Assessing the nutritional value of mussel (Mytilus galloprovincialis) biodeposits as a possible food source for deposit-feeding holothurians in Integrated Multi-Trophic Aquaculture

INTRODUCTION Previous studies have demonstrated that biodeposits –the collective term for feces and pseudofeces– from suspended mussels tend to accumulate in shallow coastal areas with low hydrodynamism, generating organic enrichment and negative impact in the benthic community beneath the culture units (Cranford et al., 2009). Deposit-feeding sea cucumbers have been suggested as candidates for reducing the benthonic impact derived from mussel farming in Integrated Multi-Trophic Aquaculture (IMTA) systems (Slater and Carton, 2007; Slater et al., 2009). IMTA is a recycling concept in which the waste products of one species (e.g. bivalve biodeposits) are extracted by another species from a different trophic level (e.g. holothurians) cultured in close proximity. The reutilization of mussel biodeposits by co-cultured sea cucumbers could be of great ecological and economical interest for coastal areas with high density of bivalve farming like the Galician Rías (NW Spain). Galicia is the third largest producer of mussel Mytilus galloprovincialis in the world, with more than 250,000 tons year-1 (Labarta et al., 2004). Biodepositions in the Galician Rías are mainly comprised of feces, as the natural low seston loads (Mytilus galloprovincialis biodeposits to determine whether they might represent a potential nutritional food source for deposit-feeding sea cucumbers. Biodeposits from cultured mussels were obtained over the course of five different seasons at two rafts located in the inner and outermost regions of a raft polygon in Ría Ares-Betanzos (Galicia), with the aim of evaluating if the nutritional value of the biodeposits varied seasonally and with the geographical location of the culture units. MATERIALS AND METHODS This study was conducted on two mussel rafts from the Lorbé raft polygon (Ría Ares-Betanzos, Galicia, NW Spain). The first raft was located on the innermost side of the polygon at 14 m depth and 500-700 m N of the nearest coastline. The second raft was moored at the outer side of the polygon at 16 m depth and 700-1000 m N of the coast. Rafts were visited during the summer (July 2010), autumn (October 2010 and 2011 for any inter-annual variability), winter (February 2011) and spring (May 2011). Mussels of homogeneous size (50-60 mm shell length) were placed in a 19 l mesocosm to collect the feces. Seawater was pumped from 3 m depth into a header tank and sieved through a 50 µm mesh to eliminate large particles before being distributed to the mesocosm (Filgueira et al., 2006). The mesocosm consisted of three replicate tanks, each divided into 16 compartments, each containing 3-4 mussels. Six replicate mussel feces samples were collected from each tank 3 to 4 h after being harvested, each replicate comprised feces from 12 individuals. Three replicate samples of seawater were collected from a tank left empty as a control to determine the biochemical composition of the seston. Seston was filtered through pre-washed and pre-combusted Whatman GF/F filters (ø 0.7 µm) and rinsed with isotonic ammonium formate. Feces samples were filtered on Whatman GF/F filters and rinsed like seston samples. The particulate organic matter (POM) of the seston and organic content (OC) of the feces was calculated as the difference between the dry weight (DW) and the ashed weight. Proteins were determined following Lowry et al. (1951) after alkaline hydrolysis with 0.5 N NaOH at 30°C for 24 h. Carbohydrates were quantified with the phenol-sulphuric acid method (Strickland and Parsons, 1968) using glucose as the standard. Lipids were extracted following the Bligh and Dyer (1959) method modified by Fernández-Reiriz et al. (1989) and colorimetrically determined using tripalmitin as a standard (Marsh and Weinstein, 1966). Results were expressed as relative %DW and as energy content in Joules l-1 (seston) or Joules mg-1 (feces), after conversion with the factors proposed by Beukema and De Bruin (1979). Seasonal and spatial differences in composition were tested with a two-way ANOVA followed by Tukey HSD as a post-hoc test. Normality and homogeneity of variance were checked prior to ANOVA with Shapiro-Wilk and Levene test, respectively. RESULTS Seston had an average of 62.68 ± 2.57 %POM and 5.87 ± 1.15 total J l-1. Proteins accounted as the major component (24.26 ± 13.36 % DW), after carbohydrates (10.40 ± 5.51 % DW) and lipids (10.31 ± 4.87 % DW). The %POM, total energy content and biochemical substrates showed higher values in spring than winter. Seston at the outer raft presented higher %POM, proteins, carbohydrates and lipids during both autumns and winter compared with the inner raft, excepting for the higher energy content measured at the inner raft during autumn 2011 and winter (Tukey’s HSD, PPerna canaliculus biodeposits with 28 % OC, 5.1 % proteins, 19.6 % carbohydrates and 1 % lipids. The integrated culture of sea cucumbers with mussels could provide an additional crop while potentially biomitigating some of the benthic impact (i.e. decreased infaunal diversity) in the Galician Rías (Tenore and González, 1975; López-Jamar et al., 1984). In fact, the holothurian Aslia lefevrei is one of the most abundant epifaunal organisms on M. galloprovincialis ropes and reaches high biomass (21-24 g dry weight m-2) under rafts in the Galician Rías (Román and Pérez, 1979; Olaso, 1979), suggesting that feces represent a preferential food input for this deposit-feeder. The culture of holothurians associated with mussel rafts could be subjected to seasonal and spatial variability in biodeposits composition. Biodeposition varied seasonally with seston quality and quantity. Feces production peaked during winter storms, when resuspension of the seafloor and increased material runoff increased the total suspended particles but diluted the digestible %POM by almost 4-folds compared to spring (Irisarri et al., 2013; Zuñiga et al., submitted). Resuspension events were more accused at the shallower inner raft in winter and resulted in significantly higher biodeposition rates (220.70 ± 74.86 mg ind-1 day-1) compared to the raft further from the coast (99.97 ± 31.38 mg ind-1 day-1). The higher nutritional quality of the seston during the spring upwelling bloom resulted in the egestion of feces with significantly higher %OC, energy and amount of biochemical compounds. The increased amount of highly digestible phytoplankton in spring increased the absorption efficiency of organic material in the gut (Irisarri et al., 2013) and, consequently, reduced the biodeposition rate in comparison with the winter period. Previous studies estimated that holothurians Stichopus japonicus and Australostichopus mollis require 1.82 and 140 g of biodeposits ind-1 day-1, respectively (Zhou et al., 2006; Slater et al., 2009). Nonetheless, dietary demands may vary depending upon the sea cucumber species, shellfish and holothurian stocking density, dispersion of the biodeposits by current speed and the spatial and seasonal variations in the natural seston and feces composition demonstrated in this study. Paltzat et al. (2008) observed a cessation of the feeding activity of co-cultured sea cucumber when oysters’ biodepositions were highest but with significantly lower OC, suggesting that the nutritional value could be more determinative than the amount of biodeposits. Overall, winter will be the best season for holothurian culture with respect to the quantity (160 mg ind-1 day-1 total) and quality (24.18 mg ind-1 day-1 organic) of biodeposits, even if feces in spring had the highest relative % OC and may also represent a high energy source for sea cucumbers. In conclusion, the results indicated the potential for using mussels’ biodeposits as a highly nutritional diet for holothurians underneath shellfish rafts in future IMTA farms in the Galician Rías.