Effect of iron on the growth rate of fishes.

To determine the nutritional requirment for iron in fish growth, the effect of daily addition of supplemental ferrous sulfate on the growth of Xiphophorus helleri and Xiphophorus maculatus (the swordtail and the platyfish) was measured by determination of weight gain and hematocrit levels. Both variables increased as a result of the added iron. The effectiveness of the treatment diminished as sexual maturity was approached. Treatment with ferrrous sulfate led to an increase in growth rate even in fish fed maximally with live brine shrimp. Ferric salt was not judged effective. Addition of ferrous iron also significantly decreased the mortality from hatching to maturity. Little is known of the mineral require ments, or of the mineral metabolism of fishes. Lovelace and Podoliak (1) showed that calcium is absorbed through the gill of the brook trout, and Phillips and his co-workers (2) have reported dietary re quirements for growth in trout, although their main concern was with the organic constitutents of prepared hatchery feeds. They determined calcium, phosphorus, and magnesium levels in their diets, but did not systematically vary these. Cheprakova (3) reported some effects of iron salts on developing eggs of the loach, the sig, and the perch, but the data reported in that paper are not at all conclusive. In connection with studies in our lab oratory concerned with respiratory rate and other factors in the swordtail (Xipho phorus helleri}, the platyfish (Xiphophorus maculatus') and the hybrid helleri X macu latus (4), we noted that the addition of ferrous sulfate to water in which the fishes were raised appeared to have a stimulatory effect on the growth of these fishes, as compared with untreated controls. Ac cordingly studies were made in which we systematically varied the amount and kind of iron available to newly hatched fry and to older fishes, and compared weight changes and changes in hematocrit values of treated to untreated animals.