Jean-Claude Brêthes

Organisation trophique de quatre peuplements de substrats rocheux selon un gradient de pollution industrielle (Golfe de Fos, France)

This study describes the trophic organization of the fauna of four superficial rock-bottom assemblages localized along a gradient of industrial pollution (Gulf of Fos, Mediterranean Sea). Six stations were studied over a period of 13 or 15 mo between May 1974 and August 1975. The faunal species (188 species) were classified into four major trophic groups: suspension-feeders, detritivores, herbivores and carnivores, which were all present in significant numbers in all four assemblages. A fifth group, the omnivores, was poorly represented. Qualitatively, the herbivores dominated by number of species (mainly Crustacea and Polychaeta). Quantitatively, the trophic groups were distributed in decreasing numbers along an increasing pollution gradient. Statistical analysis revealed a destabilization of the balance existing between the four major trophic groups along an increasing pollution gradient. Abundance of the four trophic groups exhibited spatio-temporal variations that evolved homogeneously during the annual biological cycle. When an environmental disturbance occurs, a particular trophic group will dominate, depending on the type of disturbance. At the reference station, located in relatively unpolluted waters, the three trophic groups suspension-feeders, herbivores and carnivores seemed to be well-balanced, with a slight dominance of the latter; the two other groups were distributed equally. At moderately polluted stations, suspension-feeders dominated. The most polluted stations were dominated by herbivores. Occurrence of detritivores seemed to be negatively related to the degree of thermal pollution, which provoked massive mortality and important detritus production resulting from internal stress caused by rapid temperature variations. At the population-structure level, such temperature variations are accompanied by important recruitment in a limited number of dominant species. Examination of the trophic organization of rock-bottom assemblages for which structural evolution is already well known provided complementary information on the modality of dysfunction in time and space of certain trophic-web levels, as related to progressive alterations in the rock-bottom assemblages. Such dysfunction can appear very early, even before evolution of the assemblages physiognomical characteristics. Trends in trophic organization reflect the influence of certain edaphic factors such as particle-rich currents, the impact of local thermal pollution, chemical pollution, or low salinity.

Enzymatic Activity as an Index of Trophic Resource Utilization by the Snow Crab Chionoecetes Opilio (O. Fabricius)

ABSTRACT In order to assess the food utilization of Chionoecetes opilio, we measured the activity of 5 enzymes in the digestive gland: 2 metabolize animal products (chitinase and phosphorylase-a), and 3 digest vegetal products (cellulose, alpha-amylase, and laminarinase). The analysis was made on 18 morphometrically mature males and 2 mature females. Chitinase had a high activity level (mean = 6.728 ± 1.689 units/mg of protein). Phosphorylase-a had a much lower activity level (mean = 0.114 ± 0.039 unit/mg). Alpha-amylase, while present in every crab, had a low activity level (mean = 0.102 ± 0.031 unit/mg). Cellulase was present in 85% of the crabs (mean = 0.115 ± 0.089 unit/mg). Laminarinase was hardly detected (maximum 0.002 unit/mg). The presence of active enzymes digesting vegetal products indicates that the snow crab is not a strict carnivore but is able to metabolize energy from various trophic sources, including vegetal detritus.

Homing and group cohesion in Atlantic cod Gadus morhua revealed by tagging experiments

Homing behaviour and group cohesion in Atlantic cod Gadus morhua from the northern Gulf of St Lawrence were studied based on tagging-recapture data from two periods, the 1980s and a recent period from 1996 to 2008. Two or more tags from a single tagging experiment were frequently recovered together in subsequent years. The null hypothesis was tested that the frequency of matching tag recoveries occurred by chance only through random mixing of tagged G. morhua before their recapture by the commercial fishery. The alternative hypothesis was that non-random, positive association (group cohesion) existed among tagged individuals that persisted through time and during migrations. Results show that the G. morhua population exhibits a homing behaviour, with temporal stability across seasons and years: 50% of recaptured fish in the recent period were caught <34 km from their mark site, even 3 years after release. In the 1980s, G. morhua were located at <10 km from their release site 1 year after tagging during summer and at <16 km during spring and autumn combined. Despite the increasing distance between the mark and recapture sites over time, the difference was not significant. In addition, occurrences of two or more tagged fish from the same release event that were caught together indicated a non-random association among individual fish for periods of one to several years and through migrations over several hundred kilometres. Hence G. morhua showed group cohesion in addition to site fidelity. These two interacting behaviours may be fundamental for the rebuilding and conservation of depleted fish stocks.

Variability in trophic dominance of crustaceans along a gradient of urban and industrial contamination

The trophic organization of the crustacean fauna belonging to a photophilic assemblage exposed to an urban and industrial contamination gradient in the Gulf of Fos (South of France) was studied over a period of 16 mo (May 1974 to August 1975). An increasing destabilization of the four major trophic groups from clean waters towards those most polluted was noted. Carnivores dominated at the relatively cleanwaterCystoseira stricta station; at the second, moderately pollutedC. stricta station, carnivores were replaced by suspension-feeders. Deposit-feeders dominated theCorallina cf.mediterranea stations affected by the warm-water effluent of a power plant. Other polluted stations, dominated byMytilus galloprovincialis andUlva cf.rigida, were dominated by grazers. The faunal destabilization was related to changes in the structural composition of the photophilic algal assemblage which were, in turn, related to the industrial contamination gradient along this coast.

SUBSTRATE EFFECTS ON SURVIVAL, GROWTH AND DISPERSAL OF JUVENILE SEA SCALLOP, PLACOPECTEN MAGELLANICUS (GMELIN 1791)

Abstract In the scope of expanding seeding operations of juvenile scallops (Placopecten magellanicus) around Îles-de-la-Madeleine (Quebec, Canada), this study assesses the influence of substrate on growth, survival, dispersal and predation rates in controlled experiments. Three substrates were tested: gravel, sand and sand with empty scallop shells (to investigate the feasibility of modifying the substrate as a management strategy). Dispersal rates were studied with a substrate choice experiment. Predation experiments with two major scallop predators, the rock crab Cancer irroratus and the sea star Asterias vulgaris, were carried out on different substrate types. No significant difference in growth was observed among substrates and no mortality was observed during the experiment. This suggests other factors, such as hydrodynamics, may be more important than substrate. The dispersal rate was significantly greater on sandy substrate, whereas gravel and sand–shells had similar results. Crab predation was lower on the more heterogeneous substrate (i.e., gravel) whereas, sea star predation was lower on sand. Modifying the natural sand substrate by adding scallop shells could improve seeding success because it diminishes dispersal and predation by crabs.

Behavior, Growth and Survival of Stage V Lobsters (Homarus Americanus) in Relation to Shelter Availability and Lobster Density

The effects of 2 levels of shelter availability (8 or 32 shelters) and of 3 levels of intraspecific density (5, 15, or 30 individuals per 0.5 m 2) on the behavior, survival and growth of stage V lobsters over a period of h 25 days were examined through controlled laboratory experiments. Dominance interactions were apparent in all treatments and only certain lobsters gained access to shelter in the medium- and high-density treatments. Lobsters spent more than 95% of their time in shelter when possible, and expressed significantly greater fidelity to one shelter in the 8-shelter treatment compared to the 32-shelter treatment. Shelter availability and lobster density had no effect on the incidence of molting or on the mean size of lobsters at experiments end. The proportion of injured lobsters was independent of shelter availability, but was positively related to density. Lobsters were most likely to die after having molted and percent mortality was significantly greater at low density and in the 32-shelter treatment than in other treatments, probably due to a stronger and more consistent dominance hierarchy. Overall, the findings suggest that settlers may interact, that the frequency and intensity of interactions may be modified by relative shelter availability and lobster density, and that such interactions may contribute to determine a cohorts fate.

The Canadian Atlantic groundfish experience and the constraints to the conservation of fisheries resources : a perspective

Most of the world groundfish fisheries are fully exploited or overexploited. Numerous constraints to conservation exist and are described from the Canadian Atlantic situation. Both harvesting and processing capacity increased significantly in the recent past and remain latent during the current fishing closures. Local economies are dependent on fishing influence, inducing a pressure on the management system. Fluctuations in the natural environment influence the regenerating capacity of populations. Several scientific knowledge gaps are appearing: overestimation of stocks, inability to assess and predict the effects of the environment. The institutional system is complex with conflicting views and numerous interactions which jeopardise coherent actions. The system is changing, and we cannot predict the actual effect of fishing closures. Changes must be made to develop a conservation strategy for the long time. A systemic approach must be developed to better understand the functioning of the system and to implement efficient acceptable conservation measures. Building a conservation capacity appears necessary.