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What is the effect of bait when sampling with remote underwater video systems?

By Nicholas Schmidt (SAEON Elwandle; Rhodes University), Albrecht Götz (SAEON Elwandle), Anthony Bernard (SAIAB) and Elodie Heyns-Veale (SAEON Elwandle)
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The Tsitsikamma National Park Marine Protected Area (MPA), established in 1964, is South Africa’s oldest marine conservation area. It plays a vital role in the protection of many terrestrial and marine species and is considered integral within South Africa’s protected area network.

The Park is situated along the Garden Route in the Eastern Cape of South Africa and is a well-known tourist attraction. Tsitsikamma is a Khoisan word meaning “place of much water”.

The need for protection and data

Marine resources are under threat due to the effect of modern-day fishing practices and other direct and indirect anthropogenic impacts. Habitat characteristics such as benthic invertebrates and depth, influence the type, abundance and number of fish species found at a particular location.

The degradation of habitats can have long-term effects on the benthic invertebrate community and the fish species associated with them. Therefore, it is vital to identify habitats important to all life stages of fish and ensure these habitats are adequately protected.

No-take MPAs such as Tsitsikamma are considered to be the most useful tool in the holistic protection of marine ecosystems and the maintenance of spawner biomass (the proportion of a population that is reproductively mature). However, only a small percentage of the South African coastline is completely protected (no-take) from fishing.

As a result, there has been a continued drive to establish more MPAs in the country to ensure that our marine ecosystems, resources and biodiversity are adequately managed. In order to support this process, there has been a call for more scientific data to see to it that the selection of areas for protection ensures optimal protection of ecologically significant habitats.

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Baited (L) and unbaited (R) stereo-BRUVs on the sea floor. The bait container was left empty in the unbaited deployment.

Gathering data

It is vital that a considerable amount of thought and planning is put into the design of scientific studies in order to minimise methodological biases, optimise sampling effort and increase the accuracy of, and confidence in the findings of the study. In order to generate such high-quality data archives, SAEON and the South African Institute for Aquatic Biodiversity (SAIAB) initiated a study to further optimise sampling with baited remote underwater stereo-video systems (stereo-BRUVs).

To obtain data with low variability and high species richness, bait is used to attract fish. The use of bait potentially affects the way in which fish habitat associations is interpreted and thus requires further research to tease apart potential biases.

The effect of bait on the association between fish and benthic habitats was tested over a fine spatial scale (hundreds of metres). The study aimed to determine the relationship between fish and their habitat, and if the presence of bait potentially biased this observed relationship.

Such fish-habitat associations are important for our understanding of how the variability in benthic community structure can influence the abundance and diversity of sampled fish populations. Furthermore, determining the habitat requirements of commercially targeted fish species can facilitate important fisheries management decisions that lead to the long-term sustainability of resource populations.

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Photos of the benthic invertebrate growth taken with the drop camera set-up.

Fish data

Stereo-BRUVs and RUVs (unbaited stereo-BRUVs) were deployed to obtain data on the fish community.

Stereo-BRUVs use a dual camera system to film the fish community on the sea floor. Systems are dropped into the ocean off the side of a boat and left on the bottom for 60 minutes while attached to buoys on the surface.

The method is regarded as one of the best ways to conduct cost-effective and non-destructive sampling of subtidal reef fish. Video footage is subsequently analysed by identifying, counting and measuring all fish species observed in the videos.

Habitat data

Habitat data were collected by means of a drop camera, which is deployed off the side of a boat. This method also involves the use of a camera, but here the camera is facing downwards, taking pictures of the sea floor.

For this particular study, pictures of the sea floor were taken every 30 m across a grid over a 1x1 km area. Photos were then analysed by identifying the invertebrate species seen in the photo. Data obtained from this information were used to create a detailed bathymetric map of the sampled area using GIS software.

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Generated bathymetric map of the study area showing areas of different habitat types.

A fine scale analysis of the associations between the fish and habitat data is currently underway. Two of the anticipated results of this research will be the optimal spatial scale needed to obtain sufficient data in order to understand fish-habitat interactions and if the use of bait during sampling significantly alters the relationship between fish and their habitat.

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