Gj Purser

Effect of holding temperature on ovulation, egg fertility, plasma levels of reproductive hormones and in vitro ovarian steroidogenesis in the rainbow trout Oncorhynchus mykiss

Abstract Sexually mature female rainbow trout were held at temperatures ranging from 9–21 °C for up to 3 months prior to the natural time of ovulation, in experiments conducted over three different spawning seasons. The majority of fish held at 9 and 12 °C ovulated, variable numbers ovulated at 15 and 18 °C and only one fish ovulated at 21 °C. Egg production was similar at 9, 12 and 15 °C, significantly lower at 18 °C and near zero at 21 °C. Egg survival to the eyed stage after incubation at 11 °C was similar at 9, 12 and 15 °C, and nil at 18 and 21 °C. Histological examination of oocytes from fish held at 12, 15 or 18 °C for 1 or 2 months showed no evidence of gonadal atresia. Plasma levels of gonadotropin (GtH) were measured in samples taken 1, 2 and 3 months after introduction to temperatures of 9, 12, 15, 18 or 21 °C, and showed no differences between temperatures at any time. Plasma levels of testosterone (T) and 17β-oestradiol (E 2 ) were similarly unaffected. Repeat measurement of T and E 2 in another spawning season also showed that holding temperature had no effect on plasma steroid levels. In contrast, in vitro basal steroidogenesis by isolated ovarian follicles was generally lower at 18 °C than at 12 or 15 °C. Follicles from fish held at 18 °C for 2 months did not retain responsiveness to stimulation with steroid precursors or GtH, whereas those from fish held at 12 and 15 °C did. The results indicate that elevated autumn holding temperatures have a deleterious effect on ovulation, egg production and fertility but have equivocal effects on endocrine parameters associated with vitellogenesis. This suggests that the effects are exercised on processes associated with final maturation and ovulation rather than vitellogenesis.

EFFECTS OF SALINITY AND TEMPERATURE ON EGGS AND YOLK SAC LARVAE OF THE GREENBACK FLOUNDER (RHOMBOSOLEA TAPIRINA GUNTHER, 1862)

Abstract The greenback flounder ( Rhombosolea tapirina ) is considered to have potential as an aquaculture species in southern Australia. Experiments were conducted to determine the optimal salinity for fertilisation and buoyancy of eggs, the optimal temperature and salinity combination for incubation of eggs, and the optimal temperature for yolk absorption. Experiments were carried out in 25 and 200 ml beakers and 31 containers. Optimal fertilisation rates occurred at salinities of 35–45‰ and eggs were buoyant at salinities above 28‰. The optimal temperature for egg incubation was approximately 12 °C and salinity had no effect if maintained between 15 and 45‰. Yolk absorption appeared to be most efficient at 15 °C as this temperature resulted in the largest larvae at complete yolk absorption and the fastest growth rate. First feeding occurred at the end of yolk absorption but before the oil droplet was absorbed.

Integrated open-water mussel (Mytilus planulatus) and Atlantic salmon (Salmo salar) culture in Tasmania, Australia

Abstract In a field experiment, Tasmanian blue mussels (Mytilus planulatus) were cultured within an Atlantic salmon (Salmo salar) farm in North West Bay (NWB), Tasmania to test if enhanced mussel performance and reduced organic enrichment resulted from integrated mussel–salmon culture. The performance of mussels grown within the fish farm lease (70 and 100 m from the fish cages) was not appreciably different from that of mussels grown distant to the farm (500 and 1200 m from the fish cages). After culturing mussels for 14 months, no difference among culture sites was indicated for any measured parameter except shell length (P

Effect of rapid changes in temperature and salinity on availability of the rotifers Brachionus rotundiformis and Brachionus plicatilis.

Abstract Euryhaline rotifers are an important food for rearing marine fish larvae. Their availability to fish larvae in the water column may be reduced if they are transferred to fish larval rearing tanks with different temperatures and salinities. The rotifers, Brachionus rotundiformis (formerly called S-type) and Brachionus plicatilis (formerly called L-type), were mass-cultured at 23°C and 35‰ salinity, and then abruptly transfered into tanks at different temperatures (18°C, 23°C, 28°C) and salinities (10‰, 15‰, 20‰, 25‰, 30‰, 35‰). Their availability in the water column was then measured in non-turbulent beakers over time. Both species showed initial transfer shock. The number of rotifers 1 h after stocking was approximately 50% of that potentially available, but increased to approximately 75% after 6 h. Availability was reduced when rotifers were transferred from 23°C to 18°C, but was not affected by transfer from 23°C to 28°C and remained relatively constant over time. B. rotundiformis was slightly more tolerant at higher temperatures than B. plicatilis. Salinity had a greater effect on availability than temperature. Availability of rotifers decreased as salinity was reduced. The availability of rotifers increased over time indicating that some rotifers had acclimated to the transfer conditions. B. plicatilis was slightly more tolerant at lower salinity than B. rotundiformis. Rotifers should be cultured at lower temperatures and similar salinities to the fish larval rearing tanks or acclimated for at least 6 h to larval rearing conditions before transfer.

The effect of decreased ration on feeding hierarchies in groups of greenback flounder (Rhombosolea Tapirina: Teleostei)

The aims of the present study were to investigate food consumption, growth and the dynamics of feeding hierarchies in the same groups of juvenile greenback flounder ( Rhombosolea tapirina: Teleostei) fed either high or low rations. Differential food consumption by individual greenback flounder held in groups of 20 showed the presence of feeding hierarchies in which individuals consumed between 0 and 22% of the available food. Inter-individual differences in food consumption resulted in different growth rates leading to growth depensation (measured as an increase in the coefficient of variation for weight). Intra-individual differences in food consumption tended to be larger for flounder which consumed a lower share of the available food. When the group ration was reduced the inter-individual and intra-individual variations in food consumption increased. Consequently, the relationship between inter-individual and intra-individual variation in food consumption tended to become stronger at lower food availability and indicated an increase in the strength of the feeding hierarchies. This study suggests that individual differences in food consumption, mediated through exploitation competition, contribute to growth depensation in this species of flatfish.

Prey selection by greenback flounder Rhombosolea tapirina (Günther) larvae

Abstract The potential for greenback flounder ( Rhombosolea tapirina ), as a new species for aquaculture in temperate Australia, is currently under investigation. Isolation of an endemic temperate rotifer, Testudinella sp., provided the opportunity to assess this genus against standard live prey species used in aquaculture. Greenback flounder larvae were offered a range of rotifer prey species ( Testudinella sp., Brachionus rotundiformis , Brachionus plicatilis ) and the brine shrimp Artemia , either in isolation or combination, in order to test the hypothesis that factors besides prey size affect prey selection. From first-feeding to approximately day 13 post-hatching, greenback flounder larvae preferentially consumed the rotifer Testudinella sp. over the larger width rotifer B. plicatilis and to a lesser extent the rotifer of similar width dimension, B. rotundiformis . From day 15 post-hatching, consumption (number of prey consumed per larva per unit time interval) and selection ( α ) of B. plicatilis was significantly higher than of Testudinella sp. and Artemia , but by day 18 post-hatching, there was a shift of feeding preference to Artemia . This indicated that size of prey ingested increased with age, likely reflecting an ontogenetic increase in larval sensory function, mouth gape and prey capture and handling ability. Significant selection of Testudinella sp. over the larger rotifer B. plicatilis occurred despite greenback flounder larvae being able to consume B. plicatilis . This along with significant selection (day 11 post-hatching) and consumption (days 11 and 12 post-hatching) of Testudinella sp. over the rotifer of similar width B. rotundiformis , indicates that prey size is not the sole determinant of prey consumption and suggests that greenback flounder larvae exhibit both species-specific and size-specific prey selection. Understanding the basis of prey selection by larval fish predators is essential for enhancing prey consumption in culture, especially in the context of ontogenetic shifts in prey species preference.

An experimental system for small-scale experiments with marine fish and crustaceans

Abstract A small-scale experimental larval rearing unit was designed for controlled experiments on marine fish and crustacean larvae. The system uses 3 litre hemispherical, black tanks in sets of three with their own biofilter. Alternatively the whole system can be integrated to allow the use of 18 tanks. The system has successfully been used for work on the greenback flounder ( Rhombosolea tapirina ). The main problems have been preventing the formation of surface oil films when using live feeds and solid removal at weaning. However the aims of compactness, low cost and versatility were successfully achieved.

Gravid Spot Predicts Developmental Progress and Reproductive Output in a Livebearing Fish, Gambusia holbrooki.

In most livebearing fish, the gravid spot is an excellent marker to identify brooding females, however its use to predict progress of embryonic development, brood size, timing of parturition and overall reproductive potential of populations remain unexplored. Therefore, to understand these relationships, this study quantified visual attributes (intensity and size) of the gravid spot in relation to key internal development in Gambusia holbrooki. Observations show that the colour of the gravid spot arises from progressive melanisation on the surface of the ovarian sac at its hind margin, rather than melanisation of the developing embryos or the skin of the brooding mother. More importantly, the gravid spot intensity and size were closely linked with both developmental stages and clutch size, suggesting their reliable use as external surrogates of key internal developmental in the species. Using predictive consistency of the gravid spot, we also determined the effect of rearing temperature (23°C and 25°C) on gestation period and parturition behaviour. The results show that gestation period was significantly reduced (F = 364.58; df = 1,48; P˃0.05) at 25°C. However there was no significant difference in average number of fry parturated in the two temperature groups (P<0.05), reaffirming that gravid spot intensity is a reliable predictor of reproductive output. The parturition in the species occurred predominantly in the morning and in contrast to earlier reports, tails of the fry emerged first with a few exceptions of head-first, twin and premature births. This study demonstrates utility of the gravid spot for downstream reproductive investigations in a live-bearing fish both in the field and laboratory. The reproducibility of the relationships (intensity with both developmental stage and clutch size), imply that they are also relevant to wild populations that experience varying temperature climes and stressors, significant deviations of which may serve as indicators of environmental health and climate variability.