The Effects of the Orientation of Clutch Material on the Setting of the Sydney Rock Oyster

Abstract

In 1945–46 in Port Hacking, N.S.W. there was some spawning over the entire year, but successful spat-falls occurred only over the summer, with the greatest number in November and again in March. On every 12-in. length of fibrocement slat approximately 24 oyster spat were present (on all faces) 12 months after the slats were set out. Thus on the average a four foot slat carried 96 oysters or approximately four oysters every five square inches of surface. A greater number of oysters settled when the slats were less than half an inch apart than when the slats were an inch apart. When the slats were one inch apart the central portion of the slats carried more oysters than the ends, but when the slats were within half an inch of each other the numbers at the ends and on the middle section were about the same. When the slats are within half an inch those oriented with the long axes parallel with the current show greater catches than those perpendicular to the current. However, when the slats are further apart the reverse is the case. The horizontal portion of the broad surface supplies the highest catches, but the catch is more evenly distributed when they are vertical, and this would permit better growth. The ratio of the catches on upper horizontal, lower horizontal, and vertical surfaces is similar to the ratio found by Schaeffer for O. gigas, which is a similar type oyster to O. commercialis. There is a horizontal gradient on the vertical surface across the batteries such that the number of oysters on the surface facing a particular direction increases on the columns more remote from that direction. There is a vertical gradient on the vertical and upper surfaces such that the number of oysters caught increases on the lower rows of vertical surfaces, but the evidence is anomalous for the horizontal surfaces. The discussion of the possible factors causing the pattern of set reveals that the reaction to light could be an explanation. But there are other explanations which are equally valid, and the experimental design was not such as to reveal the true causes. The practice in Australia of using closely bundled cultch material is a good one; but there is a possibility that better total catches would ensue if the cultch set out in spring were removed to growing areas in January and new cultch set out to catch the autumn spat fall.