for the three study areas in Shark Bay, Western Australia

	Environment				Maturity								Growth
Location	Depth (m)	SST (^oC)	Salinity (PSU)	Sex	L 50	L 95	L max	n	A 50	A 95	A max	n	t 0 (years)	k (per year	L ∞	r ^2	n
Denham Sound	Max. c.20	Min. c. 19	35–38	Females	401	601	750	482	5.5	9.9	19	202	−0.03	0.14	762	0.85	384
		Max. c. 26			(384–417)	(572–629)			(4.9–6.1)	(8.4–11.8)			(−0.17 to 0.11)	(0.13–0.16)	(728–795)
				Males	276	397	730	380	2.7	7.1	17	139	−0.06	0.18	660	0.84	297
					(259–295)	(369–423)			(1.6–3.3)	(5.5–8.9)			(−0.19 to 0.09)	(0.15–0.21)	(615–722)
				Combined									−0.08	0.15	728	0.85	681
													(−0.17 to 0.02)	(0.14–0.17)	(694–762)
Eastern Gulf	Max. c. 16	Min. c. 16	38–44	Females	348	484	752	689	3.2	5.6	17	309	0.04	0.18	755	0.91	523
		Max. c. 26			(330–365)	(464–500)			(2.7–3.5)	(5.0–6.0)			(−0.05 to 0.15)	(0.17–0.19)	(732–782)
				Males	243	403		599	1.6	4.2	15	225	−0.07	0.17	751	0.93	385
					(218–267)	(374–429)	800		(1.0–2.2)	(3.1–5.2)			(−0.13 to 0.01)	(0.16–0.19)	(724–784)
				Combined									−0.02	0.17	755	0.92	908
													(−0.07 to 0.04)	(0.17–0.19)	(735–774)
Freycinet Estuary	Max. c. 12	Min. c. 15	40–50 +	Females	420	566	785	580	4.5	8.4	29	213	0.13	0.17	773	0.94	392
		Max. c. 27			(392–452)	(542–587)			(3.8–5.1)	(7.4–9.2)			(0.04–0.2)	(0.16–0.18)	(759–791)
				Males	330	465	790	522	2.7	5.9	31	178	0.08	0.17	766	0.95	399
					(301–352)	(436–489)			(2.0–3.3)	(4.4–7.5)			(0.01–0.15)	(0.16–0.18)	(756–780)
				Combined									0.1	0.17	770	0.95	791
													(0.05–0.16)	(0.16–0.18)	(761–781)
Note: Mean lengths (fork length, mm) and ages (years) at which 50% (L 50, A 50) and 95% (L 95, A 95) of individual female and male fish were expected to be mature. von Bertalanffy growth parameters derived from length-at-age data. Bootstrapped 95% confidence intervals in parentheses. SST, sea surface temperature; n, sample size; t 0, hypothetical age at zero length; k, growth coefficient; L ∞, mean asymptotic length; r ^2, coefficient of determination; L max, maximum fork length; A max, maximum age.

Likelihood-ratio tests indicated that female length at maturity in the Eastern Gulf was significantly different to that in Denham Sound (χ²=46.69, df = 2, P<0.001) and Freycinet Estuary (χ²=30.66, df = 2, P<0.001); however, differences between females in Denham Sound and Freycinet Estuary were not significant (χ²=5.64, df = 2, P=0.059). Male length at maturity in the Eastern Gulf was also significantly different to that in Denham Sound (χ²=11.33, df = 2, P=0.003) and Freycinet Estuary (χ²=22.73, df = 2, P<0.001), whereas differences between males in Denham Sound and Freycinet Estuary were again not significant (χ²=4.30, df = 2, P=0.117). Mean ages at maturity for both females and males differed significantly amongst the three areas (P<0.001 in all instances).

Timing and duration of spawning season

Mean monthly GSIs for both sexes rose sharply from April and peaked between May and July before declining to a base level in September–October in both Denham Sound and the Eastern Gulf (Fig. 3). In the Freycinet Estuary, GSIs rose sharply slightly later, from May, peaked around August–September and declined to a base level in November. Spawning females were recorded between April and September and were dominant in the respective samples between May and July in Denham Sound and the Eastern Gulf (Fig. 4). In the Freycinet Estuary, spawning females were not recorded until June, were present through to October and were dominant in the samples in August–October.

Fig. 3  Monthly gonadosomatic indices (mean±1 SE) for Pagrus auratus in Denham Sound, Eastern Gulf and Freycinet Estuary, Western Australia, 1997–2004, based on fish with lengths greater or equal to their length at 50% maturity. Closed rectangles on x-axis represent winter and summer months; open rectangles, spring and autumn months (numbers are sample sizes).

Fig. 4  Monthly percentage frequencies of gonad maturity stages of female Pagrus auratus from Denham Sound, Eastern Gulf and Freycinet Estuary, Western Australia, 1997–2004, based on fish greater or equal to their length at 50% maturity. Closed bars indicate females in spawning condition.

Growth

The von Bertlanffy growth curves fitted the length-at-age data from each area reasonably well; coefficients of determination (r ²) were generally high and the mean extrapolated ages at zero length (t ₀) were close to zero in most instances (Table 1, Fig. 5). Examination of the residuals indicated no systematic bias for fish from the Eastern Gulf and Denham Sound; however, some deviation was apparent with younger individuals (<3–4 years) of both sexes from the Freycinet Estuary. An a posteriori analysis indicated that the length-at-age data were fitted better by a Schnute & Richards (1990) five-parameter growth model compared with the three-parameter von Bertalanffy model (P<0.001).

Fig. 5  von Bertalanffy curves fitted to lengths at age of female and male Pagrus auratus from Denham Sound, Eastern Gulf and Freycinet Estuary in Shark Bay, Western Australia.

The maximum ages and corresponding fork lengths for both sexes were greatest in the Freycinet Estuary. Older fish (20 + years) were not represented in samples from the Eastern Gulf and Denham Sound in contrast with the Freycinet Estuary, where individuals approaching 30 years of age were sampled in most years of this study. Length at age estimates for both sexes were comparable among the three areas up to c. 3 years after which growth of both sexes in Denham Sound was slower compared with the Eastern Gulf and the Freycinet Estuary (Table 2). Likelihood-ratio tests indicated no significant difference in growth curves between females and males in the Eastern Gulf (χ²=4.70, df = 3, P=0.195) and Freycinet Estuary (χ²=3.73, df = 3, P=0.292) but a highly significant difference between sexes for fish from Denham Sound (χ²=11.42, df = 3, P=0.009). When both sexes were compared amongst the areas, growth of both females and males were not significantly different between the Eastern Gulf and Freycinet Estuary (females, χ²=9.48, df = 3, P=0.024; males, χ²=6.94, df = 3, P=0.074) but growth differed significantly for both sexes between these areas and Denham Sound (P<0.001 in all instances).

Table 2  Fork lengths (mm, mean±1 SE) at 5, 10 and 15 years of age for female and male Pagrus auratus from the three study areas inside Shark Bay, Western Australia, including size range of fish within each age group.

	Female			Male
Location	Mean len5 yr	Mean len10 yr	Mean len15 yr	Mean len5 yr	Mean len10 yr	Mean len15 yr
Denham Sound	388 (±13)	559 (±16)	631 (±31)	407 (±10)	584 (±29)	—
Size range	292–482	503–610	600–662	263–512	523–666	—
n	21	6	2	35	5
Eastern Gulf	477 (±9)	633 (±11)	—	458 (±16)	614 (±9)	712 (±38)
Size range	368–551	540–730	—	298–571	533–716	674–749
n	28	23		20	19	2
Freycinet Estuary	430 (±14)	627 (±10)	710 (±14)	455 (±16)	640 (±10)	716 (±9)
Size range	305–565	558–720	675–761	258–569	568–730	706–743
n	23	15	5	22	18	4
Note: n=sample size.

Discussion

Variation in life history characteristics of snapper, a large, potentially mobile sparid, was found to occur at a much finer spatial scale (less than tens of kilometres) inside Shark Bay than has been shown elsewhere within the species range or with any closely related marine sparid (e.g. Pagrus pagrus; Potts & Manooch 2002). This variation, while unusual for such a teleost inhabiting a marine environment with no obvious physical barriers, could be linked to Shark Bay's marked environmental gradients, historic differences in levels of fishing mortality and the nil or very low levels of mixing at any life-history stage between snapper inhabiting the closely adjacent areas of inner Shark Bay (Edmonds et al. 1999; Bastow et al. 2002; Gaughan et al. 2003; Moran et al. 2003; Nahas et al. 2003).

Snapper in the Eastern Gulf and Denham Sound mostly spawned in May–July, whereas fish in the Freycinet Estuary mostly spawned later in the year, between August and October. Monthly sea surface temperature data indicate that optimum water temperatures for spawning by snapper (c. 16–21°C, Scott & Pankhurst 1992) exist for c. 5–6 months in the Freycinet Estuary compared with only c. 3–4 months in Denham Sound and the Eastern Gulf. The longer spawning season in the Freycinet Estuary likely reflects both the duration of optimal water temperatures and the estuary's greater overall environmental variability. Waters in the Freycinet Estuary are more saline (c. 40–50 + PSU) and the annual temperature range is greater (c. 12°C) than in the Eastern Gulf and Denham Sound (Table 1). Seasonal spawning patterns inside Shark Bay are consistent with those observed in other lower latitude snapper populations such as in southern Queensland (Sumpton 2002) where environmental conditions are similar to Shark Bay. In contrast, snapper from higher latitudes such as South Australia, Victoria and New Zealand, spawn later in the year, in late spring to late summer (Scott & Pankhurst 1992; Coutin et al. 2003; McGlennon 2003).

There were significant differences in the mean length and age at 50% maturity between sexes with females being consistently larger and older than males in all three areas. Based on estimates derived using reproductive data pooled over the 8-year study, both sexes matured at significantly smaller lengths and younger ages in the Eastern Gulf compared with Denham Sound and the Freycinet Estuary. The smaller lengths and younger ages at maturity may reflect some phenotypic compensatory response to a significant decline in population size in the Eastern Gulf. Although there was no evidence of any major variation in environmental conditions (water temperature, salinity) unique to the Eastern Gulf over the study period, significant recruitment overfishing is acknowledged to have occurred there during the early to mid-1990s, when recreational boat-based fishers caught large numbers of spawning fish in the vicinity of the main spawning aggregation off Monkey Mia (Stephenson & Jackson 2005). An egg-production-based survey (Jackson & Cheng 2001) and age-based stock assessment modelling (Jackson et al. 2005) indicate that spawning biomass in the Eastern Gulf had declined to possibly as low as 10% of the unexploited level by c. 1997–1998. Following the introduction of a moratorium on catching snapper in the Eastern Gulf, which remained in place for almost 5 years (June 1998–March 2003), spawning biomass was estimated to have recovered to around 60% of the unexploited level by 2005 (Jackson et al. 2005).

While reduced intraspecific competition is a likely cause, the smaller lengths and younger ages at maturity in the Eastern Gulf may also be linked to some genotypic change resulting from heavy exploitation of larger, older spawners (Trippel 1995). Baudains (1999) reported that a number of rare alleles that were present in snapper from the Eastern Gulf that were collected in 1984 (c. 10 years before the acknowledged period of recruitment overfishing), as part of an earlier allozyme-based study (Johnson et al. 1986), were absent in samples collected in 1997 (c. 12–13 years after the period of recruitment overfishing). Reasons for the loss of these alleles were unclear; however, evidence of loss of genetic diversity within another heavily fished snapper population has been shown in Tasman Bay, New Zealand (Hauser et al. 2002).

Snapper in inner Shark Bay are moderately long-lived with a maximum age of at least 31 years. This longevity is similar to that found in snapper from South Australia (McGlennon et al. 2000) and Victoria (Coutin et al. 2003), but significantly lower than for fish from the cooler waters of New Zealand (Francis et al. 1992). Growth rates were lower for both sexes in Denham Sound compared with the Eastern Gulf and Freycinet Estuary. Differences in growth rates among separate stocks of snapper in New Zealand have been attributed to water temperature, food availability and genetic diversity (Francis 1994). Sarre & Potter (2000) found variation in growth rates with the closely related sparid, Acanthopagrus butcheri, among estuaries along the lower west coast of Western Australia, which was attributed to differences in salinity, diet and stock density. Research with juvenile snapper in Shark Bay (30–200 mm FL; Tapp 2003) and mature fish in South Australia (W. Hutchinson, South Australian Research and Development Institute, personal communication) found that growth was unaffected at salinities between 35–44 PSU but declined significantly at salinities of 48 PSU or greater. While salinities can exceed 50 PSU in waters at the southern margins of the Freycinet Estuary, most fish reside in waters of salinities c. 40–46 PSU (G. Jackson unpublished data). That growth rates were higher in the Freycinet Estuary and Eastern Gulf suggest that the differences in snapper growth are not wholly explained by differences in abiotic factors.

While there are genetic differences amongst the three areas (Johnson et al. 1986; Whitaker & Johnson 1998; Baudains 1999), differences in growth may be linked to density-dependent factors such as food availability or competition. Lower growth rates in Denham Sound snapper may reflect reduced prey biomass linked with commercial trawl fisheries for scallops and prawns that operate in these waters but not in the Eastern Gulf or Freycinet Estuary (Sporer et al. 2008). Such trawling has the capacity to remove large quantities of prey species taken by snapper and thereby potentially limit individual growth. Additionally, stock densities over the period of this study were estimated to be greater in Denham Sound compared with the other two areas (Jackson et al. 2005). Growth coefficients for Shark Bay (0.14–0.18/year) were higher than those reported for snapper elsewhere in Australia (Table 3) with the exception of fish from southern Spencer Gulf, South Australia (0.18/year), where environmental conditions (water temperature, salinity, bathymetry, prey species) are comparable with Shark Bay. In contrast, growth coefficients were lower for snapper from New Zealand (0.10–0.16/year) where water temperatures are generally cooler.

Table 3  Comparison of von Bertalanffy growth parameters for Pagrus auratus from Australia and New Zealand estimated from lengths at age based on sectioned otoliths.

Location	L ∞ (mm)	k (per year)	t 0 (yr)	Source
Western Australia
Shark Bay, Denham Sound	728	0.15	−0.08	This study
Shark Bay, Eastern Gulf	755	0.17	−0.02	This study
Shark Bay, Freycinet Estuary	770	0.17	0.10	This study
South Australia
Spencer Gulf, northern	984	0.12	−0.35	Fowler et al. (2004)
Spencer Gulf, southern	632	0.21	−0.11	Fowler et al. (2004)
Gulf St Vincent, northern	907	0.16	−0.08	Fowler et al. (2004)
Gulf St Vincent, southern	895	0.13	−0.06	Fowler et al. (2004)
Victoria
Port Phillip Bay	857	0.12	NA	Coutin et al. (2003)
New South Wales
Various locations	574	0.14	−2.29	Ferrell (2004)
Queensland
Various locations	792	0.08	−2.45	Sumpton (2002)
New Zealand
Various locations	720	0.11	−0.75	Francis et al. (1992)
Hauraki Gulf	588	0.10	−1.11	Gilbert & Sullivan (1994)
West Coast (North Island)	667	0.16	−0.11	Gilbert & Sullivan (1994)
West Coast (South Island)	696	0.12	−0.71	Ministry of Fisheries, unpublished data
East Northland	462	0.12	−1.93	Davies et al. (2003)
Note: Values are for sexes combined. NA, not available.

Fish of 15 years of age or more were more common in the Freycinet Estuary where fish of up to 25 years of age were consistently observed in recreational catches up to 2004. The presence of the older fish in this area reflects the greater survival of these fish when they first recruited to the fishery (around 3–4 years of age), when levels of exploitation were lower in 1970s and/or pulses of better than average recruitment over the last 20–30 years. While stocks in Shark Bay's inner gulfs appear to be self-recruiting (Nahas et al. 2003), and differences in annual recruitment strength among the three areas cannot be discounted, it is more likely that differences in observed maximum age and age composition amongst the three areas reflect historic differences in fishing pressure. Recreational catches of snapper are thought to have increased only gradually between the 1970s and early 1980s and then more rapidly, between the late 1980s and mid-1990s (Stephenson & Jackson 2005). This increase in recreational exploitation is attributed to improved vehicle access to the region and increased fishing efficiency linked to fish finding technology (colour sounders and GPS) (Jackson et al. 2005). The progressive introduction of stricter management from c. 1998 onwards, including a 5-year moratorium in the Eastern Gulf (1998–2003), a 6-week spawning closure in the Freycinet Estuary (from 2000 onwards), and the introduction of a Total Allowable Catch for each snapper stock in 2003, has resulted in significantly reduced recreational snapper catches in each area (Jackson et al. 2005).

Moderately long-lived species such as snapper can potentially have 20–25 age classes coexisting in a population such as in northern Spencer Gulf, South Australia (Fowler et al. 2005). In inner Shark Bay, fish of more than 20 years of age were consistently represented only in samples from the Freycinet Estuary. The lower maximum age and absence of older fish in both the Eastern Gulf and Denham Sound could possibly reflect the effects of higher exploitation during the 1970s–1980s compared with the Freycinet Estuary.

While the importance of understanding stock structure has been long recognised, stock identification is now receiving greater attention at smaller geographic scales for a number of reasons including increased emphasis on biodiversity and a shift towards more cautious management (Stephenson 1999). Fisheries management has typically assumed population homogeneity over large geographic scales (ocean-wide) with potentially mobile species. However, there is increasing evidence of population structuring for marine species at scales of hundreds of kilometres and less such as for red snapper, Lutjanus campechanus (Salliant & Gold 2006). Differences in life history characteristics such as growth and length/age at maturity are being used to support the management of fish populations at the finer spatial scale, e.g. orange roughy (Hoplostethus atlanticus) in the Tasman Sea off New Zealand (Smith et al. 2002), red snapper in the Gulf of Mexico (Fischer et al. 2004), carpenter (Argyrozona argyrozona) off South Africa (Brouwer & Griffiths 2005), and snapper and West Australian dhufish (Glaucosoma herbraicum) on the west coast of Western Australia (Wise et al. 2007).

The results of this study are consistent with those from previous stock identification research and add further support for the fine spatial scale at which local snapper stocks and the recreational fishery in the inner gulfs of Shark Bay are managed (Stephenson & Jackson 2005). The variation in important biological characteristics demonstrated in this study supports the need to manage snapper in the Eastern Gulf, Denham Sound and the Freycinet Estuary as separate management units, both to maintain stock productivity and genetic diversity, in particular given the region's World Heritage status.

Acknowledgements

The research was funded by the Natural Heritage Trust (Fisheries Action Program, Projects 973720 and 973340), the Fisheries Research and Development Corporation (Project 2003/066) and the Department of Fisheries, Western Australia. We are grateful to the many colleagues and recreational and commercial fishers of Shark Bay who helped to collect the snapper samples. Thanks to John McKinlay, Monty Craine and Adrian Thomson for assistance with the statistical analyses. This study represents part of PhD-thesis research at the Centre for Fish and Fisheries Research, Murdoch University, by GJ. The authors thank Rod Lenanton, Mike Moran and Stephen Newman, and many anonymous reviewers for constructive comments on earlier drafts of the manuscript.