Spatial variability in the distribution, abundance, growth, mortality and age structures of tropical snappers (Pisces: Lutjanidae) in the Central Great Barrier Reef, Australia

Newman, Stephen John (1995) Spatial variability in the distribution, abundance, growth, mortality and age structures of tropical snappers (Pisces: Lutjanidae) in the Central Great Barrier Reef, Australia. PhD thesis, James Cook University.

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Abstract

The aim of this thesis was to determine the distribution and abundance of the three major families of fishes of commercial and recreational importance in the central Great Barrier Reef (Lutjanidae, Lethrinidae and Serranidae), with particular emphasis on the snappers (Lutjanidae), and to examine the age structures, growth rates and mortality rates of two snappers, Lutjanus adetii and L. quinquelineatus. Data were collected on distribution and abundance at four spatial scales : cross-continental shelf, among individual reefs, among reef zones within reefs and with depth; and two temporal scales : night versus day and two-monthly intervals over a 12 month period.

Fish traps were used to quantify the distribution and abundance of the Lutjanidae and Lethrinidae on reefs across the central Great Barrier Reef. The assemblages of fishes on inshore reefs were distinctive from those on midshelf and outershelf reefs. There were significantly fewer individuals of the Lutjanidae and Lethrinidae inshore. All species examined displayed significant cross shelf changes in abundance. This variation in abundance was due to an absence or low abundance of individuals at one or more cross shelf locations. Possible causes of these patterns are discussed. The genera Aprion, Lutjanus, Macolor, Symphorichthys, Symphorus, Gnathodentex, Gymnocranius, Lethrinus and Monotaxis were all characteristic of the shallow shelf waters less than 100m. In contrast, species of the genera Paracaesio, Pristipomoides and Wattsia were characteristic of the intermediate depths (100-200m) and the deeper outer reef slope waters in excess of 200m were characterised by species of the genus Etelis.

Visual censuses were used to quantify the distribution and abundance of the Lutjanidae, Lethrinidae and Serranidae in three reef zones (windward reef slope, lagoon, leeward back reef slopes) of three reefs on the mid-shelf and three reefs on the outer continental shelf in the central region of the Great Barrier Reef. The assemblages of species of the Lutjanidae, Lethrinidae and Serranidae displayed distinct patterns of distribution and abundance within the shallow waters of the central GBR at three spatial scales : between locations (outershelf and midshelf communities); between reefs (high abundance vs. low abundance reefs); and within reefs (characteristic communities within zones on individual reefs). Significant spatial variability was identified in the abundances of many species and species of the Lutjanidae, Lethrinidae and Serranidae were found to occur in assemblages which were characteristic of major zones (windward reef slopes, lagoons and leeward back reef slopes) and this pattern was consistent within and among shelf locations. Location on the continental shelf accounted for a high proportion of the variation in community structure. The Lutjanidae and Serranidae were more abundant on the midshelf while the Lethrinidae (in particular Gnathodentex aurolineatus and Monotaxis grandoculis) were more abundant on the outershelf reefs. Additionally, a large proportion of the species recorded were relatively rare within a given zone, reef or location. Care will need to be taken in determining the appropriate spatial scales of sampling in any future experiments (eg. manipulations of fishing pressure) to ensure that the effects of smaller scale spatial differences are not confounded when larger scale comparisons are made.

Spatial and temporal patterns in the distribution and abundance of the Lutjanidae and Lethrinidae were examined among midshelf reefs using a combination of visual censuses and fish traps. Visual censuses were used to survey the shallow water assemblages, while fish traps were used to survey deeper water assemblages below diveable depths. The assemblages of the Lutjanidae and Lethrinidae were found to vary significantly spatially among reefs and between depths and also between diel sampling periods. Significant among reef differences were observed in the shallow water assemblages for both the Lutjanidae and Lethrinidae. The deeper water assemblages of the Lutjanidae and Lethrinidae, sampled by traps varied more between depths and between diel sampling periods than among reefs or over the 12 month sampling period. Lutjanus carponotatus, L. fulviflamma and Lethrinus miniatus were significantly more abundant in the shallow set traps (12-18m), whereas Lutjanus adetii, L. russelli, L. sebae, L. vitta, Gymnocranius audleyi, Lethrinus sp.2 and Abalistes stellaris were all significantly more abundant in the deep set traps (30-40m). Additionally, Lutjanus adetii, L. fulviflamma, L. quinquelineatus, L. russelli, L. sebae, L. vitta and Lethrinus miniatus were all significantly more abundant in night set traps. In contrast, Lethrinus sp. 2, Abalistes stellaris and Plectropomus leopardus were all significantly more abundant in day set traps. The diel variability in trap catches is consistent with what is known of the feeding behaviour of the species examined. There was seasonal variability in mean species abundance in both visual census counts and trap catch data but significant trends were identified for only two species, Lethrinus obsoletus and Gymnocranius audleyi. Peaks in mean species abundance occurred generally in the June through September sampling periods.

The age and growth of Lutjanus adetii and L. quinquelineatus from the central Great Barrier Reef were determined from studies of annuli in sectioned otoliths (sagittae). The periodicity of formation of the annuli (ages) was validated through a field study involving oxytetracycline labelling of tagged fishes. Validation was obtained from tagged fishes which were recaptured after 12 months or more at liberty. This is the first time that the direct validation of ages has been achieved in 'Lutjanus species. A single opaque and translucent zone (viewed under transmitted light) was found to be formed once per year with the opaque band (annulus) formed during the winter months, May - August. Otolith (sagittae) eight was found to be strongly correlated with the age of individuals of both species. There was significant differential growth between the sexes in observed length-at-age and weight-at-age for both L. adetii and L. quinquelineatus. Males were larger than females in both species. The von Bertalanffy growth functions were as follows:

L. adetii (m) : Lt= 269.1 ( 1 - e^-0.165(^t + 6.12) ) L. adetii (f) : Lt= 315.1 ( 1 - e ^0.029(^t + 40.29) ) L. quinquelineatus (m):Lt=214.5 (1 - e^0.2599(^t + 3.427) ) L. quinquelineatus(f): Lt=204.3 (1 - e^0.1664(^t + 7.552) )

The oldest individuals examined were a male L. adetii 24 years of age and a female L. quinquelineatus 31 years of age. The shape of the growth curve of both of these lutjanid species was initially quite steep over the first few years and then became essentially asymptotic. This form of asymptotic growth curve suggests that curves describing natural mortality as a function of age will also be initially steep (high mortality) and then flatten substantially over the asymptotic growth period (low mortality) before increasing again with senility. This is in contrast to most classical fisheries models which assume that natural mortality (M) is constant and low over a wide range of ages. The annual instantaneous rate of total mortality (Z) was estimated' to be 0.300 for L. adetii, representing an annual survivorship of approximately 74%. The annual instantaneous rate of total mortality for L. quinquelineatus was estimated to be 0.154, representing an annual survivorship of approximately 86%. Regression methods used to produce estimates natural mortality rates such as those of Pauly (1980) and Ralston (1987) were found to produce overestimates of natural mortality for these long lived species and hence underestimate survivorship. Regression methods used to produce estimates of total and natural mortality rates such as those described by Pauly (1980), Hoenig (1983) and Ralston (1987) should be applied with caution. The slow growth, protracted longevity and low natural mortality rates imply that both L. adetii and L. quinquelineatus are vulnerable to overfishing despite their small size.

There was significant variability in the growth, mortality and age structures of L. adetii and L. quinquelineatus at the spatial scale of individual reefs. Significant differences in the mean length, age and weight of both species were observed among reefs independent of the sex of the fish. There were also significant differences in observed weight-at-length among reefs for both species. The age structures of both L.adetii and L. quinquelineatus were also significantly different among reefs. Peaks in abundance of year classes were variable from reef to reef. Comparisons of the von Bertalanffy growth curves indicated that the pattern of growth in individuals of L. quinquelineatus was significantly different among reefs, while the pattern of growth in individuals of L. adetii was not significantly different among reefs. However, there were no significant differences in the mean length of the early age classes of either species among reefs. The mortality rates and hence survivorship of both L. adetii and L. quinquelineatus among reefs were highly variable. It is hypothesised that the varying age structures and mortality rates of both of these species at the spatial scale of individual coral reefs is determined by the non-equilibrial balance of variable recruitment interacting with density independent mortality. Hence the effect of good recruitment years may persist in the age structure of populations over time.

Item ID: 27274
Item Type: Thesis (PhD)
Keywords: tropical snappers; Pisces: Lutjanidae; Central Great Barrier Reef; GBR; distribution; abundance; growth rates; age structures; spatial variability; estimated survivorship
Date Deposited: 20 Jun 2013 07:11
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 33%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050299 Environmental Science and Management not elsewhere classified @ 33%
SEO Codes: 96 ENVIRONMENT > 9605 Ecosystem Assessment and Management > 960507 Ecosystem Assessment and Management of Marine Environments @ 51%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 49%
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