Jean Boucher

Is reproduction mainly regulated by temperature or photoperiod in Pecten maximus

Summary The reproduction of a population of the scallop Pecten maximus in Brittany (France) has been studied for two years using two different methods: (i) determination of the gonad index; (ii) measurement of oocyte diameters by image analysis of histological sections. A difference of 15 days with respect to an increase in the gonad index was observed between the two years, which correlates well with a similar difference registered from the increase in seawater temperature during the spring. No differences were observed in the time of oocyte growth as measured directly by method 2 which suggests possible photoperiodic control.

A new multivariate mapping method for studying species assemblages and their habitats: Example using bottom trawl surveys in the Bay of Biscay (France)

Abstract This new numerical approach proposes a solution to a fundamental and difficult question in ecology, consisting of the correct geographical representation of multidimensional structures. Firstly, transformation was applied to the original matrix (n sites x q variables) in order to satisfy the condition of multinormality. Then, a hierarchical cluster analysis was used and each hierarchical level was studied and characterised by a certain probability level. For each cut off level an algorithm based on the computation of the Bayesian probabilities produced a smaller matrix (n sites x c groups). These conditional probabilities measure the chance that each site has in belonging to a predefined group of sites. Spatial distributions of these probability values for each group of sites were mapped using kriging interpolation. Finally, the maps were used to define homogenous zones on a single map by superimposing one map on the other. The maximal value of interpolated probability was used as criterion to assign each point of the map to the zones predefined by this classification. This method was applied to map demersal fish habitats by using a dataset from bottom trawl surveys in the Bay of Biscay (France) during October 1990. The boundaries between habitats wae identitied objectively. Then, the indicator species and species assemblages characterising the different habitats were identitied by using an indicator value index. This index integrates the speciticity and the fidelity quantities calculated for each species in each habitat. The obtained results showed that this method presented a robust tool to describe the habitat of exploited species. The obtained habitats were validated by their correspondence with depth strata, sediment type and also by the biological characteristics of the indicator species. The proposed method is useful in the study of temporal variations of habitats with regards to species assemblages and can also be generalised to other multivariate databases of different descriptors (physical, chemical, biological, etc.).

Temporal variability of total biomass in harvested communities of demersal fishes

The biomass variability of the demersal fish communities of the Bay of Biscay and the Gulf of Lions is analysed on a decadal scale using scientific surveys and fisheries data. The coefficient of variation of the total biomass is lower than the coefficient of variation of species biomass in both areas. Previous analyses from the Scotian Shelf have demonstrated that the total biomass of demersal fish is quite stable relative to individual species biomass. This result was interpreted as demonstrations of interactions between species due to energy limitation of systems. Moreover, the total biomass variability is lower in the Bay of Biscay than in the Gulf of Lions. As the distribution of the coefficients of variation of species in the Bay of Biscay does not differ from that in the Gulf of Lions, the difference of total biomass variability is not explained by the variability of species. Further, harvesting is investigated as a source of difference of total biomass variability between the Bay of Biscay, the Gulf of Lions and the Scotian Shelf. The mean of total biomass is the lowest in the area where the harvesting level is the highest (Gulf of Lions) and is the highest where harvesting is the lowest (Scotian Shelf), while the total biomass variability increases with the harvesting level. A hypothesis on fishing impact on the community dynamics is discussed and the total biomass variability is proposed to be a broad indicator of the fisheries impact on the demersal fish communities.