Effects of Food Limitations On Crawfish Reproduction

William R. Mcclain  |  11/4/2005 11:35:04 PM

Females with young.

Louisiana Farm & Ranch 1(9):29-31

Crawfish culture in the 130,000 acres or so of ponds in Louisiana uses no hatcheries and little or no formulated feeds. Instead, producers rely on natural reproduction from introduced or indigenous adults, and the nutritional needs of crawfish are met from the natural food chain, fueled by rice plants or other types of vegetation.
 
When crawfish populations are high, food resources can become depleted too soon, resulting in crawfish broodstock entering summer burrows with low energy reserves. Low energy reserves in pre-reproductive females have been theorized to result in low reproduction, but this has not been clearly demonstrated. Therefore, a preliminary study was conducted at the LSU AgCenter’s Rice Research Station to investigate this potential cause-and-effect relationship.

Mature female red swamp crawfish were collected in June from experimental ponds differing in food resources; they were held in the lab in simulated burrows for spawning. Experimental treatments at the time of collection consisted of ponds that were:

  1. Depleted of nearly all forage substrates.
  2. Replete in rice forage (consisting of both dead and green material).
  3. Depleted of forage but fed with a 25% crude protein formulated feed.
  4. Depleted of forage but fed whole raw soybeans (high in protein and energy).

The supplemental feeds were offered at 35 lb/acre/day, 5 days/week for 3 weeks before collection of crawfish. Crawfish were monitored in the lab for survival and reproduction over a 6-month period.

Survival during the burrow period averaged 74%, with only a slight advantage among the fed crawfish; however, among the surviving females, those that received supplemental feed before entering simulated burrows had greater reproductive success (a higher rate of spawning and more offspring). While only 9% of surviving crawfish from the forage-depleted treatments spawned, 29% of those from the forage-replete ponds spawned, and 51% and 58% of surviving crawfish from the pellet-fed and soybean-fed ponds, respectively, spawned. The number of offspring per female was 71, 225, 157 and 160 (or 5, 9.5, 9 and 8 per gram of female weight) for those respective treatments.

It was highly apparent that limited food resources near the end of the crawfish production season resulted in lower reproduction. Moreover, short-term feeding with high protein/high energy feeds in forage depleted ponds before crawfish burrowing seemed to mitigate the food shortages and even increased the spawning rate over those crawfish from ponds rich in forage. There appeared to be little difference due to feed type: whole, raw soybeans offered about the same amount of benefit as did the more expensive formulated feeds.

Further studies are needed to better corroborate this apparent cause-and-effect relationship. These results do not necessarily imply that supplemental feeding of broodstock should automatically be a recommended practice. In a productive permanent pond where carry-over crawfish are plentiful and ponds often tend to be overcrowded, improved reproductive performance is not always needed or desired. For crawfish that will be used as stockers in new ponds, it is also equally important to ascertain whether feeds offer a significant benefit.

While it is highly desired that survival and reproductive performance of introduced broodstock in new ponds be maximized, it is uncertain whether supplemental feeding of ponds is the most cost-effective means of obtaining maximum reproductive performance. Given an adequate period of time from crawfish stocking to draining of the new pond, crawfish may be able to gain sufficient energy reserves in the relatively food-rich environment of a new pond or rice crop without the use of supplement feeds. This deserves further study.

The preliminary findings of this experiment clearly suggest the importance of continued work in the area of crawfish nutrition and reproduction. This study was only the first in a series of similar studies, and the LSU AgCenter will continue to investigate these and other important areas of crawfish aquaculture.

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