Factors influencing the spawning and setting of oysters in Galveston Bay, Texas.




Hopkins, A.E.

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(1) The spawning season of oysters in Galveston Bay in 1929 was observed to begin at the end of March. At the end of August larvae were still in the water and oysters still milky, indicating a spawning season of at least six months. (2) Spawning started when the average daily water temperature in Offatts Bayou was about 25C. When 20C. was reached the oysters were just beginning to develop mature eggs and sperms, causing them to appear milky. (3) Thermograph records of water temperature in the bayou showed that at the end of February the winter temperature was 14 to 15 C., while during the spring and summer it rose to between 30 and 33 C. (4) Because of increased river discharge in spring and early summer the salinity of the water in Galveston Bay remained low for a considerable time. In early spring the salinity in the bayou was 22 to 24 parts per thousand, while early in June it was down to 7 parts per thousand. At the same time in West Bay it was only 4 parts per thousand. In East Bay late in June it was below 2 parts per thousand. During July and August the salinity rose to between 20 and 30 parts per thousand. (5) The characteristic pH of the water in Galveston Bay was between 8 and 8.4, fluctuating between these levels. In August in the bayou it rose to 8.9. (6) Setting was irregular and took place during short, isolated periods. The first spat were not obtained until a month and a half after spawning started. (7) In Offatts Bayou two chief setting periods occurred, one at the middle of May, and the other beginning at the end of August. In addition, two very light sets were obtained at one place in the bayou; at the end of May and at the beginning of July, respectively. (8) In West Bay at the Deer Islands two setting periods were observed: a short one early in July, and a more prolonged one beginning at the first of August. At the latter time setting was very heavy and up to 3,500 spat were obtained daily per bag of shells. (9) On Hanna Reef, East Bay, a light set was obtained at the beginning of May and a fairly heavy one at the end of July and first part of August. (10) The setting periods appear to have been correlated with periods of high salinity, suggesting that in some manner the larvae depend, either directly or secondarily, upon a salinity above about 20 parts per thousand, in order to develop to the setting stage. (11) Most of the larvae taken in the collections were in the straight-hinge stage, with the exception of short periods, until August when many umbo larvae were taken. It is probable that setting was not more continuous because early in the season few reached the setting stage. (12) Of 252 adult oysters from different reefs only 51 were males and 178 were females. The remaining 23 specimens were either too completely spawned out or immature to permit ready determination of sex. (13) Diatoms in the surface plankton collections were most abundant in early spring and slowly decreased in numbers taken so that in July and August the water was almost free of them. It was observed that during periods of low temperature of the water more diatoms were taken than during times of high temperature. This was the case for temperature changes occurring over a few days as well as over the entire period during which collections were made. (14) Use of wire bags of shells as cultch was tested in Galveston Bay waters and found to be highly satisfactory as a means of collecting spat for developing oyster grounds.


p. 57-83.


oyster fisheries; reproduction; spawning; temperature variations; salinity gradients; spat