Investigating Dissolved Oxygen Challenges in Aquaculture Systems (Fish Welfare Initiative)
This report contains an executive summary. Here it is, reproduced exactly as requested.
Executive Summary
From August 18th to October 13th, Fish Welfare Initiative conducted an important exploratory study on six ponds known for historically low dissolved oxygen (DO) levels (AJU1, AJU2, CRP, GRA, NRO, and VMS). The study's primary aim was to assess DO and its potential causes in each selected pond (the study was not designed to compare data or assess correlations across ponds). This initiative aimed to enhance our understanding of the extent of DO issues and identify specific causes in each targeted pond. Quantitative measurements were recorded in each pond once every two days, complemented by a qualitative analysis of farmer behaviors, achieved through regular interactions and close observation of their daily practices. The results were deliberated in reference to FWI-established water quality ranges deemed necessary for the well-being of the species, as supported by existing literature. A high-level overview of the quantitative data is shown in the table below.
Table with the Frequency DO, pH, and ammonia were inside of optimal and required ranges
| Parameter | No. Measurements | % of measurements within range | |
|---|---|---|---|
| Optimal (The ideal range for the species) |
Required (The acceptable range for the species) |
||
| DO (Morning) | 100 | 11% | 23% |
| DO (Evening) | 104 | 16% | 32% |
| pH | 205 | 9% | 63% |
| Ammonia | 101 | 0% | 42% |
| Chl-a | 195 | 25% | 37% |
The main findings were:
- In five out of six ponds, DO was a significant welfare concern. On average, morning DO was within our required range only 23% of the time, with one pond recording 0% of morning DO measurements within range.
- In three out of six ponds, a notable correspondence was observed between fish biomass and DO levels. High biomass in AJU1 before harvesting (an industry term for removing fish from the pond to be sold) was connected with low DO levels. As fish biomass steadily increased throughout the study in both VMS and NRO, a consistent decline in DO levels was observed. A lower biomass in GRA was related to balanced DO levels. This suggests that fish biomass may play an important factor in managing DO levels, a factor previously undervalued by FWI.
- The relationship between phytoplankton and DO was complex. In some ponds like AJU1 and CRP, a clear link was evident, while in others, it was less apparent. In general, phytoplankton's impact on DO seemed dependent on fish biomass, with high fish biomass exerting a stronger impact on DO than phytoplankton biomass.
- The Chlorophyll-a (Chl-a) range defined by FWI might be set too low. In CRP and NRO, despite Chl-a levels being within the prescribed range, DO levels remained suboptimal. In GRA, Chl-a levels above our optimal range were linked with good DO levels.
- Fish gasping events (where fish will rise to the water's surface to “gasp" for air) tended to coincide with DO levels below 1 mg/L and ammonia levels above 0.5 mg/L. These stressors likely act to magnify each other, as ammonia toxicity impairs respiratory function, making fish more susceptible to low oxygen conditions. This is likely one of the most stressful experiences fish commonly experience within the pond.
- Ammonia levels posed a significant welfare issue, staying within the required range in only 42% of the measurements. Various factors, including heavy rainfall, fertilization, unbalanced feeding ratio, and pond bed disturbances, were identified as potential causes of increased ammonia levels.
- All farmers in the study reported larger pond sizes than actual, with discrepancies ranging from approximately 1 to 11 acres. This raises concerns about the accuracy of reported stocking densities and suggests that fish may be more densely stocked than previously thought.