Shifting reef fish assemblages along a depth gradient in Pohnpei, Micronesia

Mesophotic coral ecosystems (MCEs) continue to be understudied, especially in island locations spread across the Indo-Pacific Ocean. Pohnpei is the largest island in the Federated States of Micronesia, with a well-developed barrier reef, and steep slopes that descend to more than 1,000 m. Here we conducted visual surveys along a depth gradient of 0 to 60 m in addition to video surveys that extend to 130 m, with 72 belt transects and 12 roving surveys using closed-circuit rebreathers, to test for changes in reef fish composition from shallow to mesophotic depths. We observed 304 fish species across 47 families with the majority confined to shallow habitat. Taxonomic and trophic positions at 30 m showed similar compositions when compared against all other depths. However, assemblages were comprised of a distinct shallow (<30 m) and deep (>30 m) group, suggesting 30 m as a transition zone between these communities. Shallow specialists had a high probability of being herbivores and deep specialists had a higher probability of being planktivores. Acanthuridae (surgeonfishes), Holocentridae (soldierfishes), and Labridae (wrasses) were associated primarily with shallow habitat, while Pomacentridae (damselfishes) and Serranidae (groupers) were associated with deep habitat. Four species may indicate Central Pacific mesophotic habitat:Chromis circumaurea, Luzonichthys seaver, Odontanthias borbonius,and an undescribed slopefish (Symphysanodon sp.). This study supports the 30 m depth profile as a transition zone between shallow and mesophotic ecosystems (consistent with accepted definitions of MCEs), with evidence of multiple transition zones below 30 m. Disturbances restricted to either region are not likely to immediately impact the other and both ecosystems should be considered separately in management of reefs near human population centers.

Size-related habitat use in juvenile Atlantic salmon: the importance of intercohort competition

In stream-living salmonids, an underlying mechanism for the critical period after emergence has generally been assumed to be size-dependent swimming capacity constraining fry (age-0) to low-velocity habitats with reduced food availability and intense competition. A further plausible mechanism is that intercohort habitat exclusion confines fry to marginal habitats. This possibility was tested using a seminatural stream with 16 test arenas, each comprising one high-velocity, deep habitat and one low-velocity, shallow habitat. We observed groups of newly emerged Atlantic salmon (Salmo salar) fry, either alone or in sympatry with one or two age-1 salmon. Salmon fry used high-velocity areas (42.2 ± 0.4 cm·s−1) most extensively in the absence of intercohort competition, where they obtained more food than in low-velocity areas (3.3 ± 0.3 cm·s−1), even though foraging efficiency was lower (though not significantly so). In sympatry with older cohorts, fry increased their use of the low-velocity habitat, with a reduced foraging activity, suggesting that strong older cohorts in natural populations may have the potential to influence the strength of the recruiting cohort by negative density dependence due to interference competition for habitat.

Behavioural transition probabilities in dugongs change with habitat and predator presence: implications for sirenian conservation

Although not widely appreciated, prey can manage predation risk by modifying the sequence of their behavioural states. We explored this phenomenon in dugongs (Dugong dugon) subject to spatially and temporally variable risk of tiger shark (Galeocerdo cuvier) predation in Shark Bay, Australia. Dugong behaviour was assayed using focal follows and organised into sequences of foraging, resting, and travelling bouts. We used log-linear analysis to test for sequence differences in relation to habitat (deep, shallow) and predation danger (sharks present, largely absent). Dugongs modified their behavioural sequences between periods of high and low shark abundance: those at risk alternated more frequently between foraging, which constrains anti-predator vigilance, and travelling, which facilitates predator detection. Dugongs also avoided continuous series of resting bouts, during which awareness is reduced, when sharks were present. These changes were only observed in relatively dangerous shallow habitat, which is hunted disproportionately by tiger sharks; behavioural responses to sharks in deep habitat were modest. We conclude that dugongs in risky habitat resort to safer behavioural sequences in response to sharks. Given that human disturbance and predators are perceived similarly by many species, some forms of vessel interaction could compromise the fitness of sirenians by eliciting similar behavioural adjustment.

Benefits and costs of vertical migration by the freshwater copepod Skistodiaptomus oregonensis: testing hypotheses through population comparison

The benefits and costs of vertical migration behaviour of the freshwater lacustrine copepod, Skistodiaptomus oregonensis, is explored through the study of two migrating and two nonmigrating populations. The association of vertical migration with the presence of pelagic threespine stickleback (Gasterosteus aculeatus) is consistent with the hypothesis that the adaptive benefit of vertical migration by S. oregonensis is avoidance of predatory stickleback. The hypothesis of avoidance of juvenile sockeye salmon (Oncorhynchus nerka) predation is not supported. Skistodiaptomus oregonensis do not migrate in the lake with the highest juvenile sockeye abundance but do migrate in the lake where juvenile sockeye are absent. A foraging efficiency hypothesis does not explain migration behaviour; neither food abundance nor food distribution distinguish lakes where S. oregonensis migrate from lakes where they do not migrate. Neither a bioenergetic efficiency hypothesis nor a thermal advantage hypothesis explain migration behaviour; temperature structures are similar in all four lakes examined. Vertical migration appears to be the result of a trade-off between predator avoidance and resource acquisition. Phytoplankton food is less concentrated in the deep habitat where S. oregonensis reside during the day. Furthermore, migrators contain less phytoplankton food in their guts than nonmigrators.

Age structure of Panulirus ornatus in two habitats in Torres Strait, Australia

The lobster population in Torres Strait, Australia, is made up of juvenile and sub-adult lobsters up to two years old (1+ and 2+), with some 3-year-old (3+) male lobsters. These lobsters live either on shallow reefs or in the deeper areas between reefs. Intensive surveys of lobsters in these two habitats showed that 1+ lobsters live only in the deep habitat, 2+ lobsters live in the deep habitat and on shallow reefs, and 3+ male lobsters live mainly on shallow reefs. In the deep habitat, lobsters were found mainly in areas that had rock and rubble substratum. There may be little movement of lobsters onto or off the shallow reefs during the winter months and 1+ lobsters probably recruit onto the shallow reefs so