Growth, Phenology, Agar Quality and Food Quality For Abalone of the Red Seaweed Gracilaria Sordida.

Abstract

An experimental study was carried out on the effect of environmental factors on the growth and quality of the agarophyte Gracilaria sordida W.A. Nelson, (Gracilariales, Rhodophyta), a seaweed with potential for commercial cultivation in New Zealand. "Quality" was measured as (1) the yield, gel strength and sulphate content of extracted agar, and (2) the food preferences and growth of abalone Haliotis iris fed on various Gracilaria diets. A laboratory-scale investigation is reported in Part One, in which G. sordida plants were cultured under different regimes of nitrogen source, nitrogen concentration, temporal nitrogen availability, salinity and irradiance. Growth of plants where N was added to the culture medium (as NO3- or NH4+) was faster than those without added-N. When plants had sufficient time to take up the entire weekly N dose from the culture medium their growth rates were similar regardless of N source, but when nutrients were supplied as weekly "pulses" of 6 hours duration the growth on nitrate was slower than that on ammonium. Ammonium levels above 214 μM inhibited plant growth. There was a decline in agar yield for all N sources as nutrient enrichment increased. The gel strength of agar was inversely related to the growth rate of plants, while the % sulphate content of agar was directly related to growth rate. The frequency of nutrient pulses (over timescales from once to four times per fortnight) had a strong influence on plant growth, and pulse concentration had a lesser influence. Only when treatments were pulsed at least twice per fortnight did growth become a function of the total N supply. Both pulse frequency and pulse concentration affected the levels of epiphyte species found attached to Gracilaria thalli, though frequency was generally the more significant factor. Their effects could be combined into a single factor, the "total N supply". Gracilaria plant growth was most rapid at intermediate salinities. The optimum range for "Manukau" plants was 23, 33 and 38 o/oo, while "Mokomoko" plants grew best at 23 and 33 o/oo. The latter ecotype was especially intolerant of the highest salinity. "Manukau" was the faster-growing variety at all salinities, though the difference was only significant at the highest salinity used. The best agar yields were obtained at low salinities. The gel strength of agar was highest at the salinity extremes, and could in some instances be explained by low values for the % sulphate content of these agars. Gracilaria growth was linearly related to irradiance. There was no significant trend in agar yield with light level, while the gel strength of agar was highest at the two extremes of irradiance. The % sulphate content of agar was inversely related to irradiance and plant growth rate, but was not related to gel strength. A field study is reported in Part Two, in which various parameters of a natural G. sordida population at Mokomoko Inlet, Southland, were monitored over a full year. Seasonal changes in the density, abundance, size and agar characteristics of plants were estimated for different lifecycle stages. The seasonal trends in plant growth rate were monitored for two plant ecotypes and at two different cultivation sites. Various methods of enhancing plant growth and density were trialed in experimental plots. Data for growth and density were used to estimate dry-matter production for extensive cultivation of plants. Production was also estimated for an intensive spray-cultivation system. The density of plants at the field site with a permanently-immersed rocky bottom was always much greater than that at the periodically-exposed mudflat site, and was greatest over summer. The abundance of fertile plants did not vary significantly between seasons, while sterile plants became very numerous in mid-summer. The % yield, of agar followed no particular seasonal trend and was no different between sexes. There was no difference in the gel strength of agar between cystocarpic and tetrasporic plants. However, agar gels from spermatangial plants were much weaker than the other stages. Plants harvested during summer produced the strongest agar gels, and these agars also had the lowest % sulphate content. Net plant growth occurred from October to March when plants were permanently immersed, and "Manukau" plants often grew significantly faster than "Mokomoko" plants. Periodically-exposed plants lost weight except for short periods of slow growth in August-September and March-April. The beginning and end of the growth season was closely related to seasonal patterns in water temperature and sunshine hours. Estimates of productivity for G. sordida plants at Mokomoko were over the range of 4.5 - 7.0 t (dry) ha-1 yr-1. The agar from plants treated with alkali during extraction formed much stronger gels compared with untreated plants, with no reduction in agar yield. Agar from alkali-treated "Mokomoko" plants was stronger than that from "Manukau” plants, while untreated agars were no different The growth of plants threaded into ropes was faster than plants attached to rocks. However much higher plant densities were possible with the latter method, which meant productivity was higher. Other culture methods were tried, but were not very successful. Growth of plants cultivated under a seawater spray was slow, though plants remained epiphyte-free using this culture method. A laboratory study described in Part Three looked at the effect of quality differences in G. sordida plants on the food preference and growth of abalone. Three plant growth rates, two plant ecotypes and two levels of plant epiphytes were compared. Abalone growth was faster when fed either fastest-growing Gracilaria plants, or plants of the "Manukau" variety, or plants bearing the epiphytic alga Audouinella sp.. In the latter two cases, the diet promoting fastest abalone growth was not the diet eaten in largest quantities. There appeared to be selection behaviour by abalone for the diet with Audouinella epiphyte, but all other diets were apparently encountered at random and diet choice was exercised by a process of "rejection" in that less would be eaten if a diet was less preferred.