If you enjoy wetting a line in the estuaries that abound along South Africa’s east coast, then you have undoubtedly spent some time trying get your paws on a spotted grunter. And when success alludes you, as it does us all at some point, I’m sure you have wondered, where do the grunter go? So in the hopes of quelling such curiosities, I’ve edited a report recently written by a few of my colleagues that provides some insights into that very question.
Original words by Dr Paul Cowley, Dr Taryn Murray and Matt Parkinson of SAIAB.
In 2004 we (read – fish nerds from the South African Institute for Aquatic Biodiversity, aka SAIAB) started looking into the secret lives of spotted grunter, more specifically, how they use their environment. Since then over 120 fish have been tagged with acoustic transmitters. This work started off in a single Eastern Cape estuary, with the focus on juveniles. With time, we expanded the scope to cover movements between neighbouring estuaries. Currently, we are looking at longshore movements and stock segregation of adults.
Information from conventional dart tagging (the yellow plastic tags most people think of when we talk about tagging fish) collected through the Oceanographic Research Institute’s Cooperative Fish Tagging Project showed that 95% of all recaptured grunter were caught within 3.5 km of their release site, with only 2% moving more than 100 km. Our early tracking work confirmed this residency behaviour, with juveniles using relatively small home ranges in both permanently open and intermittently open estuaries. Now that we have tagged many more adult grunter, we are starting to come to grips with just how much this species relies on estuaries through all of its life stages.
“A lot changed in the world in those 1163 days. The UK decided to get a divorce, the Don and Kim-Jong shook on it, Elon’s rocket came back and the Western Cape almost dried up. But this grunt didn’t care.”
An example of just how truly “home-loving” spotted grunter can be was highlighted by the recapture of one of the adults we tagged several years back. The fish was originally caught and tagged with an acoustic transmitter in the Sundays Estuary on 24 January 2015 at 61 cm fork length (FL), roughly 4.3 kg. On 01 April 2018 it was recaptured in the same estuary. That’s 38 months later. A lot changed in the world in those 1163 days. The UK decided to get a divorce, the Don and Kim-Jong shook on it, Elon’s rocket came back and the Western Cape almost dried up. But this grunt didn’t care. It wasn’t going anywhere. Despite growing 11 cm and gaining a couple of kilos, the fish rarely left the estuary. Only occasionally did it venture out to sea. When it did go, it rarely went for long, with only two forays lasting longer than 30 days. Despite never being recorded on the receivers moored in Algoa Bay, this grunterwas logged when visiting the neighbouring Swartkops Estuary and Coega Harbour during one of its short expeditions. It left the Sundays Estuary on 12 November 2016 (18h15) and was recorded in the Swartkops Estuary on 19 November 2016 (14h00). It remained in the estuary for a number of days before leaving on 01 December 2016 (15h00). It was briefly recorded in the Coega Harbour on 02 December 2016 (around 14h00) and on 05 December 2016 it ventured back to the Sundays Estuary.
Interestingly, over every Christmas period since tagging, this grunter went out to sea. It was absent from 19 to 31 December 2015, 12 December 2016 to 14 January 2017 and again from 19 December to 30 January 2018. We know that boat noise can disturb fish so it might be that holidaymakers revelling on the water were behind these sea trips, much like the exodus of Capetonians from the Mother City during the same period. But it is equally possible that these were spawning related movements. We have seen similar patterns of high attachment to a single estuary with marine excursions happening mostly during summer months from adult grunter tagged in other estuaries.
In SA the spotted grunter is important in both recreational and subsistence fisheries, and thankfully, is prohibited from commercial exploitation. However, given the high levels of estuarine dependency we are seeing, along with high fishing pressure within these systems, this species is regarded as over-exploited.
Despite catch restrictions on size (minimum 40 cm total length) and number (daily bag limit of 5 fish per person per day) things are still not looking great at a national level. An in depth survey conducted by our group on the Sundays Estuary only added to the woes. The results showed that 30% of the spotted grunter that were kept were below the legal size limit, and even more worryingly, in only 0.1% of the recorded fishing trips did the fisherfolk actually reach their daily limit of 5 fish. That is once in a thousand trips! All of these findings point to the urgent need for improved management if we are to help the status of this estuarine icon. Improved law enforcement and a reduction of the daily bag limit would go a long way towards achieving this goal. But so too would the promotion of catch and release practices across all recreational sectors. Furthermore, long-term fidelity to single estuaries means that Estuarine Protected Areas and local management arrangements on a system-by-system approach would be highly beneficial.