Many marine fish and invertebrates show a dual life history where settled adults produce dispersing larvae. The planktonic nature of the early larval stages suggests a passive dispersal model where ocean currents would quickly cause panmixis over large spatial scales and prevent isolation of populations, a prerequisite for speciation. However, high biodiversity and species abundance in coral reefs contradict this panmixis hypothesis. Although ocean currents are a major force in larval dispersal, recent studies show far greater retention than predicted by advection models. We investigated the role of animal behavior in retention and homing of coral reef fish larvae resulting in two important discoveries: ( i ) Settling larvae are capable of olfactory discrimination and prefer the odor of their home reef, thereby demonstrating to us that nearby reefs smell different. ( ii ) Whereas one species showed panmixis as predicted from our advection model, another species showed significant genetic population substructure suggestive of strong homing. Thus, the smell of reefs could allow larvae to choose currents that return them to reefs in general and natal reefs in particular. As a consequence, reef populations can develop genetic differences that might lead to reproductive isolation.

While evidence is mounting that larval reef fish are active participants in the process of dispersal and settlement, the sensory and behavioural mechanisms by which these fishes disperse and return from their oceanic phase to the reefs remain unknown. On One Tree Island (Great Barrier Reef, Australia), we tested freshly collected animals in a large choice-flume on the shore. Here, we present the first evidence that larval reef fish (primarily apogonids) approaching the time of settle- ment are capable of detecting differences between ocean and lagoon water and prefer lagoon water. We also demonstrate that they sniff actively with well-innervated noses and that attraction to lagoon water was not affected by warmer or colder temperatures. We conclude that they used chemical sig- nals to orient toward lagoon water. Finally, we describe ebb tide plumes of lagoon water that extend many kilometers from reefs. Such plumes could provide chemosensory cues for dispersal and settle- ment stages of reef fish as they develop swimming efficiency. We argue that fishes may imprint to reef odour as embryos and/or early larvae and that this could facilitate both retention near the natal reef and navigation toward reefs from greater distances.

A review of the literature suggests that river discharge plumes strongly influence fish larvae and may play a significant role in the recruitment of local fishes. Some rivers drain large land masses to discharge shallow, turbid and nutrient-rich plumes that interact with ocean currents as well as with local oceanography and meteorology; these plumes may extend hundreds of kilometres offshore and alongshore. The frontal, or mixing, zone between plume and ocean waters is characterized by strong physical and biological processes. Physical dynamics, e.g. hydrodynamic convergence, and abundant nutrients (both river derived and upwelled) in the vicinity of discharge plumes often generate large stocks of phytoplankton, zooplankton and fish larvae, as well as high rates of primary and secondary production. Physical dynamics not only act to accumulate (and probably retain) biomass in frontal waters, but also transport organisms onshore, offshore and along the frontal boundary. The mechanisms through which river plumes may influence recruitment are not clear. In considering the potential effects of scale of river discharge on recruitment, three alternative hypotheses are discussed. The short-food-chain hypothesis states that recruitment will be enhanced in the vicinity of river plumes because fish larvae experience superior feeding conditions, grow faster and thus experience a shorter stage duration and survive better. The total-larval-production hypothesis is that trophic conditions support such high total production of fish larvae that specific dynamics of growth and mortality are not relevant. The third hypothesis is that plumes facilitate the retention of fish larvae within a limited area, and it is the physical retention rather than production that explains the variation in recruitment. If one or a combination of these hypotheses explains the influence of river plumes on recruitment, then the greatest potential to affect fish recruitment may be possessed by large mid-latitude rivers carrying high suspended-sediment and nutrient loads that discharge into shelf waters to create well defined plumes where primary and secondary production are high.

The importance of top‐down effects in structuring ecological communities has been widely debated by ecologists. One way in which to examine these processes is to study the secondary effects of predator removal on communities. This study examined the role of predatory fishes in structuring communities of coral reef fishes, by using a network of marine reserves (the Great Barrier Reef Marine Park) as a natural experiment. We hypothesized that reefs with high densities of piscivores (marine reserves) would have distinct fish communities from those where piscivores have been depleted through fishing, due to variation in predation pressure. We predicted that predator depletion would result in “prey release”, and a corresponding increase in prey densities along a gradient of fishing intensity, causing a change in the community composition of reef fishes. To address this, fish counts and habitat surveys were conducted at four locations on the Great Barrier Reef. At each location, comparisons were made amongst three marine park zones that varied in their exposure to fishing practices; no‐ take marine reserves, limited fishing areas, and open fishing areas.The density and biomass of predators varied consistently among zones at each location. Furthermore, we found strong evidence for prey release at all four locations, resulting in distinct fish assemblages amongst zones. Reefs open to fishing had much lower densities of piscivores, and higher densities of prey and herbivorous fishes compared to marine reserves. This broad pattern was consistent amongst locations, and persisted at the level of species, trophic groups, families and communities. Habitat characteristics did not vary significantly amongst zones in a consistent manner amongst locations. Although habitat relationships were strong for specialist species such as butterflyfishes, densities of predators were stronger predictors of prey density for most species, and the trophic composition of reef fish communities differed significantly amongst zones at all locations. Results from this study support the concept that top‐down effects can be strong divers of prey populations and influence community structure in highly diverse systems. These data emphasize the vital role of predators, and reinforce the importance of preserving and restoring top‐down trophic interactions in ecological systems.