Hoedt, Frank Edward (1994) A comparative study of the habitats, growth and reproduction of eight species of tropical anchovy from Cleveland and Bowling Green Bays, North Queensland. PhD thesis, James Cook University.

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Abstract

In this thesis the habitat preferences, growth and reproductive biology were examined for eight species of tropical anchovies from Townsville, North Queensland. The species examined were: Encrasicholina devisi; Stolephorus insularis; Stolephorus carpentariae; Stolephorus nelsoni; Stolephorus commersonii; Thryssa aestuaria; Thryssa setirostris and Thryssa hamiltoni. The study species grew to a wide range of maximum sizes (encompassing most of the size-range found amongst tropical clupeoid species), and therefore provided an opportunity to undertake a detailed comparative study of biological parameters and life-histories.

Samples were collected at three nearshore tropical habitats; mangroves, beaches and subtidal waters at 3-12m depth. Beach seines and gill nets were used to sample the first two habitats and an otter trawl at the subtidal habitat. The age-specific habitat preferences of each species were qualitatively investigated by comparing the catches of juveniles and adults in each habitat. In the subtidal waters of Cleveland Bay, anchovy were most common at depths of less than 7m. Juveniles of all species were common in shallow waters along beaches and the juveniles of some species were also common in mangrove estuaries. Age-related habitat differences were found to occur in Stolephorus commersonii, Thryssa aestuaria, Thryssa setirostris and Thryssa hamiltoni. In these species juveniles predominantly occurred in shallow nearshore habitats (beaches and mangroves) and adults occurred in the subtidal habitat at depths to 12m. S. nelsoni, E. devisi and S. insularis occurred in shallow water off beaches and in shallow subtidal waters throughout their life-cycle. S. carpentariae was almost exclusively caught along beaches and in mangroves (depths less than 3m) at all life stages.

Seasonal changes in habitat were exhibited by species in the present study. Adult E. devisi and S. insularis occurred in shallow waters off beaches in the autumn and winter months but were rare in summer. This suggests that these species move to deeper water in summer. Catches of all species in subtidal trawl samples in summer were also lower, suggesting that a change in distribution occurs at this time, possibly movement to deeper water.

Growth was estimated in the study species using three methods: counting primary increments in the sagittal otolith; analysing length-frequency data; and in Thryssa spp. from counts of seasonal growth rings in the sagittal otolith. Prior to using primary otolith increments to age the study species, experiments were conducted to determine the periodicity of growth increment formation. The study species proved extremely sensitive to handling. However, evidence of daily periodicity of growth increment formation was obtained for four species: E. devisi; S. carpentariae; S. nelsoni and T. aestuaria. Evidence was also given that the seasonal growth rings in sagitta from Thryssa spp. were deposited annually.

Growth curves (length versus. age) were constructed for each species from primary otolith increment counts. Length versus age plots were linear in E. devisi and S. insularis while von Bertalanffy growth curves were fitted to this data for the remaining species. These findings suggest that growth in very small stolephorid anchovies may not conform to the conventional von Bertalanffy growth curve. Consequently, some standard techniques for analysing length-frequency data which are based on von Bertalanffy growth parameters should be employed with caution on small tropical anchovies. This study of primary otolith increments indicated that the longevities of the stolephorid anchovies ranged from several months to just over one year. These structures also indicate a 1-2 year lifespan for Thryssa aestuaria. The larger species of Thryssa could not be aged beyond a certain size with confidence using these structures.

Length versus age plots from primary otolith increment counts were linear for juvenile-sized fish in all of the study species. A comparison of the linear growth rates in the study species showed that growth rate was directly related to the maximum size attained by a species. This indicates that larger species of tropical anchovy have faster growth rates than small species. Primary otolith increment age data was also used to compare the patterns of growth in weight (plots of weight versus age) between the study species. These were found to exhibit one of two patterns of exponential growth in weight. The larger species, S. commersonii, T. setirostris and T. hamiltoni had a common faster rate of weight-growth than S. insularis, S. carpentariae S. nelsoni and T. aestuaria. The weight-age data in these smaller species followed a similar exponential pattern with a slower rate of increase. The similarity in weight-growth for groups of species show that growth can be conservative at an inter-species level in tropical anchovies. Furthermore, the difference in weight-growth between small and large species suggests that weight-growth may be another distinguishing feature of the life-history strategies of small and large tropical clupeoids (Lewis 1990).

An important outcome of the analyses of primary otolith increments and seasonal growth rings in this study was that the von Bertalanffy growth curve suitably described the length-age relationship for the three species of Thryssa. Growth in length in larger tropical anchovies therefore follows a pattern which is common to most other marine fishes. In the stolephorid anchovies, length-age plots did not exhibit a marked asymptote and consequently, a modified form of the equation had to be used. This finding indicates that length-frequency analysis methods that are based on the von Bertalanffy growth function may be more suited to large rather than small species of tropical anchovy. Growth parameters obtained from otoliths compared favourably with those from length-frequency analyses in the genus Thryssa, further confirming this view.

Counts of seasonal growth rings in otoliths gave estimates of longevity for the genus Thryssa. The oldest T. hamiltoni individual was 4 years of age, and the oldest individuals of T setirostris and T. aestuaria were 2 and 1 years respectively. For T. hamiltoni, von Bertalanffy growth curves fitted to plots of length against age from seasonal growth rings indicated growth differences between male and female fish; females grew faster and larger than males. Length-frequency data were analysed for all of the study species. Two methods of analysis were employed these being the estimation of growth rates in juveniles from modal progressions, and the estimation of growth parameters using the ELEFAN computer software. Length-frequency histograms indicated that growth differed between males and females in all Thryssa spp., supporting the findings of otolith-ageing. Growth rates estimated visually for juvenile fish from modal progressions were consistent with those calculated from primary otolith increments for several species. This indicates that modal progressions can provide meaningful growth estimates in tropical anchovies. However, this analysis indicated that the residency-time for cohorts (or modes in length-frequency histograms) in populations of small species is short highlighting the importance of obtaining frequent and representative samples to allow analysis of modal progressions.

Von Bertalanffy growth parameters calculated from otoliths and ELEFAN analyses compared favourably in S. nelsoni and the three species of Thryssa indicating that these techniques give meaningful age estimates for larger clupeoids. Values of K computed from otolith-derived length-age data were generally higher than those computed by ELEFAN for the remaining stolephorid anchovies and this supports recent literature in the view that ELEFAN may yield biased growth estimates for small tropical clupeoids.

The following reproductive parameters were estimated for the study species: spawning season; length and age-at-maturity; fecundity and spawning frequency. Two types of spawning seasonality were observed. E. devisi and S. insularis spawned over most of the year while the remaining species spawned during the warmer part of the year between September and January. Length-at-maturity occurred at around 70% of maximum length in all species suggesting that this ratio may be conservative in tropical anchovies. Ages-at-maturity varied from several months in small species to around one year in larger species. Fecundity for all species combined was related to both length (exponential curve) and weight (linear regression), indicating that the relationship between these parameters is conservative at an inter-species level in this group. Spawning frequency was estimated as three days for S. nelsoni and between 2 and 5 days in T. hamiltoni. The pattern of oocyte-diameter frequencies differed between stolephorid anchovies and Thryssa spp. indicating possible differences in spawning biology.

In the present study it was shown that fecundity and growth rate were related to maximum species-size at an inter-species level. Furthermore, longevity was shown to be directly related to maximum species-size for a wide range of tropical clupeoids. These findings indicate that it may be possible to estimate certain biological parameters based on maximum species size for tropical clupeoid species. Another finding of this study was that growth and reproductive parameters in large tropical clupeoids were comparable to those in some short-lived temperate species. Beverton's (1963) ratio of 1/KTmax was also similar in some large tropical clupeoids to that in temperate anchovies. Following Beverton's (1963) reasoning, the similarity in this ratio in these two groups suggests that these may exhibit similar population and fishery dynamics.

Item ID:	24109
Item Type:	Thesis (PhD)
Keywords:	Australian Institute of Marine Science; AIMS; anchovies; Cape Bowling Green Spit; Chunda Flats; Encrasicholina; growth; habitat selection; Houghton River; otoliths; Pallarenda; reproduction; Ross River; seasonal variation; Stolephorus; Strand; Thryssa; Townsville
Date Deposited:	13 Dec 2012 13:14
FoR Codes:	06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50% 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060803 Animal Developmental and Reproductive Biology @ 50%
SEO Codes:	96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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