No abstract

A system has been designed and evaluated for mass rearing of brachyuran crab larvae. Cancer magister larvae were reared through five developmental stages without the use of antibiotics. The physical design which may be flow‐through or recirculating emphasizes low maintenance and unlimited scale‐up potential. The system concept involves a slow upwelling water column which tends to offset passive sinking and creates a constant flowing, well oxygenated, growing environment. The system and evaluation study will be discussed.

An interdisciplinary team of scientists (a chemist, an engineer, and two biologists) at the University of Delaware is conducting an intensive study on the potential of recirculating seawater systems for use in mariculture. The Sea Grant‐supported project is underway in a relatively large (900 m2) laboratory at the mouth of Delaware Bay. The facility is unique in that it allows the degree of environmental control usual in small laboratory situations while allowing experimentation at the pilot plant scale. The team is attempting to culture clams and oysters from egg to commercial size in a recirculating system. A product is expected 18 months after spawning. Hatchery techniques are standard, but the postlarval animals are not planted in the field. Instead they are introduced to one of six recirculating systems for subsequent growth to commercial size. Twenty thousand newly set animals have been placed in an 8,000 liter system and are fed a mixture of several species of cultured algae. A new apparatus has been developed to separate algae from their culture media for subsequent feeding to the mollusks. Each circulating seawater system was specially designed for the culture of filter feeding mollusks and consists of an 8.6 m2 biological filter, a carbon filter, and a UV treatment station. Protein skimmers and biological monitoring systems are being optimized for future incorporation into the system. A careful record was kept of changes in nitrates, nitrites, ammonia, reactive phosphorous, calcium, magnesium, heavy metals, chloride, dissolved oxygen, pH, alkalinity, salinity, and suspended matter in the recirculating systems during start‐up and subsequent operation. Chemical information is used in engineering new refinements into the recirculating system as well as in interpreting biological phenomena in the system.

Diets consisting only of the diatoms Skeletonema costatum, Thalas‐siosira pseudonana and Phaeodactylum tricornutum were tested on Cancer magister larvae. Each algal species was fed singly and in combination with the others. Unfed controls died after 10 days without molting to the second stage. Those larvae receiving P. tricornutum lived only slightly longer than the starved animals and none reached second stage. S. costatum and T. pseudonana fed singly sustained the larvae 40 and 48 days and allowed development to the third and fourth stages respectively. Survival to 57 days and development into the fifth stage was reached on a diet consisting of a mixture of S. costatum and T. pseudonana. Mixing P. tricornutum with either of the other species did not improve survival beyond those used singly, and mixture of all 3 species was less beneficial than the best 2‐part diet.