By Dr Paul Cowley, principal scientist at the South African Institute for Aquatic Biodiversity (SAIAB) & Bruce Mann, senior scientist at the Oceanographic Research Institute (ORI)
The Two Oceans Aquarium has channelled additional funds raised through the sale of the Woolworths Ocean Promise reusable bags to another research project – this time an acoustic telemetry study on leervis/garrick (Lichia amia) migrations. Such studies use small sound-emitting tags (transmitters placed in the animal) and hydrophone receivers (listening stations) to track the movements of fish remotely. For this study, twenty transmitters will be purchased and used to tag large leervis in KwaZulu-Natal (KZN). The project is a joint initiative between the Oceanographic Research Institute (ORI) in Durban and the South African Institute for Aquatic Biodiversity (SAIAB) in Grahamstown.
The leervis, which is a no-sale recreational species and subsequently coded as red on the SASSI list, is an iconic, highly sought-after fishery species targeted in estuarine, coastal and ski-boat fisheries throughout its South African distribution (False Bay to Sodwana). Data collected by the ORI’s Cooperative Fish Tagging Project (ORI-CFTP) suggests that adult leervis undertake an annual migration from Cape waters to KZN during winter. There is strong evidence to suggest that the migration is associated with spawning and possibly aided by food availability during the annual sardine run.
Information gathered from the recapturing of previously tagged and released fish indicates that adult leervis are more vulnerable in KZN waters due to the increased fishing effort during the sardine run and the seasonal targeting of this species. The high catchability, and hence vulnerability, of this voracious predator in KZN waters during the 2013 migration created considerable media attention as periods of favourable weather conditions at sites like Thukela River Mouth and Richards Bay, lead to large-scale catches by many unscrupulous recreational anglers. There were many reports of daily bag limits being exceeded and also illicit selling by recreational users. If repeated, the consequences of such actions, without management intervention, could negatively impact the sustainability of this important coastal fishery species.
In an attempt to improve our understanding of leervis migrations, the study will use acoustic telemetry to assess the coastal movements and migrations of adult leervis. The movement behaviour of this species is currently the subject of considerable research attention and it is felt that the acoustic study will complement the other studies on this species, which include:
- Long-term data collected through the ORI-CFTP. The information analysed by Daniel Smith (MSc thesis, 2008) was recently updated and the publication of two scientific manuscripts is imminent (Dunlop et al. in prep.; Maggs et al. in prep.)
- Data collected from detailed telemetry studies on estuarine-dependent juveniles tagged in the Kowie Estuary (Eastern Cape) and Goukou Estuary (Western Cape), which are currently being analysed by Taryn Murray (PhD student based at SAIAB)
Furthermore, this project will integrate with a broader study on the migrations of other estuarine-dependent fishery species (i.e. dusky kob, white steenbras and spotted grunter) that will be initiated by SAIAB researchers together with collaborators from Norway. Collectively these studies fill a gap in our understanding of the migration ecology of leervis and provide much-needed information that can be used to assess the potential effectiveness of spatial and temporal management measures of this species. It is anticipated that the telemetry study will create much-needed awareness and shed new light on the movement patterns of leervis. For example, it will identify the role of estuaries during the migration and indicate whether all fish undertake a migration every year, or if some fish overwinter in Cape waters.
The study will also reveal whether tagged fish return to the same sites each year (i.e. exhibit homing or philopatry). Based on the findings of other telemetry studies, we should also get better estimates of mortality rates and identify the areas where migrating fish are most vulnerable.
Additionally, the collection of multiple year information on adult leervis tagged at different locations along the coastline will provide insights into stock structure and the possible existence of metapopulations (sub-populations) throughout its distributional range.
The proposed work plan for this study follows. The collaborative efforts by ORI (KZN-based) and SAIAB (Cape-based) researchers will facilitate the tagging of adult fish at distinct sites along the coastline during different phases of their annual migration. The identified sites are Mossel Bay, Algoa Bay, Transkei (Port St Johns) and the KZN north coast. A total of 20 adult fish will be caught at each of these sites and tagged with multiple-year acoustic transmitters over a two-year period.
The tagged fish will be recorded on the recently established Acoustic Tracking Array Platform (ATAP) that consists of a series of acoustic receivers (listening stations) deployed in coastal waters (and selected estuaries) from Hout Bay to Ponto do Oura. The instrumentation deployed off our coastline, which will be maintained until 2018, has created a unique “laboratory” that provides an unprecedented opportunity to study fish migrations.
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