Prepared by Taryn Murray, Bantony Ziko, Dinah Mukhari & Amber Child of the SA Environmental Observatory Network.
Estuaries are highly productive ecosystems, serving an important nursery function for many estuary-associated species through provision of abundant food sources, habitats for shelter, and protection from predators. Unfortunately, these ecosystems are amongst the most
impacted by anthropogenic activities, with things like infrastructure development and water abstraction leading to habitat transformation and destruction. Additionally, many estuaries are also overfished, with climate exacerbating all these activities.
Small-bodied and juvenile fish communities dominate these environments, including numerous mullet species. The mullet family, Mugilidae, is a diverse group of fishes consisting of 70 species and 20 genera, which occur in various coastal habitats across the world. The high abundance of mullet in many estuarine systems and coastal habitats globally is attributed to their broad tolerance of environmental variables such as salinity, temperature, dissolved oxygen and turbidity regimes, which fluctuate throughout the tidal cycles. Mullet generally spawn in the marine environment and the larvae are transported by currents into the nearby estuaries which they use extensively as juveniles (as nurseries) and as adults.
Mullet play an important ecological role in the environments in which they occur. Not only do they serve as food sources with larger predatory species including piscivorous fish and birds, but are also important to subsistence, recreational and commercial fishers who either rely on them for food or being used as live bait for targeting larger predatory species. Their
broad environmental tolerances and relative ‘hardiness’ (of some species) also make them suitable candidate species for fishery and aquaculture ventures (e.g., flathead mullet Mugil cephalus in Greece, Taiwan, etc.).
Fifteen species of mullet occur in South African estuaries in generally greater abundance relative to other smaller bodied species. Despite their generally high abundances, some species of mullet are facing overexploitation in South Africa, mostly through illegal gillnet fisheries; for example, striped mullet Chelon richardsonii along the West Coast.
Current regulations for all mullet in South Africa allow for bag sizes of 50 fish per person per day for recreational and subsistence activities, and an unlimited bag limit for commercial fisheries,
with no minimum size or closed season for all fisheries.
Movement behaviour of aquatic animals can be studied in multiple ways including using mark-recapture, acoustic telemetry, genetics, otolith microchemistry, etc. Acoustic telemetry has gained popularity over the past two decades and is currently the primary tool used worldwide to study fish movements and behaviour of submerged aquatic animals in numerous environments, including inland lakes and rivers to the high seas, and from polar regions to the tropics (Hussey et al. 2015, Matley et al. 2022). This methodology comprises two main pieces of equipment; acoustic transmitters or tags that (in this study) are surgically inserted into aquatic animals, and acoustic receivers that are deployed in the environment in which you wish to study the movements of the tagged species. The transmitter sends out a sound signal which is then recorded by the deployed receivers, which then record the unique ID code of the transmitter, along with the date and time. By putting these little detection pieces together, researchers can track the movements of each fish as they swim past the deployed receivers and piece together the movement puzzle which explains each fish’s movement behaviour through space and time.
While the expected battery life of acoustic transmitters can range from three months to 10 years, acoustic telemetry still only collects a snapshot of an animal’s movement behaviour over a set period of time, particularly for smaller bodied animals that can only be tagged with smaller acoustic transmitters, which equates to shorter monitoring periods. Natural tags, such as otoliths microchemistry, on the other hand, allows researchers to assess the lifetime use of different environments by individual animals. Otolith microchemistry involves investigating the chemical composition of fish otoliths.
Otoliths are calcified structures found in the inner ear of fish generally used for hearing and balance. The shape and size of each otolith is unique to each fish species. The otoliths grow incrementally throughout a fish’s life (like rings of trees) and as they grow, trace elements in the surrounding water body are incorporated into the otolith. While researchers commonly use otoliths to age fish, the chemical composition in the
otoliths also reflects the water chemistry (i.e., freshwater or marine water) in which the fish has lived throughout its lifetime. Combining the two methodologies provides a comprehensive overview of the space used by individual fish overtime, allowing for an assessment of similarities and/or differences among movements and methodologies.
Over the last 30 years, scientists have developed methods that can infer a fish’s past movement and migration history by using biological markers. This includes using otoliths (ear bones). Otoliths are calcified structures that are composed mainly of calcium carbonate and various trace element.