Habitats as surrogates of taxonomic and functional fish assemblages in coral reef ecosystems : a critical analysis of factors driving effectiveness

Species check-lists are helpful to establish Marine Protected Areas (MPAs) and protect local richness, endemicity, rarity, and biodiversity in general. However, such exhaustive taxonomic lists (i.e., true surrogate of biodiversity) require extensive and expensive censuses, and the use of estimator surrogates (e.g., habitats) is an appealing alternative. In truth, surrogate effectiveness appears from the literature highly variable both in marine and terrestrial ecosystems, making it difficult to provide practical recommendations for managers. Here, we evaluate how the biodiversity reference data set and its inherent bias can influence effectiveness. Specifically, we defined habitats by geomorphology, rugosity, and benthic cover and architecture criteria, and mapped them with satellite images for a New-Caledonian site. Fish taxonomic and functional lists were elaborated from Underwater Visual Censuses, stratified according to geomorphology and exposure. We then tested if MPA networks designed to maximize habitat richness, diversity and rarity could also effectively maximize fish richness, diversity, and rarity. Effectiveness appeared highly sensitive to the fish census design itself, in relation to the type of habitat map used and the scale of analysis. Spatial distribution of habitats (estimator surrogate's distribution), quantity and location of fish census stations (target surrogate's sampling), and random processes in the MPA design all affected effectiveness to the point that one small change in the data set could lead to opposite conclusions. We suggest that previous conclusions on surrogacy effectiveness, either positive or negative, marine or terrestrial, should be considered with caution, except in instances where very dense data sets were used without pseudo-replication. Although this does not rule out the validity of using surrogates of species lists for conservation planning, the critical joint examination of both target and estimator surrogates is needed for every case study

Copernicus Ocean State Report, issue 6

The 6th issue of the Copernicus OSR incorporates a large range of topics for the blue, white and green ocean for all European regional seas, and the global ocean over 1993–2020 with a special focus on 2020

Définition de réseaux d'aires marines protégées en Nouvelle-Calédonie en fonction des relations poissons-habitats

La prise en compte de critères biologiques lors de la mise en place de réseaux d'aires marines protégées (AMP) permet d'augmenter leur efficacité mais nécessite des inventaires coûteux et localisés. Dans cette étude centrée sur le site de Port-Bouquet en Nouvelle-Calédonie, nous proposons dans un premier temps d'évaluer si les habitats peuvent servir de substituts efficaces à la diversité taxonomique et fonctionnelle des communautés de poissons en milieu corallien. 11 niveaux de caractérisation d'habitats et 9 échelles spatiales différentes ont été testés par analyse de corrélation et de substitution, de manière à déterminer les conditions optimales pour lesquelles la richesse, la diversité, et la rareté des habitats permettent une bonne représentation de la richesse, la diversité, et la rareté des groupes taxonomiques et fonctionnels de poissons. Notre étude suggère que l'approche par substitution est la plus appropriée lorsqu'il s'agit d'établir un réseau d'AMP, puisque s'appui directement sur les algorithmes de sélection des sites candidats. Les algorithmes dits de « Richesse-complémentarité » et de « Rareté » utilisés dans cette étude se sont avérés sensibles aux effets du hasard et de l'échantillonnage. Des résultats encourageant ont en revanche été obtenus pour un scénario de «Diversité-complémentarité» et a pu être utilisé pour proposer un réseau d'AMP, établis sur la base de la topographie des habitats, efficace d'un point de vue de la représentativité taxonomique et fonctionnelle des poissons. (Résumé d'auteur

Understanding consequences of adaptive monitoring protocols on data consistency : application to the monitoring of giant clam densities impacted by massive mortalities in Tuamotu atolls, French Polynesia

During long-term monitoring, protocols suitable in the initial context may have to change afterward because of unforeseen events. The outcome for management can be important if the consequences of changing protocols are not understood. In Tuamotu Archipelago atolls, French Polynesia, the density of giant clams (Tridacna maxima) has been monitored for 12years, but massive mortalities and collapsing densities forced to shift from a line-intercept transects and quadrats (LIT-Q) method to a belt-transect (BT) method. We investigated with a simulation approach the conditions (density, size structure, aggregation of giant clam populations) under which the two methods provided different results. A statistical model relating the BT density to the LIT-Q density was validated using new field data acquired on the same sites with both protocols, on two atolls. The BT method usually provided higher estimates of density than the LIT-Q method, but the opposite was found for very high densities. The shape of the relationship between measurements depended on population size structure and on aggregation. Revisiting with the model the historical LIT-Q densities suggested that densities have been underestimated in the past but previously detected trends in population trajectories remained valid. The implication of these findings for management are discussed

Scaling tropical island conservation planning to the regional level can lead to unbalanced ecological representation and poor social equity among islands

The effectiveness of Marine Protected Areas (MPA) to manage natural resources has been undermined in small insular lagoons due to massive mortalities triggered by climatic events that have hit some lagoons but not others. To minimize the future risk of ineffective management efforts, it has previously been argued that management should focus on a multi-island conservation target (regional scale), rather than on individual lagoons (local scale). However, it is unclear how a MPA network designed to meet objectives at a regional scale would impact on the management of resources at the local scale. In particular, it is necessary to understand if a regional plan might incidentally maintain conservation objectives at the local scale, without disproportionately affecting, or relying on particular islands. This study used the population of the giant clam (Tridacna maxima) in a fishery context to explore the distributions of conservation features and socio-economic costs for regional networks (computed within a set of islands), compared to individual islands. Designing a MPA network at regional scale led to unbalanced representation of conservation features among atolls and incidentally missed the targeted level of protection for conservation features at local scale. Moreover, the regional network generated inequitable costs for fishermen between islands, which is likely to lead to poor perceived equity. This study suggests that perceived equity and the representation of local conservation objectives will be major factors to consider, if the French Polynesian authorities follow the path of implementing MPAs in each atoll for a regional-scale resource management plan

Consequences of an uncertain mass mortality regime triggered by climate variability on giant clam population management in the Pacific Ocean

Despite actions to manage sustainably tropical Pacific Ocean reef fisheries, managers have faced failures and frustrations because of unpredicted mass mortality events triggered by climate variability. The consequences of these events on the long-term population dynamics of living resources need to be better understood for better management decisions. Here, we use a giant clam (Tridacna maxima) spatially explicit population model to compare the efficiency of several management strategies under various scenarios of natural mortality, including mass mortality due to climatic anomalies. The model was parameterized by in situ estimations of growth and mortality and fishing effort, and was validated by historical and new in situ surveys of giant clam stocks in two French Polynesia lagoons. Projections on the long run (100 years) suggested that the best management strategy was a decrease of fishing pressure through quota implementation, regardless of the mortality regime considered. In contrast, increasing the minimum legal size of catch and closing areas to fishing were less efficient. When high mortality occurred due to climate variability, the efficiency of all management scenarios decreased markedly. Simulating El Nifio Southern Oscillation event by adding temporal autocorrelation in natural mortality rates increased the natural variability of stocks, and also decreased the efficiency of management. These results highlight the difficulties that managers in small Pacific islands can expect in the future in the face of global warming, climate anomalies and new mass mortalities

Conservation and resource management in small tropical islands : trade-offs between planning unit size, data redundancy and data loss

Resource management and conservation require the definition of planning units (PUs), i.e., the spatial domain where management decisions are applied. PUs are either pre-established in size and shape following management constraints or are data driven(DDPUs) by overlay of multidisciplinary data layers. The trade-offs between these two approaches have not been investigated previously for small tropical islands and their characteristics. Here, we use resource density, fishing pressure and susceptibility to mortality for a giant clam fishery in a small French Polynesia atoll to discuss the suitability and impact of the two approaches in conservation management. Aggregation to pre-established PU grids highly affected data even for PU as small as 2500 m(2), with higher loss of spatial information for density and fishing effort. By contrast, DDPU rendered well small scale patterns of interest but reduced redundancy. Our results stress the importance of considering the initial patterns of data in the definition of planning units, and we suggest a 3 steps process to identify adequate trade-offs between PU size, PU redundancy and data loss to properly draw practical recommendations for small islands

Habitats as surrogates of taxonomic and functional fish assemblages in coral reef ecosystems : a critical analysis of factors driving effectiveness

Species check-lists are helpful to establish Marine Protected Areas (MPAs) and protect local richness, endemicity, rarity, and biodiversity in general. However, such exhaustive taxonomic lists (i.e., true surrogate of biodiversity) require extensive and expensive censuses, and the use of estimator surrogates (e.g., habitats) is an appealing alternative. In truth, surrogate effectiveness appears from the literature highly variable both in marine and terrestrial ecosystems, making it difficult to provide practical recommendations for managers. Here, we evaluate how the biodiversity reference data set and its inherent bias can influence effectiveness. Specifically, we defined habitats by geomorphology, rugosity, and benthic cover and architecture criteria, and mapped them with satellite images for a New-Caledonian site. Fish taxonomic and functional lists were elaborated from Underwater Visual Censuses, stratified according to geomorphology and exposure. We then tested if MPA networks designed to maximize habitat richness, diversity and rarity could also effectively maximize fish richness, diversity, and rarity. Effectiveness appeared highly sensitive to the fish census design itself, in relation to the type of habitat map used and the scale of analysis. Spatial distribution of habitats (estimator surrogate's distribution), quantity and location of fish census stations (target surrogate's sampling), and random processes in the MPA design all affected effectiveness to the point that one small change in the data set could lead to opposite conclusions. We suggest that previous conclusions on surrogacy effectiveness, either positive or negative, marine or terrestrial, should be considered with caution, except in instances where very dense data sets were used without pseudo-replication. Although this does not rule out the validity of using surrogates of species lists for conservation planning, the critical joint examination of both target and estimator surrogates is needed for every case study

Alert thresholds for monitoring environmental variables : a new approach applied to seagrass beds diversity in New Caledonia

Monitoring ecological variables is mandatory to detect abnormal changes in ecosystems. When the studied variables exceed predefined alert thresholds, management actions may be required. In the past, alert thresholds have been typically defined by expert judgments and descriptive statistics. Recently, approaches based on statistical power were also used. In New Caledonia, seagrass monitoring is a priority given their vulnerability to natural and anthropic disturbances. To define a suitable monitoring strategy and alert thresholds, we compared a Percentile Based Approach (PBA) and a sensitivity analysis of power (SAP). Both methods defined statistically relevant alert thresholds, but the SAP approach-was more robust to spatial and temporal variability of seagrass cover. Moreover, this method characterized the sensitivity of threshold values to sampling efforts, a useful knowledge for managers

Best management strategies for sustainable giant clam fishery in French Polynesia Islands : answers from a spatial modeling approach

The giant clam Tridacna maxima has been largely overexploited in many tropical regions over the past decades, and was therefore listed in appendix II of the Convention of International Trade in Endangered Species (CITES) in 1985. In French Polynesia, several atolls and islands harbor the world's highest stocks of giant clams in very shallow and accessible areas, which are therefore highly vulnerable to fishing pressure. The local fishery authority (i.e., Direction des Resources Marines or "DRM") implemented several management schemes in 2002 to control and regulate fishing pressure. However, for further decisions DRM was missing a sensitivity analysis on the effectiveness of the possible management actions. Here, we report on the use of a deterministic Viable Population Analysis (VPA) and spatially-explicit age-based population model that simulated the 30-year trajectory of a Tridacna maxima stock under different management approaches. Specifically, given various scenarios of intra-island larval dispersal, we tested which of No-take-Areas (NTAs), rotational closures, size limits, quotas, and restocking schemes would lead to the highest future stocks in Tubuai and Raivavae, two exploited islands of the Austral archipelago. For both islands, stock abundances were estimated in 2004/2010 and 2005/2010 respectively, and natural mortalities were assessed previously only in Tubuai. When compared to field data, the model successfully predicted the 2010 stocks for Tubuai, but proved to be less reliable for Raivavae, where natural mortality rates may well be different from those on Tubuai. For Tubuai, the spatial model suggested that reducing fishing effort (through fixed quotas) and banning fishing below the 12 cm size limit (as currently implemented) were the most effective management actions to sustain T. maxima populations into the future. Implementing NTAs was of poor effectiveness. NTAs increased giant clam stock inside the protected area, but also increased overfishing in the neighboring areas, and were ineffective overall