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INFLUENCE RELATIVE DES FACTEURS ECOLOGIQUES ET DE LA PECHE SUR LA STRUCTURATION DES STOCKS DE POISSONS RECIFAUX DANS SIX PECHERIES DU ROYAUME DES TONGA (PACIFIQUE SUD)
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Two fishing grounds were selected in each of the three archipelagos of Tonga, according to fishing pressure (high and low). In each ground, socioeconomic surveys provided an evaluation of fishing pressure. Reef fish stocks were assessed through underwater visual censuses along 241 transects and their habitat was described with a novative method (Medium Scale A p p r o a c h ) , better adapted to their life territories. The analysis of ecological and fishing data showed a variation between global fishing pressure at the archipelago level, which was responsible, in combination with ecological factors, for more differences than between the pair of sites within each archipelago. The fishing factor explains globally less variance of fish populations (1.6 to 5.7%) than factors acting at micro-scale (depth, hard substrate and live coral coverages, heterogeneity and topographic complexity) and at meso-scale (oceanic influence), that explain 23.3 to 34.3% of variance. The study confirmed fishing effects already known (such as reduction of fish populations average size, compensatory increase of density in the small size classes) and showed, at least for scarids, a “shifting dominance” phenomenon, based on the decrease of large-size target species, which benefits to small size species, less vulnerable to fishing. Clustering of species according to diet or life history traits revealed gradual changes of density and biomass in specific groups according to fishing pressure gradient. This result provides potential for setting up indicators of stock status for better management of reef fish resources.
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... We assumed in our study that the habitat variables were a major structuring factor of consumed fish populations, as shown by several authors (e.g. Jennings et al., 1996), in particular if we compare it with fishing effects (Clua, 2004). Other structuring factors are known to have significant effects, such as recruitment (Sale, 1991;Hixon and Webster, 2002), inter-specific predation (Caley, 1993), fishing (Russ and Alcala, 1998;Jennings and Kaiser, 1998) or temporal variability (Galzin, 1987;Friedlander and Parrish, 1998b;Thompson and Mapstone, 2002). ...
... As far as we know, the literature does not mention these variables as usual structuring variables of fish populations, except for "branching coral" in a study conducted in Hawaii (Friedlander et al., 2003). On the other hand, many authors have already shown the structuring role of environmental variables such as "live coral" (Bell and Galzin, 1984;Legendre et al., 1997) or "hard bottom" and "sand" (Labrosse, 2000;Clua, 2004), as revealed by the LIT method in our study on biomass by size classes and trophic groups. ...
Habitat characteristics play a critical role in structuring reef fish communities subjected to fishing pressure. The line intercept transect (LIT) method provides an accurate quantitative description of the habitat, but in a very narrow corridor less than 1 m wide. Such a scale is poorly adapted to the wide-ranging species that account for a significant part of these assemblages. We developed an easy-to-use medium scale approach (MSA), based on a semi-quantitative description of 20 quadrats of 25 m2 (500 m2 in total). We then simulated virtual reef landscapes of different complexities in a computer, on which we computed MSA using different methods of calculation. These simulations allowed us to select the best method of calculation, obtaining quantitative estimates with acceptable accuracy (comparison with the original simulated landscapes: R2 ranging from 0.986 to 0.997); they also showed that MSA is a more efficient estimator than LIT, generating percentage coverage estimates that are less variable. A mensurative experiment based on thirty 50-m transects, conducted by three teams of two divers, was used to empirically compare the two estimators and assess their ability to predict fish–habitat relationships. Three-factor multivariate ANOVAs (Teams, Reef, Methods) revealed again that LIT produced habitat composition estimates that were more variable than MSA. Canonical analyses conducted on fish biomass data successively aggregated by mobility patterns, trophic groups, and size classes, showed the higher predictive power of MSA habitat data over LIT. The MSA enriches the toolbox of methods available for reef habitat description at intermediate scale (< 1000 m2), between the scale where LIT is appropriate (< 100 m2) and the landscape approach (> 1000 m2).
... A study on fishing practices and household fish consumption was conducted for each fishing ground in parallel with the underwater censuses. This study used a qualitative approach which confirmed that there was no critical difference among the five fishing grounds in terms of gear types (hooked lines, spearfishing, and nets) and main target species (Clua 2007). This approach did not, however, allow an accurate assessment of the fishing pressure. ...
... Fish caught by Tongan fishers are home-consumed for one third, and bargained or sold in local markets for the other two thirds. Scarid species are, before Serranids and Acanthurids, the main targeted coral reef fishes in the three archipelagos of the Kingdom of Tonga, with no differential preference for any given species between archipelagos (Clua 2007). This allowed us to consider the ranking of villages, which was established by two independent approaches using all species, to be applicable to Scarids. ...
The present paper analyzes data collected between 2001 and 2002 on 81 reef fish species targeted by fishers at 5 sites in the Kingdom of Tonga (South Pacific). We first ranked the sites with respect to fishing pressure using two independent methods: (i) Tongan demography and reef surfaces available for fishing, and (ii) the differential effects of fishing on the whole set of 81 species grouped by their life history traits (LHT). We then focused on Parrotfish (Scaridae), which are heavily targeted in coral reef fisheries. We used the identified gradient of fishing pressure to study the effect of fishing on the community structure and test the hypothesis of "shifting dominance" amongst the 20 Scarid species present in the surveys. In addition to the classical effect of decreasing fish size in a family strongly targeted by fishers, the shifting dominance phenomenon includes a decrease in the abundance of the large-bodied and highly targeted species, favouring their replacement by smaller-bodied species from the same family, which are less impacted by fishing. In a context of interspecific competition amongst Scarids, the stress of fishing appears as a factor favouring the replacement of species with large maximum size, and LHT promoting low resilience, by smaller species with the opposite attributes. The discussion focuses on the various processes that can explain the shifting dominance phenomenon. The total density of resilient species, which increased along the gradient of increasing fishing pressure, can be used as an indicator of the over-exploitation of fish communities for reef fisheries management.
... L'habitat a en effet une part importante dans la structuration des assemblages de poissons et celle-ci doit être prise en compte dans toute mesure de gestion spatialisée. De nombreuses méthodologies différentes existent pour déterminer des variables d'habitat (Charbonnel et al., 2002 ;Clua, 2004 ;García-Charton et al., 2004 ;Ferraris et al., 2005) (Harvey et al., 2001 ;Harvey et al., 2004 ;Tessier et al., 2005). Quelle que soit la méthode utilisée, il faut toujours s'assurer au préalable qu'elle n'induit pas d'effet sur la variabilité spatiale par l'introduction d'éventuels biais (e.g. ...
Marine Protected Areas' (MPAs) and Artificial Reef's (ARs) management requires complex assessment and monitoring programmes, dealing with different sources of variability. We studied and developed experimental designs and analysis methods suited for the establishment of a monitoring of MPAs and ARs. This methodology is developed from existing data sets in the Northwestern Mediterranean. We build multi-criteria indicators allowing a statistically testable diagnosis of the impact of MPAs and ARs on reef fish assemblages. Using ecological performance indicators permits to monitor and to give an image of the assessed system to managers. It was possible to show the global response of the fish assemblages to the protection by a MPA. This response was evidenced by increases in abundance, species richness or diversity, gradually through space, time and among various taxonomic groups or fish individual sizes. Large fishes reacted faster to protection and shallow habitats were more sensitive to the existence of a MPA. Our results can be useful for the implementation of new MPAs or for the immersions of ARs and for the development of their management plans. La gestion des Aires Marines Protégées (AMP) et des Récifs Artificiels (RA) nécessite des évaluations complexes avec des suivis adaptés, incorporant différentes sources de variabilité. Nous avons étudié et développé des protocoles expérimentaux et des méthodes d'analyse appropriés à la mise en place de suivis récurrents des AMP et des RA. Cette méthodologie est développée à partir de données existantes sur différents cas d'étude en Méditerranée nord-occidentale. Nous avons construits des indicateurs multiparamétriques permettant un diagnostic statistiquement testable de l'impact des AMP et des RA sur les assemblages de poissons démersaux-benthiques. Disposer d'indicateurs de la performance écologique de telles structures permet de suivre et de restituer aux gestionnaires les évolutions du système observées lors de l'évaluation. Il fut possible de montrer la réponse globale du peuplement de poissons à la mise en place d'une AMP. Cette réponse, se traduisant par des augmentations d'abondance, de richesse spécifique ou de diversité, est graduelle dans le temps, l'espace et selon les groupes taxonomiques considérés ou la taille des individus d'une même espèce. Les grands individus réagissent le plus vite à la mise en protection et les habitats de faibles profondeurs sont plus sensibles à la présence de la réserve. Nos résultats peuvent servir à la mise en place de nouvelles AMP ou à l'immersion de RA et au développement de leur plan de gestion.
In many biological studies of water quality, a diversity index is calculated in 'bits per individual' by using Shannon's Approximation to Brillouin's Formula. Difficulties associated with such use of Shannon's Formula and its associated parameters are discussed. Recent research has indicated that diversity indexes can be improved if (a) biological sample collection and analysis are standardized prior to use for among various aquatic systems diversity indexes and their associated parameters, (b) the diversity index is measured in 'sits per individual' rather than the presently used unit of 'bits per individual,' and (c) the equation e=(H - H(min)/(H(max) - H(min)) is used to evaluate the uniformity of distribution of individuals among species in a sample where e equals relative evenness. Relative evenness, a ratio, is an expression in which Brillouin's and Shannon's Equations are not arbitrarily mixed. Values of diversity indexes using Brillouin and Shannon Formulas, both in bits and sits per individual, and relative evenness are given for 16 hypothetical samples.
Butterflyfish communities vary in richness longitudinally from highs of >30 species to lows of 2-3 species. We investigated this gradient because it provides an opportunity to address some basic questions relating to determinants of community structure, e.g., are richness and abundance functions of geography or of local conditions? How are richness and density of species populations related? Do community-level processes, such as competition, determine community structure? In a broader sense, to what extent are community properties internally deterministic as opposed to being functions of higher-level processes? From 1981 through 1997 we counted kinds and number of butterflyfishes on over 2000 10-min transects distributed from the eastern Caribbean to the western Indian Ocean and Red Sea. We grouped transects at the same site into stations, stations in the same local area into islands, and islands into 18 geographic regions, thus organizing the data into four spatial levels. For each station, extent of live coral cover and degree of open-ocean influence were estimated. Richness is highest in the Philippine-Bornean-New Guinean region and declines radially from that center. Total number of individual butterflyfish also declines over that gradient. On more local scales, increased open-ocean exposure and live-coral cover enhance both richness and numerical abundance. Because species richness declines more rapidly with distance from the richness center than does abundance, across the same gradient average abundance per benthivore species rises. In peripheral, species-poor reaches of the Indopacific, the Eastern Tropical Pacific, and the Caribbean the communities are dominated by one or a few very common species. Butterflyfishes respond to changing richness of their communities by adjusting the abundance of individual species. Use of space by individual species decreases with increasing richness. Global richness is determined chiefly by geography; within local areas local factors are more important. Community structure is pre dominantly organized by richness, hence first by geography and then by habitat characteristics.
