Outdoor rearing facilities of free spawning calanoid copepods for turbot larva can host a bank of resting eggs in the sediment

It is well established in Denmark to rear calanoid copepods in outdoor tanks for use as live feed during turbot ( Scophthalmus maximus ) larval production. However, the copepod assemblages, composed of a mixture of all development stages and therefore body sizes, vary over time and do not always match the larval needs. When turbot larvae reach metamorphosis and are transferred indoor for weaning, the outdoor tank sediments may reveal vast amounts of copepod eggs undergoing dormancy. Here, we report a copepod species succession firstly among Centropages hamatus and then Acartia spp. both with resting eggs as part of their life cycles as a result of two different nutrients treatments and a control. We found a tendency to a higher egg production and indeed more eggs in the sediment of nutrient amended tanks. In fact close to 5 million eggs per square meter, making up to 400 million eggs per tank was found in the sediment after one production. Instead of discarding the sediment between production batches, we propose to collect it and generate an egg bank. These eggs can be stored for months to a year, however, according to the results, a large loss rate occurred, which could be potentially decreased by the optimization of storage conditions. Those procedures will enable hatchery managers to apply newly hatched copepod nauplii exactly when the turbot larvae start feeding which would potentially solve the, often occurring, mismatch between the time of start-feeding turbot larvae and actual available prey field.