ALBERT

Notes: Chloroform-methanol (2:1 v/v), absolute methanol, and 90% acetone were evaluated for their effectiveness as extractants of chlorophyll a from samples of phytoplankton communities collected from catfish ponds. Chloroform-methanol consistently extracted more chlorophyll a than either 90% acetone or methanol. Precision for the methanol extraction was also unacceptably low, with an average coefficient of variation of 17%. Average coefficients of variation for the chloroform-methanol and 90% acetone extraction procedures were 6 and 5%, respectively. Filtered samples should be steeped in chloroform-methanol for at least 4 h to obtain maximum chlorophyll extraction, and the addition of MgCO3 to the extractant as a buffer is not necessary.

Notes: Channel catfish fingerlings were stocked into 16 0.04-ha ponds at a rate of 24,700 fish/ha on 5 May 1992. Four replicate ponds were used for each of the following treatments: 1) fed once daily at 0830h; 2) fed once daily at 1600h; 3) fed once daily at 2000h; and 4) fed on demand using demand feeders. Fish on the first three treatments were fed to satiation. All fish were harvested 145d after stocking. Results from this study showed that when channel catfish raised in ponds were fed once daily to satiation, time of feeding had no significant impact on water quality, feed consumption, feed conversion, weight gain, or body proximate composition. Fish fed on demand consumed more feed than fish fed once daily to satiation, but difference in weight gain was not significant. These data indicated that feeding time may not be critical for channel catfish production as long as fish are fed when dissolved oxygen is sufficient. Although feeding at night was not detrimental in this study, night feeding is not recommended on large ponds unless sufficient aeration is available to quickly provide oxygen in an emergency—and even then it would be problematic.

Notes: A data set describing annual variation of water quality in ten commercial channel catfish Ictalurus punctatus ponds was subjected to exploratory statistical analysis to infer ecological processes affecting pond water quality. Two factors explained 67% of the variation in concentrations of water quality variables. The first factor (Factor 1) explained 49% of the variance and was associated with a large negative loading by total ammonia-nitrogen and large positive loadings by total nitrogen, total phosphorus, chemical oxygen demand, and chlorophyll a. Factor 1 was interpreted with respect to factor loadings to represent the effect of phytoplankton biomass. The second factor (Factor 2) explained an additional 18% of the variance and was associated with a large negative loading by soluble reactive phosphorus and large positive loadings by nitrite-nitrogen and, to a lesser extent, nitrate-nitrogen. Factor 2 was interpreted to be related to variation in pond sediment oxygenation. Although factor analysis indicated the overwhelming effect of phytoplankton biomass on water quality, opportunities for management of phytoplankton communities in large commercial aqua-culture ponds are limited. However, maintenance of an oxidized sediment-water interface may improve water quality by limiting the diffusion of reductant-soluble phosphorus from sediment to water and increasing sediment nitrification rates.

Notes: Three pairs of brood channel catfish Ictalurus punctatus were induced to spawn in aquaria supplied with flowing water from a reservoir filled with ground water (calcium hardness = 110 mg/L as CaCO3= 44 mg/L as calcium). Fertilized egg masses were allowed to incubate in aquaria for 6 h and each mass was then split into five portions. The five portions were then allowed to hatch and the resulting sac fry developed in waters with calcium hardnesses of 0, 1, 5, 10, or 100 mg/L as CaCO3 (0, 0.4, 2, 4, or 40 mg/L as calcium). Test waters were prepared from distilleddeionized water and reagent-grade chemicals; tests were conducted using static-renewal conditions. Survival from hatch to onset of exogenous feeding (“swim-up”) averaged 62% in calcium-free water and 98% at all other calcium hardness levels. Wet weight gain, dry weight loss, and resistance to environmental hypoxia were significantly affected (P 〈 0.05) by environmental calcium levels: best growth, yolk utilization rate (indicated by changes in dry weight and visual observation), and tolerance to low dissolved oxygen concentrations were found at calcium hardnesses of 10 and 100 mg/L as CaCO3. Based upon these results, a minimum calcium hardness of 10 mg/L as CaCO3 (4 mg/L as calcium) is recommended for channel catfish hatchery water supplies.

Notes: Cyanobacteria (blue-green algae) in the genera Anabaena, Aphanizomenon, Microcystis, and Oscillatoria often form extensive and persistent blooms in freshwater aquaculture ponds. Bloom-forming cyanobacteria are undesirable in aquaculture ponds because: 1) they are a relatively poor base for aquatic food chains; 2) they are poor oxygenators of the water and have undesirable growth habits; 3) some species produce odorous metabolites that impart undesirable flavors to the cultured animal; and 4) some species may produce compounds that are toxic to aquatic animals. Development of cyanobacterial blooms is favored under conditions of high nutrient loading rates (particularly if the availability of nitrogen is limited relative to phosphorus), low rates of vertical mixing, and warm water temperatures. Under those conditions, dominance of phytoplankton communities by cyanobacteria is the result of certain unique physiological attributes (in particular, N2 fixation and buoyancy regulation) that allow cyanobacteria to compete effectively with other phytoplankton. The ability to fix N2 provides a competitive advantage under severe nitrogen limitation because it allows certain cyanobacterial species to make use of a source of nitrogen unavailable to other phytoplankton. The ability to regulate cell buoyancy through environmentally-controlled collapse ad reformation of intracellular gas vacuoles is perhaps the primary reason for the frequent dominance of aquaculture pond phytoplankton communities by cyanobacteria. Cyanobacteria that can regulate their position in the water column gain a distinct advantage over other phototrophs in poorly mixed bodies of water. In addition to the physicochemical interactions that influence phytoplankton community dynamics, cyanobacterial-microbial associations may play an important regulatory role in determining community structure. Cyanobacteria are always found in close association with a diverse array of microorganisms, including eubacteria, fungi, and protozoans. These associations, which in the past have often been viewed as antagonistic, are increasingly seen as mutualistic and may function in a positive manner during bloom development.

Notes: This study was conducted to compare water quality and channel catfish production in earthen ponds located in two dissimilar physiographic regions of the southeastern United States and supplied with water of disparate quality. Ponds at Auburn, Alabama are on acidic Piedmont soils and filled with poorly mineralized runoff water; ponds at Stoneville, Mississippi are on slightly alkaline alluvial clays and filled with groundwater of high total alkalinity and hardness. Channel catfish were stocked at 8,750 fish/ha, fed daily, and provided nightly aeration in 0.04-ha ponds at both sites. Ponds were managed as similarly as possible. Minimum daily water temperatures and pH were higher at Stoneville than at Auburn, and there were greater concentrations of suspended clay turbidity, dissolved inorganic phosphorus, total ammonia-nitrogen, and nitrite-nitrogen at Auburn than at Stoneville. The taxonomic composition of the phytoplankton community was broadly different between the two sites. Taste tests revealed off-flavor in fish at both sites, but there were no significant differences (P 〉 0.05) in flavor scores between sites. The quality of flavor was somewhat different between sites, and these differences in quality were thought to result from observed differences in the taxonomic composition of phytoplankton communities. All differences in water quality seemed to be directly or indirectly related to the dissimilarity in the quality of the water supply and soils at the two locations. Although some water quality variables differed between sites and changed over time at both sites, environmental conditions never deteriorated enough at either site to cause serious stress or mortality in fish. There were no significant differences (P 〉 0.05) in average net fish production, survival, weight of individual fish at harvest, or feed conversion ratios. Average net fish production and feed conversion ratios, respectively, were 4,905 kg/ha and 1.27 at Auburn and 5,286 kg/ha and 1.27 at Stoneville. The results of this study demonstrate the need for site-specific investigations when conducting certain types of aquaculture research.

Notes: Selected water quality variables were measured at monthly intervals for 1 yr in 10 commercial channel catfish ponds in northwest Mississippi. Temporal changes in most variables appeared to be related to seasonal periodicity of phytoplankton abundance. Phytoplankton standing crops and total organic matter were highest in summer months when primary production was favored by warm water temperatures, high solar irradiance, and large inputs of nutrients resulting from high summer fish feed allowances. As day length, water temperature, and feed inputs decreased in autumn and winter, phytoplankton abundance and organic matter concentrations decreased. Seasonal changes in total nitrogen and total phosphorus concentrations were similar to phytoplankton abundance because much of the total nitrogen and phosphorus was contained within phytoplankton cells. Contrasting to the seasonal trend for total nitrogen, concentrations of dissolved inorganic nitrogen were lowest in the summer and highest in the cooler months. Rapid assimilation by phytoplankton served to maintain relatively low concentrations of dissolved inorganic nitrogen during the summer despite highest nitrogen loading rates during that period. Low water temperatures and generally less favorable conditions for phytoplankton growth decreased rates of nitrogen assimilation in the winter and ammonia, nitrite, and nitrate accumulated. Soluble reactive phosphorus concentrations were low throughout the year because physico-chemical processes, such as precipitation and adsorption to bottom muds acted to continually remove inorganic phosphorus from the water column.

Notes: Abstract.— Two studies were conducted to evaluate the potential use of limnocorrals (in situ enclosures) for evaluating the effects of phytotoxic compounds on phytoplankton community structure and water quality. Limnocorrals consisted of open-bottomed, fiberglass cylinders that were 2.44 m in diameter and 1.53 m high. The enclosures were placed in an aquaculture pond and allowed to settle 10–20 cm into the bottom mud, forming a watertight seal that isolated approximately 5.5 m3 of pond water. The first study evaluated the effect of water mixing on environmental conditions within limnocorrals. Mixing was accomplished by injecting air through airstones suspended inside the enclosures. Conditions in unmixed limnocorrals rapidly deviated from conditions in the pond and in aerated enclosures, with overall phytoplankton biomass decreasing while abundance of cyanobacteria and concentrations of soluble reactive phosphorus increased. Conditions in limnocorrals with continuous mixing also deviated from conditions in the pond. Environmental conditions among replicate mixed enclosures were, however, relatively consistent and stable for at least 2 wk. The second study evaluated the use of limnocorrals for testing the effects of phytotoxic compounds on phytoplankton community structure and water quality. A commercial chelated copper algicide was added to randomly selected, mixed limnocorrals at the label-recommended rate. The algicide killed nearly all phytoplankton in the treated enclosures within 1 wk; however, treated limnocorrals were rapidly recolonized by green algae and diatoms. Conditions in untreated limnocorrals remained relatively stable and consistent among replicates for 16 d, after which total phytoplankton biomass began to decrease, possibly due to nutrient depletion within the enclosures. Although conditions inside the enclosures deviated with time from those in the surrounding pond water, mixed limnocorrals appear to provide a convenient and reliable method for short-term studies of algicides and other water quality manipulations.

Notes: Channel catfish fingerlings Ictalurus punctatus were exposed to copper sulfate or a commercial chelated copper product in a series of static toxicity tests conducted using waters with a wide range of total alkalinity and hardness values. Estimates of mean 96 h LC50 values were 0.05, 0.73, 0.95, and 0.98 mg/L as Cu for copper sulfate and 0.06, 1.51, 1.97, and 1.74 mg/L as Cu for the chelated copper product in waters having total alkalinities of 16, 76, 127, and 240 mg/L CaCO3 respectively. On a copper basis, the chelated product was significantly (P 〈 0.05) less toxic to fish than copper sulfate in all waters except that of the lowest total alkalinity. Highly significant (P 〈 0.01) linear relationships were found between LC50 values for copper from copper sulfate and pH, log [total alkalinity], and log [total hardness], of test waters. These results cast doubt on the validity of the formula commonly used to calculate practical copper sulfate pond treatment rates, which is based upon a simple linear relationship between application rate and total alkalinity.