Copepods are an essential live prey item for the larviculture of many marine ornamental fish species. Copepods are generally small, nutritious, and exhibit movement patterns thought to elicit feeding behaviors in larval fishes. Larval fish must be able to efficiently capture live prey items to ensure ample energy reserves for growth and development. However, most copepod species have strong escape swimming behaviors. The parameters of copepod escape swimming, such as speed and response latency, can be species-specific and influenced by a myriad of environmental variables. The goal of these experiments was to suppress copepod escape responses to improve prey capture by larval fishes.
Pilot studies were conducted to determine copepods’ lethal limits to acute exposure to environmental stressors. Treatments were selected based on the following factors: ease of use, cost effectiveness, and relative safety to fish larvae. Three sets of experiments examined the copepods’ behavioral effects to temperature, salinity, and water viscosity (Table 1). The viscosity experiments tested two common live feed enrichment products, “Easy DHA Selco” and “AlgaMac 3050.”
For each experiment, three containers with clean saltwater and a 35 micron nested screen were stocked with 100,000 newly hatched nauplii. Nauplii were transferred to a treatment cup for a set period of time and returned to the recovery cup with clean saltwater. Copepod escape behaviors were recorded hourly using an Edgertronic high-speed video camera (500fps) paired with a 4x long working distance objective lens, piezoelectric stimulus probe (predator mimic), 10 MHz pulse generator, and fiber optic light. Videos were recorded of copepod nauplii responding to the stimulus probe to capture how and when nauplii reacted. For each escape response recorded (n=30), the nauplii’s trajectory over time is measured using ImageJ software. From that data, the average and maximum speed, total distance travelled, response latency, and total escape duration are measured. Survival and response percentages will also be reported.
The most successful treatment(s) will show reduced speeds and/or distances and longer response latencies, with these affects lasting for a long period of time. These factors should improve prey ingestion rates by larval fishes and will be tested in future experiments.