The influence of ontogeny and prey abundance on feeding ecology of age-0 Lake Whitefish (Coregonus clupeaformis) in southeastern Lake Michigan

A shift towards oligotrophic conditions in Lake Michigan has led to concern that altered trophic pathways are leading to lower early life survival and recruitment for Lake Whitefish (Coregonus clupeaformis). This study evaluated ontogenetic shifts in age-0 Lake Whitefish diets and evaluated how feeding ecology and the amount of food eaten varied with prey abundance and composition at a site in southeastern Lake Michigan during 2014–2017. Although prey densities varied among years, cyclopoid copepods were overall the most abundant prey available. In turn, cyclopoids were the predominant prey item in diets each year, particularly for the smallest larval Lake Whitefish. However, there was a tendency for the importance of cyclopoids to decline somewhat in each diet index as fish grew and other prey such as calanoid copepods, Bosminidae, Daphniidae and/or chironomids increased in importance. High zooplankton abundance, especially high cyclopoid abundance, available to the small size groups of Lake Whitefish (<21 mm) in 2014 was associated with high food mass/fish, high number of zooplankton eaten/fish, and low incidence of empty stomachs compared with 2015–2017. As fish grew, the impact of food abundance on prey consumption diminished somewhat, indicating that the relationship between fish feeding ecology and the prey environment can change quickly with fish size during the early life period.     

Ontogenetic shifts in prey selection and foraging behaviour of larval and early juvenile alligator gar (Atractosteus spatula)

The early foraging ecology of alligator gar (Atractosteus spatula) is poorly documented, with little information available on young‐of‐year food habits or ontogenetic diet shifts. We conducted laboratory experiments to quantify prey selection and foraging behaviours of larval and early juvenile alligator gar (16–80 mm TL) simultaneously offered zooplankton, chironomid larvae and one of three densities of fish prey. The smallest size groups of alligator gar consumed zooplankton almost exclusively, but with increasing size, selection for zooplankton declined and selection for fish prey increased. At higher densities of fish prey, alligator gar exhibited lower selection for zooplankton and positively selected for fish at smaller sizes. Ingestion efficiencies for chironomids were considerably lower than for zooplankton or fish prey, resulting in low rates of consumption and negative selection for chironomids by all size groups of alligator gar. Fish prey elicited a different foraging response from alligator gar than zooplankton or chironomids, as alligator gar pursued and struck at fish over longer distances than for other prey types. With increasing size, alligator gar used a wider vertical range of the water column for foraging, changed their strike tactics and exhibited decreased handling times for zooplankton and fish. These observations indicate that alligator gar undergo several functional and behavioural changes during early ontogeny that facilitate a rapid transition to piscivory, but fish prey density strongly affects prey consumption patterns and the size at which alligator gar transition to piscivory.     

黄海南部不同体长鳀对食物粒级的选择

基于2000年6月和2014年11月黄海南部采集的浮游动物和不同生长阶段鳀(Engraulis japonicus)的胃含物(饵料)组成分析资料,研究鳀的饵料粒级分布与摄食粒级选择性及其与环境生物组成的关系,旨在推进粒级在传统摄食生态分析中的应用。结果显示:黄海南部鳀的饵料种类组成与环境中生物种类组成有关;各体长组中,鳀的饵料粒级多样性与种类多样性分布趋势相似;体长为110 mm左右的鳀的饵料种类多样性和粒级多样性水平最高;鳀的饵料粒级均值随鱼体体长增加而增加;体长约为30 mm和≥70 mm的鳀有明显的饵料粒级转换。根据上述结果,认为在鳀胃含物分析过程中,整合环境中饵料生物的相对组成和粒级大小,可有效评价鱼类对饵料的粒级选择性。     

Prey partitioning and use of insects by juvenile sockeye salmon and a potential competitor,threespine stickleback,in Afognak Lake,Alaska

Freshwater growth of juvenile sockeye salmon (Oncorhynchus nerka) depends upon the quality and quantity of prey and interactions with potential competitors in the foraging environment. To a large extent, knowledge about the ecology of lake‐rearing juvenile sockeye salmon has emerged from studies of commercially important runs returning to deep nursery lakes, yet information from shallow nursery lakes (mean depth ≤ 10 m) is limited. We examined seasonal and ontogenetic variation in diets of juvenile sockeye salmon (N = 219, 30–85 mm) and an abundant potential competitor, threespine stickleback (Gasterosteus aculeatus; N = 198, 42–67 mm), to understand their foraging ecology and potential trophic interactions in a shallow Alaska lake. This study revealed that adult insects made up 74% of all sockeye salmon diets by weight and were present in 98% of all stomachs in Afognak Lake during the summer of 2013. Diets varied temporally for all fishes, but small sockeye salmon (<60 mm) showed a distinct shift in consumption from zooplankton in early summer to adult insects in late summer. We found significant differences in diet composition between sockeye salmon and threespine stickleback and the origin of their prey indicated that they also separated their use of habitat on a fine scale; however, the two species showed overlap in size selectivity of zooplankton prey. Considering that aquatic insects can be a primary resource for juvenile sockeye salmon in Afognak Lake, we encourage the development of nursery lake carrying capacity models that include aquatic insects as a prey source for sockeye salmon.     

Impact of environmental and innate factors on the food habit of Chinese perch Siniperca chuatsi (Basilewsky) (Percichthyidae)

Laboratory and field investigations were conducted to study the food habit of Chinese perch Siniperca chuatsi (Basilewsky) from first feeding through adult stage. Only fish larvae were consumed by Chinese perch larvae (2–21 days from hatching), and the presence of zooplankton did not have any significant effect on their survival rate. The ability of Chinese perch to feed on zooplankton is clearly limited by some innate factor. Instead of gill rakers, Chinese perch larvae have well‐developed sharp teeth at the first feeding stage, and are well adapted to the piscivorous feeding habit unique to the larvae of Chinese perch, e.g. they bite and ingest the tails of other fish larvae. At the first feeding stage (2 days from hatching), daily rations were both very low, either in light or complete darkness. Although early‐staged Chinese perch larvae (7–17 days from hatching) could feed in complete darkness, their daily rations were always significantly higher in light than in complete darkness. Late‐staged Chinese perch larvae (21 days from hatching) were able to feed in complete darkness as well as in light, similar to the case of Chinese perch yearlings. Chinese perch yearlings (total length, 14–16 cm) consumed prey fish only and refused shrimp when visual cues were available (in light), but they consumed both prey when visual cues were not available (in complete darkness), suggesting that prey consumption by Chinese perch yearlings is affected by their sensory modality in predation. Both prey were found in the stomachs of similar‐sized Chinese perch (total length, 14–32 cm) from their natural habitat, suggesting that shrimp are consumed by Chinese perch at night. Prey selection of Chinese perch with a length >38 cm, which consumed only fish in the field, appears to be based upon prey size instead of prey type. These results suggest that although environmental factors (e.g. light intensity) affect prey detection by Chinese perch, this fish is anatomically and behaviourally predisposed to prey on live fish from first feeding. This makes it a difficult fish to cultivate using conventional feeds.     

Effects of enriched Artemia by fish and soybean oils supplemented with vitamin E on growth performance,lipid peroxidation,lipase activity and fatty acid composition of Persian sturgeon (Acipenser persicus) larvae

This study aimed to evaluate the effects of enriched Artemia by fish and soybean oils supplemented with vitamin E on growth performance, lipid peroxidation, lipase activity and fatty acid composition of Persian sturgeon (Acipenser persicus) larvae. For this purpose, five experimental diets including non‐enriched Artemia (control diet), Artemia enriched with soybean oil supplemented with 15% and 30% vitamin E (S15 and S30 diets) and fish oil supplemented with 15% and 30% vitamin E (F15 and F30 diets) were used. The larvae were fed to apparent satiation four times per day for 22 days. The results indicated that fish fed enriched Artemia had no significant differences compared with those fed non‐enriched Artemia in terms of growth and survival, but increase in vitamin E levels from 15 to 30% improved growth performance of larvae. Vitamin E content in fish fed S15 and S30 diets was significantly higher. Fish fed non‐enriched Artemia had significantly higher thiobarbituric acid and lower lipase activity. The highest HUFA and n‐3/n‐6 ratio were observed in fish fed F15 and F30 diets. Our results demonstrated that fish oil can completely replace with soybean oil in larval diets. Therefore, using S30 diet is recommended for feeding of Persian sturgeon larvae.     

Feeding selectivity in early life stages of Rhamdia voulezi under experimental conditions

The feeding selectivity of laboratory‐reared larvae of Rhamdia voulezi was assessed to investigate the preferred preys in early life stages. Three experiments were conducted at different larval development stages, starting soon after the absorption of the yolk sac (on fourth day after hatching – DAH), using zooplankton from fish ponds as food. Differences were detected in the patterns of prey selection among development stages. At first, the larvae prefer small preys, such as rotifers and cladocerans. Later, when they are more developed, they prefer to feed on large preys, such as copepods. On the fourth DAH, larvae in the pre‐flexion and initial flexion stage strongly selected rotifers and cladocerans (Diaphanosoma spinulosum, D. brevireme, Moina sp., M. micrura and M. minuta). At the other stages, cladocerans continued to be strongly selected. However, on the eighth DAH, larvae in flexion stage selected less strongly copepods Argyrodiaptomus azevedoi, Metacyclops mendocinus and Termocyclops decipiens. On the tenth DAH, larvae in postflexion stage selected more strongly copepods Argyrodiaptomus furcatus, Notodiaptomus cf. spinuliferus and M. mendocinus. As larvae grow, they tend to specialize in feeding of a particular prey. Prey selection of R. voulezi larvae was based both on species and zooplankton size. The size of the zooplankton ingested was related to the mouth gape size of larvae.     

Use of endoscopy for gender and ovarian stage determinations in Russian sturgeon (Acipenser gueldenstaedtii) grown in aquaculture

The Russian sturgeon (Acipenser gueldenstaedtii) is native to the Caspian Sea, the Black Sea and the Azov Sea. It used to breed in the main incoming rivers, until dam construction in the mid 20th century blocked upriver spawning migration. Aquaculture of Russian sturgeon has only recently begun, prompted by their declining populations in natural habitats and the rise in meat and caviar prices. However, information on their gonadal development and puberty under culture conditions is incomplete.

Because sturgeons have no external sexual dimorphism and there are no external markers for sexing, internal examination of the gonads must be employed for gender identification as well as to monitor their development. The present study describes endoscopic and histological observations of the gonads of young Russian sturgeons aimed at identifying gender and monitoring ovarian developmental stage in females up to the age of 6 years, when they enter their first puberty cycle, as well as at 7 years of age, when they have completed vitellogenesis, under culture conditions. This information, as related to fish age and size, is of vital importance to commercial farming of Russian sturgeon for caviar production and reproduction.

For gonadal observations in both sexes, we used an endoscopic system consisting of a 4 mm, 14 cm long cystoscope sheath incorporated with fiber-optic light transmission, connected to a halogen cold light source and a miniature videocamera with a control unit attached to a color monitor. This system allowed us viewing of the fish's abdominal organs, and to save pictures of selected areas of the gonads on a computer as the fish's personal record. Ovarian biopsies were taken in parallel for histology at typical stages of gonadal development.

Gender could be identified with this system as early as at 3 years of age and the sex ratio under culture conditions of females, males and unidentified gender were 55, 40 and 5%, respectively.

Not only did large differences occur in the developmental stages of female of the same age group, but also ovarian development was highly asynchronous at the early vitellogenic stages. In late vitellogenesis, at the “gray egg” stage (1600–2600 μm diameter), the oocytes were quite regular in size and color, and remained so until the final stages of maturity.

Our study suggests that endoscopy is an efficient method for both gender identification at an early age, and for determination of gonadal development stage in sturgeon aquaculture. The ability to see the whole intact gonads of anesthetized fish can reveal important management and research information, with minimal damage or stress to the fish.     

Effects of Feeding Rate on Habitat Quality in Fish Rearing Ponds

Manufactured feeds are commonly added to earthen ponds to enhance growth and survival of juvenile fish. However, excessive feeding may decrease fish production efficiency and yields by causing hypoxia (dissolved oxygen, DO < 2 mg/L) and stimulating excessive phytoplankton, filamentous green algae, and vascular plant growth. In this study, we quantified the effects of manufactured feed addition (no feeding, 1%, or 3% body‐weight/day, BW/d) on DO and inorganic phosphorus (P) and nitrogen (N) concentrations, plant abundance, and invertebrate prey production in ponds stocked with age‐0 channel catfish, Ictalurus punctatus. We found that the 3% BW/d ponds had lower DO concentrations and greater infestation by filamentous green algae (Rhizoclonium spp.) as compared to the 1% BW/d and no‐feeding ponds. Using stable N and carbon (C) isotopes to trace the fate of feed‐derived N and C in ponds, as well as analysis of zooplankton abundance, we determined that the supplied feed did not support or enhance production of natural invertebrate prey. To improve fish production efficiency, we recommend that managers leverage natural prey support of growth during early life, then adjust feeding levels to enhance growth of older fish and maintain suitable habitat quality.     

Spatio‐temporal,ontogenetic and interindividual variation of age‐0 diets in a population of yellow perch

Dietary niches of fishes have traditionally been evaluated at the population level, with diet pattern central tendencies compared spatio‐temporally among habitats and populations. More recently, however, studies have emphasised the importance of within‐population diet variation and niche partitioning. Several studies have examined diets of young yellow perch (Perca flavescens) at the population level and have described an ontogenetic transition from zooplankton to benthic prey during the first year of life. However, independent of ontogenetic diet shifts, intrapopulation variation of young yellow perch diets remains largely unexplored. We quantified patterns of diet composition in age‐0 yellow perch collected from Saginaw Bay, Lake Huron, USA during July–October, 2009 and 2010. We observed substantial variation in diet composition among individuals across and within sites, but found relatively weak evidence indicating an ontogenetic diet shift. Zooplankton were the dominant prey for age‐0 yellow perch on most occasions, and individual diets were composed primarily of either zooplankton (e.g. Daphnia spp., Calanoida) or benthic (i.e. Chironomidae larvae, Chydoridae) prey. These patterns were not simply attributable to differences in prey availability and ontogenetic diet shifts, because a) not only diet composition, but also prey selectivity (Chesson's α) varied among sites and b) individual and spatial diet differences were evident independent of ontogeny. Within‐cohort differences in diet composition may be an important, but often overlooked, phenomenon with implications for cumulative trophic interactions and intracohort growth and survival among young fish.     

Requirements for n-3 highly unsaturated fatty acids in feeding juvenile Iranian sturgeon (<Emphasis Type="Italic">Acipenser persicus</Emphasis>) and its effects on growth,carcass quality,and fatty acid composition

A 60-day feeding experiment was carried out on juvenile Iranian sturgeon (Acipenser persicus) to evaluate the effects of different percentages of canola oil and fish oil containing n-3 highly unsaturated fatty acids (n-3 HUFA) on fish growth and fatty acid composition. The requirement for n-3 HUFA of juvenile Iranian sturgeon (48.4 ± 1.98 g) was studied by feeding the fish with various diets containing six different percentage of n-3 HUFA ranging from 1.56 to 17.25 (% of total fatty acids). Neither the weight gain, feed conversion ratio, condition factor, specific growth rate nor the protein efficiency ratio showed any significant differences between the dietary treatments nor in the body composition of juvenile Iranian sturgeon (P > 0.05); also there were no significant difference with respect to the effect of the dietary treatment (P > 0.05) on the blood parameters, for the content of plasma protein, glucose, cholesterol, and triglyceride. The fatty acid composition of the carcass of the Iranian sturgeon fed with the diets containing various levels of n-3 HUFA was reflected by the dietary fatty acid composition. The content of n-3 HUFA in the fish increased with an increase in dietary n-3 HUFA levels. The results indicate that the dietary n-3 HUFA had no effect on the growth of juvenile Iranian sturgeon.     

Effects of exogenous dietary enzyme on growth, body composition, and fatty acid profiles of cultured great sturgeon Huso huso fingerlings

The effects of enzyme supplementation on growth, body proximate, and fatty acid composition of great sturgeon Huso huso fingerlings were investigated. Multienzyme (Kemin^?) was added to diet in different concentrations (0, 250, 500, 750, and 1,000 mg/kg). Fingerlings were randomly stocked in tanks and fed with the experimental diets three times a day at 5–10% body weight for 46 days. Fingerlings fed diets containing 250 mg/kg exogenous enzyme exhibited higher weight gain and specific growth rate. This level of enzyme in the diet significantly improved feed conversion ratio. Body protein and moisture decreased slightly when fish fed with enzyme-supplemented diets while fat increased. Fingerlings fed with 500 mg/kg enzyme in diet had higher contents of n-3 essential fatty acids and lower n-6/n-3 fatty acid ratio. Optimum growth performance in great sturgeon fingerlings can be obtained by 250 mg/kg multienzyme in the diet.     

Growth performance,lipid requirement and antioxidant capacity of juvenile Russian sturgeon Acipenser gueldenstaedti fed various levels of linoleic and linolenic acids

To evaluate the effects of dietary linoleic (LA) and linolenic acids (LN) on growth, enzyme activities to lipid addition and antioxidant capacity of the Russian sturgeon, Acipenser gueldenstaedti, 10 diets with different sources of lipid were formulated. Coconut oil in the basal diet was replaced by sunflower oil to make three diets (LA0.5 + LN0, LA1.0 + LN0 and LA2.0 + LN0) with dietary LA at 0.5%, 1.0% and 2%, by perilla oil to obtain three diets (LA0 + LN0.5, LA0 + LN1.0 and LA0 + LN2.0) with dietary LN at 0.5%, 1.0% and 2%, and by the combination of sunflower oil and perilla oil to provide three diets (LA0.25 +LN0.25, LA0.5 + LN0.5 and LA1.0 + LN1.0) containing LA and LN at 0.25, 0.50 and 1.00%. Coconut oil was used as the control diet at 10%, but without LA and LN (LA0 + LN0). After feeding for 56 days, the fish whole body, the serum and the liver of sturgeon were sampled. Fish fed LA0 +  LN0 and LA0 + LN2.0 gained lowest weight but was not significantly different from the fish fed LA0 +  LN1.0. Fish fed LA1.0 + LN1.0 gained highest weight among all groups. Feed conversion ratio was the best in fish fed LA1.0 +  LN1.0. Survival, condition factor and viscerosomatic index did not differ among treatments. Total body n‐6 fatty acids increased with the levels of n‐6. Total body n‐3 fatty acids also increased with dietary n‐3, and fish fed 2.0% LN had the highest content of total body n‐3 fatty acids among all groups. Triglyceride and cholesterol in the serum of fish fed LA1.0 +  LN1.0 were lowest. Fish fed LA1.0 +  LN1.0 showed the highest lipoprotein lipase and lipase activities, but the lowest malate dehydrogenase activity. Fish fed LA1.0 +  LN1.0 showed higher catalase, superoxide dismutase activity and total antioxidant capacity than fish fed other diets. This study indicates that diets containing both LA and LN are best for the growth of Russian sturgeon. The level of LA1.0 + LN1.0 in the diet is most beneficial for growth performance and antioxidant capacity in juvenile Russian sturgeon.     

Size-dependent predation on marine fish larvae by Ctenophores, Scyphomedusae, and Planktivorous fish

Size-dependent predation rates on marine fish larvae by the ctenophore Mnemiopsis leidyi , scyphomedusa Chrysaora quinquecirrha , and planktivorous fish Anchoa mitchilli were estimated via experiments in 3.2 m³ me-socosms. Predation rates on larvae of the goby Gobio-soma bosci were estimated in relation to 1) length of larval prey, 2) presence or absence of alternative < 1 mm zooplankton prey, and 3) a predator-prey interaction between the scyphomedusa and ctenophore. The consumption rate of larvae by the three predators generally declined as larval length increased. The ctenophore ate fewer (1.0 d^-1 per predator) larvae than did the medusa (7.8 d ^-1 per predator) or bay anchovy (4.0 d ^-1 per predator), but it consumed larvae in all size classes tested (3.0–9.5 mm SL). Predation by bay anchovy and the medusa was more size-dependent: 3.0–5.0 mm larvae were most vulnerable. Fewer larvae were eaten by the ctenophore and bay anchovy when zooplankton alternative prey was available, but predation on larvae by the medusa was not influenced by alternative zooplankton prey. Consumption rate of fish larvae by the medusa was reduced 20–25% when the ctenophore was present as its alternative prey. An encounter-rate model was developed and its parameters estimated from the experimental results. Model simulations indicated that the relationship between larval size and vulnerability is dependent on size, swimming speed, and behavior of both predators and larvae, and that bigger or faster-growing fish larvae are not always less vulnerable to predation.     

Two species of seabirds foraged in contrasting marine habitats across the cold-water belt along the coast of northern Hokkaido in the southwestern Okhotsk Sea

To understand the environmental factors affecting the density of foraging seabirds across the cold-water belt in the southwestern Okhotsk Sea, we conducted a 1-day (180-km transect length) shipboard seabird survey off the northeastern coast of Hokkaido during summer in 2019, along with acoustic observations of potential prey (zooplankton and fish) biomass, thermosalinograph measurements, and CTD observations. Planktivorous short-tailed shearwaters Ardenna tenuirostris (66% of total seabirds) and piscivorous rhinoceros auklets Cerorhinca monocerata (28%) were predominant, but foraged in contrasting habitats. A large foraging flock of shearwaters was observed in the cold-water belt zone, including its front with coastal Soya Warm Current Water and the offshore Fresh Surface Okhotsk Sea Water, where surface chlorophyll a concentrations were the highest but not related to their prey (zooplankton) biomass at any spatial scale between 4.6 and 9.2 km. In contrast, the density of auklets was high in the coastal Soya Warm Current Water, where the acoustically determined fish biomass was large, and showed a positive relationship with the fish biomass especially in the lower layer (29–104 m depth) at any spatial scale. This species-specific difference in response to prey biomass might be related to prey-searching behaviors; i.e., rhinoceros auklets search prey underwater visually, but short-tailed shearwater can use both visual and olfactory cues to locate zooplankton patches from the air.

     

Mouth size and predicted food size preferences of larvae of three cyprinid fish species

Mouth size was examined in larvae and juveniles of three cyprinid fish species: grass carp (Ctenopharyngodon idella Val.), silver carp (Hypophthalmichthys molitrix Rich.) and bighead carp (Aristichthys nobilis Rich.). A linear relationship was found between mouth size and the total length of fish, from the initial exogenous feeding stage up to 20–30 mm. Based on the mouth size, the size of the prey which could be consumed was calculated assuming 45° of mouth opening for optimum prey width and 90° for maximum prey width. Food particle size considered to be suitable for commencement of feeding amounted to 50–90 μm for silver carp larvae, 90–150 μm for grass carp larvae and 150–270 μm for bighead carp larvae. These criteria can be applied to moving rotifiers and nauplii as well as to the motionless particles of compound, dry diets.     

Fishmeal replacement by soybean,rapeseed and cottonseed meals in hybrid sturgeon Acipenser baerii ♀ × Acipenser schrenckii ♂

A 12‐week feeding trial was conducted to evaluate the effects of replacing fishmeal (FM) with soybean meal (SBM), rapeseed meal (RM) and cottonseed meal (CSM) on growth, feed utilization and body composition of juvenile hybrid sturgeon Acipenser baerii ♀ × Acipenser schrenckii ♂ (initial body weight, 8.63 ± 0.24 g). Five isonitrogenous and isoenergetic diets were formulated as follows: a control diet (FM60) containing 600 g/kg FM and four other diets (FM45, FM30, FM15 and FM0 containing 450, 300, 150 and 0 g/kg FM, respectively) where protein from FM was substituted by a mixture of SBM, RM and CSM. Fish fed FM0 and FM15 had poorer growth performance, feed utilization, apparent digestibility coefficients of dry matter, protein, lipid and gross energy, and fed FM0 had poorer hepatosomatic index and survival compared with the fish fed FM60. The whole body lipid in fish fed FM0 was significantly higher than that in fish fed FM60 and FM15. This study indicates that 300 g/kg of FM can be replaced with a mixture of SBM, RM and CSM in the diet of juvenile hybrid sturgeon without compromising growth performance, feed utilization and body composition.     

Applying the ontogeny of digestive enzyme activity to guide early weaning of pigfish,Orthopristis chrysoptera (L.)

Weaning marine fish larvae from live prey to a dry microdiet is an important step towards optimizing the commercial production, but early weaning is constrained by the lack of sufficient digestive enzymes at first feeding. This study quantified the activity of five digestive enzymes throughout the larval period of pigfish (Orthopristis chrysoptera [L.]) to assess ontogenetic changes in digestive abilities, and then trials were conducted that determined the optimal time for weaning. The activity of all digestive enzymes was low or undetectable at first feeding (3 days post hatching, dph; 2.5 mm standard length, SL). A substantial increase in activity occurred at 5.7 mm SL (17 dph), 6.9 mm SL (21 dph), 7.7 mm SL (23 dph), 8.4 mm SL (25 dph) and 11.2 mm SL (30 dph) for bile salt‐dependent lipase, trypsin, chymotrypsin, amylase and acid protease respectively. During the weaning experiment, larvae were co‐fed live prey and microdiet beginning 15 dph (4.8 mm SL). Live prey was withdrawn from the diet at 24, 28, 32 or 36 dph, with the control receiving live prey and microdiet throughout (to 43 dph). There were no significant differences in mean final SL among treatments, but survival was significantly lower when larvae were weaned at 24 dph compared to 32–43 dph. Based on the digestive enzyme activity and survival, weaning larval pigfish at 32 dph (11.7 mm SL) when reared at 24°C is recommended.     

Effects of Spirulina and plant oil on the growth and lipid traits of white sturgeon (Acipenser transmontanus) fingerlings

The aim of this research was to evaluate the efficiency of diets with Spirulina and plant oils (POs) inclusion for white sturgeon weaning and their effects on the fatty acid (FA) composition of fish flesh. Three isoproteic (45%) and isoenergetic (21 MJ kg^?1 DM) diets were formulated: one fish meal‐based diet integrated with fish oil (FMO) and two 40% Spirulina meal‐based diets integrated with corn (SPC) or soybean (SPS) oils respectively. One hundred and thirty‐five white sturgeon fingerlings (mean weight 17.5 g) were stocked randomly in nine fibreglass tanks. At the end of the trial, which lasted 71 days, the growth performance traits and somatic indexes were determined. The chemical composition, gross energy and FA profile were determined on the fish fillets. No significant effects were observed for the growth performances or fillet chemical composition. The FA profile of the fillets reflected those of the diets. In particular, the fillets of the fish fed with the SPC and SPS diets were lower in n‐3 FA, due to the substitution of fish oil (FO) with POs. It is possible to replace FO and meal in sturgeon; therefore, Spirulina meal integrated with POs could be a good alternative to sturgeon diet.     

Embryonic and larval development in the semelparous Nereid polychaete Hediste diversicolor (OF Müller, 1776) in Norway: Challenges and perspectives

Knowledge of broodstock manipulation, gametogenesis, artificial fertilization and larval nutrition is a prerequisite to reach a large‐scale production of the polychaete Hediste diversicolor. In this study, the characteristics of oogenesis and spermatogenesis and embryonic and larval development were observed. Moreover, the effect of temperature on embryonic development and effect of diets and sediment on earlier juvenile development were tested. Mature oocytes were characterized by a spherical shape with a diameter of about 200 μm. They showed radial symmetry with a large centric nucleus surrounded by four distinctive layers. Embryo cleavage started between 4 and 10 hr after fertilization, and developed to the swimming trochophore stage after 5–6 days after fertilization at 11°C. The rate of embryonic development increased with a temperature increase from 6.1 to 21.2°C. The trochophore larvae increased in length from day 9 and commenced differentiation into 3‐setiger stage larvae at day 12. The length growth of larvae until 7‐setiger stage was 0.43 mm/day when fed with fish feed, while those fed shellfish diet and smolt sludge both grew around 0.21–0.23 mm/day. The results suggest that fish feed is a superior diet compared to shellfish diets and smolt sludge for the early larval stage of H. diversicolor.