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    • Pyramids, built by the Egyptians and reversed by sharksby Pierre Labourgade, Valentin Santanbien and Morgan Schler

      Published by Charlotte Recapet the October 5, 2020 on 8:28 AM

      The case of a extreme inverted trophic pyramid of reef sharks supported by spawning groupers in Fakarava, French Polynesia

      Predators play a key role in the structure and functioning of ecosystems (Paine 1966; Begon et al. 2006). Through food webs, the relationship between preys and predators is crucial in order to maintain a balance, including in marine ecosystems (Woodson et al. 2018). A trophic pyramid is a graphic representation designed to show the biomass at each level of the food chain. The lowest level starts with decomposers and the pyramid ends with top predators. This is called a pyramid because generally, the biomass in the lower levels turns out to be much higher than in the upper levels (Figure 1 A). However, in the marine environment, and in some remote and almost unoccupied areas, predators may dominate in terms of biomass, generating an inverted pyramid (Figure 1 B).


      Figure 1 Diagram of a normal (A) and inverted (B) trophic pyramid

      Aggregations of grey reef sharks, Carcharhinus amblyrhynchos are observed on some reefs in the Indo-Pacific (Robbins 2006) (Figure 2). The southern pass of Fakarava atoll in French Polynesia has a population of around 600 individuals of this species (Mourier et al. 2016) (Figure 3). This makes it one of the few places to present such a large grouping. With such a large population on a reef channel of just over 1 kilometer, the area has up to three times the biomass per hectare documented for any other reef shark aggregation (Nadon et al. 2012). The biomass of predators is then much greater than preys, thus generating an inverted trophic pyramid. During this study, scientists tried to understand how those large group of sharks can survive when prey biomass is insufficient.


      Figure 2. Aggregation of grey reef sharks


      Figure 3. Panoramic view of Fakarava atoll

      During the study period, video-assisted underwater visual surveys conducted across the pass allow the researchers to find that sharks population can represent up to 700 individuals. Then, scientists use bioenergetic models based on known value of parameters that influence energetics needs of shark-like “asymptotic length”, “growth rate” or “proportion of fish in the diet” to determine prey biomass needed for all the individuals. According to bioenergetic models, the food requirements to maintain that large population is approximately 90 tons of fish per year, which is not provided by the environment as it is. However, the pass is used as a breeding ground for many fish species, thereby reducing the prey-shark ratio. This means that the prey biomass will be much higher than that of sharks during these reproduction periods (Mourier et al. 2016), leading to frenetic predation behavior in the shark that will allow it to meet its energy needs (Robbins and Renaud 2016; Weideli, Mourier, and Planes 2015). Furthermore, the continuous presence of prey aggregation is ensured by the successive migration of different species to this site, in order to meet the metabolic demands of the shark population present (Craig 1998). With simulation-based on researcher bioenergetic model, sharks would not have enough energetic income after 75 days if other prey species didn’t migrate to the pass. There is, therefore, an idea of metapopulation where the exchange of individuals between populations in normal and inverted trophic pyramids ensures that the energy needs of each individual are met (Figure 4). This exchange of individuals between populations will allow the long-term maintenance of the species and, in the case presented here, of the shark.


      Figure 3. Diagram of the transfer of potential prey for the shark between two normal pyramids and one inverted trophic pyramid via migratory flows

      The temporal aspect in the movement of individuals between populations is therefore important to be considered during the development of management and conservation measures. Indeed, if we want to ensure the sustainability of the grey reef shark in this pass, we must not only protect the habitat on-site, but also the original habitat of different species that come to reproduce in the pass. These species are indeed essential for the survival of sharks since they represent the only source of energy available during certain periods of the year.

      Read the full study: Mourier, J., Maynard, J., Parravicini, V., Ballesta, L., Clua, E., Domeier, M.L., and Planes, S. (2016). Extreme inverted trophic pyramid of reef sharks supported by spawning groupers. Current Biology 26 (15): 2011–2016.

      Other cited articles:

      Begon, Michael, Colin R. Townsend, et John L. Harper. 2006. Ecology: from individuals to ecosystems. Sirsi i9781405111171.

      Craig, P. C. (1998). Temporal spawning patterns of several surgeonfishes and wrasses in American Samoa. Pacific Science, 52(1), 35-39.

      Nadon, M. O., Baum, J.K., Williams, I.D., McPherson, J.M., Zgliczynski, B.J., Richards, B.L., Schroeder, R.E., and Brainard, R.E. (2012). Re-creating missing population baselines for Pacific reef sharks. Conservation Biology 26 (3): 493–503.

      Paine, R.T. (1966). Food web complexity and species diversity. The American Naturalist 100 (910): 65–75.

      Robbins, W. D., and Renaud, P. (2016). Foraging mode of the grey reef shark, Carcharhinus amblyrhynchos, under two different scenarios. Coral Reefs 35 (1): 253–260.

      Robbins, W.D. (2006). Abundance, demography and population structure of the grey reef shark (Carcharhinus amblyrhynchos) and the white tip reef shark (Triaenodon obesus)(Fam. Charcharhinidae). PhD Thesis, James Cook University.

      Weideli, O. C., Mourier, J., and Planes, S. (2015). A massive surgeonfish aggregation creates a unique opportunity for reef sharks. Coral Reefs 34 (3): 835–835.

      Woodson, C. B., Schramski, J.R., and Joye, S.B. (2018). A unifying theory for top-heavy ecosystem structure in the ocean. Nature communications 9 (1): 1–8.

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    • Shad, those endangered travelersby Alicia Dragotta and Claire Valleteau

      Published by Charlotte Recapet the April 6, 2020 on 1:52 PM


      Photograph by MRM association

      Shad are migratory fish that travel great distances between sea and river in order to reproduce. These long journeys are the source of great energy expenditure, particularly to find the most favourable spawn environment. These species are considered bio-indicators of our waterways. Their presence or absence indicates the ecological state of the water. Migratory distance was governed by energetics, behaviors, maturation, and upstream delays at dams. Individual adult migrant American shad (Alosa sapidissima) ascend the Connecticut River and spawn, and survivors return to the marine environment. Theodore Castro-Santos and Benjamin H. Letcher presented a simulation model of these behaviors.

      The purpose of this model is to evaluate the effects of biological and physical variables on adult spawning success and survival. Only energy devoted to migration has been taken into account in the model. Physiology and energetics strongly affected distribution of spawning efforts and survival into the marine environment. Delays to both upstream and downstream movements had dramatic effects on spawning success. Other factors influencing migratory distance included entry date, body length, and initial energy content. Furthermore, dams alter reproductive success and have an impact on migration (delay).

      This model suggests shad that spend more time in the river have greater spawning success but are more likely to die of energy depletion.  Many important factors in the models presented here remain enigmatic. Perhaps the most important question is what causes shad to reverse direction and migrate downstream. Do both energetics and maturation play a role ?

      Answering this question could be difficult but may be possible using, say, a combination of physiological telemetry (e.g., Hinch et al. 1996) and data on reproductive status, especially of downstream migrants.  The purpose of this paper was to develop a management tool to evaluate the relative importance of biological and physical factors on shad reproduction and survival. Restoring access to spawning habitat by providing fish passage has been a central management strategy. Ecological continuum is very important to preserve species, including these migratory fish. Dams for example, were built for many reasons, at the origins in order to mill operations, and today for hydraulic energy exploitation. We have to reconsider the interest of these dams, remove those which are useless and adapt the others. This process has been under way for several years, opening the door to restoring access to the rivers.

      Read the full study: Castro-Santos, T. and Letcher, B.H. (2010) Modeling migratory energetics of Connecticut River American shad (Alosa sapidissima): implications for the conservation of an iteroparous anadromous fish. Canadian Journal of Fisheries and Aquatic Sciences. 67(5): 806-830. https://doi.org/10.1139/F10-026

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    • What does the future has in store for red salmon in a context of global climate change?by Camille Sestac and Amandine Tauzin

      Published by Charlotte Recapet the November 1, 2019 on 1:18 PM

      Pacific salmon have extremely complex life histories and may be threatened by global climate change, as Peter S. Rand and colleagues investigate in their recent study.


      Life cycle of Sockeye Salmon

      Among all species, fishes must adapt to face disruptions caused by global climate change. Sockeye salmon (Oncorhyncus nerka), an anadromous species of salmon found in the Northern Pacific Ocean and rivers discharging into it, has a complex life cycle. As a migratory species, their energetic demands are high during spawning migration. Climate change might have important impacts on populations and their migration via variation of river discharge, increase of water temperature and decline of growth conditions. Aiming to better understand the impacts of these disruptions on the migratory performance of this species of salmon, Peter S. Rand from Wild Salmon Center teamed up with researchers from British Columbia. Their goal is to evaluate the effects of past and future trends in river discharge and temperature on the migratory performance of Sockeye Salmon in the Fraser River.

      In a context of global climate change, it is crucial to understand the effects of disruptions on ecosystems and the populations living in them. Indeed, it is important to know the impacts of these disruptions on every stage of their life cycle (the juvenile freshwater period, the estuarine period, and the subadult marine period) so that we can maintain the populations stock. It’s especially important for fishery management because the fishing quota has greatly increased over the last decades and has threatened populations of Pacific salmon, particularly during their spawning migration. That’s why with three main objectives, these scientists used analysis to improve the understanding of how changes in river conditions can affect the energy use and the mortality rate in Sockeye salmon population. To do so, they used several models: one to search a link between energetic conditions of individuals and en route mortality, one to simulate the energy use during spawning migration and one to hindcast and forecast energy use by simulating fish’s behaviour and migration conditions (for more information, a tip, read the article!).


      Long-range forecasts of lower Fraser river temperature during the summer of 2018

      Using these friendly models, Rand and his colleagues proved that energy reserves and energy depletion of early Stuart Sockeye salmon are major factors that can affect their ability to reach their spawning grounds. They also stated that energy depletion is a function of both river temperature and discharge. Therefore, this population is structured by condition-dependant mortality. Nevertheless, this group of researchers brought to light a mechanism that allows fishes to cope with some environmental variability, providing a certain degree of resilience over time. Therefore, even if energetic demands and migration mortality increase as a result of exposure to warmer temperatures, it will be compensated by reduced time travel to the spawning ground as the river flow will be lower.

      However, increase of temperature means increase of diseases appearing and developing and that stress added may be a direct cause of increased mortality during migration. Finally, as if it wasn’t already bad enough for our salmons, ocean productivity can be affected by climate change and thus affect their river migration success. In fact, this can lead to a decrease of body size and body energy content. It implies that individuals will start their migration with lower energy densities and will be more likely to exhaust their energy stock before even reaching the spawning grounds.


      Salmon jumping over a weir
      According to the US-Canada Commission, a 21° C temperature spike was measured on the Fraser River in 2009. However, sockeye salmon show signs of physiological stress and migratory difficulties above 19°C and from 20°C, the first signs of illness and death appear. But migration of Sockeye salmon is not only threatened by climate change. In fact, migration of salmon specially is impacted by humans or natural obstacles. Dams and weirs form large obstacles for this migratory species and can be very difficult to cross. Many studies have already proved that this kind of obstacles, even when equipped with crossing devices, delay their migration and thus jeopardize their reproduction. This can lead to a decline of the population and in some cases to its extinction, as it happened in Belgium.

      So, whilst some questions have been answered, it seems that more studies need to be carried out to improve our knowledge about the impact of global change which seems to be another sword of Damocles hanging over the head of Sockeye salmon.

      Cited paper: Rand, P.S. et al. (2011) Effects of River Discharge, Temperature, and Future Climates on Energetics and Mortality of Adult Migrating Fraser River Sockeye Salmon. Trans. Am. Fish. Soc. 135(3), 655-667. https://doi.org/10.1577/T05-023.1

      Featured images: Life cycle of Sockeye salmon by Camille Sestac, graph from https://www.pac.dfo-mpo.gc.ca/science/habitat/frw-rfo/index-eng.html , Sockeye Salmon from www.ryanvolberg.com

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    • A damned energy loss for migratory fishes: dams!by Manon Salerno

      Published by Charlotte Recapet the June 10, 2019 on 9:42 AM

      Many species of fish grow in the sea and breed in rivers. These migratory fish are called anadromous. When a migratory fish is ready to breed, it leaves the sea and up a river to lay watershed upstream. It will find the optimum conditions to reproduce and allow the development of its offspring. But to do so, they spend a lot of energy on the upstream and sometimes, obstacles like dams in their path does not make it easy for them. This is the case of American Shad in the Connecticut River in the United States. Since the 1970s, 4 hydroelectric dams have been built in the river. Even if they are equipped with fish ladders, these obstacles require the Shad more energy to cross them than if they were not present. We know energy availability can be a limiting factor in migration. Thus, in 1999, scientists wanted to understand energy management in these fish, especially when it is modified by the presence of such.

      Any organism needs energy to perform the movements / migrations necessary for its life cycle. When they are heading into a period that will not allow them to feed (overwintering, migration), some species store energy, such as the bear before hibernating. For American Shad, this stock has to be created before migration because it will not feed during this move. First, scientists have found these are subcutaneous lipid reserves and skin constitute a special tissue for energy storage, which is rather unusual. Salmon, for example, usually mobilizes lipids from muscles and viscera. In contrast, for migration, somatic tissues (red and white muscles and skin) provide about 90% of the energy required in shad.

      According to this study, crossing dams is expensive in energy, especially for females. In fact, American Shad is a species able to reproduce itself several times in its life, but if migration requires too much energy, it will only happen once. It is therefore easy to understand a multitude of dams can have an influence on the reproduction of these fish and therefore on population size, even if they are equipped with systems allowing fish to pass. Not to mention some fish do not even find the fish ladder. These are more likely to be stressed, eaten by predators such as birds, or competing with other fish and unlikely to breed.

      Although fish ladders are quite efficient at the upstream for the American Shad, it is sometimes not suitable for other species. In addition, the outmigration can also present risks of mortality (water retention, drop height etc ...). It is therefore essential to remove the dams for which their function is not provided anymore. But in the United States, the erasure of small dams often meets opposition from local communities. Even though many dams have been removed, they represent a strong historical or landscape value for the inhabitants, creating tensions between the supporters of the restoration and the local communities. This situation reminds the context existing in France, where the aesthetic and historical arguments are very powerful. Many dams are attached to mills and water plants of olden times are therefore seen as a "living historical landscape" very characteristic of their landscape. Because of the local character of each operation, an opposition not necessarily collective but influential and well directed, is enough to block some sites.

      Cited study: J. B. K. Leonard and S. D. McCormick (1999) Effects of migration distance on whole-body and tissue-specific energy use in American shad (Alosa sapidissima). Canadian Journal of Fisheries and Aquatic Sciences 56(7), 1159-1171

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