IMIS - Marine Research Groups

Sometime between the fourth and the fifth the larvae lose the free-swimming habit and settle to the bottom. It is to be expected that catches of larvae made during June and early July will be made up largely of first and second-stage larvae. As the season advances the older larvae are expected to predominate in the catches, although it as been shown that first-stage larvae can be taken during the latter part of August. Purpose: a) this year's survey is planned as the first of a series of annual surveys which it is hoped will enable us to determine the relative abundance of lobster larvae from year to year. If reliable estimates of larval abundance can be made they may prove extremely valuable in predicting, several years in advance, the abundance of commercial-sized lobsters. b) if reliable estimates can be made of the relative numbers of the four larval stages present in any one year, then it will be possible to estimate the mortality that occurs during this free-swimming period. This information is essential to a thorough understanding of the lobster's life history and is of direct application in evaluating lobster hatcheries and rearing stations. Gear: - a net 18 ft in length by 12 ft in width by 2 ft in depth - attached to a frame made of ¾ inch galvanized pipe - frame was 12x2' at mouth and net tapered off to 16 inches at cod end. - Large bottle attached at cod end to collect larvae Mesh: Marquisette material, 8 meshes to the inch. Ordinary towing done with upper bar of the net above the surface of the water. Net attached to the boat with block and tackle from the boom which was set at 20 to 25 degrees so that it projected out over the side of the boat. It was towed about 60 feet behind the boat in a wide circle to keep it out of the propeller wash. Duration: Half hour tows made at each station at an approximate speed of 1.5 knots Method: On complete ion of each tow the contents of the net were washed down into the bottle at the cod end of the net. This was done by working the net up and down in the water before taking it into the boat. In the boat the contents were emptied into a tub and the small end of the net was washed again and examined carefully before making the next tow. The contents of the tub were then strained trough cheesecloth and checked carefully. The lobster larvae were sorted into first, second, third and fourth stages and preserved in pint self-sealer jars with 5% formalin These jars were shipped to the Atlantic Biological station where they were re-examined, counted and sorted.