Back in 2016, we set out to tag adult dusky kob (Argyrosomus japonicus) in the Breede Estuary with the aim of understanding their movements and distribution within the known range for this species, which is from False Bay to northern Kwa-Zulu Natal. Current management of this iconic estuarine and coastal fishery species considers the entire population as a single, well-mixed, stock however there is some evidence suggesting that adult movements may be more limited than previously believed.
The Breede Estuary is significant for the conservation of this species as it is one of the last estuarine systems in the country where recreational anglers consistently catch large individuals. The species is prized in angling circles as it can grow to massive sizes, exceeding 50 kg. Yet these giant fish are shrouded in mystery as their elusive nature often leaves anglers questioning the sanity of their endless pursuit. The truth of the matter is that dusky kob stocks are in a dismal state. More than 20 years ago the adult stock was believed to be below 5% of pristine levels and the intervening period has yielded no sign of recovery. There are many factors that have contributed to the demise of the species but the biggest is overfishing. Dusky kob are extremely vulnerable to overfishing for many reasons, the most significant of which are their slow growth and late maturation (they take roughly 7 years to mature and can live for more than 40 years) and their reliance on estuaries (much of the population resides almost entirely in estuaries until they reach maturity and in these restricted environments they are heavily exploited). This brings us to the pressing need for this study, as an improved understanding of the spatial structure of the population can allow for finer-scale management initiatives, which could greatly aid population recovery.
To understand the movements of this elusive fish we employed some of the latest fish tracking technologies. Captured fish were fitted with uniquely identifiable acoustic transmitters (or tags), which are surgically implanted into the fish’s body cavity through a small incision that is sutured shut before release. The tags have a life span of approximately eight years. Tagged fish are then detected when they swim within 500 m of an acoustic receiver (listening station) that records the date and time along with the unique identification code of the tag (fish). Sixteen acoustic receivers were deployed along the length of the Breede Estuary and four more at sea in the area surrounding the river mouth. The receivers are serviced twice a year so that the stored data can be downloaded. Movements of tagged fish out of this area are monitored by the Acoustic Tracking Array Platform (ATAP) consisting of over 100 acoustic receivers moored at strategic locations along the coastline. Forty adult dusky kob were tagged in the summer of 2016/17 ranging in size from 114 cm to 172 cm and during 2017 an addition 10 juveniles (65-75 cm) were tagged. Since then the project has expanded. In 2018 the marine receiver array was extended to cover the neighbouring De Hoop Marine Protected Area (MPA).
The aim of the expansion into De Hoop was to better understand the refuge role the MPA plays for the dusky kob population in the region as well as to examine exchange between the MPA and the Breede estuary. In addition to deploying more receivers, 27 dusky kob were also tagged from the shore within the MPA during the summer of 2018/19 ranging in size from 85 – 152 cm.
The findings of this project thus far have shed new light on the vulnerability of this species. To date, coastal movements have been limited to the area between De Hoop and the Gouritz River mouth. No fish has yet moved further east into Mossel Bay. This would suggest that the adult population has considerable spatial structure along with its distribution and is likely comprised of discreet sub-units that rarely mix. In the area of the Southern Cape where this study is based, it would appear that the home range of most adult fish is limited to less than 200 km. Furthermore, what has become abundantly clear is the major role that Breede Estuary plays in the ecology of this local stock. Every year more than 80% of the tagged fish returned to the Breede during the spring and summer months. Even fish that appear to reside closer to Mossel Bay have shown repeated movements to the Breede at very specific times each year. It is highly likely that these movements are linked to spawning behaviour in the proximity of the estuary mouth. Alarming results on the fishing pressure this dusky kob stock faces have also emerged. Since tagging, 40% of the juveniles tagged in the Breede Estuary have been recaptured. For adults, this figure currently stands at 11%. Early results on the expansion into De Hoop have shown no movements from juveniles beyond the MPA but 17% of adults have already visited the Breede Estuary, suggesting a critical linkage between the two areas.
As the tags have another five to seven years of battery life remaining, the information gathered through this study will continue to grow, and hopefully, contribute to the improved management of this estuarine icon both in this region and at a national level.