Fish Health Notes - Archives Mar/Apr 2000

Runt recovery worth a look

ITHACA, NY - Separating out runts the little ones from your tanks and ponds may be worth the effort, according to a recently published study.

Researchers at the University of California at Davis (UC Davis) examined the fate of sturgeon runts at a commercial fish farm and found that a substantial proportion of these "poor growers" recovered during a three-month study.

As part of their findings, they described the cost-effectiveness and conditions under which raising runts in separate tanks might be profitable.

In a follow-up study, the same researchers M. P. Georgiadis, R. P. Hedrick, W. O. Johnson, and I. A. Gardner also showed that once sturgeon runts recovered and started growing, they grew as well as the "never been a runt" group.

Before I start summarizing the details of this work, I need to caution you that the UC Davis team's conclusions are only applicable to the population studied. Still, I think this "first-of-its-kind" epidemiological investigation is worth a look.

Making the grade

Most of you are aware of the practice of grading fish. Grading is based on the assumption that small fish will grow better when their larger tankmates are removed. But, why does the size variation become so great in the first place? Scientists have a term for this called growth depensation.

This is definitely stepping outside my area of expertise, but in reviewing several articles in the fisheries literature, I came up with this list of possible reasons why certain fish end up on the short side in aquaculture settings:

Most of the time, the difference in growth rates is caused by large fish preventing small ones from gaining access to food (competition);
Dominant fish can also cause social stress in small subordinates, leaving the little guys so stressed that they eat less even when they can get at the feed and have reduced food conversion efficiency;
Small fish that spend more time evading large dominant fish may have higher energy costs and reduced growth rates;
Improper feed particle size can lead to the stunting of the smaller individuals because they either reject the larger particles or spend more time breaking them up, which can be another competitive disadvantage;
Aggressive tankmates can inflict injuries that cause the smaller victims to pay for tissue repair costs out of their growth budgets;
Genetic differences in metabolic rates could also affect growth rates; and
Fish that survive a disease outbreak or are chronically infected may be more likely to be at a size disadvantage because of the energy they have expended fighting to regain their health.

Diseased runts

Differences in disease susceptibility may not be as important as the other factors, like competitive interactions and social status, but endemic viral infections were a factor in the sturgeon runt study.

The UC Davis investigators found that sturgeon suffering from viral infections may have increased mortality beginning at two to four weeks of age and that some of the surviving fish end up as runts.

The sturgeon in this study were graded between the 11th and 13th week after stocking. A sturgeon was considered a runt if its muscles were atrophied, which the researchers determined by looking at the muscle mass behind the head to see if it was concaved. The runts were often slower moving and found near the top of the water column.

One of the runt recovery factors the UC Davis folks analyzed was prior mortalities in the original growout tank. The proportion of runts created was related to the earlier viral outbreaks, but the exact mechanism by which these viruses stunt the growth of sturgeon is not known.

The losses that occurred after the separation of the runts were higher if the fish had come from tanks with the viral infections. Even though these runts had higher mortality, they also showed better recovery and were more likely to return to normal growth rates.

Although there were fewer runts from the tanks without the high mortality viral outbreaks, stunting was evident, which indicated that there was more than one mechanism responsible for the runting.

Worth the trouble

This epidemiological study showed that a substantial proportion of runt sturgeon recover if they are separated from their dominant brothers and sisters.

The number of runts that recovered during the second and third months was significantly higher than those in the first month after grading. The stress of handling and moving the runts may account for the low initial recovery rates.

Water quality differences and stocking density did not seem to affect mortality or recovery rates.

The financial analysis concluded that as long as the price of sturgeon was above a certain threshold level, it is more profitable to raise the runts than to cull them and use the tanks for good-growing fish.

Follow-up

A follow-up study looked at what happened when the recovered runts were returned to the general population and had to deal with their dominant tankmates again.

The mortality during the follow-up was negligible. Of 12 tanks containing 30 recovered runts and 30 good-growing controls each, nine tanks showed similar growth rates after four months and three tanks were found to have former runts growing faster than controls.

The authors suggest that similar studies could easily be conducted at other farms with different species.

For instance, researchers in Wisconsin found that 10% of their cultured walleye stopped eating and slowly starved to death, even though they appeared healthy. If these walleye runts were separated out into their own tanks, maybe they would show the same type of recovery rates as the sturgeon.

The management problem created by growth depensation (remember that term?) won't go away, but research like this may help put money back in your pockets.

A final word of caution is necessary, particularly if the runts you return to health have survived a disease episode on your farm. The risk of disease carriers, even after an outbreak has subsided, is always a complication and should be taken into consideration whenever management changes are contemplated.

Rod Getchell works in the Aquatic Animal Health Program at the Cornell University College of Veterinary Medicine.