Report Job Roy B. Johnson Marine Biologist
Project No. MO-R-5 Project Name;
Dates
May 12, 1964
Survey of Oyster Populations and Associated Organisms January 1, 1963 to December 31, 1963 Job. No. 2
Period Covered;
Life History Study of the Commercial Oyster in the Lower Laguna Madre Abstract; South Bay oyster population samples of one standard bushel (culled of dead shell) contained a low of 27 market oysters in January and a high of 116 market oysters in May. Market oysters were generally small in size. A peak spat set occurred from May to July, 1963, as indicated by data from population samples and spat collectors. This peak occurred after a rise in temperature and during a period of reduced salinities in the bay. Growth rates averaged 2.8 millimeters per month for market oysters and 5,4 millimeters per month for seed oysters. Oysters grew faster during the spring and winter. Condition tests indicated that market oysters were in poor condition dur ing the fall of 1963, possibly due to unusually high salinities. There was no incidence f Dermocystidium marinum. Objectives; To determine the factors affecting spawning, spat set, growth and condition of commercial oysters in the lower Laguna Madre. Procedures; Monthly oyster population samples of one standard bushel with all dead shell removed were taken from a private lease at the south end of South Bay. Two samples were collected in June. All samples were taken by hand. Each live oyster was measured to the nearest millimeter. Oysters from one to 25 millimeters were considered spat, those from 26 to 85 millimeters seed, and those 86 millimeters and larger were considered market. Six market oysters were selected at random from each sample and were test ed to determine the condition factor, shell factor, and percent solids. The standard volumetric and weight relation procedures used by Hofstetter (1959) were employed. A tray containing 40 seed and market oysters was maintained in South Bay for growth rate studies. Each oyster was measured to the nearest millimeter and the average monthly growth was determined. Spat collectors consisting of two 4-inch by 12-inch pieces of asbestos siding were wired to lathe and kept beneath the oyster platform. Each month exposed collectors were replaced with clean siding and returned to the labora tory. Each oyster which had set during the month was measured. The number of spat was divided by the number of days in the water to obtain the "spat set per day" which is graphed in Figure 1. Dermocystidium marinum samples were made and senttfco Rockport for analysis
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Salinity was determined at the time of each sampling using specific gravity hydrometers and Knudsen's Hydrographic Tables. Water temperatures presented are monthly averages from 16 stations in the lower Laguna Madre, described in the hydrographic report (MF-R-5, Job No. 19). Findings and Discussions The numbers and percentages of spat, seed and market oysters for each sample are shown in Table 1. The percentage of market oysters ranged from 0.9 percent in January to 6.9 percent in Dec ember 1963, decreasing during periods of heavy spat set. Market oysters constituted 5 percent or more of the population in March, May, October, and December. The numbers of market oysters per bushel were fairly high throughout the study period, ranging from 27 per bushel in January to 116 per bushel in May. Most of the market oysters were under 4 1/2 inches in length. The percentage of spat decreased steadily from January to April 1963, associated with a corresponding increase in percentage of seed and market oysCers. Data from spat collectors (Figure 1) and from population samples indicated continual spawning throughout the study period, with a large peak of spat set from May to July. Water temperatures increased rapidly from 10.0C. in February to 26.0C. in April. Monthly salinities decreased from 46.4 ppt in January to 38.3 ppt in March. Monthly salinities aver aged 40.0 from ppt March to June, and increased to 48.0 ppt in August. The peak of spat set, therefore, occurred shortly after a rise in water temperature to near 25.0C, and during a period of relatively low salini ties in the bay. Condition tests (Table 2) indicated that market oysters were in poor condition during September, October and November, 1963, possibly due to high salinities which averaged near 45.0 ppt during those months. Salini ties were much lower in 1962, as the highest recorded in South Bay was 39.7 ppt.. Other than these, condition indices and environmental conditions for 1962 and 1966 were comparable. Average monthly growth of South Bay oysters, by 15 millimeter groups, for 1962 and 1963 are shown in Figure 2. The growth-tray oysters were missing in June, and the tray had been damaged. The tray was repaired and returned to South Bay in July, and a new series of measurements was begun. Growth rates varied depending upon the size of the oyster and also upon the season. The growth rate for seed oysters decreased in most cases as they approached market size. For the entire study period, market oysters from 86 to 140 millimeters averaged approximately 2.8 millimeters per month, whereas seed oysters from 30 to 80 millimeters averaged approximately 5.4 millimeters per month. Growth rates were considerably higher during the spring and winter than the summer and fall* There was no incidence of Dermocystidti.um marinum at any time.
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Prepared by: Roy B. Johnson Marine Biologist
Robert P. Hofstetter Project Leader
Approved by:
^sd2!S!~
Coordinator
Ernest G. Simmons Regional Supervisor
Literature Cited: Hofstetter, Robert P. 1959. Survey of the Abundance and Condition of Market Oysters. Texas Game and Fish Commission, Project Report, 1958-1959.
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Table I Number and Percentage of Market, Seed and Spat Oysters Collected in South Bay Samples Number Date 1/9/63 2/20/63 3/12/63 4/9/63 5/14/63 6/6/63 6/23/63 7/22/63 8/21/63 9/17/63 10/22/63 11/13/63 12/9/63 Market 27 84 104 56 116 88 76 38 65 40 52 62 54 Seed 676 708 700 896 488 460 371 487 544 364 320 338 464 Spat 2160 1204 872 604 1652 2976 2960 1481 1249 1158 682 596 262 Market 0.9 4.2 6.2 3.6 5.1 2.5 2,2 1.9 3.5 2.6 4.9 6.2 6.9 Percentage Seed 23.4 35.5 41.8 57.6 21.6 13.1 10.9 24.3 29.3 23.3 30.4 33.9 59.5 Spat 75.7 60.3 52.0 38.8 73.3 84.4 96.8 73.8 67.2 74.1 64.7 59.9 33.6
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