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Each year millions of larval and 0+ juvenile fishes are recruited into estuarine fish populations around the world. For several decades the roles of littoral aquatic and emergent macrophyte habitats as nursery areas for many of these species have been studied and debated at length. This review attempts to collate the published literature and provide a synopsis of the varying, and sometimes conflicting, views on this topic. A large number of studies have shown that a range of species and an abundance of juvenile fishes are associated with littoral macrophytes in estuaries, some of which are found almost exclusively within particular plant habitats. Other studies have shown the movement of certain juvenile fishes from one type of littoral plant habitat to another as they grow and develop new feeding strategies and dietary requirements. Overall, it would appear that seagrass beds and mangrove forests are particularly favoured by fishes as nursery areas in both estuaries and the nearshore marine environment, and that the loss of these habitats leads to a decline in juvenile fish diversity and abundance. Salt marshes and reed beds generally have a lower diversity of fishes than seagrass and mangrove habitats, possibly due to the more temperate location of salt marshes and the dense structure of some reed beds. Stable isotope studies in particular are providing increasing evidence that carbon assimilated by juvenile fishes in mangrove, marsh and reed habitats is not primarily derived from these macrophytes but comprises a mixture of these sources and a diverse range of macro- and microalgae, particularly epiphytic, epipsammic, epipelic and epilithic diatoms and algae found in these areas. The closest trophic link between the macrophyte food chain and associated fishes occurs in seagrass habitats where a significant portion of the overall macrophyte leaf biomass often consists of epiphytic algae and diatoms. Structurally, mangrove forests, salt marshes and reed beds provide more substantial and complex habitats for juvenile fish refuge, but some of these habitats are constrained with regard to nursery provision by being fully exposed at low tide. Under such circumstances the small fish are sometimes forced into creeks and channels where larger piscivorous fishes are often present. Overall, in terms of a broad ranking of the four habitats as potential fish nursery areas, seagrass meadows are ranked first, followed by mangrove forests, salt marshes and then reed beds. This ranking does not imply that the lower ranked habitats are unimportant, since these plants perform a myriad of ecosystem services that are not related to the provision of fish nursery areas, e.g. bank stabilization. It is also emphasized that the protection of specific plant species should not be encouraged because it is important to have an ecosystem approach to conservation so that the diversity of habitats and their connectivity for fishes is maintained.

Our ability to predict invasions has been hindered by the seemingly idiosyncratic context-dependency of individual invasions. However, we argue that robust and useful generalisations in invasion science can be made by considering “invasion syndromes” which we define as “a combination of pathways, alien species traits, and characteristics of the recipient ecosystem which collectively result in predictable dynamics and impacts, and that can be managed effectively using specific policy and management actions”. We describe this approach and outline examples that highlight its utility, including: cacti with clonal fragmentation in arid ecosystems; small aquatic organisms introduced through ballast water in harbours; large ranid frogs with frequent secondary transfers; piscivorous freshwater fishes in connected aquatic ecosystems; plant invasions in high-elevation areas; tall-statured grasses; and tree-feeding insects in forests with suitable hosts. We propose a systematic method for identifying and delimiting invasion syndromes. We argue that invasion syndromes can account for the context-dependency of biological invasions while incorporating insights from comparative studies. Adopting this approach will help to structure thinking, identify transferrable risk assessment and management lessons, and highlight similarities among events that were previously considered disparate invasion phenomena.

An influential paradigm in coral reef ecology is that fishing causes trophic cascades through reef fish assemblages, resulting in reduced herbivory and thus benthic phase shifts from coral to algal dominance. Few long-term field tests exist of how fishing affects the trophic structure of coral reef fish assemblages, and how such changes affect the benthos. Alternatively, benthic change itself may drive the trophic structure of reef fish assemblages. Reef fish trophic structure and benthic cover were quantified almost annually from 1983 to 2014 at two small Philippine islands (Apo, Sumilon). At each island a No-Take Marine Reserve (NTMR) site and a site open to subsistence reef fishing were monitored. Thirteen trophic groups were identified. Large planktivores often accounted for >50% of assemblage biomass. Significant NTMR effects were detected at each island for total fish biomass, but for only 2 of 13 trophic components: generalist large predators and large planktivores. Fishing-induced changes in biomass of these components had no effect on live hard coral (HC) cover. In contrast, HC cover affected biomass of 11 of 13 trophic components significantly. Positive associations with HC cover were detected for total fish biomass, generalist large predators, piscivores, obligate coral feeders, large planktivores, and small planktivores. Negative associations with HC cover were detected for large benthic foragers, detritivores, excavators, scrapers, and sand feeders. These associations of fish biomass to HC cover were most clear when environmental disturbances (e.g., coral bleaching, typhoons) reduced HC cover, often quickly (1–2 yr), and when HC recovered, often slowly (5–10 yr). As HC cover changed, the biomass of 11 trophic components of the fish assemblage changed. Benthic and fish assemblages were distinct at all sites from the outset, remaining so for 31 yr, despite differences in fishing pressure and disturbance history. HC cover alone explained ~30% of the variability in reef fish trophic structure, whereas fishing alone explained 24%. Furthermore, HC cover affected more trophic groups more strongly than fishing. Management of coral reefs must include measures to maintain coral reef habitats, not just measures to reduce fishing by NTMRs.

Habitat structural complexity influences biotic diversity and abundance, but its influence on marine ecosystems has not been widely addressed. Recent advances in computer vision and robotics allow quantification of structural complexity at higher-resolutions than previously achieved. This provides an important opportunity to determine the ecological role of habitat structural complexity in marine ecosystems. We used high-resolution three-dimensional (3D) maps to test multiple structural complexity metrics, depth and benthic biota as surrogates of fish assemblages across hundreds of meters on subtropical reefs. Non-parametric multivariate statistics were used to determine the relationship between these surrogates and the entire fish assemblage. Fish were divided into functional groups, which were used to further investigate the relationship between surrogates and fish abundance using generalized linear models. Fish community composition and abundance were strongly related to habitat complexity metrics, benthic biota and depth. Surface rugosity and its variance had a significant positive influence on the abundance of piscivores and sediment infauna predators, and a negative effect on the abundance of predators, herbivores, planktivores and cleaners. Final models for fish functional groups explained up to 68% of the variance. The best metrics to explain the variance in fish abundance were benthic biota (25 ± 7.5% of variance explained, mean ± SE) and complexity metrics (16 ± 6.6%, mean ± SE). Our results show that high-resolution 3D maps and derived metrics can predict a large percentage of variance in fish abundance and potentially serve as useful surrogates of fish abundance across all functional groups in spatially dynamic reefs.

Intraguild predation interactions make fish communities prone to exhibit alternative stable states with either piscivore or prey fish dominance. In the Baltic Sea, local declines of coastal piscivores like perch (Perca fluviatilis) have been observed to coincide with high densities of sticklebacks (Gasterosteus aculeatus). Mechanisms behind this shift between piscivore and stickleback dominance were studied both experimentally and in field. Results showed that predation by sticklebacks has a strong negative effect on perch larvae survival, but this effect rapidly decreases with increasing perch size, likely due to gape limitations and digestion constraints in sticklebacks. Large spatial and temporal variations in patterns of stickleback migration into perch spawning sites were observed. Whether or not high density of sticklebacks will cause declines in coastal piscivore populations is suggested to depend on the availability of spawning sites in which sticklebacks do not migrate into or arrive late in the reproduction season of coastal piscivores.

Lake ecosystems are exposed to a range of anthropogenic pressures, particularly eutrophication, and in some cases also stocking and/or overfishing of top-predator fish species, all factors that have implications for the food web structure and which could lead to dominance of nuisance cyanobacteria. Restoration of degraded lakes demands insight into the relative role of top-down for bottom-up regulating forces. While knowledge about these forces in temperate lakes is extensive, comparatively little is known of their role in subtropical lakes where the importance of herbivorous and benthic feeding fish is higher. Here, we analyzed a long-term monitoring data set on subtropical, shallow Lake Taihu, China, and applied random forests regression to examine how phytoplankton was related to environmental variables and biotic assemblages. Our results indicate that the biomass and density of phytoplankton increased with increasing biomass of benthivorous and zooplanktivorous fish and decreased with increases in ammonium concentrations, the nitrogen to phosphorus ratio, and zooplankton biomass, while the response to climate fluctuations and changes in the biomass of piscivores was weak. Effects of higher trophic levels explained as much of the variance in phytoplankton biomass as did nutrients and climatic factors. Moreover, the remarkably reduced ratio of zooplankton to phytoplankton biomass and the decline in cladoceran individual biomass emphasized the increasing importance of top-down control in regulating the phytoplankton following extensive stocking. Our findings offer insight into how fish management may be combined with catchment-level restoration measures to conserve and enhance water quality.

Fish biologists have long assumed a link between intestinal length and diet, and relative gut length or Zihler’s index are often used to classify species into trophic groups. This has been done for specific fish taxa or specific ecosystems, but not for a global fish dataset. Here, we assess these relationships across a dataset of 468 fish species (254 marine, 191 freshwater, and 23 that occupy both habitats) in relation to body mass and fish length. Herbivores had significantly relatively stouter bodies and longer intestines than omni- and faunivores. Among faunivores, corallivores had longer intestines than invertivores, with piscivores having the shortest. There were no detectable differences between herbivore groups, possibly due to insufficient understanding of herbivorous fish diets. We propose that reasons for long intestines in fish include (i) difficult-to-digest items that require a symbiotic microbiome, and (ii) the dilution of easily digestible compounds with indigestible material (e.g., sand, wood, exoskeleton). Intestinal indices differed significantly between dietary groups, but there was substantial group overlap. Counter-intuitively, in the largest dataset, marine species had significantly shorter intestines than freshwater fish. These results put fish together with mammals as vertebrate taxa with clear convergence in intestine length in association with trophic level, in contrast to reptiles and birds, even if the peculiar feeding ecology of herbivorous fish is probably more varied than that of mammalian herbivores.

1. Seagrass and seaweed habitats constitute hotspots for diversity and ecosystem services in coastal ecosystems. These habitats are subject to anthropogenic pressures, of which eutrophication is one major Stressor. Eutrophication favours fast-growing ephemeral algae over perennial macroalgae and seagrasses, causing habitat degradation. However, changes in top-down control, caused by, for example, overfishing, may also have negative impacts on such habitats by decreasing grazer control of ephemeral algae. Meanwhile, systematic analyses estimating top-down effects of predator manipulations across a wide range of studies are missing, limiting the potential use of top-down control measures in coastal management. 2. Here, we review the literature on experiments that test top-down and bottom-up controls in seagrass Zostera marina and seaweed Fucus spp. food webs in the North Atlantic. Using meta-analysis and meta-regression, we compare effect sizes of consumer and nutrient manipulations on primary producers, grazers and mesopredators. 3. Presence of mesopredators on average doubled the biomass of ephemeral algae through trophic cascades, mainly mediated via negative effects on amphipods and isopods. Of the grazers, gastropods had twice as strong a negative effect on ephemeral algae as amphipods/isopods, but responded weakly to both predators and fertilization. In accordance with theory, top-down effects became stronger with eutrophication. 4. Across studies, top-down effects on ephemeral algae at all trophic levels are on par with eutrophication effects. However, the few studies manipulating piscivorous fish make estimates of their top-down effects uncertain. 5. Synthesis and applications. Consistently strong top-down effects in coastal ecosystems call for an integrated ecosystem perspective. Management should consider measures to improve stocks of predatory fish and reduce mesopredators for restoration and conservation of essential seagrass and seaweed habitats, thereby increasing the long-term viability of ecosystem services from coastal habitats.

We considered the limnological literature for an overview of biomanipulation methods that were implemented to avoid or reduce cyanobacterial bloom development in ponds and lakes. For this purpose, we reviewed 48 publications representing 34 whole-lake and large-scale case studies of different biomanipulation approaches clearly mentioning the extent of a cyanobacteria bloom problem and the cyanobacteria taxa involved. This delivered complementary information to the suite of review papers already providing elaborated syntheses on biomanipulation and associated ecotechnological measures as a restoration tool for overall eutrophication reduction and control. We considered nature-based solutions such as fish removal and associated water drawdown, addition of piscivorous fish, filter-feeding planktivorous fish, Daphnia or bivalves, re-introduction of macrophytes and a combination of accompanying restoration methods. Reasons for success or failure to control cyanobacterial blooms of especially Anabaena, Pseudanabaena, Aphanizomenon, Aphanocapsa, Limnothrix, Microcystis , Oscillatoria or Spirulina spp. could be explained through bottlenecks encountered with fish removal, stocking densities, cascading effects, associated zooplankton grazing, diet shifts away from cyanobacteria, macrophyte recovery, nutrient or pH status. Threshold values to avoid failures are synthesized from experiments or monitoring studies and presented in a conceptual scheme about cyanobacteria reduction through (1) direct abatement of existing blooms and forcing/maximization of biotic key interactions (2) reducing risk of blooms and improving lake or pond multi-functionality and (3) avoiding blooms, balancing biotic communities and enhancing existing ecosystem services. More information will be required on temporal dynamics and abundances of cyanobacteria taxa in whole-lake pre- and post-biomanipulation conditions to better evaluate the applicability and effectiveness of such nature-based solutions.

Human impacts are forcing species towards marginal and suboptimal portions of their historical ranges. Cetaceans are now under protection, but are still threatened by fishing activities, which reduce fish stocks, alter their feeding behavior, and can cause mortality due to bycatch. Here, we investigated how different fishing activities affect cetacean population density patterns in the Mediterranean, one of the most impacted and fished seas. We collected 366 population density estimates for eight cetacean species. We then classified species into four trophic groups (planktivorous, piscivorous, teutophagous, generalist) and modelled their density as a function of both environmental and fishing variables (artisanal, demersal destructive, demersal non‐destructive with low bycatch, demersal non‐destructive with high bycatch, and pelagic fishing with low bycatch). Finally, to quantify human contribution to the observed geographic pattern of population density, we predicted and compared population density patterns under a baseline fishing and a minimum fishing scenario. The four groups of cetacean species exhibited diverse responses to environmental and fishing variables. Demersal destructive fishing consistently had a negative influence on species population density. In contrast, others, such as demersal non‐destructive fishing, showed mixed effects, including a potential attraction effect on piscivorous species. Overall, we predicted a probable change in the geographic pattern of population density of cetaceans in response to fishing activities, especially along the coasts in planktivorous species and in offshore areas in generalist species. Our study provides evidence of the negative impact of fishing activities on cetacean population density, while highlighting functional group‐specific responses to different fishing practices. These findings enhance our understanding of human‐induced changes in marine ecosystems, suggesting probable alterations to the natural population density patterns of cetaceans in the Mediterranean Sea.