Larval Walleye Research Articles

A new method for force-feeding larval fish

Knowledge of digestive function in fish larvae may provide insight into the problem of poor survival of first feeding larvae, but research approaches to investigate digestion in larvae are limited. To compare the functional development of the digestive system in altricial and precocious larvae, we developed a method for force-feeding radio-labeled nutrients to fish larvae. The method involves micro-injection of diets into the gut of anesthetized and immobilized larvae in a petri dish under a dissecting microscope. Three protocols are described illustrating the application of this method to studies of larval digestive function. To determine the development of acid digestion, pH indicator solution was injected and the gut of the transparent larvae was monitored for color change. To determine nutrient assimilation efficiency, radio-labeled nutrients were injected. After a period of time, counts of the recovery water and the larvae were made using liquid scintillation counting. Finally, to determine sites of nutrient uptake, radio-labeled nutrients were force fed to larvae that were subsequently examined by microscopic autoradiography. The utility of this method is illustrated with observations of protein, polypeptide and free amino acid assimilation in larval walleye ( Stizostedion vitreum) and pH changes in larval striped bass ( Morone saxatilis).

Effects of Surface Water Spray, Diet, and Phase Feeding on Swim Bladder Inflation, Survival, and Cost of Production of Intensively Reared Larval Walleyes

Abstract Three experiments were conducted to determine the effects of a surface water spray and diet on survival and swim bladder inflation of intensively reared larval walleyes. Survival and swim bladder inflation were determined at 30 d posthatch. In each experiment, 275-L cylindrical tanks were stocked with 20 fry/L. In experiment 1, a spray of water was applied to the surface of three tanks, and three additional tanks received no surface spray. BioKyowa FFK-B diet was fed to fish in all tanks. Survival of fish with the spray (11.8%) and without the spray (9.1%) was not significantly different, but the incidence of swim bladder inflation was higher with (98.4%) than without the spray (51.7%). In experiment 2, larval walleyes were fed one of three commercially available larval feeds—FFK-B, San Francisco Bay Brand EPAC, and Zeigler Larval AP200—and all tanks received a surface spray. Only the fish fed FFK-B had reasonable 30-d survival (33.1%) and swim bladder inflation rates (95.6%). In experiment 3, ph...

Length reduction and dry weight loss in frozen and formalin-preserved larval walleye, Stizostedion vitreum (Mitchill)

We examined the relationships between fresh and preserved measurements of length and dry weight for larval walleye, Stizostedion vitreum (Mitchill), subjected to freezing and preservation in formalin. Length reductions for both preservation techniques were <5% and decreased with larval size. Dry weight losses ranged from 32·1 to 54·0% for frozen larvae and from 18·2 to 30·1% for larvae preserved in formalin with larger larvae losing proportionately less weight. Freezing caused significantly greater length shrinkage and dry weight loss than preservation in formalin.

Occurrence of a check in otoliths of reared and sea-caught larval walleye pollock Theragra chalcogramma (Pallas) and its relationship to events in early-life history

In frontal sections of postlarval otoliths of walleye pollock Theragra chalcogramma (Pallas), some increments were observed to be fused, forming a band structure (check) on the outer distal side. Backward counts of otolith daily growth increments showed the check was formed at different dates for different individuals, beginning from early to late in March and terminating from late in March to mid-April. A comparison between samples from various areas around Hokkaido revealed that the check was formed at almost the same body length (8–11 mm), independent of their age in days and growth patterns. Otoliths of larvae reared under constant environmental conditions also showed the band structure, appearing at the same body length as for wild fish. Comparing these observations with those published in the literature, morphological or physiological changes seem to be responsible for the check formation that the fish experience at 8–11 mm body length.

Age-specific mortality and transport of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska

A field study was conducted in May 1988 to estimate mortality rates of individual cohorts of larval walleye pollock Theragra chalcogramma in Shelikof Strait, Gulf of Alaska. Two surveys of the area of larval distribution were done 12 d apart and daily mortality rates estimated from changes in abundance of each cohort. This study was designed to minimize potential sources of bias to mortality estimates, such as larval extrusion or escapement from sampling gear, advective or diffusive gain or loss of larvae into or out of the sampling area, the uncertainty incurred by the use of length as a proxy for age, and the often erroneous assumption of constant production and mortality rates. The age- specific mortality rates from this study were low compared to rates previously estimated for larval wall- eye pollock. Four significant mortality estimates, ranging from 0.041 to 0.0f6 d-', came from among the youngest cohorts. The low larval mortality rates in 1988 coinaded with very calm ocean conditions. Larval drift rates estimated in this study were coherent with the movement of a satellite-tracked drift buoy and averaged 3.1 to 3.9 an s-', which is comparable to rates reported from other studies.

The survival and growth of larval walleye, Stizostedion vitreum, and trophic dynamics in fertilized ponds

Two fertilization methods, inorganic fertilizers (IF) versus combined inorganic and organic fertilizers (IOF), were compared in four ponds with two replicates of each treatment at a fish stocking density of 244 000 ha −1. Fish survival and total length averaged 77.9% and 38.9 mm in the IF ponds, and 25% and 49.2 mm in the IOF ponds respectively. No significant differences were found for the abundances of total Zooplankton ( P > 0.15) and phytoplankton ( P > 0.34) between the two treatments, but the mean dissolved oxygen at the bottom was significantly lower in the IOF ponds (6.8 mg O 2I −1) than that in the IF ponds (10.4 mg O 2I −1( P < 0.021). The high fish mortality in the IOF ponds may have been due to low dissolved oxygen, or to ammonia toxicity. We suggest that in hatchery ponds filled with lake water, the use of inorganic fertilizer alone (N: P = 20: 1) will provide better results than the combined use of inorganic and organic fertilizers.

Evidence for Natural Reproduction by Stocked Walleyes in the Saginaw River Tributary System, Michigan

Abstract A program of stocking walleyes Stizostedion vitreum into Saginaw Bay, Lake Huron, begun in the late 1970s by the Michigan Department of Natural Resources and sportsmen's groups, resulted in large spawning runs in tributary streams in the early 1980s as water quality in the bay improved. Natural reproduction was documented by a 2-year sampling effort for larval walleyes that revealed the presence of substantial numbers of larvae over a 15–18-d period, with maximum densities (350-3,500/1,000 m3) recorded on 21 April 1987 and 6 May 1988 in the Saginaw River and on 6 May 1988 on the Tittabawassee River. More larval walleyes were collected at night and near bottom (3 m) than during the day and near the surface (1 m). The Tittabawassee River, one of four major rivers that form the Saginaw, was a major source of walleye larvae collected in the Saginaw River, but some walleye larvae were also collected in the Shiawassee River. Over 500,000 walleye larvae passed by the study area on the Saginaw River in 2...

Tests of Drift Samplers for Estimating Abundance of Recently Hatched Walleye Larvae in Small Rivers

Abstract The efficiency of drift nets for assessing abundance of larval walleyes in small rivers was tested during the springs of 1985–1987 by capture of hatchery-reared larvae released upstream of various arrays of drift nets. Catches of larvae in 18 trials with small drift samplers (17.8-cm mouth diameter) averaged only 5.3% (95% confidence interval, 3.3–7.2%) of expected values when larvae were released 1 or 5 km upstream of the nets. Short-term, short-distance release–capture experiments (10 min) with pairs of these samplers and a barrier net indicated that near 100% efficiency could be obtained with these nets in optimal conditions. Patterns of capture by the nets suggested that larval behavior, predation, and mortality were largely responsible for the poor capture rates. Also, fluctuations in stream discharge, heterogeneous distribution of drifting larvae, and distance from the source of larvae affect the effectiveness of sampling gear.

Observation and analysis of fishery processes: larval pollock at Shelikof Strait, Alaska

ABSTRACTConditions affecting distributions of larval walleye pollock (Theragra chalcogramma) were examined at Shelikof Strait, Alaska, during springtime, 1986 and 1987. Abundance and distribution of larval pollock southwest of the Strait's southern entrance was determined with oblique plankton tows taken each year in May. Infrared images of sea surface temperature patterns were derived from AVHRR scenes obtained by NOAA satellites during each April and May. Pattern displacements between 24‐hour‐interval images were used to estimate surface motion. Each spring, measurements were taken by remote weather stations and ships, and a nearsurface current meter record was obtained during 1987. Treated as quasi‐synoptic, spatial relations between sets of surface temperature, surface flow, and larval pollock distributions show coincidences between submesoscale physical and biological features. The highest larval abundances occurred as patches within a cold plume (1986) and an eddy (1987). These confirm that physical features can retain larval pollock on the continental shelf. Observations are examined for evidence of physical and biological events that jointly can cause such coincidences and foster alternatives for survival during transport to nursery grounds. Explanations for presence of cohorts observed within the 1987 eddy are given in terms of spatial and temporal relationships evident between spawning and hatching areas, hatch date distributions, meanders, eddy generation and movement, background flow, and advection times. The observations, analyses, and results are consistent with the concept of a coupled, fluctuating biophysical process that can emulate variations in larval abundance and provide a multiplicity of system pathways for early‐life stages representations.

A comparison of two production-scale modules for the intensive culture of larval walleye

A 2·6 m 3 round-bottomed trough and a 3·3 m 3 round tank with a hemispherical bottom were tested as potential modules for a production system for walleye (Stizostedion vitreum) larvae during the first 30 days after hatching. Each module was stocked with 50 000 larvae. The larvae were reared entirely on a prepared feed. After 30 days, survival in the trough was 47% but only 13% in the tank. Fish in both modules reached a total length > 30 mm after 30 days. Swim-bladder inflation in the tank was 69% but only 16% in the trough. Excessive horizontal water flow at the surface may have impeded inflation in the trough. Water quality in each system was satisfactory. The trough required the least maintenance.

Flow Cytometric Identification of Larval Triploid Walleyes

Abstract A simple and rapid flow cytometric method of identifying triploidy in larval fish was developed. Prolarvae (4.6–8.0 mm total length) of walleye (Stizostedion vitreum) were mechanically dissociated into single-cell suspensions and stained with the metachromatic fluorescent dye acridine orange. Dissociation of the prolarvae, application of a two-step acridine orange staining procedure, and identification of ploidy level by flow cytometry was accomplished in under 10 min/sample. This procedure could be adapted for other fish species and could be used to identify other levels of polyploidy or mosaicism.

System for Maintaining Sediment Suspensions during Larval Fish Studies

Abstract A new system was developed for maintaining suspensions of inorganic solids during laboratory studies on early life stages of fish. Microfine bentonite was successfully held in suspension in specially constructed units during a 21-d fishless test, a 28-d experiment with juvenile green sunfish (Lepomis cyanellus), and four shorter experiments (5–9 d) with four species of larval fishes, white sucker (Catostomus commersoni), northern pike (Esox lucius), channel catfish (Ictalurus punctatus), and walleye (Stizostedion vitreum). Each experiment on larval fish was conducted until the yolk-sac had been absorbed. Concentrations of bentonite ranged from 0 to 728 mg/L. Each unit consisted of a holding chamber set in a stainless steel bowl and two submersible pumps that recirculated the suspension. Turbidity readings remained nearly constant throughout each experiment. Because the turbidity of suspensions was well correlated with bentonite concentration (r 2–0.989) and easy to measure, turbidity was used as ...

Electron transport system(ETS) activity as a possible index of respiration for larval walleye pollock Theragra chalcogramma.

Electron transport system (ETS) activity was measured for larvae of the walleye pollock Theragra chalcogramma, with the objective of assessing this method for estimating the respiration of fish larvae in situ. The ETS activity decreased minimally after 50 days' storage at -80°C. The ETS activity increased linearly with larval dry weight. The average dry weight-specific ETS activity of feeding stage larvae was 9.75±2.25 (S.D.) μlO2/mg•h, which was significantly higher than 5.82±0.88μlO2/mg•h for pre-feeding stage larvae. The average ratio of respiration to ETS activity was 0.47±0.16. As in laboratory raised larvae, the ETS activity in wild larvae also increased linearly with larval dry weight. The regression coefficient of laboratory raised and wild larvae were not significantly different. The average dry weight-specific ETS activity for wild larvae was 8.60±2.99μlO2/mg•h. The comparison of the ETS activity in laboratory raise and wild larvae indicates that the ETS activity method has potential to assess respiratory activity of pollock larvae in situ. Further experiments are needed to evaluate the impact of various environmental conditions to the respiration to ETS activity ratio, which is necessary to convert ETS activity data to respiration.

Effects of physical factors on the vertical distribution of larval walleye pollock Theragra chalcogramma under controlled laboratory conditions

Effects of physical factors on the vertical distribution of larval walleye pollock Theragra chalcogramma under controlled laboratory conditions

Communications: Factors Influencing Early Mortality of Walleye Fry Reared Intensively

Abstract The effects of diet, rearing conditions, and source of fish were tested in four sets of 20-d feeding trials with larval walleyes (Stizostedium vitreum) at the Bozeman (Montana) Fish Technology Center. Conicalbottom 140-L tanks with upwelling flow were used to test 13 treatments, including six commercial diets under various water temperature and light conditions. The feeding of BioKyowa B-250 diet resulted in the highest survival (as high as 15.6% after 20 d). In one trial, a significantly higher percentage (54.1 %) of fry fed Aquafauna Bio-Marine 4200 diet had inflated swim bladders than fish in other treatments (P < 0.05). Histological examination showed that cell degeneration and bacterial infection can develop in inflated swim bladders. Bacterial infections, failure of swim bladders to inflate, and cannibalism apparently caused low survival. Survival of fry from each of four sources was significantly different (P < 0.05), so the origin of the fish may influence the performance of walleye fry u...

Method for the Intensive Culture of Walleye

Abstract Techniques were developed to rear larval walleyes (Stizostedion vitreum) successfully through the first 30 d, the current bottleneck in the intensive culture of this species. Fish were reared at production densities of 60 fish/L at two temperatures, 19 and 24°C. Upwelling water flow with frequent food provision maintained food particle densities close to 100/L. Fish were fed entirely on a new experimental diet, Kyowa Fry Feed (Kyowa Hakko Kogyo Co., St. Louis), offered in graded particle sizes appropriate for larval walleyes. This diet was readily ingested by larvae from the first day of feeding. At 19°C, mean survival to 30 d was 28%, and final total lengths were 23.4 mm. Fish reared at 24°C grew to 31.4 mm, but a single catastrophic decrease in dissolved oxygen caused large losses, and final survival in this group was only 4%. Further modifications of this culture system will be needed to improve water quality and facilitate swim bladder inflation.

The effects of feeding periodicity and ration on the rate of increment formation in otoliths of larval walleye pollock Theragra chalcogramma (Pallas)

The effect of feeding conditions (periodicity and ration level provided) on otolith increment counts of laboratory reared larval walleye pollock Theragra chalcogramma (Pallas) was tested. Increments were deposited with daily periodicity for larvae growing under optimal feeding conditions (high, continuous ration). Increment counts determined by light microscopy were less than expected, assuming a rate of formation of 1 · day −1 for larvae grown at low rations. Increment deposition and length (and therefore growth rates) were positively correlated. Daily increment resolution was disrupted after 4 days of starvation. Evidence is discussed indicating that increments continue to be deposited on otoliths of slow-growing and starved larvae, but are too narrowly spaced for resolution by light microscopy. Feeding larvae more than once daily had no effect on increment deposition rates. For up to 38 days, increment counts from the sagitta and lapillus were equivalent, but for the first 20 days increment counts were easier from the lapillus, which was larger. After 20 days, the sagitta was larger and overall easier to read.

Intensive Culture of Larval Walleyes with Dry or Formulated Feed: Note on Swim Bladder Inflation

Abstract Two experiments were conducted to test the effects of modified feeding techniques and three dry or formulated diets on the survival, growth, and swim bladder inflation of larval walleyes (Stizostedion vitreum). In experiment 1, duplicate lots of 3,000 fish/tank were fed either a gelatin–casein diet (Gel), a semimoist extruded diet (Biotrainer), or dehydrated, decapsulated brine shrimp (Artemia sp.) cysts (Decaps). Mean survival to 21 d was greater for fish consuming Biotrainer (12.3%) than for fish fed Decaps (1.6%) or Gel (0.5%). Fish consuming Decaps were longer at the end of the trial (P < 0.01) than either Biotrainer- or Gel-fed groups (18.2 mm versus 16.7 and 13.1 mm total length, respectively), but the proportion with inflated swim bladders was less (44% Decaps versus 83% Biotrainer and 94% Gel; P < 0.01). In experiment 2, Biotrainer and Decaps were fed to triplicate lots of 2,000 fish/tank. Mean total length at 28 d was greater (P < 0.01) for fish consuming Decaps (21.3 versus 17.3 mm), but survival (Decaps, 6.1 %; Biotrainer, 5.3%) and swim bladder inflation (Decaps, 57%, Biotrainer, 60%) did not differ between the two treatments. The percentage of fish with inflated swim bladders, which had been low in earlier trials, seemed to increase with the use of feeding rings; the rings restricted the area of surface film caused by the feeds. The Decaps and Biotrainer were both consumed well and supported adequate growth rates.

Sensitivity differences between eggs and larvae of walleye pollock (Theragra chalcogramma) to hydrocarbons

Exposure of embryonic and larval walleye pollock ( Theragra chalcogramma ) to the water-soluble fractions (WSF) of Cook Inlet crude oil during embryonic development caused mortality and a variety of morphological abnormalities. Median WSF concentrations (1·8 ppm) which caused mortality after hatch did not differ significantly from those causing morphological abnormalities (1·6 ppm). Pollock larvae exposed to WSF did not develop abnormalities, but swimming and survival were significantly affected within 4 h of exposure. The LC 50 for larvae dropped from 1·9 ppm on day 4 to 0·9 ppm on day 10. The tissue of developing pollock larvae bioaccumulated significantly more dissolved toluene and naphthalene than those of embryos. Consequently, when exposed to equal concentrations of WSF, larval tissues are exposed to much higher hydrocarbon concentrations than embryonic tissues. On the basis of internal tissue concentrations, larvae were less sensitive than eggs, but the opposite conclusion is reached if external (WSF) concentrations are used for estimation of sensitivity.

Effects of Diet on Growth and Survival of Larval Walleyes

Abstract The effects of diet quality on larval walleye (Stizostedion vitreum vitreum) growth and survival are described. The cyclopoid copepod Diacyclops thomasi consumed larval walleyes within 10 min at dense copepod concentrations and within 1 d at lower densities (500 organisms/L). At initial feeding, larval walleyes consumed both copepods and cladocerans 500-1,100 μm total length. Postlarva-II walleyes fed four different diets (minnow larvae, brine shrimp, and two size grades of zooplankton) for 6 d averaged 4.00 mg, 3.48 mg, 2.23 mg, and 1.92 mg (dry weight), respectively. No appreciable differences in survival (70-85%) were observed on these diets. During the first 3 weeks of life, the survival of walleye larvae at optimal conditions of diet, temperature, light, space, and density was 71% (range, 52-87%). The combined influence of higher water temperatures and smaller food organisms stimulated initial feeding 3 d sooner than previously reported for larval walleyes. Sufficient food and higher temperatures may be factors in suppression of cannibalism. Artemia sp. may be the best choice of a first food, because brine shrimp support high growth and survival of larval walleyes and do not prey on the larvae as do some copepods.