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The benefit of using adaptive, hypothesis-based sampling protocols to investigate larval fish dynamics

By Paula Pattrick1, Wayne Goschen1 and Francesca Porri2
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Most coastal fishes have a planktonic3 dispersive larval stage, spending a few days to months in the pelagic environment until they settle into a juvenile or adult habitat. Larval dispersal therefore includes the time from spawning to settlement.

It is the survival of larvae during this dispersing stage which will largely determine the size and extent of adult marine populations.

Larval transport is an important component of dispersal, with a broad dispersion requiring significant larval displacement. Small-scale shallow water physical processes play an important role in larval transport.

Flow in the nearshore is far more complex than in the deep ocean due to processes such as surface gravity waves, buoyancy-driven flows, wind-forcing, surface and internal tides, large-amplitude internal waves and bores4, and boundary-layer effects. These processes have major implications for larval transport, either suppressing or enhancing it.

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Dr Paula Pattrick samples fish larvae using a bongo net

Dr Francesca Porri and MSc student Siphelele Dyantyi sort samples

Argyrosomus inodorus (Family Sciaenidae) larva

Estimating larval transport

As larval biologists and oceanographers, we are interested in how oceanographic processes transport larvae in the water. Oceanographic data is collected with an acoustic Doppler current profiler (ADCP) and CTD, which samples temperature, salinity and velocities of the water column right from the surface to the ocean floor. 

Bongo nets are towed behind a boat, sampling larval fishes from the surface to the ocean floor. By specifically targeting and sampling event-type processes like strong wind events or internal tidal bores, we will have a better understanding of larval transport, which for these events is currently unknown in South Africa.

For accurate, reliable estimates of larval transport which are often obtained from numerical modelling studies, empirical information on small-scale processes needs to be embedded into these models. Studies assessing the relative contributions of various physical transport mechanisms in larval transport are therefore important.

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An acoustic Doppler current profiler (ADCP) is concurrently used to collect physical variable data

The scales and extent of larval transport within the coastal zone is still poorly understood. Our research is therefore designed using adaptive hypothesis-driven sampling5 to determine the functional relationships between larvae and the physical coastal features as transport mechanisms.

Adaptive, hypothesis-driven sampling is used successfully to sample hydrodynamics and larval distributions. Adaptive hypothesis-based sampling is initiated in response to a real-time change in a time-dependent variable, such as temperature or wind, that is integral to the hypothesised larval transport mechanism.

Identifying how biological mechanisms interact with the dynamic coastal oceanography will allow us to predict settlement, recruitment and population dynamics. Explaining larval dispersal and connectivity can have applications for both conservation and resource management. In relation to management and conservation, this is vital for fisheries to remain sustainable.

 

1 SAEON
2 South African Institute for Aquatic Biodiversity (SAIAB)
3 Floating or drifting in great numbers in bodies of salt (or fresh) water.
4 (Pineda 1991) A predictable process within the lunar cycle where deep water is brought to the surface (upwelling) in a direction perpendicular to the coast. 
5 Sampling in response to a particular event.

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