R. J. Albieri

Reproductive biology of the mullet Mugil liza (Teleostei: Mugilidae) in a tropical Brazilian bay

The reproductive biology of Mugil liza Valenciennes, 1836 is described as a contribution to an elaborate management programm. A total of 243 specimens (89 males and 154 females) were collected in the Sepetiba Bay in south-eastern Brazil from July/2006 to June/2007. The gonadosomatic index (IG) and the sequential development of the ovaries observed through histological studies suggested that the spawning season ranged from May to August. The population reached total sexual maturity (LT100) at 550 and 570 mm total length (LT) for males and females, respectively. Females attained a larger size than males, and the sex ratio was female-biased for fish larger than 500 mm LT. The hepatosomatic index (IH) was significantly related to the IG, indicating that vitellogenesis mobilizes hepatic energy during reproduction. Mean fecundity was 3,080,000 oocytes. The presence of only two phases of oocyte development in ripe ovaries - a reserve stock and a clutch of post-vitellogenic oocytes - indicated that ovarian development is group synchronic and this species is characterized as a total spawner. The results suggest that establishing a closed fishing season from May to August and establishing a minimum size for capture of 350 mm LT would enhance stock conservation and production for future harvest seasons.

Distribution and size of the mojarra Diapterus rhombeus (Cuvier) (Actinopterygii, Gerreidae) in a Southeastern Brazilian bay

Diapterus rhombeus is one of the most abundant fish species in the Sepetiba bay, which is a 520 km2 semi-closed coastal area in Southeastern Brazil. The size and distribution of this species have been described in order to assess the use of the area as a rearing ground in both spatial and temporal (seasonal) dimensions. A fish sampling program was conducted monthly by using an otter trawl between October 1998 and September 1999, in three bay zones (outer, middle and inner), defined according to depth and salinity gradient. Highly significant differences were found for CPUEs (number and biomass) among seasons and zones. Size ranged from 50 to 230 mm TL, and three size-groups were defined according to a maturation scale (n = 1435): 1 - immature ( 100 mm TL). Immature and L50 individuals were more abundant (number and biomass) during Autumn in the inner zone, while adults (L100) predominated during Summer in the outer zone. Evidences of movements of young-of-the-year (50-90 mm TL) individuals from the inner to the outer zone were detected as they reach larger sizes (180 mm TL) in the second year of life. Condition (k) was higher in larger sized individuals in the outer zone during Spring, when they are apt to start the reproductive process. Two cohorts were detected according to modal progression: the first (smaller size) showing faster growth than the second, evidencing a slower growth rate as they reach larger size. Distinct size classes occupying different bay zones suggest that the age-groups optimize their coexistence by partitioning the available resources, avoiding intra-specific competition.

Size distribution of the jack Chloroscombrus chrysurus (Linnaeus) (Actinopterygii, Carangidae) in a tropical bay at Southeastern Brazil

Chloroscombrus chrysurus (Linnaeus, 1766) e o representante mais abundante da familia de Carangidae na Baia de Sepetiba, uma area semi-fechada com 450 Km2, localizada no sudeste do Brasil. Foram analisadas as CPUEs de numero, peso e tamanho, alem dos dados ambientais, a fim de avaliar parte do ciclo de vida desta especie num ambiente de baia. As amostras foram realizadas mensalmente em triplicatas atraves de rede de arrasto de fundo, em tres zonas da baia (interna, central e externa), definidas de acordo com um gradiente espacial de profundidade e salinidade. Juvenis foram registrados primeiramente na zona interna da baia, medindo entre 30-90 mm de comprimento total - CT (moda = 60 mm CT) na primavera. Este mesmo grupo de tamanho desloca-se para as zonas central e externa da baia durante o verao-outono quando atingem CT = 120-180 mm, juntando-se posteriormente aos adultos no inverno. Jovens do ano nao utilizam as praias arenosas da Baia de Sepetiba como area de recrutamento, embora a desova desta especie ocorra na plataforma continental, com ovos e larvas penetrando em direcao a parte mais interna da baia. Pelo menos duas classes anuais de tamanho coexistem na baia durante os primeiros dois anos. O gradiente ambiental de temperatura, salinidade, transparencia, oxigenio dissolvido e profundidade foram pobres preditores para o numero e o peso dos peixes, e a correlacao negativa significante com salinidade e a profundidade reflete somente as altas abundâncias de jovens na parte mais interna da baia onde predominam maiores influencias de drenagem continentais e maiores deposicoes de sedimento.

Seasonal response of fish assemblages to habitat fragmentation caused by an impoundment in a Neotropical river

Changes in fish assemblages between the zones above and below Funil dam in Southeastern Brazil were investigated to evaluate the possible impacts of this impoundment in two contrasting seasons: summer/wet and winter/dry. We expect differences in fish assemblage structure and in environmental conditions between seasons and between the reservoir and the zone downriver of the dam. A total of 3,579 individuals comprising 38 species, including six non-natives, were collected. As expected, the comparatively high habitat complexity and water flow regime of the downriver zone favored a richer and more abundant fish assemblage compared with the reservoir, especially in the wet season. In this period, water covers part of the riparian vegetation, increasing habitat availability and nutrient input. Additionally, the dam prevents upriver migration of rheophilics fish species such as the Characiformes Prochilodus lineatus and Leporinus copelandii, and the Siluriformes Pimelodus fur and Pimelodus maculatus, thus increasing shoals below the dam. Although the reservoir represents a simplified ecosystem highly influenced by non-native top predator species (e.g. the Perciformes Cichla kelberi and Plagioscion squamosissimus), seasonal processes (e.g. water level fluctuations and flood pulses) seem to play a role in structuring of the fish assemblage. Environmental variables, mainly turbidity, temperature, and conductivity were significantly associated to spatial-temporal patterns of fish assemblage. In this freshwater tropical reservoir, the spatial scale, rather than the seasonal changes in environmental variables, was the dominant factor structuring fish assemblage in the reservoir and in the zone downriver of the dam.

Assessing fish assemblages similarity above and below a dam in a Neotropical reservoir with partial blockage

Damming rivers disrupts the water flow and changes the ichthyofauna organisation. We investigated an impoundment with permanent connection and homogeneous environmental conditions between the zones above and below the dam. Temperature was comparatively higher during wet season irrespective of zone, and both zones had higher dissolved oxygen, conductivity and transparency in the dry season. A total of 1687 individuals comprising 27 species were collected in the downriver zone, while the reservoir had 879 individuals and 23 species. Each zone had different fish assemblage composition and structure, but assemblages were not explained by the examined environmental variables (r2 = 0.08; p = 0.307). Migratory species such as Pimelodus maculatus, Pimelodus fur, Leporinus copelandii and Prochilodus lineatus were the most affected, and probably are prevented to perform upriver migrations. On the other hand, lentic adapted species such as G. brasiliensis, Hoplias malabaricus and Hoplosternum littorale successfully colonised the reservoir. Therefore we conclude that the presence of the lateral hydrological connectivity alone does not guarantee the ecological connectivity since fish assemblage similarity differed between the two zones. Fish passage facilities should be monitored and managed to evaluate and improve their functionality.

Opportunistic reproductive strategy of a non-native fish, the spotted metynnis Metynnis maculatus (Kner, 1858) (Characidae Serrasalminae) in a tropical reservoir in south-eastern Brazil

We investigated the reproductive strategy developed by the non-native fish Metynnis maculatus (Kner, 1858) to explain its success in establishing in a tropical reservoir in south-eastern Brazil. A total of 198 specimens were examined, 100 males (total length, TL = 110–172 mm) and 98 females (TL = 110–191 mm), collected using gill nets from September 2005 to August 2006. Sex ratio is balanced, although females significantly outnumber males in size larger than 160 mm TL. Six stages of oogenesis and five of spermatogenesis are described, based on staining aspect of the nucleus and cytoplasm structure with photonic microscopy. A protracted reproductive period from August to January was found with females producing several large clutches of small eggs. Oocyte mean diameter ranged from 19.6–48.3 μm in pre-vitellogenic ovarian stage to 83.4–149.4 μm in vitellogenic ovarian stage whereas in spawning stage ovary, oocyte diameter varied from 300 to 500 μm. In ripe stage, oocytes can reach diameter up to 2.00 mm. Batch fecundity ranged from 670 to 16,333 oocytes (mean fecundity = 7056 oocytes) for females having TL ranging from 145 to 191 mm and total weight ranging from 75 to 176 g. A tendency for opportunistic strategy was detected as indicated by a long spawning period, high fecundity, small oocytes and small body size that have proved to be effective in the Lajes reservoir.