Update
GOURITSMOND NEWS - The number of dusky kob - Argyrosomus japonicus, boerkabeljou in Afrikaans - is declining at an alarming rate.
"Less and less dusky kob is being caught in our fishing waters," said Org Nieuwoudt, who runs a fishing charter from this town.
According to him the problem is excacerbated by the lack of policing of firstly, the use of the slipway for the launching of boats by people who do not pay the levy, and secondly, of the number and size of this species which is being caught off the Hessequa coastline.
"Witsand and Stillbaai have wardens who keep an eye on illegal launching and bag numbers, but that is not the case here. When illegal fishing is reported, police from Mossel Bay must attend, but by the time they arrive, the culprits are long gone."
Prof. Stephen Lamberth of the department of Agriculture, Fisheries and Forestry (DAFF) provided enlightenment on the issue.
According to him, the four main species of kob harvested in South African waters are dusky kob Argyrosomus japonicus, silver kob A. inodorus, squaretail kob A. thorpei and west coast dusky kob A.coronus. “The current ‘official’ distribution of dusky kob A. japonicus is South Africa (False Bay to Kosi Bay), eastern Africa, across the northern Indian Ocean to Australia. However, the most recent still unpublished, genetic work, indicates that our A. japonicus is likely a distinct endemic kob species only occurring in South African waters.”
What percentage of dusky cob remains?
The most recent stock assessments all indicate that the number of fish that could potentially breed, is less than 2% of what it would have been under ideal circumstances.
“This means that the stock/population is well and truly collapsed and in a critical state. Long-term catch records (decline in catch rates), historical data (more then than now) and population genetics (loss of genetic diversity) all independently confirm this assessment. Our thresholds are that spawner biomass < 40 % of pristine unfished levels is over-exploited, < 25 % is collapsed.” (< = less than; > = more than)
Effective population size is determined by genetics work. In the past 10 years, effective population size of dusky kob has been less than 500 individuals being able to breed in any year. Effective population size on the south coast has been < 50 fish per year. This means that most of the young are from a few parents.
An increase in hybridisation is also indicative of population decline. Dusky kob males are hybridizing with silver kob (A. inodorus) females. Silver kob are also collapsed (<
9%) and the most likely reason for this hybridization is that the few large > 110 cm dusky kob males are more attractive to silver females than 40 cm silver males. Dusky males are also louder and easier to find in the sea.
How is that determined?
Stock assessments rely on fisheries dependent (catch records and trends) and independent (biological sampling, research surveys & catches). Genetic studies help confirm findings by quantifying loss of genetic diversity and how many parents actually contributed to the young swimming around. All the data are fed into population (statistical, mathematical). On an international scale our assessments are comparatively very robust as we’re fortunate to have good data e.g. the National Marine Linefish System (NMLS) is the largest georeferenced species database globally.
Why did the numbers decline?
Fishing, development and environmental factors have contributed to decline but fishing carries most of the blame. Estuarine and surf-zone angling (recreational and small-scale subsistence) are currently responsible for the bulk of the catch, whereas commercial line-fish catches are a fraction of that in the past. The latter is due to stock depletion and unavailability, not to changes in targeting towards other species.
Illegal fishing, especially with gill nets in estuaries is having a devastating impact on dusky kob and other species throughout it’s SA range. The average size of dusky kob caught in confiscated illegal gill nets is 30 cm.
This indicates that in estuaries where illegal gill netting takes place, most dusky kob are caught and killed before they reach one year of age. This situation is exacerbated by dusky kob being an obligate estuary-dependent species that have to spend their first few years of life in estuaries; there are no alternative nursery areas for it.
Among others, environmental influences (anthropogenic and natural) include catchment development, changes in freshwater flow, pollution (sound and water quality). Reduced freshwater flows have seen reduction in cues for juveniles entering estuaries and loss of essential turbid habitat (juvenile kob require very muddy turbid habitat in their first year of life and are adapted to this habitat in which their predators struggle to survive).
Adolescent kob move into slightly clearer waters. Only 5% of total SA estuary habitat is very muddy and turbid, whereas 50% comprises clearer waters preferred by adolescents).
Flow reduction (abstraction, climate change) and increased storminess building up sandbars, is also seeing an increase in the frequency and duration of mouth closure of temporarily open/closed estuaries, especially during the peak spring recruitment period, so larvae and juveniles struggle to find estuaries available to recruit into.
Lastly, kob are soniferous fish that uses sound to communicate, especially during shoaling, finding a mate, courtship and spawning. Changes in water quality (including ocean acidification) are seeing changes in formation of otoliths (ear stones), drumming muscles and swim bladders and therefore changes in vocalisation. Power boat and other noise is also having an impact on vocalization, energy expenditure and growth.
How can it be turned around?
Communication and strict enforcement of, and compliance with, fisheries regulations and a concerted effort to remove all illegal gill netting from estuaries and the sea.
The minister has already approved (not yet gazetted) a slot limit for dusky kob to protect adult and juvenile fish. The slot limit is that no kob under 50 cm or greater than 110 cm will allowed to be caught and kept. This and other regulations (daily bag limit of 1 per person per day) require enforcement. The prohibition on night-fishing in the Breede estuary has also been approved for implementation in estuaries countrywide.
Environmental water requirements need to be determined and adhered to, to protect and maintain estuarine nursery habitat. Most estuary management plans have estuary specific measures for fish and fishing. “There was a surprisingly high level of support for the Breede night-fishing prohibition during the comment period.”
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