Effect of water motion on the cultivation of the economic seaweed Gracilaria parvispora (Rhodophyta) on Molokai, Hawaii

A cage culture system was previously developed for the red alga Gracilaria parvispora Abbott on Molokai, HI; however, yields have shown marked variation, even among cages with identical stocking rates and fertilization treatments. Water motion, which can be affected by location and arrangement of the cages in the grow-out area, was hypothesized to be a factor contributing to the variation in yield. To examine this, the growth rates of thalli and the development of sporelings in relation to water motion were determined in replicated trials both in tanks and in small-scale field experiments.Generally, water motion had a substantial effect on both thallus growth rate and spore development. In the tank cultures of thalli, water velocities ranged up to 13.7 cm s−1, and relative growth rates (RGRs) ranged from 2.8% to 8.9% day−1. In the lagoon, water velocities ranged between 3.6 and 11.6 cm s−1. Relative growth rates of the thalli in the lagoon trials were 0.02–10.3% day−1. Sporeling density, a measure of spore development, was also significantly affected by water motion. In the tank trials, water motion ranged from near 0 to 6.50 cm s−1, and sporeling densities ranged from 1.4 cm−2 at lower water motion levels to 11.6 sporelings cm−2 at higher levels. Similar results were obtained in lagoon trials, with sporeling densities ranging from 0.2 to 6.7 cm−2. However, sporeling length was not significantly correlated with water motion. In previous trials, we have been able to achieve high sporeling densities, but elongation of sporelings has been inhibited. In the present trials, we were able to break the apparent sporeling dormancy by incubating the sporelings in tanks enriched in nutrients supplied by fish cultures. Consideration of the effects of water motion is important in designing culture systems for species of Gracilaria and other marine algae. The results also suggest that nutrients play a key role in regulating the early development of G. parvispora sporelings.