A three-frequency method to determine the abundance and the size of two euphausiid species (Euphausia superba and Euphausia crystallorophias)

Euphausia superba and Euphausia crystallorophias dominate the biomass and play a key role in the Ross Sea pelagic ecosystem (Marr, 1962; Azzali et al., 1999). To estimate the abundance of each stock it is necessary to discriminate E. superba from E. crystallorophias aggregations and to obtain information about the mean size (length) of the organisms in each aggregation.
In principle the abundance of the two populations can be estimated by single-frequency acoustics, using the standard echo integration method. This implies that species can be identified indirectly, from visual analysis of echograms, and directly from net samplings. The mean size of the crustaceans can be estimated from “in situ” target strength measurements, using the relationship between target strength and size, and from the catches . Unfortunately the precision and credibility of this method are often insufficient when applied in complex environmental and biological conditions, as in the Ross Sea.
Both crustaceans can create aggregations with similar shape, therefore visual separation of their echograms is difficult. The possibility of net samplings depends on the environmental conditions. In the Ross Sea such a limitation is due mostly to the presence of ice-cover. Moreover uncertainties of net samplings arise due to the selectivity of the net and to the net avoidance (Wiebe, 1972; Everson and Bone, 1986). This last bias affects particularity the hauls carried out in “day-time” when the light is intense. Ross Sea was investigated during the austral summer, in absence of darkness. There are two methods for “in situ” target strength measurements “split beam” and “dual beam”. The fundamental condition to apply both methods is the detection of an echo coming from an isolated organism, with no interference from noise or other targets. It means that organisms must occur in low concentrations and echo level from a single organism must be greater by the detection threshold than the total level noise. For small organisms as the Euphausiids, living in a noisy environment (storms, ice) and in dense concentrations, the detection of single targets from a ship hardly ever can be done. Therefore, in the Ross Sea the standard echo integration method can be applied sporadically, only in particularly favorable conditions.
In order to solve the above problems, since the first Italian expedition to the Ross Sea (1989-90), a two – frequency method for the recognition of E. superba aggregations have been applied (Azzali et al., 1999). In the last two expeditions of 1997-98 and 1999-2000, a three-frequency method for euphausiids discrimination and size estimation has been developed .
This paper explores applications of the multi-frequency method using data from three expeditions to the Ross Sea (1980-90; 1997-98 and 1999-2000), where the environmental conditions, the sampled areas, the instrumental and the sampling strategies varied.
First, on the basis of the echo-integrations, made simultaneously either at two or at three frequency, and of the results of net samplings, the thresholds and the decision criteria to recognize the two species are established. Next the acoustic estimates of euphausiid lengths, derived from the fluid sphere model, are compared with lengths collected from net samplings.
Finally, the developed criteria and algorithms are effectively applied to estimate E. superba biomass found in the area of the Ross Sea investigated in December 1997 and in January-February 2001. The results are compared with those obtained from the standard method.