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Shedding light on nocturnal fish communities at rocky reefs in Algoa Bay
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The predictable day-night (or diel) changes in ambient light affect the ability of animals to access their required resources and carry out basic activities such as feeding.
The way in which a species is influenced by ambient light is dependent on a number of species-specific characteristics, such as their body morphology and ecological position (or feeding guild).
The differences in species’ characteristics and the consequent array of responses to the changes in ambient light therefore cause diel variation in the observed community structure, in other words, which and how many species are present (species diversity) and how many individuals per species are present (abundance).
Diel variation in animal community structures is a widespread phenomenon, with most of the research being focused on terrestrial animals. Despite this, the majority of studies investigating marine fish communities have been restricted to day-time sampling.
Global knowledge gap
This trend of neglecting nocturnal marine fish communities is prevalent in South Africa and many other regions in the world. As a result, our global understanding of marine fish communities is based almost entirely on day-time observations. Such studies may therefore provide information that is not truly representative of night-time communities, since the structure of fish communities is likely to differ between day and night.
From this, it is clear that there is an incomprehensive ecological understanding of marine fish communities and a gap in the current knowledge.
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Sampling in the dark
We owe the relatively slow progression of diel research focused on marine fish communities (compared with terrestrial animals) to the logistical challenges associated with sampling this environment in darkness. Baited remote underwater stereo-video systems (stereo-BRUVs) provided an opportunity to investigate both diurnal (active at day) and nocturnal (active at night) marine fish communities in an efficient and comparative manner.
The method uses bait to attract fish into the camera’s field of view where they can be identified, counted and measured. When fitted with an artificial light source, stereo-BRUVs have been effective at sampling deep (50-200m) and nocturnal fish communities.
To our knowledge, this was the first time stereo-BRUVs were used to investigate South Africa’s nocturnal reef fish communities.
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Diverse day-time assemblage
A unique and diverse assemblage was recorded on shallow reefs at day. The assemblage was made up of many specialised species, which are generally more efficient at extracting energy from resources and are therefore considered to be superior competitors. These species included herbivores through to top predatory teleost species, many of which belonged to the family Sparidae (sea breams).
At night, many of these sea breams were not recorded, and from this it was hypothesised that these species display strict diurnal activity that is restricted to shallow reefs. It was further hypothesised that at night, these species may enter sheltered spots within the reef to avoid the period of the diel cycle that offers unfavourable feeding conditions for these visual feeders.
Species that are not adapted to carrying out their activities in the dark, may therefore trade less-efficient feeding at night for rest as an energy conservation strategy.
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Additional sense for operating in darkness
The available niche is exploited by those species that are capable of carrying out activities in darkness. Such nocturnal species included demersal bottom-feeding sharks belonging to the catshark family and dogfish sharks.
These species all possess morphological adaptations that act as an additional sense. For example, barbels, as the name implies, possess barbels around their mouth that are used for tactile probing, while all sharks possess an electrosensory system that provides an additional sense for prey detection.
The adaptations have enabled these species to exploit the productive shallow reef ecosystem at night when competition is low.
Different responses
Nocturnal species showed different responses to the change in ambient light. Those species that possess elongated bodies, adapted for manoeuvrability and entering small crevices, did not show evidence for movement. These species included barbels, hagfish and catsharks.
It was therefore hypothesised that these species adopt a similar behaviour to those diurnal species. It is likely that catsharks, hagfish and barbels enter sheltered habitat within the reef during the day when competition is high.
Other species with body shapes that are better adapted to sustained swimming, showed evidence for commuting, which involves the movement between different habitats in response to the diel change in ambient light. For example, the sand soldier (a sea bream) showed evidence for movement onto shallow reefs at night.
Previous studies have shown that this species occupies shallow sandy flats adjacent to reefs during the day and predominantly feeds at night. It was therefore hypothesised that the sand soldier may avoid reefs at day due to the high levels of competition, taking advantage of the productive shallow reefs at night when a less diverse and competitive community is present. Similar movement patterns were observed for dogfish sharks and panga (another sea bream).
In this study, stereo-BRUVs were used in a novel approach to shed light on South Africa’s poorly understood nocturnal reef fish communities, which highlighted the importance of including nocturnal sampling in studies investigating reef fish communities. The communities undergo distinct diel shifts in community composition, species richness and abundance, with endemic and exploited sea breams no longer being the dominant family on rocky reefs at night.
Findings
The findings of this study have provided a holistic review of the complex patterns that have evolved in Algoa Bay’s rocky reef fish, which have allowed multiple species to coexist and exploit the productive reef ecosystem by partitioning their resources according to the diel cycle, reef depth and habitat type.
