No abstract

Experiments were conducted to determine the performance of larval walleye (Stizostedion vitreum) reared in clear, 0.2 nephelometric turbidity units (NTU), and turbid water, 20 or 50 NTU, on formulated feed. Larvae were cultured for 21, 28, or 30 days posthatch in four trials with 3 or 4 replicate tanks per treatment (clear and turbid) in each trial. Duration of each trial was dependent on the arrival of newly hatched larvae for subsequent trials. The desired turbidity levels were obtained by pumping a solution of clay to the culture tanks every 20 (trial 1) or 30 min (trials 2, 3 and 4). Other than turbidity, all environmental conditions (dissolved oxygen, alkalinity, pH, total ammonia, un‐ionized ammonia, nitrate, nitrite, chloride, and hardness) and rearing techniques were similar between treatments. Larvae were stocked at 20/L and fed formulated feed (Fry Feed Kyowa B‐400 and B‐700) every 3 to 7 min, 24 hours per day. Significant differences in feed acceptance and total length between larvae in the clear and turbid water were observed as early as 7d posthatch. Larvae in turbid water began eating the formulated diet one to two days before those in clear water. In all trials, survival, final length, and final weight of larvae reared in water of high turbidity were significantly greater than for larvae reared in clear water. Mean survival (±SE) for all four trials was 27.7 ± 5.6% in high turbidity water and 5.9 ± 1.3% in clear water. At the end of the trials, mean total length of the larvae reared in turbid water was at least 3.2mm (15%) greater than that in the clear water. Mean final weight of the larvae from turbid water was 2.25 times greater than larvae from clear water over the four trials. In two of the four trials, gas bladder inflation (GBI) of larvae reared in high turbidity was significantly greater than for fish reared in clear water, but the difference in GBI was not significant in the other two trials. In this study, performance of larval walleye was greatly enhanced by water with a turbidity of at least IS NTU.

Calcium chloride was used to increase total hardness of hatchery well water from 40 mg/L to 200 mg/L, and two experiments were conducted to evaluate effects on egg diameter and hatch rates of eggs from striped bass Morone saxatilis. In the first experiment, fertilized eggs were divided into two lots; one lot was incubated in untreated well water with a total hardness of 40 mg/L, the other lot was incubated in water hardened to about 200 mg/L. The eggs that were incubated in treated water hatched at a significantly higher rate (70.3%) than the control eggs (53.9%). The mean egg diameter of the treatment eggs (2.45 mm) was significantly smaller than the control eggs (3.14 mm). In a second experiment, eggs were incubated in hardened water for different durations. Fertilized eggs were incubated for 3, 6, or 48 h in hardened water then transferred to well water. Eggs incubated for 48 h in hardened water hatched at a significantly higher rate (44.7%) than those incubated for 3 h (25.3%) or 6 h (31.9%). No significant difference was detected in numbers of eggs per liter between treatments. The higher hatch rate will permit meeting production goals while using fewer broodstock and incurring lower production costs.

Thyroid hormones, 3,5,3',5'tetraiodothyronine (T4) and 3,5,3'-triiodothyronine (TJ) have been found in the eggs of several teleost species and are potential regulators of larval development, growth, and survival. The purpose of the present study was to determine whether natural variation of TJ and T4 in the eggs of six stocks of walleye, five wild stocks from Kansas, Iowa, Wisconsin, Minnesota, and North Dakota, and a semidomesticated stock from an Ohio fish hatchery, have an effect on larval performance in mass culture. Immersion studies were conducted with samples of larvae from four of the same stocks at exposure concentrations of 0.01.0.05, and 0.1 ppm of T3 and T4. Natural egg concentrations of TJ (range from 0.70 to 1.5 ng/g wet weight of egg) were not significantly different among stocks. Although means of T4 concentration among stocks were significantly different (range 0.53-9.27 ng/g), the difference was caused by the exceptionally high concentration for the Wisconsin stock (9.27 ng/g f 2.20). Performance measures of the Wisconsin stock (Mississippi River), were not related to T4 concentration in that stock. In spite of similarity in concentrations of T4 and T3 in the eggs, there were significant performance differences among the stocks (survival to 21 d, gas bladder inflation, viability, cannibalism, and growth). The immersion studies, however, revealed a significant difference in incidence of cannibalism and temperature units (TU) to 50% mortality (i.e., survival) between the control groups and treatment groups exposed to 0.01 to 0.1 ppm T, and T4. Survival was extended more than 2-fold longer in larvae immersed in T3 compared with T4. The immersion study indicated that thyroid hormones are potentially regulators of walleye development, but further investigations are needed to determine reasons for differences in larval performance based on natural concentrations in the egg and artificial exposure (immersion).The thyroid hormones (TH), namely thyronine (3,5,3'-triiodo-L-thyronine, T,), and thyroxine (T4) have metabolic, structural, central nervous system, and behavior effects in vertebrates (Gorbman 1969). Of these two hormones, thyronine (T,) is considered to be the more active form (Eales 1985). Furthermore, studies have shown that thyroid hormones affect development and growth of larval fish (Brown et al. 1987).

In 1992 at Rathbun State Fish Hatchery, experiments were conducted to evaluate the effects of stocking density on performance of larval walleyes (Stizostedion vitreum) in small (278-L) cylindrical tanks, and to compare fry performance in production-scale (6 79-L) and small tanks. Hatched fry were reared for 21 d and fed Fry Feed Kyowa B400 and B700 in all experiments. I>ensilies of 20, 30, and 40 fry/L were evaluated in two trials, and densities of 20, 50, and 60 fry/L were evaluated in another trial. Fry performance did not differ significantly among density treatments of 20, 30, or 40 fry/L. In the other trial, however, survival was significantly greater for fish stocked at 20 fry/L than for fish stocked at 50 or 60 fry/L, but the number of fry produced (yield) was 216% greater in the tanks stocked at 60 fry/L. Growth rate was not affected by density in any trial. For comparisons between 278-L and 679-L tanks, three 679-L tanks were stocked at 20 fry/L in each of the three trials. Survival and viability differences between tank sizes were inconsistent, but the highest survival was an average of 74.9% to 21 d posthatch in the three 679-L tanks in the second trial. Cannibalism and gas bladder inflation were more common in the small tanks. Incidence of cannibals was as high as 7.5% of the original number stocked in the small tanks, but did not exceed 3.5% in the large tanks. Over all three trials, from 98.5 to 100% of the survivors in the 278-L tanks, and from 88 to 96.3% in the 679-L tanks, had inflated gas bladders. The outstanding rate of gas bladder inflation is attributed to the use of a surface spray, which seems to clear the water surface of oil and debris. These experiments demonstrate the feasibility of mass culture of larval walleyes in production-scale tanks.