Molecular tools for studying puberty in the grey mullet, Mugil cephalus: cloning of cDNAs that regulate reproductive function

The cDNAs coding for the brain GnRHs (AY373449-51), pituitary GH, SL and PRL, and liver IGFs (AY427954-5) were isolated. Partial cDNA sequences of the brain (Cyp19b) and gonadal (Cyp19a) aromatases have also been obtained. These tools would be utilized to study the endocrine regulation of puberty in the grey mullet.

Temporal expression of G-protein-coupled receptor 54 (GPR54), gonadotropin-releasing hormones (GnRH), and dopamine receptor D2 (drd2) in pubertal female grey mullet, Mugil cephalus.

The G-protein-coupled receptor 54 (muGPR54) cDNA was cloned from the brain of the grey mullet, and its expression level, as well as those of the gonadotropin-releasing hormones (GnRH1, GnRH2, GnRH3) and dopamine receptor D2 (drd2), in the brain, pituitary and ovary of pubertal fish (early, intermediate, advanced) were determined by real-time quantitative RT-PCR (QPCR). The muGPR54 cDNA has an open reading frame of 1140 bp with a predicted 380 amino acid peptide, containing seven putative transmembrane domains and putative N-glycosylation and protein kinase C phosphorylation sites. QPCR results showed that the early stage of puberty in grey mullet is characterized by significantly high levels of expression of GPR54, GnRH and drd2 in the brain relative to the intermediate and advanced stages, except for GnRH1 that increased at the advanced stage of puberty. In the pituitary, drd2 expression declined significantly at the advanced stage relative to levels at the intermediate stage. Ovarian expression of GPR54 significantly increased from the intermediate stage of puberty relative to the early stage while that of GnRH1 acutely increased at the advanced stage of puberty. The ovarian expression of drd2 decreased as puberty progressed, but the changes were not significant. The results suggest the possible role of GPR54 and GnRH in positively regulating pubertal development in grey mullet and the dopaminergic inhibition of reproductive function mediated by drd2.

Investigations into the potential for mixed cultures of banana prawns Penaeus merguiensis, sea mullet Mugil cephalus and rabbitfish Siganus nebulosus for bioremediation of aquaculture waste

To experimentally investigate the potential of mixed species polycultures for bioremediation of nutrient rich prawn farm effluent, a series of experiments was performed with banana prawns Penaeus (Fenneropenaeus) merguiensis, sea mullet Mugil cephalus and rabbitfish Siganus nebulosus to determine their compatibilities during particular life stages. Rabbitfish demonstrated a high tendency to prey upon banana prawn juveniles when no other food was available. Mullet of various sizes did not appear to prey upon banana prawn postlarvae (PL16) or juveniles in a fed or unfed environment. The study confirms the good potential for mullet and banana prawn polycultures.

New SNPs for population genetic analysis reveal possible cryptic speciation of eastern Australian sea mullet (Mugil cephalus)

Sustainable management of sea mullet (Mugil cephalus) fisheries needs to account for recent observations of regional-scale differentiation. Population genetic analysis is sought to assess the situation of this ecologically and economically important fish species in eastern Australian waters. Here, we report (i) new population genetic markers [single nucleotide polymorphisms (SNPs) and potential microsatellites], (ii) first estimates of spatial genetic differentiation and (iii) prospective power tests for designing more comprehensive studies. Six DNA samples from three sampling regions (North Queensland, South Queensland and central New South Wales) on the eastern coast of Australia were used to prepare restriction site associated DNA (RAD) tag libraries from genomic DNA digested with EcoRI and MseI. A pooled sample of regional RAD tag libraries was sequenced using the Roche GS-FLX Titanium platform. A total of 172837 raw reads (17.4Mbp) were retrieved, 95500 of which were used to discover 1267 SNPs and 1417 microsatellites. A subset of 161 SNPs was validated based on 63 additional DNA samples genotyped using the Sequenom MassArray (iPLEX Gold chemistry). Altogether 92 SNPs (57%) were confirmed, with 40% of these marking fixed variants between northern and southern sampling regions. Our preliminary findings indicate a multispecies fishery stock of M. cephalus in eastern Australian waters, but suggest that strong genetic differentiation occurs north of major fishing grounds. Low potential differentiation within major fishing grounds (e.g. FST=0.0025) can be resolved with a likely power 67% by using standard sample sizes of 50 and validated subsets of available markers.

Hazard Function Models to Estimate Mortality Rates Affecting Fish Populations with Application to the Sea Mullet (Mugil cephalus) Fishery on the Queensland Coast (Australia)

Fisheries management agencies around the world collect age data for the purpose of assessing the status of natural resources in their jurisdiction. Estimates of mortality rates represent a key information to assess the sustainability of fish stocks exploitation. Contrary to medical research or manufacturing where survival analysis is routinely applied to estimate failure rates, survival analysis has seldom been applied in fisheries stock assessment despite similar purposes between these fields of applied statistics. In this paper, we developed hazard functions to model the dynamic of an exploited fish population. These functions were used to estimate all parameters necessary for stock assessment (including natural and fishing mortality rates as well as gear selectivity) by maximum likelihood using age data from a sample of catch. This novel application of survival analysis to fisheries stock assessment was tested by Monte Carlo simulations to assert that it provided unbiased estimations of relevant quantities. The method was applied to the data from the Queensland (Australia) sea mullet (Mugil cephalus) commercial fishery collected between 2007 and 2014. It provided, for the first time, an estimate of natural mortality affecting this stock: 0.22±0.08 year −1 .

Aspects of the reproductive biology of Mugil cephalus (Linnaeus, 1857) in Bonny Estuary

Aspects of the reproductive biology of Mugil cephalus in the Bonny estuary (Nigeria) were studied between January and December 1996. Males were observed to be more slender than females while the females have deeper bodies. The male:female ratio (1:0:95) was not significantly different. The minimum size at maturity was 16.6cm (0.5 yr). Fish matured at 24.3cm TL(1.76 yr) with median maturity size of 19.5cm TL(0.71 yr). Median maturity for male and female fish were 16.4cm TL(0.41 yr) and 18.2cm TL(0.60 yr) respectively. Breeding occurred once a year between September and December, from late rainy season to early dry season. Mean absolute fecundity was 1,403, 808 eggs (range 107, 729-4,445, 423 eggs) for fish of 17.0.29.5cm TL (mean 22.5cm TL). Fecundity correlated positively with fish total weight, length, ovary weight and age

Age and growth of Mugil cephalus (Linnaeus, 1758) (Perciformes: Mugilidae) in Bonny Estuary

The age and growth of Mugil cephalus was investigated in Bonny Estuary, Nigeria, from January, 1995 to December, 1996. Length-weight relationships were isometric with length exponents of 2.84 (males), 2.90 (females) and 2.88 (overall). Modal length at age were 12.0cm, 20.9cm, 25.0cm, 28.4cm and 30.2cm TL for ages 0+, 1+, 2+, 3+ and 4+ respectively. Corresponding total weights were 20.01g, 78.93g, 173.12g, 217.61g and 247.50g, respectively. Asymptotic length (Lo) was estimated 33.2cm TL, asymptotic weight (W sub(o)) was 484g, growth coefficient K=0.55847 super(-1) and hypothetical age at zero length To = 0.152yr. Longevity, Tmax, was 5.0yr, length and weight growth performance indices were Q super(1)=2.79 and Q = 1.44, respectively. Total mortality, natural mortality and fishing mortality were z = 1.02yr super(-1), M=0.607yr super(-1) and F=O. 3129yr super(-1), respectively. The exploitation ratio E was 0.4048 and exploitation rate U = 0.2302yr super(-1)

Sexual differentiation and gonad development in striped mullet (Mugil cephalus L.) from South Carolina estuaries*

This study examined the sexual differentiation and reproductive dynamics of striped mullet (Mugil cephalus L.) in the estuaries of South Carolina. A total of 16,464 specimens were captured during the study and histological examination of sex and maturity was performed on a subsample of 3670 fish. Striped mullet were sexually undifferentiated for the first 12 months, began differentiation at 13 months, and were 90% fully differentiated by 15 to 19 months of age and 225 mm total length (TL). The defining morphological characteristics for differentiating males was the elongation of the protogonial germ tissue in a corradiating pattern towards the center of the lobe, the development of primary and secondary ducts, and the lack of any recognizable ovarian wall structure. The defining female characteristics were the formation of protogonial germ tissue into spherical germ cell nests, separation of a tissue layer from the outer epithelial layer of the lobe-forming ovarian walls, a tissue bud growing from the suspensory tissue that helped form the ovary wall, and the proliferation of oogonia and oocytes. Sexual maturation in male striped mullet first occurred at 1 year and 248 mm TL and 100% maturity occurred at age 2 and 300 mm TL. Female striped mullet first matured at 2 years and 291 mm total length and 100% maturity occurred at 400 mm TL and age 4. Because of the open ocean spawning behavior of striped mullet, all stages of maturity were observed in males and females except for functionally mature females with hydrated oocytes. The spawning season for striped mullet recruiting to South Carolina estuaries lasts from October to April; the majority of spawning activity, however, occurs from November to January. Ovarian atresia was observed to have four distinct phases. This study presents morpholog ical analysis of reproductive ontogeny in relation to size and age in South Carolina striped mullet. Because of the length of the undifferentiated gonad stage in juvenile striped mullet, previous studies have proposed the possibility of protandric hermaphrodism in this species. The results of our study indicate that striped mullet are gonochoristic but capable of exhibiting nonfunctional hermaphroditic characteristics in differentiated mature gonads.

Growth, recruitment, and abundance of juvenile striped mullet (Mugil cephalus) in South Carolina estuaries

Growth, recruitment, and abundance of young-of-the-year (YOY) striped mullet (Mugil cephalus L.) in estuarine habitats in South Carolina from 1998 to 2000 were examined and compared to historical data (1986–91) of growth, recruitment, and abundance. Daily growth increments from the sagittal otoliths of juvenile striped mullet were validated by using fish immersed in oxytetracycline hydrochloride (OTC) for five hours from the Charleston Harbor Estuary system. The distribution of back-calculated birthdates indicated that striped mullet spawn from October to late April and estuarine recruitment occurs from January through May. Juveniles were more abundant in mesohaline and polyhaline salinity regimes but were found throughout the estuary. Juvenile growth after recruitment into the estuary can be described by the relationship Total length (mm) = 0.341 (Age)1.04 (r2=0.741, P=0.001). Growth of juveniles according to the analysis of size-frequency data from historical surveys (1986 to 1991) in the same estuaries gave the relationship Total length (mm) = 8.77 (month)1.12 (r2=0.950, P=0.001). The similarity in the growth curves for both groups of fish suggests that juvenile striped mullet in South Carolina have consistent annual growth during the first year of life.

Fecundity and spawning season of striped mullet (Mugil cephalus L.) in South Carolina estuaries

Fecundity in striped mullet (Mugil cephalus) from South Carolina correlated highly with length and weight, but not with age. Oocyte counts ranged from 4.47 × 105 to 2.52 × 106 in 1998 for fish ranging in size from 331 mm to 600 mm total length, 2.13 × 105to 3.89 × 106in 1999 for fish ranging in size from 332 mm to 588 mm total length, and 3.89 × 105 to 3.01 × 106 in 2000 for fish ranging in size from 325 mm to 592 mm total length. The striped mullet in this study had a high degree of variability in the size-at-age relation-ship; this variability was indicative of varied growth rates and compounded the errors in estimating fecundity at age. The stronger relationship of fecundity to fish size allowed a much better predictive model for potential fecundity in striped mullet. By comparing fecundity with other measures of reproductive activity, such as the gonadosomatic index, histological examination, and the measurement of mean oocyte diameters, we determined that none of these methods by themselves were adequate to determine the extent of reproductive development. Histological examinations and oocyte diameter measurements revealed that fecundity counts could be made once developing oocytes reached 0.400 μm or larger. Striped mullet are isochronal spawners; therefore fecundity estimates for this species are easier to determine because oocytes develop at approximately the same rate upon reaching 400 μm. This uniform development made oocytes that were to be spawned easier to count. When fecundity counts were used in conjunction with histological examination, oocyte diameter measurements, and gonadosomatic index, a more complete measure of reproductive potential and the timing of the spawning season was possible. In addition, it was determined that striped mullet that recruit into South Carolina estuaries spawn from October through April.

Observations on the artificial fertilization of eggs and larval rearing of the grey mullet, Mugil cephalus L.

The results are presented of attempts to artificially fertilize Mugil cephalus eggs in the Philippines. Embryonic development is outlined and rearing of the larvae described. Mass mortality occurred during week 3 of rearing.

Culture of grey mullet, Mugil cephalus, Linnaeus in brackishwater ponds at two stocking densities

The culture potential of hatchery-produced grey mullet (Mugil cephalus, Linnaeus) stocked with average weight of 3.7 g and at 2,500 (treatment I) and 3,000 (treatment II) fingerlings/ha in six 350m brackishwater ponds following the lab-ab method of culture was studied.