1164
Age-based demography and reproduction of hapuku, Polyprion
oxygeneios, from the south coast of Western Australia:
implications for management
Corey B. Wakefield, Stephen J. Newman and Brett W. Molony
Wakefield, C. B., Newman, S. J., and Molony, B. W. 2010. Age-based demography and reproduction of hapuku, Polyprion oxygeneios, from the
south coast of Western Australia: implications for management. – ICES Journal of Marine Science, 67: 1164 – 1174.
The hapuku, Polyprion oxygeneios, inhabits deep (.100 m) continental slope waters of Western Australia. In all, 1352 P. oxygeneios
were collected from the waters along the south coast of Western Australia (ca. 358S) from 2004 to 2008. The species is gonochoristic,
and spawns during the austral winter (May– September). Ages were estimated from counts of opaque zones from thin-sectioned sagittal otoliths. Classification analysis of the outer margin of sectioned otoliths indicated that a single opaque zone is deposited annually.
Female P. oxygeneios (n ¼ 630; 535 – 1114 mm total length, TL) ranged in age from 2 to 35 years and males (n ¼ 691; 521 – 1004 mm
TL) from 2 to 52 years. von Bertalanffy growth models for male and female P. oxygeneios were statistically, but not biologically, different
(,5% difference in mean and estimated lengths-at-age). Estimates of the lengths and ages at which 50% of the females and males in
the population reached sexual maturity were 760 and 702 mm TL and 7.1 and 6.8 years. The instantaneous rate of natural mortality
(M ) was estimated to be 0.09. Estimates of the instantaneous rate of fishing mortality (F ) were low (0.01 – 0.05). Harvest rates in 2005
and 2006 were close to estimated sustainable levels. Monitoring of any future increases in catch and effort in continental slope waters
in both State- and Commonwealth-managed fisheries is required in order to assess impacts to stock sustainability. Sustainable management would also benefit from improved understanding of possible pan-oceanic recruitment of the species among southern hemisphere populations.
Keywords: age, growth, hapuku, mortality, Polyprion oxygeneios, reproduction, Western Australia.
Received 2 November 2009; accepted 14 February 2010; advance access publication 26 March 2010.
C. B. Wakefield, S. J. Newman and B. W. Molony: Western Australian Fisheries and Marine Research Laboratories, Department of Fisheries,
Government of Western Australia, PO Box 20, North Beach, WA 6920, Australia. Correspondence to C. B. Wakefield: tel: +61 8 9203 0111;
fax: +61 8 9203 0199; e-mail: corey.wakefield@fish.wa.gov.au.
Introduction
The current taxonomy of the family Polyprionidae has two genera,
Polyprion and Stereolepis, each represented by two species
(Roberts, 1986; Nelson et al., 2004; Smith and Craig, 2007;
Gomon et al., 2008). There appears to be some conjecture over
the validity of P. yanezi (de Buen 1959), which in this case
has been treated as a junior synonym of P. oxygeneios
(C. D. Roberts, pers. comm.; Nelson, 2006; Eschmeyer, 2007).
The Polyprionidae is polyphyletic, but the limits and relationships
of the family have not been studied in detail (Smith and Craig,
2007), and the biology of the four species in this family is
poorly known. Generally, all four species attain remarkably large
size (.150 cm and .70 kg), are long-lived (ages .60 years),
are late to mature (Peres and Klippel, 2003), and probably form
spawning aggregations (Beentjes and Francis, 1999; Domeier,
2001). Such life-history attributes typically render species highly
vulnerable to fishing exploitation (Sedberry et al., 1999; Koslow
et al., 2000; Sadovy, 2003; Cornish, 2004). For example, the vulnerability of S. gigas to fishing in the Pacific Ocean has been recognized by the International Union for the Conservation of Nature
and Natural Resources (IUCN) as being critically endangered
with extinction (Cornish, 2004). Similar concerns have been
expressed for the conservation status of P. americanus, which has
recently been given a data-deficient risk status by the IUCN
(Sadovy, 2003).
Hapuku are widely distributed throughout all southern oceans
between 288S and 438S (Paxton et al., 1989), predominantly in
temperate mid-shelf to upper slope waters in depths ranging
from ca. 50 m to a maximum recorded depth of 854 m
(Williams et al., 1996; Barreiros et al., 2004). Polyprion spp. have
a unique life-history strategy that involves an extended pelagic
juvenile stage of up to 4 years in oceanic waters, with juveniles
attaining sizes of up to 670 mm total length (TL) for P. oxygeneios
(Roberts, 1996; Francis et al., 1999) and 650 mm TL for
P. americanus (Sedberry et al., 1999; Machias et al., 2003). In comparison, the settlement process of the polyprionid, S. gigas, involves
a pelagic larval stage of 1 month (Gaffney et al., 2007), which is
typical of many broadcast spawning teleosts (Neira et al., 1998).
Genetic analyses (mtDNA and microsatellite) have identified
separate northern and southern hemispheric populations of the
congeneric P. americanus. The connectivity between southern
populations of that species was less pronounced, but notably the
studies lacked representatives from southern Indian Ocean populations, e.g. southern Africa and Western Australia (Sedberry et al.,
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Age-based demography and reproduction of Polyprion oxygeneios from Western Australia
1996; Ball et al., 2000). A long pelagic life stage, coupled with a
strong association with floating objects (Roberts, 1996; Massuti
et al., 1999; Riera et al., 1999), is believed to facilitate pan-oceanic
mixing within the separate northern and southern hemisphere
populations of P. americanus (Sedberry et al., 1996; Ball et al.,
2000). Throughout their circumglobal distribution in the southern
hemisphere, the two species of Polyprion appear to be sympatric
(Roberts, 1987; Williams et al., 1996, Barreiros et al., 2004).
Moreover, considering that P. oxygeneios also has an extended
pelagic juvenile stage that is strongly associated with floating
objects (Roberts, 1996), it is very likely that they also undertake
pan-oceanic mixing between populations. Such large geographic
connectivity underscores the need for an understanding of population demographics for the sustainable management and exploitation of both species.
Hapuku are highly valued by commercial and recreational
anglers throughout their distribution (Beentjes and Francis,
1999; Francis et al., 1999; Barreiros et al., 2004). Commercial landings of P. oxygeneios were first recorded in the state-managed fisheries of Western Australia in 1986 (derived from compulsory catch
statistics provided by commercial fishers to the Department of
Fisheries, Government of Western Australia). Since then, annual
commercial catches have slowly increased, with an average
annual catch of ca. 19.3 t between 1986 and 2008 from Western
Australia. Catches from the state-managed fisheries along the
south coast of Western Australia averaged 13 t during the same
period. The Commonwealth-managed Western Deepwater Trawl
Fishery has operated farther offshore for more than 20 years and
has recorded an additional 0 –344 kg year21 between 2000 and
2006, a period of relatively low levels of effort (Moore et al.,
2007). The total annual commercial catches in waters of Western
Australia are low compared with those reported for New
Zealand. Catches of P. oxygeneios in NZ waters were first recorded
in the 1930s, peaked in 1983/1984 (at 2700 t for P. oxygeneios and
P. americanus combined), and have since been managed to an
annual quota of 1500 t (Francis et al., 1999). Therefore, catches
of P. oxygeneios in Western Australia are relatively small, likely
representing differences in demography (e.g. a smaller stock
size) and/or ecology (low productivity because of the dominant
south-flowing Leeuwin Current in Western Australia).
Recently in Western Australia, there have been concerns regarding a possible shift in fishing effort by both recreational and commercial fishers to deeper offshore waters (.100 m), following a
marked decline in the abundances of many nearshore fish
species and management targeting a 50% reduction in inshore
catches (Wise et al., 2007). The shift in fishing effort to target
fish in deeper water appears to be a global phenomenon. The generally lower resilience of species in deeper water to fishery exploitation owing to their relatively low productivity renders them less
likely to recover following overexploitation than continental shelf
species (Morato et al., 2006). Improved understanding of the
biology of P. oxygeneios would provide a valuable basis for assessing appropriate levels of sustainable exploitation in Western
Australia, along with improving the knowledge of the population
demographics of hapuku and for polyprionids in general. The
aims of this study that examined the age-based demographics
and reproduction of P. oxygeneios from the south coast of
Western Australia were to (i) determine spawning periodicity,
(ii) validate the frequency of opaque-zone deposition in otoliths
using margin classification to facilitate age estimates of the
species, (iii) describe the length and age compositions and
1165
determine growth rates for the species from the south coast of
Western Australia, (iv) estimate the lengths and ages at which
females and males attain sexual maturity, and (v) estimate the
rates of natural and fishing mortality. This information will be
crucial to manage sustainably this deeper water fish resource
from the south coast of Western Australia.
Methods
Reproductive variables
Samples of hapuku were obtained from commercial catches taken
using hook and line from 2004 to 2008 between 115830′ E and
125800′ E off the south coast of Western Australia (Figure 1). All
fish were caught using multihooked vertical drop lines. The TL
for each P. oxygeneios was measured to the nearest 1 mm, and
the gonads were removed and weighed (WG) to the nearest
0.01 g, the sex was determined, and the gonads were assigned a
macroscopic stage (Table 1). Gonadosomatic indices (GSIs) were
derived such that GSI ¼ (WG × 100)/WW, where WG is the
gonad weight (g) and WW the whole weight of fish (g). Data on
the developmental stages of gonads and GSIs in each corresponding calendar month of the year were pooled. The monthly prevalence of fish in spawning condition was estimated as the
proportion of mature P. oxygeneios (≥L50) possessing gonads at
stages III and IV. The trend in monthly spawning prevalence was
compared with that of the average monthly GSIs.
Age and growth
The sagittae of each hapuku were removed, cleaned, and stored in
paper envelopes. The right sagitta of each fish was embedded in
epoxy resin and sectioned transversely through its primordium
in a direction perpendicular to the sulcus acusticus, using a slowspeed saw with a diamond-tipped blade. Otolith sections were cut
thin, i.e. between 0.11 and 0.15 mm, to improve growth-zone
clarity, as suggested by Peres and Haimovici (2004) for the congeneric P. americanus. Sections were mounted on glass slides with a
cover slip, using casting resin. The opaque zones on each otolith
section were counted along an axis from the primordium to the
crista superior (dorsal rim of the sulcus acusticus; Figure 2)
Figure 1. Polyprion oxygeneios were caught by commercial fishers
between 115830’E and 129800’E on the south coast of Western
Australia. The common depth range of the species of 100 –1000 m is
shown (grey shading).
1166
C. B. Wakefield et al.
Table 1. Macroscopic characteristics of ovaries and testes of Polyprion oxygeneios.
Stage
I. Immature/Resting
II. Developing
III. Developed
IV. Ripe/Spawning
V. Spent
Ovaries
Occupy up to one half of length of ventral cavity.
Cylindrical, blood capillaries visible and pink to orange
Occupy up to two-thirds of ventral cavity. Blood
capillaries and oocytes visible
Occupy full length of ventral cavity. Oocytes clearly
visible but not hydrated. Blood capillaries more
conspicuous
Similar in size to stage IV. Hydrated oocytes (translucent)
visible throughout ovarian lobes or concentrated in
oviduct
Ovaries reduced in size, flaccid, and red in areas
Testes
Occupy up to one half of ventral cavity. Flat and white
Occupy up to two-thirds of ventral cavity and white colour
more apparent
Testis much larger occupying up to full length of ventral cavity.
No milt discharged when slight pressure is applied to lobes
or abdomen
Similar in size to stage IV. Milt discharged when slight pressure
is applied to lobes or abdomen
Testes reduced in size, flaccid, and red in areas
Trends in the growth of females and males of P. oxygeneios were
determined from the mean length at each age. A von Bertalanffy
growth equation was fitted to the lengths-at-age (.5 years, see
below) of P. oxygeneios, such that Lt = L1 (1 − e−k(t−t0 ) ), where
Lt is the predicted mean TL (mm) of fish at age t (years), L1 the
asymptotic mean length (mm), k the growth coefficient
(year21), t the estimated age (years), and t0 the hypothetical age
(years) at which fish would have zero length. The growth rates
of females and males were compared using a likelihood ratio test
(Cerrato, 1990).
Estimates of sexual maturity
Figure 2. Transverse section of the right sagitta of a male Polyprion
oxygeneios of 940 mm TL with 24 delineated opaque zones (white
circles). The scale (white bar) represents 1 mm.
under reflected light at a magnification of ×20 –40, without any
knowledge of the size of the fish to which they belonged. The
margin of each otolith was categorized as either thin translucent
(i.e. ,50% of the width of the preceding translucent zone),
wide translucent (i.e. .50% of the width of the preceding translucent zone), or opaque. The proportions of each category of the
otolith margin in each calendar month were pooled across years
and used to determine the timing of opaque- and translucent-zone
deposition in the otoliths.
The primary reader (CBW) examined each otolith, and a secondary reader (SJN) examined a subsample of the otoliths on
one occasion (n ¼ 104). To establish the level of confidence that
can be placed in the interpretation of the otolith structure, the precision of counts between readers was assessed. The counts from
both readers were compared, and the precision of age estimates
calculated using the Index Average Percent Error (IAPE;
Beamish and Fournier, 1981).
The age of each P. oxygeneios was estimated using a combination of the birthdate, the time of year when the opaque zones
on the otoliths of the majority of P. oxygeneios become delineated,
and the number of opaque zones. The birthdate of P. oxygeneios
was considered to be the period immediately before peak spawning
(1 June), as determined from the annual trends in GSIs and
gonadal development (derived from macroscopic staging).
The lengths and ages at which P. oxygeneios reached maturity were
determined using data for the months when spawning was evident,
i.e. when at least some fish contained developing and/or developed gonads (stages II– V; Table 1). The lengths at which 50%
of female and male P. oxygeneios attained sexual maturity (L50)
were determined using a reparameterized form of the logistic
equation (Punt and Kennedy, 1997; Hesp et al., 2004; Wakefield
et al., 2007) to describe the relationship with length of the probability that a fish during the spawning period possessed gonads
at stages II –V. Therefore, it was assumed that during the spawning
period, fish with gonads of those stages would have had the potential to spawn, were spawning, or had recently spawned, and that
fish with gonads at stage I would have remained immature. The
reparameterized logistic equation used was PL ¼ 1/{1 +
exp[ – loge(19)(L 2 L50)/(L95 2 L50)]}, where PL is the proportion
of mature P. oxygeneios at a length L, and L50 and L95 are the estimated lengths at which 50 and 95% of P. oxygeneios have attained
sexual maturity, respectively. Values of L50 and L95 and their 95%
confidence intervals (CIs) were determined by bootstrapping, with
replacement, of each dataset to generate 2000 estimates of the parameters of the logistic equation. The parameters and 95% CIs for
the reparameterized logistic equation were calculated as the
median and the upper and lower 2.5 percentiles of the 2000 bootstrap estimates of each length class. Estimates of sexual maturity
with respect to age were calculated using the same equation, but
with A50 and A95 substituted for L50 and L95, respectively.
Estimates of mortality
Estimates of the instantaneous rate of total mortality (Z) were calculated using catch-curve analysis of the age compositions of
P. oxygeneios in 2005 and 2006. The procedure involved fitting a
linear regression equation to the natural logarithm of the frequencies of fish from a minimum age 1 year older than the maximum
Age-based demography and reproduction of Polyprion oxygeneios from Western Australia
1167
frequency, and up to a maximum age before two consecutive frequencies of zero (Ricker, 1975; Kritzer et al., 2001). Values of Z and
95% CIs were determined by bootstrapping, with replacement, the
age composition for each year to generate 2000 estimates. Z and
the 95% CIs were calculated as the median and the upper and
lower 2.5 percentiles of the 2000 bootstrap estimates.
The instantaneous rate of natural mortality (M) was derived
using the general regression equation of Hoenig (1983) for fish,
where logeZ ¼ 1.46 2 1.01 logetmax (tmax is the maximum age in
years, noting that the Hoenig (1983) equation provides a reasonable approximation of M in demersal fish (Hart and Russ, 1996;
Newman et al., 1996, 2000). Reference points for the target
(Ftarget, optimal) and threshold (Fthreshold) rates of fishing mortality were calculated for P. oxygeneios using the estimate of M
where Ftarget ¼ 2/3 M and Fthreshold ¼ M (Patterson, 1992).
Fishing mortality, estimated by subtraction (from F ¼ Z 2 M)
was then compared with the estimates of Ftarget and Fthreshold.
Results
Reproduction
The reproductive development of gonads of P. oxygeneios ≥L50 at
maturity (estimates given below) from the south coast of Western
Australia displayed a similar annual trend for both females and
males. From October through April, almost all possessed gonads
at stage I (immature/resting; Figure 3). Gonadal recrudescence
commenced in both females and males in May, with the first
appearance of gonads at stages II and III. Ovaries contained
hydrated oocytes (stage IV) from July to September, whereas
testes were in spawning condition in June too. Gonadal regression
(stage V) was observed mainly in September in both sexes
(Figure 3).
The values of GSI for both female and male P. oxygeneios ≥L50
at maturity were highest in the months when developed (stage III)
or ripe/spawning (stage IV) gonads were present (Figure 4). GSI
values of female P. oxygeneios increased progressively from 1.09
(+0.13 s.e.) in May to 7.66 (+1.27 s.e.) in August, before declining to 2.37 (+0.51 s.e.) in September, and to 0.89 (+0.07 s.e.) by
October. The GSI values for male P. oxygeneios followed a similar
trend to those of females (Figure 4). Hence, the spawning period of
P. oxygeneios off the south coast of Western Australia is between
May and September, with the majority of spawning from June
to August (i.e. the austral winter).
Growth-zone periodicity in otoliths
Sectioned otoliths of P. oxygeneios with a wide translucent zone
(i.e. .50% of the width of the preceding translucent zone) at
their outer margin were almost exclusively in fish caught from
January to May (Figure 5). Sectioned otoliths with an opaque
zone on their outer margin first appeared in June and then in all
months until October. The percentage of sectioned otoliths with
an opaque outer margin increased from 9.9% in June to a
maximum of 12.9% in July, before gradually declining during
the following 3 months (Figure 5). The appearance of sectioned
otoliths with a thin translucent outer margin (i.e. ,50% of the
width of the preceding translucent zone) was also first found in
June (14.4%). The percentage of such otoliths gradually increased
to a maximum of 74.6% in November, 4 months after the
maximum frequency of those with an opaque outer margin,
before precipitously declining over the following 2 months.
Figure 3. Percentage frequency histograms of macroscopically staged
ovaries of Polyprion oxygeneios from the south coast of Western
Australia. Data are pooled for corresponding months (sample sizes
shown) and limited to fish ≥L50 for corresponding sexes.
The trends exhibited by the monthly percentage contributions
of each classification of the outer margin of sectioned otoliths
demonstrated that a single opaque zone is deposited annually in
the otoliths of P. oxygeneios, beginning in some fish in June and
completed in all fish by the end of October, i.e. from winter to
mid-spring. Therefore, counts of opaque zones from sectioned
otoliths can be used to age hapuku from the south coast of
Western Australia.
Precision of age estimates
The precision of opaque-zone counts for P. oxygeneios was high,
with an IAPE of 1.95% (n ¼ 104). There was good agreement
among counts across all age classes, with 69.2% of the two
counts being identical. No counts differed by more than two
rings (3.8% of repeat readings), with most counts differing by
only one ring (26.9% of repeat readings). As such, most otolith
sections were fair or good to read, with relatively minor and
1168
C. B. Wakefield et al.
inconsistent differences between sections. The high level of precision among independent otolith readings indicates that both
readers interpreted the otoliths of P. oxygeneios in a similar
manner.
Length and age compositions and growth
In all, 642 female and 698 male hapuku were collected. The length
frequency distributions of females paralleled that of males
(Figure 6), displaying essentially two modes, peaking at ca.
630 mm and, to a lesser extent, 850 mm TL. Females ranged in
TL from 535 to 1114 mm and in age from 1.7 to 35.1 years.
Males ranged in TL from 521 to 1004 mm and in age from 2.4
to 51.8 years (Figure 6). The mode representing larger fish, i.e.
850 mm TL, most likely reflects large numbers of fish .10
years old, which corresponds to the period when growth reaches
an asymptote (Figures 7 and 8). Females and males were represented in all age classes up to 37 years (Figure 6), and the age frequency distribution of females paralleled that of males, with most
fish being between 3 and 5 years old (Figure 6).
The growth rates of females and males varied throughout their
life. The mean length of females and males increased between ages
2 and 5 by up to 50 mm (Figure 7), at a growth rate of
15 mm year21. Between 5 and 9 years old, the growth rate of
both females and males increased to 40 mm year21, with an
increase in the mean length-at-age of 150 mm (Figure 7). This
Figure 4. Mean monthly GSIs +1 s.e. (lines) and the percentage
with gonads at stages III (white bars) and IV (grey bars) of female
(top) and male (bottom) Polyprion oxygeneios from the south coast
of Western Australia. The data have been pooled for corresponding
months (sample sizes shown) and are limited to fish ≥L50 for each
sex separately. On the x-axis, open rectangles represent winter and
summer, and black rectangles spring and autumn.
Figure 5. Monthly percentage contribution of three classifications
describing the outer margin of sectioned otoliths of Polyprion
oxygeneios (sample sizes are shown above each month). White bars,
thin translucent (,50% of the width of the preceding translucent
zone); grey bars, wide translucent (.50% of the width of the
preceding translucent zone); black bars, opaque.
Figure 6. Age (above) and length (below) frequency histograms for
female (white bars, above x-axis) and male (grey bars, below x-axis)
Polyprion oxygeneios from the south coast of Western Australia
collected from 2004 to 2008. The dashed lines represent the
corresponding ages and lengths-at-50%-maturity (A50 and L50) for
females and males.
Age-based demography and reproduction of Polyprion oxygeneios from Western Australia
1169
Figure 7. Mean length (+1 s.d.) and variance (lower panel) at age
for female (above) and male (middle) Polyprion oxygeneios from the
south coast of Western Australia.
period of fast growth also corresponded to the greatest levels of
variance in TL-at-age (Figure 7). Given that the IAPE between
the two readers was low, this greater variance is most likely a reflection of variable growth among fish over the period when the
species matures (Figures 9 and 10). The growth of females and
males then slowed substantially at ages ≥10 years to rates of
4 mm year21, for both sexes.
The von Bertalanffy growth curves were fitted separately to
females and males at ages ≥5 years (Table 2), because this was considered to be the youngest age at which most fish had undergone
their transition from pelagic to deep benthic environments
(Figure 6). Likelihood-ratio tests showed that growth between
the two sexes was significantly different (p , 0.05), with females
attaining a slightly larger size than males (Figure 8). However,
the differences in the estimated lengths of females and males
from the von Bertalanffy growth equations were always ,5%
(,40 mm), and similarly the mean lengths at corresponding
ages of females and males was always ≤5% (Table 3). Hence, the
statistical difference in growth between females and males
should be considered to be of limited biological significance. As
a consequence, the von Bertalanffy growth curve was fitted to
length-at-age data for females and males combined (Table 2).
Length- and age-at-maturity
The smallest mature female and male (i.e. with gonads of stages
II –V) caught during the spawning period was 744 and 638 mm
Figure 8. Age –length relationship of female (solid line) and male
(dashed line) Polyprion oxygeneios from the south coast of Western
Australia, fitted with von Bertalanffy growth curves (fish ≥5 years).
TL, respectively. The prevalence of mature females increased
from 33% in the 700–749 mm TL class to 70% in the 750–
799 mm TL class, and to .90% in all length classes .800 mm
TL (Figure 9). The prevalence of mature males followed a
similar trend, but at a slightly smaller size; whereby 2% were
mature in the 600–649-mm TL class, increasing to .94% in all
length classes .700 mm TL (Figure 9). The maturation of
females at a larger size than males was also reflected in the estimates at which 50% of fish attained maturity (L50), i.e. 760 and
702 mm TL, respectively (Figure 9).
The youngest mature female and male caught during the
spawning period was 4 years old. The prevalence of mature
females and males in sequential age classes was similar, increasing
from 5% in fish 4+ years old to 100% in all fish .10 years
(Figure 10). Likewise, the estimated ages at which 50% of
females and males attained maturity, i.e. A50, were similar, at 7.1
and 6.8 years.
Mortality estimates
The estimates of Z were slightly higher in 2005 than in 2006, i.e.
0.14 year21 (95% CI of 0.13 and 0.17) vs. 0.10 year21 (95% CI
of 0.09 and 0.12; Figure 11). The estimate of M derived from
Hoenig’s (1983) equation based on the maximum recorded age
for P. oxygeneios from this study was 0.09 year21. F was estimated
1170
C. B. Wakefield et al.
Figure 9. Percentage frequency of gonads of immature (stage I,
white bars) and mature (stages II –V, grey bars) Polyprion oxygeneios
in sequential 50 mm length classes (sample sizes noted above) of
females (above) and males (below) collected during the spawning
season (June– September) from the south coast of Western Australia.
Lines represent the expected percentage of mature fish and the 95%
CI determined from logistic regression analysis.
to be 0.01 –0.05, Ftarget to be 0.06 (i.e. 2/3 M ), and Fthreshold to be
0.09 (i.e. M ). These results indicate that current harvest rates are
between 1 and 5% of the available stock of P. oxygeneios, that
just 6% can be harvested annually in a sustainable manner, and
that such rates should not exceed 9% of the stock size.
Discussion
Reproduction
There was no evidence of sexual dimorphism or dichromatism in
P. oxygeneios. The length and age compositions of female and male
fish were similar throughout life. The sex ratio was equivalent and
gonads were easily sexed macroscopically, at all ages. These traits
Figure 10. Percentage frequency of gonads of immature (stage I,
white bars) and mature (stages II –V, grey bars) Polyprion oxygeneios
in each age class (sample sizes noted above) of females (above) and
males (below) collected during the spawning season (June–
September) from the south coast of Western Australia. Lines
represent the expected percentage of mature individuals and the
95% CI determined from logistic regression analysis.
provide evidence that, in accordance with the conclusions of
Roberts (1989), P. oxygeneios is gonochoristic. This is the same
mode of reproduction recorded for the congeneric P. americanus
(Roberts, 1989; Peres and Klippel, 2003).
The trends exhibited by macroscopically staged gonads and
GSIs of females and males demonstrated that P. oxygeneios underwent gonadal recrudescence in May, spawned over 3 months
during the austral winter, i.e. June–August, and had completed
gonadal regression by the end of September. This spawning
period is the same as that recorded for P. oxygeneios in
New Zealand, at similar latitudes to this study (Beentjes and
Francis, 1999). The spawning period of the sympatric P. americanus
in the southern hemisphere is similar to that of P. oxygeneios (Peres
Table 2. von Bertalanffy growth parameters (≥5 years, upper and lower 95% CI) for females and males of Polyprion oxygeneios.
Sex
Female
Estimate
Upper
Lower
Male
Estimate
Upper
Lower
Sexes combined
Estimate
Upper
Lower
L1 (mm, TL)
k (year21)
t0 (year)
n (≥ 5 years)
905
922
887
0.23
0.29
0.18
20.20
1.08
21.48
361
877
890
864
0.22
0.26
0.18
20.47
0.55
21.48
890
900
880
0.24
0.27
0.20
20.63
0.71
20.84
n (all years)
Amax
TLmax
630
35.1
1114
399
691
51.8
1004
783
1 352
–
–
L1, hypothetical mean asymptotic length at an infinite age; TL, total length; k, growth coefficient; t0, hypothetical age at zero; n, sample size; Amax, maximum
recorded age; TLmax, maximum recorded total length.
1 517
0.08
23.73
–
1 638
0.03
216.56
–
Stereolepis gigas (Ayres 1859)
Data deficient
Stereolepis doederleini (Lindberg and Krasyukova 1969)
Data deficient
–
0.14H
–
ca. 0.2 –0.8C
31
39
TS
TS
North Atlantic
Northwest Atlantic (Blake Plateau)
Peres and Haimovici
(2004)
Sedberry et al. (1999)
Vaughan et al. 2001
Southwest Atlantic
TS
–
–
76
This study
Francis et al. (1999)
Pavez and Oyarzún (1985)
South coast, Western Australia
New Zealand
South Pacific (Juan Fernández
Archipelago)
TS
TS
?
?
52
63
?
?
Reference
0.01 –0.05
–
–
–
and Klippel, 2003; CBW, unpublished data). The spawning
locations of these Polyprion species have yet to be defined
(Beentjes and Francis, 1999), but extensive mass movements of
both species have been recorded at times relative to the commencement and completion of their spawning period, with suggestions of
the formation of spawning aggregations (Beentjes and Francis,
1999; Peres and Klippel, 2003).
To date, the length- and age-at-maturity of P. oxygeneios have
been poorly documented, so this study represents the first comprehensive description of maturity for the species. Female hapuku
matured at a slightly larger size than males, i.e. L50 values of 760
and 702 mm TL, respectively. The ages at which 50% of females
and males attained maturity were essentially similar, at around
their seventh year, suggesting that the differences in L50 are due
to small variations in growth between sexes. All P. oxygeneios ,4
years old were immature and, conversely, all individuals ≥10
years old were mature. Information on the length- and
age-at-maturity of P. oxygeneios from other populations was not
available. However, the lengths and ages at which 50% of female
and male P. americanus attain maturity in Brazil were similar, at
779 mm TL and 10.4 years for females and 749 mm TL and
9 years for males (Peres and Klippel, 2003). Similarly, Sedberry
0.09H
0.07H
0.13P
0.16P
Figure 11. Age frequency histograms (grey bars) and linear
regression (+95% CI, lines) fitted to the natural logarithms of the
age frequencies (white circles) of Polyprion oxygeneios in 2005
(above) and 2006 (below). Estimates of total mortality (Z), the
correlation coefficients (r 2), and the sample sizes are given.
Locality
Differ. (%)
2.7
3.5
3.4
3.2
3.1
Method of
ageing
M
614
789
848
867
874
Longevity
(years)
F
631
818
878
896
902
F
Differ. (%)
1.7
5.0
4.7
2.8
1.5
M
M
626
776
818
875
890
L1
k
t0
(mm)
(year21)
(year)
Z
Polyprion oxygeneios (Schneider and Forster 1801)
890
0.24
20.63
0.10 –0.14A
1 317
0.07
24.63
–
C
1 761
0.09
0.17
–
F
1 449
0.10
0.06
–
Polyprion americanus (Bloch and Schneider 1801)
1 210
0.06
26.30
–
F
637
817
858
900
877
Mortality parameters
Age (years)
5
10
15
20
25
Estimated TL (vB)
Growth parameters
Mean TL
Table 4. Comparison of growth parameters of the von Bertalanffy growth function (L1, k, t0), mortality parameters (Z, M, F), longevity, and the method of growth determination (otoliths:
TS, transverse section) for species of the Polyprionidae.
Table 3. Differences (%) in the mean and estimated (from von
Bertalanffy parameters) TLs between female and male Polyprion
oxygeneios at sequential 5-year age classes.
A, estimates of total mortality Z, derived from age composition data; H, estimates of natural mortality such that Z ¼ M for an unfished stock, derived from the equation of Hoenig (1983); C, estimates of total
mortality Z, derived from catch data; P, estimates of natural mortality M, derived from the equation of Pauly (1980).
1171
Age-based demography and reproduction of Polyprion oxygeneios from Western Australia
1172
et al. (1999) reported that P. americanus from the Blake Plateau in
the western South Pacific were mature by 8 years.
Although hapuku mainly inhabit continental slope areas, the
depth range does overlap with that of shelf and deeper-water habitats (.500 m; Koslow et al., 2000). Despite this, the onset of
maturity in P. oxygeneios closely mirrors that of shelf-distributed
species from the south coast of Western Australia, e.g. 5 –9 years
for Pagrus auratus and Nemadactylus valenciennesi (Coulson
et al., 2005; Wakefield, 2006), compared with deeper-water
species, e.g. 22 –40 years for Hoplostethus atlanticus (Francis and
Horn, 1997; Branch, 2001). Immature P. oxygeneios are only susceptible to benthic fishing for a relatively short period between
settlement and maturity, because settlement is delayed by up to
4 years and most fish are mature after 7 years of age. As such
there is a degree of resilience in the species conveyed by 4-year
period before settlement. Exploitation of this pre-settlement
phase by pelagic fisheries (e.g. purse-seine or longline) could negatively impact population viability (Sedberry et al., 1999).
Age, growth, and settlement
The precision of opaque-zone counts from sectioned otoliths of
P. oxygeneios between two readers was high, i.e. IAPE 1.95%,
though noting that only a small number of otoliths were examined
by both readers (n ¼ 104). Earlier studies have mentioned difficulties in the readability of growth zones in sectioned otoliths of P.
americanus and P. oxygeneios, which resulted in high ageing
error between readers (Francis et al., 1999; Sedberry et al., 1999).
Improved clarity of growth zones in otoliths of P. americanus
was achieved through thinner sections (Peres and Haimovici,
2004). Hence, thin sections are recommended for this genus to
improve readability and precision.
Opaque zones were deposited in the otoliths of P. oxygeneios
annually between June and October, coinciding with both the
coolest annual water temperatures and the spawning period for
this species on the south coast of Western Australia. This period
of opaque-zone deposition was similar to that determined using
oxytetracycline injections in P. oxygeneios in New Zealand
(Francis et al., 1999). A single opaque zone is also deposited
annually in the otoliths of P. americanus, but with different seasonality from that of P. oxygeneios, such that opaque zones were
deposited in April in otoliths of fish in the North Atlantic
(Sedberry et al., 1999) and from September to February in fish
from the Southwest Atlantic (Peres and Haimovici, 2004). In contrast to P. oxygeneios, the deposition of an opaque zone in the otoliths of P. americanus from the Southwest Atlantic did not coincide
with either the coolest annual water temperatures or their spawning period (Peres and Klippel, 2003). Therefore, the factors influencing opaque-zone formation in otoliths of Polyprion spp. remain
uncertain, or at least inconsistent.
High levels of precision in age estimation combined with relatively large sample sizes of P. oxygeneios over a wide age range
allowed the growth rates to be estimated from mean length-at-age
data during different periods of the life history. Although there
were no pelagic juveniles included in this study, the smallest fish
collected (,630 mm TL) were considered early post-settled,
allowing growth rates of pelagic juveniles to be extrapolated accordingly. The estimated growth rate during the pelagic life stage of
P. oxygeneios was high at 150–300 mm TL year21. Immediately
post-settlement and between the ages of 2 and 5 years, growth of
P. oxygeneios slowed substantially to 15 mm year21. The transition undertaken by Polyprion spp. from pelagic to benthic
C. B. Wakefield et al.
environments at such a size and age is associated with considerable
changes in their functional morphology, e.g. colour change, larger
eyes, and rounder body shape, which consequently suppressed
growth (in relation to length) in these early post-settled fish
(Machias et al., 2003). This would explain the slower growth rates
recorded for P. oxygeneios at ages that corresponded to settlement
(Roberts, 1996; Francis et al., 1999).
Following the transition associated with settlement and
between the ages of 5 and 9 years, which corresponds to the
period when the species matures, the growth rate increased to
40 mm year21. The growth rate then slowed to 4 mm year21,
when all fish were mature (i.e. .10 years). Given that abundances
of P. oxygeneios declined in sequential age classes .4 years, the
mortality rates were greater than settlement rates thereafter.
These estimates of the age and length at which P. oxygeneios
settles are consistent with those previously described for the
species in New Zealand (Roberts, 1996; Francis et al., 1999).
Estimates of the age of pelagic juveniles of the congeneric P. americanus have not been determined.
Previous studies using von Bertalanffy equations to describe the
growth of P. oxygeneios and P. americanus have calculated exceptionally large values of t0, i.e. 23.73 to 216.56 years (Table 4; Francis
et al., 1999; Sedberry et al., 1999; Vaughan et al., 2001; Peres and
Haimovici, 2004). As the growth of hapuku is suppressed during
settlement and settlement is likely to be size-dependent (Machias
et al., 2003), resulting in the selection of faster-growing fish from
demersal samples between these ages, the von Bertalanffy growth
curves were limited to age and length data for fish ≥5 years. This
resulted in improved estimates of t0 of between 20.20 and 20.47
years for females and males, respectively. Nevertheless, estimates
of L1 between populations are still comparable. The estimate of
L1 for combined sexes of P. oxygeneios from Western Australia
was markedly less than that for the same species in New Zealand
and the South Pacific (Pavez and Oyarzún, 1985; Francis et al.,
1999) and for P. americanus (Table 4).
Management implications
This study has only included an assessment of P. oxygeneios from
the south coast of Western Australia, but notes that the species
is also found on the lower west coast of Western Australia.
The connectivity between stocks from the two areas is unknown.
The value of M was estimated to be 0.09 (based on a maximum
age of 52 years from this study; Hoenig, 1983), and that of F to
be very low (in the range 0.01 –0.05). The fact that the estimated
F was ,Ftarget (0.06) and Fthreshold (0.09) suggests that harvest
rates in 2005 and 2006 were within sustainable limits. However,
if we were to consider the longevity of the species based on a
maximum age of 63 years from New Zealand (Francis et al.,
1999), then M would be 0.07 (Hoenig, 1983) and the range of F
(0.03–0.07) from the south coast of Western Australia would
exceed Ftarget (0.05) and equal Fthreshold (0.07). If further sampling
in Western Australia finds hapuku of ages approaching the
maximum recorded in New Zealand then it is very likely that
the harvest rates were already at sustainable limits in 2005 and
2006. These low values of M demonstrate the high inherent vulnerability of the species, at relatively low annual harvest rates, i.e. F ≤
0.07. Therefore, there is still uncertainty about the status of the
resource. Currently, both State and Commonwealth fisheries
harvest the species, and they are highly likely to be exploiting
the same stock, i.e. it is a straddling stock. Currently, the south
coast remains the only open access wetline (hook and line)
Age-based demography and reproduction of Polyprion oxygeneios from Western Australia
State-managed fishery in Western Australia, with catches averaging
19.3 t annually between 1986 and 2008. Hapuku are also taken in
small numbers (,150 kg annually since 2000) in the gillnet
catches of the West Coast Demersal Gillnet and Demersal
Longline Fishery and Joint Authority Southern Demersal Gillnet
and Demersal Longline Fishery, which targets sharks (derived
from compulsory catch statistics provided by commercial
fishers to the Department of Fisheries, Government of Western
Australia). Moreover, the Commonwealth-based Great
Australian Bight Trawl Fishery has also reported catches of
hapuku (1–14 t annually between 1995 and 2002; Lynch and
Garvey, 2003). Therefore, based on the current assessment, effort
levels should be constrained to at most 2005 and 2006 levels for
both State and Commonwealth fisheries, as a precautionary
approach to sustainable management of the species on the lower
west and south coasts of Western Australia.
Studies of the population structure of P. americanus using
genetic analyses (mtDNA and microsatellite) revealed three distinct stocks including the North Atlantic and Mediterranean,
Brazil, and South Pacific (Sedberry et al., 1996; Ball et al., 2000).
Unfortunately, populations in the southern Indian Ocean, e.g.
southern Africa and Western Australia, were not examined.
Pan-oceanic connectivity is considered to be facilitated by a
strong association with floating objects during their long pelagic
life stage (Roberts, 1996; Massuti et al., 1999; Riera et al., 1999),
assuming that their association with such objects is constant
over long periods. Given that hapuku also have an extended
pelagic juvenile stage that is strongly associated with floating
objects (Roberts, 1996) and that the two species are sympatric, it
is very likely that P. oxygeneios also undertakes pan-oceanic
mixing between populations. Therefore, the sustainable management of the Western Australian population of P. oxygeneios
would benefit from improved understanding of the contribution
of pan-oceanic recruitment between southern hemisphere populations, particularly those in the southern Indian Ocean, and the
fisheries that exploit them.
Acknowledgements
We thank the numerous managers and processors from Albany
Bait Producers, formerly Collie Seafoods, in Albany, and South
East Fisheries in Esperance, Western Australia, who were an
integral part of the collection of specimens. Thanks are also due
to Helen Mee and Sarah Cutler for sectioning the otoliths and
to Rod Lenanton for his constructive comments on an earlier
version of the manuscript. Logistic support for the study was provided by the Department of Fisheries, Western Australia.
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