Marcel Fréchette

An in situ estimation of the effect of wind-driven resuspension on the growth of the mussel Mytilus edulis L.

Previous studies have suggested that the effect of near-bottom seston depletion on individual growth in dense beds of suspension-feeding mussels Mytilus edulis L. may be alleviated by particle enhancement caused by resuspension. As a test of the relative importance of depletion and resuspension, mussels were grown at two intertidal sites, 0 and 1 m off bottom (St Lawrence Estuary, Quebec), with the same immersion time, but with differing phytoplankton concentrations and resuspended loads. Growth at both sites was measured according to two independent methods in order to distinguish between effects of resuspended particles and vertical phytoplankton gradients. The first method involved the estimation of gain in tissue weight and was resuspension-dependent since mussel growth integrated effects of wind events as well as those of calm weather. In the second method, growth was estimated from scope for growth (SFG) measurements carried out only in calm weather, to yield resuspension-free results. Growth at both sites was compared by computing the ratio of growth (0 m : 1 m) for each method, an approach not dependent on agreement of absolute growth rates between the two methods. Results indicated that the net effect of wind events and resuspension did not have a significant effect on growth during May–July and May–November study periods, and that 0 m : 1 m growth and SFG ratios were usually <1. This suggests that decreased mussel growth at the bottom of the water column was primarily caused by phytoplankton depletion near the mussels, a process which could not be ameliorated by resuspended food supplies.

On the use of self-thinning relationships in stocking experiments

Abstract We present a method for estimating optimal stocking density (OSD), via the analysis of the relationship between yield (biomass, B ) and population density ( N ) at harvest, using a B - N diagram (BND). This differs from the usual approach in aquaculture, in which yield is expressed as a function of initial population density, and B and N are analysed separately. Both methods allow estimation of OSD. In addition to OSD, however, the BND potentially allows: (1) assessment of the relative importance of competition-dependent and competition-independent mortality factors; (2) estimation of approximate OSD and maximum yield by extrapolation of results from short-term experiments; and (3) indentification of the nature of the factor regulating competition-dependent mortality. We compared the classical and BND methods using data from mussels grown in suspension cultures. The classical approach suggested that OSD was roughly 400 individuals per 30.5 cm of sleeve, that mortality patterns were the same for all stocking densities, and that competition-dependent mortality occurred only at high density. The BN approach suggested that optimal seeding density ranges between 120 and 200 mussels per 30.5 cm of sleeve, which was below the range of initial densities tested. In addition, there was evidence that mortality patterns differed between mussel groups. One group exhibited successively competition-dependent mortality and competition-independent mortality, whereas others exhibited either one type or the other. In an experiment designed to test the effect of spat origin (stock effect) on commercial yield, the classical approach suggested that there were no differences in yield and survival, despite differences in growth rate. The BN approach, however, showed that yield was constrained by self-thinning, not by intrinsic properties of the stocks. Thus the BND invalidated the experiment. The BN approach, unlike the classical approach, yielded results consistent with state-of-the-art commercial practice and general knowledge about the stocks tested.

THE CONTINGENCY PERIODOGRAM: A METHOD OF IDENTIFYING RHYTHMS IN SERIES OF NONMETRIC ECOLOGICAL DATA

(1) Analysis of series, in time or space is a subject of growing interest in ecology, but the methods hitherto available (correlogram, periodogram, spectral analysis) were restricted to the analysis of metric (quantitative) data. Many ecological series, however, are of (or contain) essential variables which are qualitative (categorical), and many quantitative variables are more efficiently sampled as rank-ordered (ordinal) variables. There are no numerical methods to analyse series of nonmetric data. (2) In this paper, the periodogram of Whittaker & Robinson is generalized to qualitative data series, using information theoretic measures. Algorithms are also described to partition rank-ordered variables into classes, in order to analyse them using the contingency periodogram. (3) The validity of the contingency periodogram is assessed by comparing its results with those of the periodogram of Schuster, using two series of metric data. Both methods identify the same periods for a series of artificial data, and also for serial measurements of the photosynthetic capacity of estuarine phytoplankton, even when these metric data are reduced to a small number of states prior to contingency periodogram analysis. (4) The contingency periodogram is also used to analyse a multivariate (multi-species) phytoplankton series. The multivariate data are reduced to a single multi-state qualitative variate by clustering the samples. At least one of the periods revealed is surprising, but can be explained.

A modelling study of optimal stocking density of mussel populations kept in experimental tanks

Abstract Measuring physiological rates in the field is a central part of carrying capacity studies, but is labor-intensive. The problem may become especially acute if many sites are to be considered. We present a strategy for estimating site-specific physiological rates based on the modelling of a reference growth experiment. Additional data are provided from monitoring of temperature or taken from the literature. To model growth in the reference experiment, we first took initial values of physiological rates from the literature. We then tuned them to simulate individual growth of a first subset of mussels from the reference experiment. Using the tuned values, we then modeled growth of a second subset of mussels to test the parameter values against their growth. We then modeled growth of mussels as a function of population density to obtain estimates of biomass-density and production-density curves for systems like the reference experiment. These curves provided much of the information usually required for managing cultured populations. We conclude that combining modelling of reference experiments in this way with particle transport models may prove useful for assessing optimal stocking density in situations where intensive field work programs are not possible.

Bivalve Feeding and the Benthic Boundary Layer

Benthic suspension feeders particularly some bivalves have strong structuring effects on rocky shore communities which are commonly dominated by the Mytilidae in most oceans (see Paine 1984). This dominance may be related to at least two important aspects of their ecology the sestonic nature of their food and their sessile mode of life. The food of seston feeders is produced in a three-dimensional environment which flows over the animals. Under favourable conditions this food is continuously replenished by currents sinking and reproduction of planktonic organisms. Thus owing to their sedentary nature and feeding behaviour there is strong coupling between the pelagic and benthic environments through bivalves. Energy from the pelagos is channeled directly to the benthos via the filtering activity of the organisms and is eventually used for various metabolic and growth activities (see Dame and Patten 1981). Given the above it can be postulated that the biomass of other feeding guilds (e.g. carnivores which depend directly on primary consumers and perhaps even grazers through the elimination of algal growth by sessile animals Dayton 1973) as well as the overall structure of intertidal communities is dependent on the “success” or “failure” of the suspension feeders. Thus given the central position of suspension feeders as structuring agents of the intertidal communities energy flow from the pelagos to the benthos can be determined by measuring energy flow through this guild. Similar principles hold for subtidal macrofaunal assemblages. Wildish and Peer (1983) for example reported similar relationships between the pelagos and benthos in the Bay of Fundy where suspension feeders contributed over 88% of the total macrofaunal production.

Fluctuating asymmetry and mortality in cultured oysters (Crassostrea gigas) in Marennes-Oleron basin

Japanese cupped oysters suffer summer mortality in many culture sites along the French Atlantic coast. To ascertain whether mortality might be associated with morphological features of the shells, we estimated fluctuating asymmetry (FA) of hatchery/nursery-produced oysters kept in culture bags in the intertidal. FA is defined as random variations from a perfect symmetry plane (bilateral or radial) in some morphological trait. FA increases with disruption of homeostasy during ontogeny and has both genetic and environmental determinants. We found significant differences in FA between dead and live oysters in two of our three study groups. Therefore, lack of developmental stability may be involved in the summer mortality syndrome of oysters. We also found indirect evidence that once acquired, FA patterns of shells are retained at least partially. Since FA likely appears early during ontogeny and is recorded in the shells, we hypothesise that it might be useful for forecasting the ability of spat to resist environmental stress.

Growth, survival and fluctuating asymmetry of Iceland scallops in a test of density-dependent growth in a natural bed

We report an experiment designed to test for overpopulation in an Iceland scallop, Chlamys islandica (O.F. Muller), slow-growing bed. Scallops were installed in pearl nets at the site of the scallop bed and in a nearby unpopulated site, near the bottom and far from the bottom. With this set- up, all groups, except that one near the bottom inside the bed, were free from potential overpopulation effects. In addition, there were two stocking densities to test for containment effects. Shell growth was higher at 2.0 m above the bottom, outside the scallop bed, than at the three other site and height combinations, which were not significantly different from each other. There was a borderline effect of site on soft tissue growth, which was probably slower at the donor site. We conclude that there was no evidence of overpopulation in the scallop bed. On both sites, soft tissue growth was slowest near the bottom, irrespective of group size, intermediate at 2.0 m height, high population density, and fastest at 2.0 m height, low population density. Survivorship was lower near the bottom, but was independent of site and of group size. Available data showed no sustained vertical patterns in phytoplankton concentration in the water column. These results suggest that food depletion occurred in the pearl nets, but that density-independent factors dominated density- dependent factors in the pearl nets near the bottom. Likely explanations involve the interaction between containment effects and current speed variations in the benthic boundary layer. Survivorship decreased with fluctuating asymmetry of the ears of the shells. Fluctuating asymmetry, therefore, may provide a means of including individual effects in growth experiments and

Estimating optimal population density for intermediate culture of scallops in spat collector bags

Abstract In a scallop restocking project in Iles-de-la-Madeleine, Quebec, we investigated the spat growing within the collector bags, instead of using the usual pearl net method. To estimate optimal population density in the collector bags, we studied yield as a function of spat population density ( B – N curves) at three depths in the water column (10, 20, 30 m) and in a nearby lagoon. The B – N curves suggest that for 1-year-old spat, biomass at 10 m depth was maximum at ca. 1900 individual spat per bag. At higher population density, self-thinning occurred and yield decreased. The B – N curve for 20 m did not reach a maximum at the population densities encountered in the experiment. At population density ca. 2000 ind/bag, yield of individuals ≥2 cm shell height was greater at 20 m than at 10 m, apparently because of higher food availability at 20 m and presence of interspecific competitors at 10 m depth. Yields in the lagoon and at 30 m depth were not satisfactory. Intermediate culture should be done at 20 m depth, with at least 2000 ind/bag. The upper limit to this estimate could not be determined experimentally.

Phytoplankton photosynthetic response to light in an internal tide dominated environment

The influence of internal tides on phytoplankton photosynthetic response to light was studied in the Lower St. Lawrence Estuary. Photosynthesis at saturating light intensity responded to variations in the vertical density gradient, which were linked to the internal tides. The photosynthetic response was lag-correlated to the vertical water stratification. This suggests that the link between photosynthesis and the internal tides may have resulted from phytoplankton light adaptation.

Hierarchical structure of bivalve culture systems and optimal stocking density.

Bivalve culture systems are hierarchical, with culture units being nested within culture gear, which are nested within farms, and so on. The possibility that processes acting at the scale of individual culture units may interact with high-level processes has been overlooked in carrying capacity models, although basin-scale patterns are generated at the scale of culture units. Here I study the effect of increasing basin-scale loading on unit-scale optimal stocking density (OSD). I find a curvilinear relationship, with OSD decreasing with basin-scale loading. Clearly basin-scale models should incorporate culture-unit effects. This may be achieved by using experimental studies of the clearance rate of whole culture units to complement estimates of ecophysiological processes of individuals. Such culture-unit information, along with knowledge of associated local phytoplankton depletion at various current speeds and culture-unit stocking levels, may be used to generate submodels to be included in basin-scale models. To facilitate experimental testing of across-scale effects, I develop a simple food-regulated growth model combining density dependence at the scale of individual culture units and at the scale of basins.