Reproduction and feeding of the mountain mullet,Agonostomus monticola,in Trinidad,West Indies

Synopsis Reproduction and feeding were studied in mountain mullet,Agonostomus monticola, in northeast Trinidad during the period January 1987 to September 1988. Macroscopic and microscopic examination of the gonads indicated a single, distinct reproductive season from June to October during the wet season. Evidence implies only one spawning event per individual during the reproductive season. Females significantly outnumbered males resulting in a sex ratio of 20.4:1. The mean size at maturity for females was 135 mm fork length (FL). Fecundity (F) is related to size by the equations F = 167.3 FL^2.7 and F = 16430.7 G + 235079 (G = gutted weight).A. monticola was found to be an omnivorous feeder. The dominant food groups found in the stomachs were insects, prawns, fruits and algae, in order of decreasing importance.     

Isolation and characterization of microsatellite loci for mountain mullet (Agonostomus monticola)

We report on the isolation of 15 polymorphic microsatellite loci from mountain mullet (Agonostomus monticola). In the two populations sampled, loci exhibited two to 21 alleles and observed heterozygosity values ranged from 0.222 to 1.000. All loci conformed to Hardy-Weinberg equilibrium expectations, and none exhibited linkage disequilibrium. Although A. monticola is an important subsistence fishery in parts of its range, little is known about its ecology and many populations appear to be experiencing declines. These microsatellite loci should prove useful in the study of population structure of A. monticola and aid in other potential conservation efforts such as the management of hatchery broodstock.     

Reproductive biology of female bigeye tuna Thunnus obesus in the western Pacific Ocean

The reproductive biology of female bigeye tuna Thunnus obesus was assessed by examining 888 fish (ranging from 84·9 to 174·4 cm fork length, L_F) caught by Taiwanese offshore longliners in the western Pacific Ocean from November 1997 to November 1998 and November to December 1999 and 258 gonad samples from these fish. The overall sex ratio of the catch during the sampling differed significantly from 0·5, but males were predominant in sizes >140 cm L_F. Reproductive activity (assessed by histology), a gonado‐somatic index, and the size‐frequency distributions of whole oocytes indicated that spawning occurred throughout the year and the major spawning season appeared to be from February to September. The estimated sizes at 50% maturity (L_F50) of females was 102·85 cm (95% c.i .: 90·79–110·21 cm) and the smallest mature female was 99·7 cm L_F. They are multiple spawners and oocytes develop asynchronously. The proportion of mature (0·63) and reproductively active (0·70) females with ovaries containing postovulatory follicles indicated that they spawn almost daily. Batch fecundity for 15 females with the most advanced oocytes (>730 µm) ranged from 0·84 to 8·56 million eggs (mean ± s.d . = 3·06 ± 2·09). The relationships between batch fecundity (F_B, in millions of eggs) and L_F (cm) and round mass (M_R, kg) were (r² = 0·84) and (r² = 0·80), respectively. The parameters estimated in this study are key information for stock assessments of T. obesus in the western Pacific Ocean and will contribute to the conservation and sustainable yield of this species.     

Life-history variations in the fluvial sculpin, Cottus nozawae (Cottidae), along the course of a small mountain stream

Life-history variations in male and female fluvial sculpins, Cottus nozawae, were studied in a small mountain stream in Hokkaido, Japan, primarily by using capture-mark-recapture methods. At three study areas established along the stream course, the majority of marked sculpins were recaptured in their original location over one or more years, indicating their long-term occupation of each restricted habitat area. Sculpin densities increased toward the upstream habitats, whereas individual growth rates were more rapid downstream. In both sexes, sculpins distributed downstream matured at a larger body size and later in life than upstream sculpins, clearly demonstrating a clinal variation in these respects. A comparison of life-history variations in C. nozawae with those in amphidromous C. hangiongensis suggests that intrapopulational life-history variations in the former might be environmentally induced, and that one of the most important determinants for the variations in Cottus species might be population density.     

Population structure and reproductive biology of Schilbe intermedius (Teleostei: Schilbeidae) in the Pendjari River,Benin

This study, the first on fish reproduction in the Pendjari River, investigated aspects of the reproductive biology of Schilbe intermedius. A total of 429 females and 239 males were collected from March 2007 to February 2008. Females were larger than males and the sex ratio was 1:1.8 in favour of females. Size at first maturity was estimated to be 14.9 cm and 16.1 cm for males and females, respectively. Absolute fecundity ranged from 1 006 to 83 980 (22 421 ± 16 083) oocytes, and mean relative fecundity was 201 ± 162 oocytes g^?1 of total body mass. Frequency distributions of oocyte diameter suggested synchronous development, with total spawning. Spawning lasted from June to November, coinciding with peaks in rainfall and flooding. Larger females began spawning about two months before smaller ones.     

Reproductive biology of Bagrus docmak in the Victoria Nile,Uganda

Aspects of the reproductive biology of Bagrus docmak in the Victoria Nile were investigated between November 2005 and October 2006. Macroscopic and histological analysis of the gonads confirmed it as an asynchronous batch spawner which spawns throughout the year with bimodal spawning peaks coinciding with rainfall seasons. The first spawning peak occurred from March to May, the second from September to November. The sex ratio did not significantly deviate from 1:1. Length at sexual maturity was 33.6 cm and 31.6 cm fork length (FL) for females and males, respectively. Batch fecundity ranged from 1 000 eggs in 34 cm FL fish to 43 000 eggs in 79 cm FL fish, and correlated linearly with FL (r = 0.72) and body weight (r = 0.79). Mean relative batch fecundity was 6 eggs g^?1 (SE 2). These results could guide research into the possibility of artificially inducing the fish to spawn, and its subsequent culture.     

Reproduction of Micropogonias funieri in a shallow temperate coastal lagoon in the southern Atlantic

The white croaker Micropogonias furnieri , in the coastal Rocha Lagoon, spawned during 5 months, in late spring and summer. It was eurythermic (gonad growth at 12·5 to 25·5° C, spawning at 20 to 27° C) and mesoxic (living at 5·2 to 9·1 mg l^-1). The spawning occurred in brackish (8–18 salinity), basic ( c . 8 pH) and oxygenated ( c . 8·0 mg l^-1) waters. The temperature appeared to be an important environmental factor affecting the timing of reproduction. The size at first maturity (19–20 cm) was 11–12 cm lower than the reported for the Río de la Plata spawning area (Uruguay). Juveniles were observed throughout most of the year suggesting that the lagoon is also a nursery area. In Brazil, M. furnieri spawns in marine areas while in Uruguay it spawns in estuaries. This is the first time that a coastal lagoon of the subtropical and temperate western coast of the South Atlantic Ocean has been shown to be a spawning area of a marine species.     

Effects of population characteristics and structure on estimates of effective population size in a house sparrow metapopulation

Effective population size (N_e) is a key parameter to understand evolutionary processes and the viability of endangered populations as it determines the rate of genetic drift and inbreeding. Low N_e can lead to inbreeding depression and reduced population adaptability. In this study, we estimated contemporary N_e using genetic estimators (LDNE, ONeSAMP, MLNE and CoNe) as well as a demographic estimator in a natural insular house sparrow metapopulation. We investigated whether population characteristics (population size, sex ratio, immigration rate, variance in population size and population growth rate) explained variation within and among populations in the ratio of effective to census population size (N_e/N_c). In general, N_e/N_c ratios increased with immigration rates. Genetic N_e was much larger than demographic N_e, probably due to a greater effect of immigration on genetic than demographic processes in local populations. Moreover, although estimates of genetic N_e seemed to track N_c quite well, the genetic N_e‐estimates were often larger than N_c within populations. Estimates of genetic N_e for the metapopulation were however within the expected range (<N_c). Our results suggest that in fragmented populations, even low levels of gene flow may have important consequences for the interpretation of genetic estimates of N_e. Consequently, further studies are needed to understand how N_e estimated in local populations or the total metapopulation relates to actual rates of genetic drift and inbreeding.     

Reproductive and feeding biology of the Natal mountain catfish, Amphilius natalensis (Siluriformes: Amphiliidae)

Aspects of the biology of the Natal mountain catfish, Amphilius natalensis, including gametogenesis, spawning season, size-at-maturity, sex ratio, diet and feeding morphology were determined from fish collected in the Songimvelo Game Reserve, South Africa, between 1989 and 1990. Female sexual maturity was established at 63 mm total length. A. natalensis is an asynchronous, iteroparous spawner, breeding throughout summer from August to February. Sex ratio of females to males was 2.2:1, with females attaining a significantly larger size than males. Gametogenesis followed a pattern similar to that of other freshwater teleosts peaking over the spawning period. Stomach content analysis and observations on feeding morphology revealed that A. natalensis was an opportunistic predator with large fish consuming larger prey from a greater variety of taxonomic groups. The most abundant prey items eaten were dipteran (particularly Chironomidae) and ephemeropteran (particularly Baetidae) larvae.     

Age Composition, Growth and Reproductive Biology of the Salamanderfish Lepidogalaxias salamandroides: A Re-examination

This study has determined the age and size compositions, growth rate and reproductive biology of Lepidogalaxias salamandroides using data for fish of known sex and age and the monthly trends exhibited by various gonadal variables, including the sizes and stages of the oocytes. The results resolve conflicting conclusions drawn by other workers concerning certain aspects of the biology of this freshwater species and expands our knowledge of this teleost. Our results are based on data derived from samples collected from small, ephemeral acidic pools in south-western Australia in 22 consecutive months. Although ca. 78% of the fish caught belonged to the 0+ age class, substantial numbers of the 1+ and 2+ age classes and some 3+ and 4+ fish were caught. Spawning occurs between late May and late August, with peak activity in late July and early August. During the first year of life, growth is initially rapid, but then ceases in the weeks immediately prior to and during the period when the habitat becomes dry and fish aestivate, before recommencing in autumn as the pools again become filled with water. The growth coefficient (K) in the von Bertalanffy growth equation was lower for females (0.63) than males (1.28), reflecting the greater growth undergone by females after the first year of life. Maturity was attained by ca. 28% of the females and ca. 26% of the males that reached the end of their first year of life and by all fish that reached the end of subsequent years of life. The L₅₀s for females and males at first maturity were 43.2 and 38.6mm TL, respectively. The mean fecundity was only 82.4, reflecting a combination of a small body size and a relatively large egg (diameter=1.8mm). Lepidogalaxias salamandroides is a multiple spawner, which increases the chances of some eggs and newly-hatched embryos encountering a period of favourable environmental conditions during the spawning period, which is characterised by bouts of heavy rainfall. The ratio of females to males was essentially parity in the 0+ and 1+ age classes and in all age classes collectively. When testes commence maturing for the first time, the anal fin of males starts to become modified to form a structure that facilitates the transfer of sperm to the female, and it remains modified throughout the rest of life.     

Population structure,demography, and dynamics of mountain baboons: An interim report

Counts of 61 baboon troops (Papio cynocephalus ursinus) at four localities in the Drakensberg mountains confirmed earlier reports of a small mean troop size. This troop size of 22.49 animals changed neither with latitude nor elevation. Data from two of the sites suggested that population density increases from south to north, while a working assumption of 2.5 animals/ km² allowed us to set the population size at 7,540 animals, living in 335 troops. Both the adult sex ratio of 2.07 females/male and the immature/ adult female ratio of 1.17 were unaffected by troop size. Repeated counts from nine known troops revealed that the population is at equilibrium. © 1995 Wiley-Liss, Inc.     

The influence of persistent individual differences and age at maturity on effective population size

Ratios of effective populations size, N(e), to census population size, N, are used as a measure of genetic drift in populations. Several life-history parameters have been shown to affect these ratios, including mating system and age at sexual maturation. Using a stochastic matrix model, we examine how different levels of persistent individual differences in mating success among males may affect N(e)/N, and how this relates to generation time. Individual differences of this type are shown to cause a lower N(e)/N ratio than would be expected when mating is independent among seasons. Examining the way in which age at maturity affects N(e)/N, we find that both the direction and magnitude of the effect depends on the survival rate of juveniles in the population. In particular, when maturation is delayed, lowered juvenile survival causes higher levels of genetic drift. In addition, predicted shifts in N(e)/N with changing age at maturity are shown to be dependent on which of the commonly used definitions of census population size, N, is employed. Our results demonstrate that patterns of mating success, as well as juvenile survival probabilities, have substantial effects on rates of genetic drift.     

Reproductive biology of two co‐occurring mugilids,Liza argentea and Myxus elongatus,in south‐eastern Australia

The reproductive biology of Liza argentea and Myxus elongatus occurring in two estuaries (Lake Macquarie and St Georges Basin) was found to differ. Gonado‐somatic index values and macroscopic staging of gonads identified the peak spawning period of L. argentea occurred between March and November in Lake Macquarie and January and April in St Georges Basin. In contrast, peak spawning of M. elongatus was concentrated between January and March in both estuaries. Spawning of L. argentea probably occurred in the lower reaches of estuaries as well as in nearshore coastal waters, whereas evidence indicated M. elongatus spawned only in ocean waters. The mean fork length at maturity (L_F50) was greater for females than males in both species, and it also occurred at a larger mean L_F in M. elongatus (males = 230 mm and females = 255 mm) than L. argentea (males = 180 mm and females = 207 mm). Estimates of total potential fecundity were also greater for M. elongatus (425 484–1 157 029) compared to L. argentea (159 933–548 954). Both species had determinate fecundity and displayed a group synchronous pattern of oocyte development, with two distinct size classes of oocytes present in mature ovaries. Liza argentea probably release the larger class of oocytes in one spawning event, but this could not be established for M. elongatus.     

Community structure and diet of roving herbivorous reef fishes in the Abrolhos Archipelago, south-western Atlantic

Community structure and diet of roving herbivorous reef fishes were analysed in 13 study sites around the five islands of the Abrolhos Archipelago, north-eastern Brazil (17°58' S; 38°42' W). This area has been part of the Abrolhos Marine National Park since 1983. Abundances and diets of fishes within the families Scaridae, Acanthuridae and Kyphosidae were compared among groups of sites differing in benthic community structure and exposure regime. The abundance of roving herbivorous fishes was higher in shallower sites than in deeper sites. At all study sites, observations of total herbivorous reef fish community structure revealed that 64% of fishes were acanthurids, 33% were scarids and only 4% were kyphosids. This pattern was predominant in all study sites. The majority of fishes examined in this study had the bulk of their diet based on both algae (35–90%) and detritus (35–65%). Among groups of algae, filamentous algae were the most commonly consumed as the majority of roving herbivorous fishes in Abrolhos feed as scrapers and excavators. The exceptions were the kyphosids, which had a diet consisting primarily of macroalgae (mainly Phaeophyta), and Acanthurus coeruleus that consumed mainly turf algae and very little detritus. The data collected indicate that roving herbivorous fishes in the Abrolhos Archipelago are more abundant at calm sites where detritus and delicate algae, their major food resources, are more abundant. The results presented here, along with underwater observations, suggest that the abundance and diet of these three fish families are associated with substratum cover.     

Chemical and isotopic composition of karstic lakes in Jamaica,West Indies

Lakes in the limestone region of Jamaica exhibit a range of chemical characteristics that reflect varying inputs from precipitation, surface runoff and groundwater, together with the subsequent evolution of the water within the limnic environment. Detailed spatial and temporal sampling was conducted on one lake, Wallywash Great Pond. Chemical data, together with D/H, ¹⁸O/¹⁶O, ¹³C/¹²C and ⁸⁷Sr/⁸⁶Sr ratios confirm that the karstic spring waters entering the lake evolve chemically through degassing, mixing with rainfall and runoff, biogenic decalcification (resulting mainly from bicarbonate assimilation by the high biomass of submerged macrophytes), and evaporation. Modern carbonate sedimentation in Wallywash Great Pond is largely of high-Mg calcite. This is consistent with Mg/Ca molar ratios >2 within much of the lake. However, aragonite forms on the adaxial leaf surfaces of Potamogeton spp. This may be explained either as a result of locally elevated Mg concentrations or a high degree of supersaturation favouring very rapid carbonate precipitation. Two small lakes to the north of Wallywash Great Pond show minor influence of the Na-Cl dominated coastal aquifer, suggesting that coastal lakes are sensitive to variations in the boundary between fresh and brackish groundwater caused by changes in climate or sea level. Their ¹³C/¹²C ratios are strongly influenced by biogenic CO₂ derived from plant respiration or decay.     

Production,population biology and diet ofNeomysis integer (Leach) in a shallow Frisian lake (The Netherlands)

TheNeomysis integer population of a shallow Frisian freshwater lake, situated in the north of the Netherlands, was studied from May until October 1980. The results were compared with observations on populations in other Frisian lakes, and with the results reported by other authors on brackish-water and marine populations of this species.Three generations were observed: one overwintering generation and two summer generations. Reproduction stopped completely during winter. During 1980 population densities ofN. integer were relatively low in all Frisian lakes. In Slotermeer the maximum population density was only 6 individuals m^–2, production amounted to 10 mg dry wt m^–2 yr^–1. The annual P/B ratio was 4.0. The diet ofN. integer consisted, in terms of biomass, of more than 95% detritus and animal food, the latter mainly consisting ofBosmina and cyclopoid copepods. Feeding intensity reached a maximum at sunset. It was still high during first part of the night and was low in the morning just after sunrise.     

Size structure, sex ratio and onset of sexual maturity of two species of Hypostomus

Length-frequency analyses showed that male Hypostomus affinis are bigger than females, but H. luetkeni showed no such differences. The sex ratio was estimated at 1:1 for both species. The temporal variation of sex ratio indicated a decrease in the male population when GSI was at its maximum value, suggesting that males are less prone to capture during the spawning season. According to the decrease of males during the reproductive period, paternal behaviour is proposed for both species. H. affinis reached sexual maturity at a smaller size than H. luetkeni. Considering the length weight relationship and the mortality rate of the reproductive females in both species, the physiological consequences of delay or precocity of the first maturation are discussed.     

Do females adjust the sex of their offspring in relation to the breeding sex ratio?

When the adult sex ratio differs between years in local populations, but still is predictable between adjacent years, it has been proposed that the best strategy would be to bias the offspring sex ratio in favour of the rare sex. We tested this hypothesis using a data set of great reed warbler offspring, sexed by molecular techniques, that were collected over 11 breeding seasons at two adjacent reed marshes. Three important assumptions for this hypothesis are fulfilled in the studied great reed warbler population. First, a substantial proportion of great reed warblers are living in small local populations where sex ratio distortions would be sufficiently large and common. Second, breeding adults and their offspring return to breed in the local population to a high degree. Third, females have a possibility to assess the breeding sex ratio before laying their eggs. At our study site, the breeding sex ratio was positively correlated between successive years. However, contrary to our prediction, female great reed warblers seemed not to adjust their offspring sex ratio in relation to the local breeding sex ratio.