In the previous post, we touched on the benefits of outdoor shrimp farming systems and how it was a dream come through for inland farmers like us. However, upon the operation of the site, we realized that the outdoor shrimp farming system has its own unique set of challenges. In this post, we will try to uncover some of the challenges that are not so obvious until you start to operate the outdoor ponds.
Water availability (TIDE BASED)
Despite being 300-400 meters from the coast, you will not be able to get good quality seawater daily. On average, you will only be able to get 12-16 days of good seawater every month due to the tidal effect. Only when the tides are high enough on days during the new moon and full moon will you be able to perform water exchange. You can check out how the tidal changes based on the calendar month by using this cool website. Using this tide chart, you can see that the tide only exceeded 2.5m on the 26th November-30 November, and it doesn't happen again until 7 December-13 December. We always try to only take in water when the tide is about 2.5m, as it allows for saltier water to enter the pond. Taking a much lower cut-off will usually mean that the water will also be less in salinity and minerals and higher in silt. It will not be a huge problem when you’re stocking at a low density where the average feeding rate is low, but you will need water exchange at a high feeding rate. So in theory, you can be stuck in a situation where you have high ammonia due to excessive feeding but have no water for your exchange. Of course, you can also use probiotics to break down excessive organic material, but it will be at an additional cost to your production. Another method is to build a pipeline into the sea and directly draw in seawater with stable water chemistry. The trade-off is a high capital expenditure. Which is usually too much for a small-medium operator.
Unwanted species
Another problem associated with the intake of raw seawater involves introducing underwater species. Despite having a filter mesh over the water supply, some of the species are still able to pass through the opening in the nets. For our first batch of outdoor ponds, we have opted to use a screen size of 800 microns to filter the incoming seawater. Although 800 micrometers (0.8mm) was enough to filter out the unwanted adult species, it was not able to filter out the larvae or juveniles. Many of the unwanted species came into the pond during the juvenile stage and proceeded to grow out into adulthood and reproduce inside the pond. Some of these species include the wild species of tiger prawn, seabass, and spear shrimp. These wild spear shrimps are commonly known as sea prawns and have a very similar resemblance to the vannamei. Among the unwanted species within the pond, these wild spear shrimp cause the biggest problem in terms of survival and FCR. Due to their aggressive nature, they can easily outcompete the vannamei for feed, increasing the FCR of the pond culture. Furthermore, they have also been known to reduce the survival rates of the pond. They also have a shorter reproduction cycle, and it is possible to reproduce very quickly in the pond. During the mid-stage of the culture, we happened to go inside the pond to fix some equipment, and we found many of these shrimp larvae in the water's surface. What we observe is that the larvae population for this spear shrimp reduced at the end of the culture cycle, we hypothesize that only the larger vannamei can hunt and feed on the spear shrimp larvae at body sizes of 20-25 grams. While you might think why don't we just use a smaller mesh size for our filter? In our case, we have tried to swap into a smaller mesh size of 200 microns, but we find it reduces clogging up the flow too much. The problem is also compounded by the fact that we have only 16 days of seawater intake in a month, with only a few hours to get good seawater. So based on this scenario, what we have done differently from this batch is not to perform any water exchange for the first 50 days. This is to ensure that we do not introduce any unwanted species into the pond. But things never go as planned when you’re farming shrimp outdoors. :)
Leaky Pond (reduction in water level)
Another problem that you might not anticipate is the ponds constructed might be leaky due to the nature of the soil. The severity of this condition depends on several factors such as the clay content of the soil, whether compaction was done, and the width of the bund, etc. In our case, it was obvious that the pond was leaky as water flowed to the adjacent pond where we just finished our crop. We intended to rotate the ponds after each harvest to allow time for proper drying of the pond to oxidize the organic material. However, if any of the adjacent ponds are leaking it will not result in proper drying of the pond. After the first harvest, we were careful not to introduce any unwanted species into the pond by placing a 200-micron mesh filter and slowly filling up the pond. We first intended not to perform any water exchange for the first 60 days to allow our shrimp to have grown to a certain size before introducing water exchange with the 800-micron filter sheet. However, we realized on day 50 the water level was too low due to the leaking into the adjacent pond. That we were forced to introduce water quickly with a wider mesh filter. The water level dropped over 35-40% for 1.5- 2 months. But the temperature swing due to the lower water level was bearable as we were in the monsoon period (no excessive temperature swings).
In traditional earthen pond farming, everybody tends to focus on the water quality, diseases, and flooding risk. But what many tend to overlook is the points that we have mentioned previously. In the next part, we will be covering some of the diseases we have encountered and what we are currently doing to battle this issue.