The Optimal Harvesting Frequency for Production on a Number of Calanoid Copepod Parvocalanus crassirostris

Abstract

Background and Objectives :  Parvocalanus crassirostris is a calanoid copepod species of significant ecological and nutritional importance, widely recognized as a premium live feed for the larviculture of marine ornamental fish. Its naupliar stage is particularly small, highly digestible, and enriched with essential polyunsaturated fatty acids (PUFAs), including docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which play critical roles in enhancing larval growth, pigmentation, neural development, and survival rates. These nutritional qualities make P. crassirostris an optimal live diet for marine fish species with small mouth openings, such as butterflyfish and angelfish, which are otherwise challenging to rear. Distributed across tropical and subtropical waters—including coastal areas of Thailand and the Hawaiian Islands—this species has been the focus of intensive research aimed at improving culture techniques, optimizing production efficiency, and meeting the increasing demand for high-quality live feeds in aquaculture hatcheries.Despite its high potential, the continuous and efficient production of P. crassirostris remains challenging, mainly due to the delicate balance required between harvesting and population stability. Over-harvesting can severely deplete populations, while insufficient harvesting may lead to overcrowding, increased competition for resources, and reduced reproductive output. Previous studies have highlighted the importance of establishing optimal harvest intervals to maintain sustainable yields and prevent fluctuations in population dynamics. This study aimed to determine the most appropriate harvesting frequency for P. crassirostris, focusing on naupliar and copepodite stages, to develop guidelines applicable to commercial-scale production systems and ornamental fish hatcheries.

Methodology : The experiment was designed using a completely randomized design (CRD). Four different harvesting strategies were tested: non-harvest (control), harvesting once every other day, harvesting once every third day, and harvesting once every fifth day. Each harvest involved the removal of 20% of the culture volume, after which the tank was replenished with filtered, sterilized seawater. Twelve units of 5-litre glass tanks were divided into 4 triplicate treatments, 3 replicates each. Copepods were sourced from stock cultures maintained at the Cha-am Research Station, Institute of Marine Science, Burapha University, and were acclimated under controlled laboratory conditions. The initial stocking density was 1.99±0.13 ind./ml^-1. In the current study, Cultures were fed daily with two microalgal species, including Isochrysis galbana & Chaetocerop sp. each at a concentration of 75,000 cells/ml^-1 to 1 time per day. The culture was maintained at room temperature, salinity of 28 ppt, and a photoperiod of 24:0 h light:dark cycle. Routine culture management included siphoning bottom sediments and performing 20% water exchanges every three days to maintain optimal water quality and reduce organic waste buildup. Population density, specific growth rate (SGR), and the proportion of developmental stages (nauplii, copepodite, and adult) were monitored every two days over a 21-day experimental period. Sampling was conducted by collecting 49 mL of water from each tank, which was preserved with 10% neutral buffered formalin prior to counting under a stereo microscope (Olympus SZ).

Main Results : The results showed that the optimal harvest frequency was every other day, yielding a total number of 6,170 copepods (60% of the production). Harvesting once every third day had an average copepod output of 2,527.78 (25%). Harvesting once every fifth day gave the lowest total output (1,511.11 copepods, 15%). In compared were consisting of the harvesting frequencies in this experiment there was no significant effect on copepod density during the production (P > 0.05). The results showed that responded copepods by increasing the maximum copepod density was (mean±SE) 3.33±0.33, 4.53±0.82, 3.43±0.23, 3.22±0.39 ind./mL^-1 at 4-6 daysbefore increasing density to below 1 ind./mL^-1(mean±SE) 0.31±0.08, 0.36±0.09, 0.33±0.07, 0.24±0.04 ind./mL^-1 at 14 days, respectively. P. crassirostris showed no significant (P>0.05) in the specific population growth rate (mean±SE)  0.06±0.125, 0.14±0.14, 0.21±0.11, 0.002±0.14 day^-1. The proportion of copepods in adult stage (36.19%, 59.04%, 71.76% and 28.11% respectively), copepodite (9.57%, 20.16%, 14.51% and 13.11% respectively), and nauplii (54.24%, 20.28%, 13.76% and 58.78% respectively) did not show statistical differences between treatments. These results suggest that harvesting frequency does not disrupt age structure or recruitment dynamics within the population, provided that harvesting is conducted in moderate volumes.

Conclusions : Harvesting P. crassirostris every other day proved to be the most effective strategy for maximizing yield while maintaining population stability. This schedule ensures regular availability of nauplii and copepodites at peak nutritional value, which is essential for larval fish culture. Less frequent harvesting (every three or five days) resulted in lower yields and less efficient population turnover. These findings agree with previous studies on calanoid copepods, which highlighted that frequent but moderate harvesting supports culture performance, reduces density-related stress, and promotes reproduction.Harvesting every other day provides an optimal balance between sustainable biomass production and stable population dynamics, making it suitable for hatcheries requiring a steady supply of high-quality live feed. Non-harvest strategies may be useful for maintaining broodstock populations for future scaling. Moreover, this study underscores the potential of semi-recirculating or intensive culture systems to further enhance production efficiency, reduce operational costs, and meet the growing demand for sustainable aquaculture feeds. The insights gained here offer practical guidelines for improving P. crassirostris culture, ensuring reliable, cost-effective live feed supply chains for marine ornamental fish hatcheries and other aquaculture industries.