Shrimp Behavior 101
It's midnight. Do you know where your shrimp are? Rod McNeil knows because he's underwater watching them with lights!
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"Lying motionless and silent on the bottom, I watch their approach, and their reaction to me. When they spot me, all their antennae go up. I move toward them, the big shrimp on the leading edge of the troop take a head high posture and keep an eye on me. I don't move in any closer than a meter and a half. They freeze, watch me for thirty or forty seconds, and then troop right over the top of me."
At Aquaculture 2001 (Orlando, Florida, USA, January 2001), I interviewed, Rod McNeil, a shrimp farming consultant that specializes in super-intensive systems. Rod has observed shrimp behavior all over the world, in the wild, in ponds, during the day, and at night with lights. He has observed all the popular farmed species, focusing on the postlarvae and juvenile stages in the wild and the juvenile and adult stages in ponds.
Shrimp News: What's a nice guy like you doing in shrimp farming?
Rod McNeil: I'm an Aquatic Ecologist. When I first became interested in shrimp farming, I looked at it from an economic point of view and came to the conclusion that farms would have to intensify to be cost-effective. Next, I figured that I better know what the shrimp were doing in the natural world before attempting to farm them in intensive systems.
In the late 1990s, I started my underwater observations of shrimp in Florida, in the estuaries along the Gulf of Mexico, and went on to observe wild shrimp behavior in Mexico, Costa Rica, Panama, Australia and Ecuador. I observed a variety of species including Penaeus duorarum, P. aztecus, P. stylirostris, P. vannamei, P. semisulcatus, P. latisulcatus, P. esculentes and P. monodon. Most of these observations were of small juvenile shrimp.
In Ecuador, I did prolonged observations of wild vannamei to see if there were variations between their daytime and nighttime behavior. I wanted to see what life was like in vannamei's natural environment.
Shrimp News: What kind of equipment did you use?
Rod McNeil: I'm a diving fanatic from way back, so I started with SCUBA gear, but found that when I got in too close, the bubbles disturbed the shrimp. So I switched to rebreather equipment and was able to get in real close. I can stay down for up to eight hours. I use a DiveCom full face mask with voice-activated recording equipment so that I can record my thoughts and observations. At night, I use infared halogen floodlights that match the color temperature of my film and allow observation without affecting the behavior of the shrimp.
Shrimp behavior changes dramatically at night, and, as expected, the biggest changes take place at sunset and sunrise. During the day, juvenile shrimp move around in troops, packed solid, as many as 250 per square meter. They move together somewhat the way a school of fish moves, in unison. When threatened by a predator, they scatter the same way a school of fish scatters, confusing the predators and making it difficult for them to zero in on individual animals. The troops become much more compact as the amount of light increases. At midday, you see the tightest troops. At night, when the predators can't see them, juveniles scatter throughout their environment. During the day, the troops will break up and scatter about if there is substrate available, like mangrove roots or sea grass.
I found that young animals, below a couple of grams, behave similarly across species. They are basically grazers, living on surfaces, the surfaces of mangrove roots, the blades of sea grass, harvesting the periphyton and copepods, which feed on the periphyton. You don't find them out in the water column looking for food. It's dangerous out there--where the predators can see them. On a blade of sea grass, their translucent bodies disappear into the background, as they munch away on the surface growth. You don't usually see juvenile shrimp on the bottom, they are usually hanging on to something. Up to five grams or so, all the farmed species gravitate to substrates and feed on detritus.
When you look at how we do shrimp farming, it seems really odd. The only surface we provide is the bottom. Relative to the surface area that could be created in the pond, it's a small fraction of what they would normally have in the wild. Same thing in the hatchery, we give them the bottom when they are looking for grass and roots.
Typically, juvenile shrimp are in shallow estuaries where the water is only five to fifteen feet deep, hanging under a root or on the backside of a sea grass blade. Viewed from above, they're out of sight.
Shrimp News: Did you also observe adult shrimp behavior?
Rod McNeil: Yes, but it's much more difficult to find adults in the wild because they are usually out at sea in deep water. In Australia, however, I observed adult monodon, merguiensis, esculentus and latislucatus in the shallows. During the day, they travel in troops of two or three hundred members.
As adults, japonicus and monodon are very benthicly-oriented, spacing themselves out in territories, hunting for polychaetes and other creatures in the pond bottom. Adult vannamei and stylirostris, on the other hand, continue to feed on vertical and suspended substrates throughout their life cycles. If you intend to grow shrimp in intensive bacterial floc systems, monodon probably would not be a very good choice. It takes less crowding and likes to dig in during the day. Species like vannamei, esculentes, latisulcatus and semisulcatus are much more detritus-oriented than monodon. They spend a lot of time on sea grass beds. If you look at the work the Australians have done, you will find that there is about 13 times the surface area on the sea grass than there is on the bottom. The shrimp population densities are high, but not on the bottom. They're on all this structure. That's one of the reasons why mangroves are so important as shrimp nursery grounds. Out of sight, juvenile shrimp graze on the periphyton and the copepods associated with them on the undersurface of the roots.
With monodon, you don't see these trooping behaviors at mid-day. They usually occur at dawn or just after dawn and at dusk. By ten in the morning, many are buried in the bottom.
Shrimp News: What did your observations of wild shrimp behavior tell you?
Rod McNeil: About five years ago, when I first got involved with shrimp, I dove some ponds in Colombia and discovered that the pond environment was quite different than the natural environment. Very, very different in fact. Look at a typical growout pond. There's nothing for the shrimp to cling to, no structure, no mangrove roots, nothing but the bottom. In the natural world, shrimp enjoy a tremendous amount of structure during most of their life cycle. Living without it probably causes stress.
All species of shrimp like structure. Some of the open water shrimp, like merguienses and duorarum, don't use it as much as other species, but even they like to associate with it, hide out in it and hover under it. So there's structural association regardless of species. With species like vannamei and stylirostris, there's an extremely strong structural response. They really like structure and they want to be on it all the time. If you put anything into a pond with stylirostris and pull it out ten minutes later, I guarantee, there will be stylirostris clinging to it. They like structure that much. They'll come out of the water before letting go of that structure.
Shrimp News: Let's talk about shrimp behavior in ponds. In most ponds, you have an earthen bottom and no structure. What are the shrimp doing in there?
Rod McNeil: Well, by the time shrimp are 5 grams and larger, their behavior in ponds is pretty much the same across species, so my comments apply to all species of farmed shrimp. Shrimp go through many physiological and behavioral changes when they are young, but by the time they are in the 2-5 gram range, they begin to walk and act like adults. While most of my observations in the wild were of postlarvae and juveniles, most of my pond observations have been of juveniles and adults.
Shrimp News: What do you see when you slip under the surface of a shrimp pond?
Rod McNeil: I might survey seventy percent of the bottom without seeing anything. Then, suddenly, as I lie motionless on the bottom of the pond, a troop of thousands of shrimp approaches. The farmer stocked at 20 per cubic meter and here they are at densities of 250 to 400 shrimp per cubic meter. A vast portion of the pond is unoccupied. It's one of the major things that I have observed in shrimp ponds and seems to occur with most farmed species. As shrimp become juveniles, they take up group social activities.
During the day, shrimp form troops and roam around the bottom of the pond looking for food. They move at a pretty good clip, especially the benthic species like stylirostris and vannamei, with the very largest shrimp at the head of the troop and the smallest shrimp at the back, or not really with the troop at all. The smallest shrimp are off in the shallows around the edge of the pond looking for food and protection.
Within the troops, the competition for food is brutal. Obviously, the guys at the front of the troop get most of the feed. So if you don't do a good job of dispersing your feed, those guys get most of it.
Shrimp News: Does the troop have a shape?
Rod McNeil: Yes, it's shaped like a teardrop, with the bulbous portion forming the leading edge, and the biggest shrimp across its front, then tapering off to the smaller shrimp at the rear.
When the sun comes up the troops are quite small, maybe only five or six shrimp in each, but as they start roaming around the bottom of the pond looking for food, they bump into other troops and unite. This process continues until you have some really big troops. I really don't know how big they get. I've seen some troops with several thousand shrimp in them. It's kind of scary when you're lying on the bottom and watching them come toward you.
Shrimp News: Have you ever done your observation when the pond was being fed?
Rod McNeil: Sure, if the feed falls within ten meters of the troop, they apparently sense it instantaneously, change direction and head for the feed. If you throw out a very uniform layer of feed ahead of the troop, say a minute ahead of its arrival, as they approach, they accelerate, and much of the feed gets trampled into the bottom, buried before any shrimp feed on it. Shrimp are opportunists, they grab the top layer and forget about what's smashed into the bottom. Another interesting thing they do is grab a pellet and continue to walk along with the troop, apparently not wanting to miss the next feeding spot, and not wanting to be abandoned by the troop. They carry the pellet with them and chew on it for awhile and then drop it. Or if they see a bigger pellet, they'll drop the first one and pick it up. So if you look at a feed tray or a mound of feed that has been poorly dispersed, you'll see a trail of pellets away from the tray that taper off in the direction of the troop's new path. Stragglers and runts show up and feed on the pellets and pieces that have been left behind by the big shrimp.
If the shrimp are in fairly small troops, they will dig for trampled pellets.
At night when the troops break up, the shrimp go back over the bottom and dig up buried pellets. You don't see digging behavior with vannamei during the day; it's mostly a nighttime behavior. Other species, like P. monodon, show strong digging behavior all day long. When they uncover a pellet, it usually explodes into a cloud of tiny particles.
Shrimp News: What about the natural feeds in the ponds. Are they feeding on them?
Rod McNeil: Yes, in fact, there's a very strong preference for them, particularly at night, when shrimp become solo hunters. During the day, when they're trooping around the pond, their antennae are swept back along their sides. At night, when the troop breaks up, and individuals move out on their own, the antennae are very active, sweeping forward like tentacles, searching for food. When the antennae lock on to a food source, they point directly at it and sometimes twitch. They can sense the extremely small electrical fields generated by the animals that live in the pond bottom. When they locate something, they move their antennae over the spot and then start digging. It's tough to see what they pull out of the bottom because they tear it apart so quickly. They're vicious little critters.
Shrimp News: What roll is the shrimp's eye playing in all of this?
Rod McNeil: I think the eyes play some roll in feeding, especially when they are right on top of something and tearing it apart. Otherwise, the eyes are probably used to detect predators coming in from above.
Shrimp News: At night, are the shrimp evenly distributed on the pond bottom?
Rod McNeil: Yes, at night, maybe an hour after dusk, the troops break up, the individual shrimp spread out across the bottom and start digging for feed source material. They don't have a strong preference for pellets at night; that's when they go after the natural feeds in the pond. They will walk right by the pellets, dig a hole and grab something, often I can't tell what. About two hours before dawn, their feeding behaviors begin to change, and they begin eating pellets again.
Shrimp News: Shrimp farmers often feed at night. You must consider that a pretty bad practice?
Rod McNeil: In general, yes, shrimp probably utilize much less commercial feed at night. I would not distribute more that a third or a quarter of the total ration at night. And feeding between 10 p.m. and 4 a.m. is probably an exercise in futility. Since they miss so much of the feed anyway, from being trampled, torn apart or missed, the amount of feed wasted in a pond is a pretty appalling thing. Feed lost in the bottom amounts to at least a third of the total feed. I think one of the strongest impressions that I come away with after lying on the bottom for a couple of hours is that there has to be a better way to feed shrimp.
Shrimp News: I get the idea that shrimp are always feeding?
Rod McNeil: That's right, that's pretty much all they do, night and day, around the clock. They are always on the move looking for food. If they really stuff themselves, they may slow down for a while, but an hour and a half later, they revert back to feeding behaviors. A shrimp's life is always on the move, always eating.
Shrimp News: What happens when you put a feeding tray into a pond to monitor feed consumption?
Rod McNeil: Because it's a large olfactory signal, a troop will eventually pick up on it and arrive at the tray. Very early in the morning, the troops are small so you get a fairly good representation of shrimp consumption. But if you're putting out trays at high noon or at two o'clock in the afternoon, stand back because you can have very large troops with thousands of members arrive at the tray at the same time. When they arrive, it's absolute mayhem. I'm typically two or three feet away, lying flat on the bottom, and it's actually scary. The big shrimp at the head of the troop get there first and swarm over the feed. As the smaller shrimp arrive, they clamor on top of the big shrimp and before you know it, the tray is covered with five or six layers of shrimp. The big ones on the bottom attempt to throw anything smaller than themselves off the tray. The big shrimp get most of the feed. They stay on the tray for fifteen or twenty minutes and then they all grab a pellet and walk off. As they walk away, they'll chew on the pellet for a while and then drop it. A smaller shrimp then may come along, pick it up, chew on it for awhile and also drop it.
If you pull a tray when the troop is on it, all the shrimp on top, probably the medium-size shrimp that could not get down to the bottom of the tray, immediately jump off. The little guys were never on the tray in the first place. So what you see when you pull up a tray is the most successful feeders on the bottom of the tray that could not get out of the way because all of their brethren were piled on top of them. From my underwater view, the tray explodes into a volcano of activity with all the shrimp doing their "to the rear" move at the same time. They crash into each other and flop about as they attempt to abandon ship. What remains when the tray arrives at the surface is just a tiny fraction of what was on it when you started the pull.
If you put a feed tray out after a major troop just walked by and if the troop was headed into the current, which is usual, you may not get any activity at that tray until another troop discovers it, which could provide false data to the person monitoring the trays.
If you deploy a feed tray just behind a major troop in the middle of the day, you're likely to get a very poor sample of feeding behavior and food utilization because only the stragglers and runts will be there. Fed trays should be out for at least two hours in semi-intensive ponds. The main troop moves around in a circle, closer to the edges of the pond than the center, heading into the current. I have no idea how many troops there are in a pond. In some 12-hectare ponds, it's hard to find the shrimp and then, suddenly, a troop of thousands of shrimp will pass right over the top of me. And then, I won't see any shrimp for an hour. It's one of the reasons that you have to go down and stay down. Lying on the bottom of the pond facing into the current, it might be thirty or forty minutes between troops, particularly on a semi-intensive farm that's stocked at 20 PLs per square meter.
Shrimp News: Will the troop pass right over you?
Rod McNeil: When I'm diving with rebreather equipment, I'm basically a lump on the bottom from the shrimp's point of view. Yup, they troop right over me.
Shrimp News: Do they nibble on any of your body parts?
Rod McNeil: Yes, although I dive with a wetsuit, my hands and head are exposed and they nibble on the skin, pinch it here and there, trying to figure out how to eat it.
Information: Rod McNeil, 14460 East Shore Road, Polson, MT 59860 USA (phone 406-883-6921, email email@example.com).
Source: Rod McNeil. Interview by Bob Rosenberry, Shrimp News International, January 2001.