Thomas Waldrop’s research while affiliated with The Conservation Fund and other places

Swimming exercise and dissolved oxygen (DO) are important parameters to consider when operating intensive salmonid aquaculture facilities. While previous research has focused on each of these two variables in rainbow trout Oncorhynchus mykiss, studies examining both variables in combination, and their potential interaction, are absent from the scientific literature. Both swimming exercise (usually measured in body lengths per second, or BL/s) and DO can be readily controlled in modern aquaculture systems; therefore, we sought to evaluate the effects of these variables, separately and combined, on several outcomes in rainbow trout including growth performance, fin health and survival. Rainbow trout fry (18 g) were stocked into 12 circular 0.5 m3 tanks, provided with either high (1.5–2 BL/s) or low (approximately 0.5 BL/s) swimming exercise and high (100% saturation) or low (70% saturation) DO, and grown to approximately 1 kg. By the conclusion of the study, higher DO was independently associated with significantly (p < .05) increased growth performance. Significant differences were not noted in other outcomes, namely feed conversion, condition factor and mortality, although caudal and right pectoral fin damage was associated with low oxygen and low swimming exercise treatments respectively. Cardiosomatic index was significantly higher among exercised fish. These results suggest that swimming exercise and DO at saturation during the culture of rainbow trout can be beneficial to producers through improved growth performance and cardiac health.

Swimming exercise, typically measured in body-lengths per second (BL/s), and dissolved oxygen (DO), are important environmental variables in fish culture. While there is an obvious physiological association between these two parameters, their interaction has not been adequately studied in Atlantic salmon Salmo salar. Because exercise and DO are variables that can be easily manipulated in modern aquaculture systems, we sought to assess the impact of these parameters, alone and in combination, on the performance, health and welfare of juvenile Atlantic salmon. In our study, Atlantic salmon fry were stocked into 12 circular 0.5 m³ tanks in a flow-through system and exposed to either high (1.5–2 BL/s) or low (<0.5 BL/s) swimming speeding and high (100% saturation) or low (70% saturation) DO while being raised from 10 g to approximately 350 g in weight. Throughout the study period, we assessed the impacts of exercise and DO concentration on growth, feed conversion, survival and fin condition. By study's end, both increased swimming speed and higher DO were independently associated with a statistically significant increase in growth performance (p < .05); however, no significant differences were noted in survival and feed conversion. Caudal fin damage was associated with low DO, while right pectoral fin damage was associated with higher swimming speed. Finally, precocious male sexual maturation was associated with low swimming speed. These results suggest that providing exercise and dissolved oxygen at saturation during Atlantic salmon early rearing can result in improved growth performance and a lower incidence of precocious parr.

There is interest in culturing Atlantic salmon Salmo salar to market-size in land-based, closed containment systems that use recirculation aquaculture systems (RAS), as this technology often enables facilities to locate near major markets, obtain permits, exclude obligate pathogens, and/or reduce environmental impacts. Use of land-based RAS to intensively culture market-size Atlantic salmon is a relatively new frontier and little information is available. Three trials were conducted to evaluate the performance of two North American strains of Atlantic salmon raised from post-smolt to market-size (4–5kg) in a near-commercial scale (260m3), land-based RAS using only freshwater. St. John River (SJR) salmon were reared during the first trial, and Cascade salmon (CS1 and CS2) were evaluated during two subsequent trials. Salmon were received as fertilized “eyed” eggs and cultured on-site through the entire production cycle. The grow-out period began at 14-16 months post-hatch when salmon post-smolt weighed 0.34 − 0.75kg on average. CS1 and SJR salmon grew 386–393g/month to a mean size of 4.1-4.2kg and CS2 salmon grew 413g/month to a mean size of 4.9kg prior to first harvest. Thereafter, weekly salmon harvests commenced for the next 6-19 weeks. The grow-out period, excluding harvest, lasted 9-10 months for each trial. Average water temperature was maintained at 15-16C. Consistently linear growth rates were achieved by each population suggesting that growth was relatively independent of fish cohort/genetic strain, fish size, and maximum biomass density, which was 35, 100, and 118kg/m3 for SJR, CS1, and CS2, respectively. Feed conversion ratios ranged from 1.07-1.10. Fish mortality (including culls) for SJR, CS1, and CS2 was 9.5, 6.6, and 7.5% of the original number of stocked fish, respectively. No obligate fish pathogens, kudoa, sea lice, or pervasive parasites were detected. Salmon were not vaccinated against specific pathogens; and no antibiotics, pesticides, or harsh chemotherapeutants were used. Hydrogen peroxide (50–100ppm) and salt (10 ppt) were occasionally used to treat fungus during pre-smolt production, and salt (2-3 ppt) was used to treat fungus or ameliorate stress after handling events. No salmon escaped the facility due to built-in fish exclusion barriers. Early male maturation was observed during each trial. Male salmon began to exhibit maturation traits (kype, darkened skin coloration) at a mean weight of 1.5–2kg and were removed from the grow-out system when they weighed 2–3kg. SJR, CS1, and CS2 populations exhibited 37.0, 38.5, and 17.0% maturity, respectively. Fillet yield and product quality of immature, market-size salmon were comparable to reported measurements for commercially available salmon reared in net pens. This research suggests that it is biologically and technologically feasible to culture Atlantic salmon from post-smolt to market-size in a land-based RAS of suitable commercial scale; however, early male maturation could represent a production barrier. As of 2016, all-female Atlantic salmon eggs are commercially available and could provide an expedient solution to the problem of early male maturation in RAS.

As global aquaculture fish production continues to expand, an improved understanding of how environmental factors interact in fish health and production is needed. Significant advances have been made towards economical alternatives to costly fishmeal-based diets, such as grain-based formulations, and defining the effect of rearing density on fish health and production. Little research, however, has examined the effects of fishmeal- and grain-based diets in combination with alterations in rearing density. Moreover, it is unknown whether interactions between rearing density and diet impact composition of the fish intestinal microbiota, which might in turn impact fish health and production. We fed aquacultured adult rainbow trout (Oncorhynchus mykiss) fishmeal- or grain-based diets and reared them under high- or low-density conditions for 10 months in a single aquaculture facility, and evaluated individual fish growth, production, fin indices, and intestinal microbiota composition using 16S rRNA gene sequencing. We found that the intestinal microbiotas were dominated by a shared core microbiota consisting of 52 bacterial lineages observed across all individuals, diets, and rearing densities. Variation in diet and rearing density resulted in only minor changes in intestinal microbiota composition despite significant effects of these variables on fish growth, performance, fillet quality and welfare. Significant interactions between diet and rearing density were only observed in evaluations of fin indices and relative abundance of the bacterial genus Staphylococcus. These results demonstrate that aquacultured rainbow trout can achieve remarkable consistency in intestinal microbiota composition, and suggest the possibility of developing novel aquaculture strategies without overtly altering intestinal microbiota composition.

Improved equipment and husbandry practices are required to effectively grade and harvest fish in large land-based culture tanks. The objective of our work was to develop and evaluate several types of relatively inexpensive, portable, and efficient fish handling equipment to reduce the labor requirement for grading and harvesting fish from large circular culture tanks. This equipment and husbandry practices also had to provide for worker safety and minimize the stress or damage to the fish. Two techniques were developed and evaluated to remove the entire population from a large and deep circular tank, i.e. (a) purse seine and (b) carbon dioxide avoidance response. Two other techniques were developed and evaluated to remove the fish from a large (150m3) and deep (2.44m) circular culture tank after they had been top-graded in situ using a 3-panel clam-shell grader: (c) an airlift fish pump and hand sorting/dewatering box and (d) a sidewall drain box for hand sorting/dewatering. Some of these technologies are new, while others (such as the purse seine) have been used in other applications. Our commercial-scale evaluation of these technologies provided insight into the advantages and disadvantages of each option. With use of the clam-shell grader, the majority of the fish in the culture tank were never lifted from the water during the self-sorting process, which minimized stress, perhaps enhancing final product quality. In contrast, harvesting the tank using the purse seine and hand brailing was much more labor intensive and increased the stress on the fish, as indicated by a nearly 10-fold increase in fish mortality compared to the mortality observed when the clam-shell type crowder/grader system and an airlift fish pump or sidewall drain box were used during fish harvest. The combination of the clam-shell crowder/grader with the sidewall drain harvest box was our preferred harvest method, because of its low labor requirement, relatively low fish mortality, and rapid harvest rate. We also think that the carbon dioxide avoidance harvest technique can be used effectively, with little labor input and practically zero mortality when the entire fish population must be removed from a fish culture tank, but not during a selective harvest using in situ grading. Ultimately, the more effective technologies and practices should help fish farmers overcome scale-up issues and improve land-based fish farm profitability.

A model partial-reuse system is described that provides an alternative to salmonid production in serial-reuse raceway systems and has potential application in other fish-culture situations. The partial-reuse system contained three 10m3 circular ‘Cornell-type’ dual-drain culture tanks. The side-wall discharge from the culture tanks was treated across a microscreen drum filter, then the water was pumped to the head of the system where dissolved carbon dioxide (CO2) stripping and pure oxygen (O2) supplementation took place before the water returned to the culture tanks. Dilution with make-up water controlled accumulations of total ammonia nitrogen (TAN). An automatic pH control system that modulated the stripping column fan ‘on’ and ‘off’ was used to limit the fractions of CO2 and unionized ammonia nitrogen (NH3N). The partial-reuse system was evaluated during the culture of eight separate cohorts of advanced fingerlings, i.e., Arctic char, rainbow trout, and an all female brook trout × Arctic char hybrid. The fish performed well, even under intensive conditions, which were indicated by dissolved O2 consumption across the culture tank that went as high as 13mg/L and fish-culture densities that were often between 100 and 148kg/m3. Over all cohorts, feed conversion rates ranged from 1.0 to 1.3, specific growth rates (SGR) ranged from 1.32 to 2.45% body weight per day, and thermal growth coefficients ranged from 0.00132 to 0.00218. The partial-reuse system maintained safe water quality in all cases except for the first cohort—when the stripping column fan failed. The ‘Cornell-type’ dual-drain tank was found to rapidly (within only 1–2min) and gently concentrate and flush approximately 68–88% (79% overall average) of the TSS produced daily within only 12–18% of the tank’s total water flow. Mean TSS concentrations discharged through the three culture tanks’ bottom-center drains (average of 17.1mg/L) was 8.7 times greater than the TSS concentration discharged through the three culture tanks’ side-wall drains (average of 2.2mg/L). Overall, approximately 82% of the TSS produced in the partial-reuse system was captured in an off-line settling tank, which is better TSS removal than others have estimated for serial-reuse systems (approximately 25–50%). For the two cohorts of rainbow trout, the partial-reuse system sustained a production level of 35–45kg per year of fish for every 1L/min of make-up water, which is approximately six to seven times greater than the typical 6kg per year of trout produced for every 1L/min of water in Idaho serial-reuse raceway systems.

The objective of this research was to evaluate the dissolved carbon dioxide stripping efficiency of two types of 1-m tall structured plastic packing (tubular NORPAC and structured block CF-3000 Accu-Pac media) that were placed separately in two full-scale forced-ventilation cascade columns that were located within a coldwater recirculating aquaculture system at the Freshwater Institute. These two structured packing types were selected because they both provide large 4–5 cm void spaces that are either vertically-continuous (e.g. the tubular NORPAC) or an open structure with zigzagging but continuous void spaces (e.g. the blocks of cross-corrugated sheet media), which should reduce the likelihood of plugging with biosolids. Water flow rates were adjusted so that each cascade column was loaded with either 87, 136 and 187 m3/h water flow per m2 of cascade column plan area (i.e. 35, 56 and 76 gpm/ft2). Air:water loading rates of 2.2:1 to 3.4:1, 5.1:1 to 5.6:1, and 9.5:1 to 9.9:1 were produced by setting the water flow rates through each column at 1.62, 2.54 and 3.48 m3/min, respectively, and then measuring the resulting air flow rate through the column under these conditions. As expected, the dissolved carbon dioxide removal efficiencies of both structured packing tested were found to depend on the volumetric air:water loading rate applied. The lowest volumetric air:water loading rate (i.e. 2.2:1 to 3.4:1) resulted in only 21–24% dissolved carbon dioxide removal. However, the dissolved carbon dioxide removal efficiencies rose to 32.4–33.6 and 35.8–37.2% for the medium and high air:water loading rates, i.e. 5.1:1 to 5.6:1 and 9.5:1 to 9.9:1, respectively. A second objective of this research was to determine if either packing would plug with biosolids after long-term operation. At the end of approximately 1 year of operation, both of the plastic packing materials were examined from the top of the packing to determine if potential fouling or plugging problems were apparent. A thin layer of brown biofilm covered both packings, but the biofilm did not appear to threaten water or airflow through the packing. In addition, no large mats of biosolids were visible from the top of either column. However, flooding at the interface of the support screen and the tubular NORPAC was suspected to have reduced air flows measured at the highest hydraulic loading rate tested (i.e. at 187 m3/h per m2), which coincided with the lowest air:water loading rates tested.