Michel Harvey

Spatial variation in growth of the bivalve Macoma balthica (L.) on a tidal flat: effects of environmental factors and intraspecific competition

Abstract A descriptive field study was conducted on a tidal flat of the Lower St. Lawrence Estuary to examine spatial variations in shell growth of the infaunal bivalve Macoma balthica (L.) in relation to tidal level, sediment characteristics, and local density. This study was based on 83 0.3 × 0.3 m 2 sediment samples randomly located on a 1.6 km 2 tidal flat where maximal amplitude of tide was 4.8 m. Mean density of Macoma balthica for the whole sampling area was 1254 m −2 ( S x = 87 m −2 ) and densities were higher and more variable near the mean water level. Variations in the effects of density and environmental factors along the tidal gradient were studied after a stratification of the stations by immersion time. Overall, shell growth rate increased with immersion time, the latter accounting for about 5 to 63% of the spatial variation of mean shell length for the six youngest generations. Fifty-two to 61% of the adjusted annual shell growth was explained by a linear relationship with tidal level. A significant but weaker linear relationship was obtained between growth and density, and between growth and sediment grain size. For the whole sampling area, a linear model with tidal level and density as independent variables explained 37 to 80% of the spatial variability of mean shell length for each generation and 59 to 77% of the spatial variability in adjusted annual growth rates. The effect of density was not significant in the lower part of the tidal flat, however, it became significant when immersion time was ≈ 50%, and had a maximum effect when immersion time was about 40%. About 65 to 85% of the spatial variability of the growth rate along the tidal gradient can be explained by stepwise multiple regressions involving density as the first independent variable. Overall, the results suggested that intraspecific competition played a major role in ≈ 67% of the population of Macoma balthica within the study area.

SPATIAL ANALYSIS OF RECRUITMENT OF MARINE INVERTEBRATES ON ARBORESCENT SUBSTRATA

Juveniles of various species were observed on the perisarc of the dead hydroid Tubularia larynx in the Gulf of St Lawrence, Canada, some at densities up to twenty times more than on other substrata. This suggested active microhabitat selection by settling larvae. Here the results are reviewed of three field experiments using arborescent substrata, which tested whether recruitment of invertebrates could be accounted for by a passive model of encounter, or whether activity of larvae, or settled juveniles, needs to be invoked to explain the distributions. The distribution of inert particles on arborescent structures was examined in a laboratory flume. Both branch diameter and branching heterogeneity of the structures were considered as independent variables. A comparison of laboratory and field results suggested that passive settlement processes (hydrodynamic) were sufficient to explain the recruitment patterns at scales greater than 3 cm. At small scales (ca 1 mm), however, the hypothesis of active larval a...

Spat settlement of the giant scallop, Placopecten magellanicus (Gmelin, 1791), and other bivalve species on artificial filamentous collectors coated with chitinous material

Abstract Based on previous studies showing that both passive (flow around filamentous and ramose organisms) and active (selection towards chitin in the perisarc of dead hydroids) processes may be involved in the settlement of scallop larvae on natural substrata, we examine here the possibility of using artificial filamentous material coated with a thin film of chitinous material to maximize spat collection of the giant scallop, Placopecten magellanicus , on artificial collectors. Chitinous material from different sources (hydroid perisarcs and prawn, crab, and lobster exoskeletons) was purified and transformed, and used to coat Netron ® netting. Two types of collectors were used in three different field experiments. The first type of collector consisted of 13 g of Netron ® netting (15 mm mesh size), coated with a thin film of chitinous material, placed in 10 mm mesh cubic Vexar cages (10 × 10 × 10 cm). This type of collector was used in the first two experiments, respectively designed to test (1) the effect of the type (source) of chitinous film as well as (2) the combined effects of the type of chitinous film, the number of layers of chitin on the netting, and the surface texture (roughness) of the chitinous film. A third experiment was carried out with collectors made of 150 g (wet weight) of perisarc of dead hydroids Tubularia larynx placed into 10 cm cubic Vexar cages. Perisarcs were processed using various physical and chemical treatments to examine the stability of the cue(s) responsible for the settlement of scallop larvae and other associated bivalve species. A total of 110 collectors was placed in the water near Tongue Shoal, Passamaquoddy Bay, New Brunswick, Canada on 8 September 1994 and retrieved on 30 October 1994. These spat collectors were successful in collecting large numbers of spat of the giant scallop (8179 individuals) and other bivalve species (65 739 individuals). There was a significant positive effect of type of chitinous films on the mean number of spat of each bivalve species collected including the giant scallop P. magellanicus . Over 66% more bivalve and 35% more giant scallop spat were collected in the Netron ® collectors coated with chitinous films compared with control collectors (Netron ® without any coating). On the contrary, in collectors other than controls, we observed no significant effect of the types of chitinous films (prawn, hydroids, crab, lobster) and no consistent pattern of variation in spat recruitment among collectors offering different numbers of layers of chitin and surface textures (roughness). Finally, spat of scallop and other bivalve species were recruited in higher numbers (N spat · g −1 ) on ‘Hydroid’ compared with ‘Netron ® ’ collectors. Among the ‘Hydroid’ collectors there was an occasional lower recruitment with bleached hydroids but no significant differences in the number of spat collected by the hydroids which were either not treated, deproteinized or boiled.

Marine Mammal Strandings and Environmental Changes: A 15-Year Study in the St. Lawrence Ecosystem

Understanding the effects of climatic variability on marine mammals is challenging due to the complexity of ecological interactions. We used general linear models to analyze a 15-year database documenting marine mammal strandings (1994–2008; n = 1,193) and nine environmental parameters known to affect marine mammal survival, from regional (sea ice) to continental scales (North Atlantic Oscillation, NAO). Stranding events were more frequent during summer and fall than other seasons, and have increased since 1994. Poor ice conditions observed during the same period may have affected marine mammals either directly, by modulating the availability of habitat for feeding and breeding activities, or indirectly, through changes in water conditions and marine productivity (krill abundance). For most species (75%, n = 6 species), a low volume of ice was correlated with increasing frequency of stranding events (e.g. R2 adj = 0.59, hooded seal, Cystophora cristata). This likely led to an increase in seal mortality during the breeding period, but also to increase habitat availability for seasonal migratory cetaceans using ice-free areas during winter. We also detected a high frequency of stranding events for mysticete species (minke whale, Balaenoptera acutorostrata) and resident species (beluga, Delphinapterus leucas), correlated with low krill abundance since 1994. Positive NAO indices were positively correlated with high frequencies of stranding events for resident and seasonal migratory cetaceans, as well as rare species (R2 adj = 0.53, 0.81 and 0.34, respectively). This contrasts with seal mass stranding numbers, which were negatively correlated with a positive NAO index. In addition, an unusual multiple species mortality event (n = 114, 62% of total annual mortality) in 2008 was caused by a harmful algal bloom. Our findings provide an empirical baseline in understanding marine mammal survival when faced with climatic variability. This is a promising step in integrating stranding records to monitor the consequences of environmental changes in marine ecosystems over long time scales.

Short-term variations in settlement and early spat mortality of Iceland scallop, Chlamys islandica (O.F. Müller)☆

Abstract A field experiment was carried out from 20 September to 9 November 1992 in the Baie des Chaleurs (Gulf of St. Lawrence) to examine the temporal pattern of Iceland scallop ( Chlamys islandica ) spat settlement on artificial filamentous collectors. One hundred sixty five cubic Vexar cages (10 × 10 × 10 cm) filled with 150 g (wet weight) of dead hydroids ( Tubularia larynx ) perisarcs were deployed and retrieved using a sequential time exposure schedule [collectors were deployed in decreasing numbers (30 to 5) every ≈ 5 days and retrieved in the reverse order (5 to 30) of their immersion]. This approach gave independent samples with respect to time and made it possible to estimate (1) near instantaneous settlement density, (2) short-term post-settlement mortality, (3) cumulative collection efficiency, and to compare (4) settlement onto substrata immersed for different periods of time. Simultaneous measurement of environmental factors (tidal range, current speed, seawater temperature and salinity) allowed an assessment of the relative influence of environmental factors in determining settlement density. The spat collectors used in this study were successful in collecting many thousands of Iceland scallop post-larvae. Settlement took place during 1 month, from the middle of September to the middle of October 1992 and collectors immersed for 10, 15, and 20 days collected significantly more spat than those immersed for 5 days. After the settlement season, spat mortality remained low for at least an additional 25 days (the end of the observation period). The daily mean density (DMD) of newly settled spat during each ≈ 5-day interval varied significantly in time and there was no significant correlation between DMD and the environmental factors. Different hypotheses concerning larval flux (no. of larvae × mean current speed) and their probability of coming into contact and being retained by a collector (retention efficiency) are proposed to explain the temporal pattern of spat collection observed in this study. Some suggestions are made to enhance collection efficiency of artificial scallop spat collectors.