Management of South African dusky kob Argyrosomus japonicus (Sciaenidae) based on per-recruit models

Abstract

The dusky kob Argyrosomus japonicus is an important linefish species that is abundant in South African waters between Cape Agulhas and the M~ambique border. The juveniles «I 070 mm) occur inshore, where they are targeted by surf-zone and estuarine anglers, and the adults are caught mainly beyond the surf by recreational and commercial linefishermen. Yield-per-recruit (YIR) and spawner biomass-per-recruit (SBIR) models were applied to the South African dusky kob stock, with accommodation for different inshore (Fj"'ho",) and offshore (FoJf'hm,,) fishing mortality. It is shown that, as a result of high juvenile mortality (Fi'uho •.•.= 4-6-fold FSB2S and 6-IO-fold FSB40)' dusky kob were exploited far beyond optimal (FSB40) and threshold (FSB25) fishing limits, and that SBIR was 1.0-4.5% of the pristine value. This is indicative of severe stock depletion and of recruitment overfishing. Based on the bag frequencies of 16 367 angler outings, it is clear that the current recreational bag limit of 10 fish•angler-I.day-I does not limit fishing effort. Three alternative management scenarios (combinations of Fi"'ho •.•.and minimum size at first capture) are discussed, which elevate SBIR above 25%SBIR",=o, increase YIR beyond current levels, and equitably apportion YIRinsho •.•. and YIRojJ,',o •.•.. Whereas it would take about 41 years for any of the alternative scenarios to attain maximum SBIR, a j-fold increase is expected after 9 years and a 6.0-6.4-fold increase after 20 years. The increase in spawner biomass associated with the alternative strategies is expected to increase both inshore and offshore yield dramatically (as a result of higher recruitment), even though YIR'n,ho'" would be 69-70% of its current value. It is concluded that, because of the susceptibility of A. japonicus to recruitment overfishing, the species may have been overexploited in fisheries throughout its wide distributional range, and that accurate life history data and assessment-based regulations are essential for successful long-term management.

A choice experiment application to estimate willingness to pay for controlling excessive recreational fishing demand at the Sundays River Estuary, South Africa

“…The stock status of the dusky kob and white steenbras is believed to be collapsed, while the stock status of the spotted grunter is considered over-exploited (Cowley et al 2009). The most recent research available on the adult dusky kob population suggests that it is between 1 and 4.5% of the non-impacted (original) population, a level that could be below the recovery threshold for this species (Griffiths, 1997). The populations of these species have declined radically during the last decade due to the popularity of the Sundays River Estuary for recreational fishing.…”

Identification of naturally occurring hybrids between two overexploited sciaenid species along the South African coast

“…Given the fact that dusky kob attain sexual maturity at a much larger size than silver kob (Griffiths, 1996b(Griffiths, , 1997b and that drumming muscles are used during courtship in sciaenid species (Griffiths, 1997c), unidirectional hybridization could be explained by hypothesizing that large dusky kob males may occasionally out-compete silver kob males by gaining access to oestrous silver kob females. Griffiths (1996b) found that sexually mature dusky kob males (but not females) could be found throughout the distribution range of the species in South African waters and Griffiths (1997b) reported a tendency for silver kob to have a skewed sex-ratio in favour of females, which could result in increased opportunities for dusky kob males to compete with silver kob males.…”

“…Although silver kob seems to have entered a recovery phase due to drastic efforts to reduce quotas in the commercial linefishery (Winker et al, 2013), wild stocks of both species have been severely depleted due to overfishing (Griffiths, 1997a, c). Specifically, spawner biomass-perrecruit has been estimated at less than 5% and 12.5% of pristine values for dusky kob and silver kob, respectively (Griffiths, 1997c), which urges a prompt evaluation of the current conservation status of these species. Following the decline of wild stocks and growing local demand for these species in the seafood market, kob farming has been initiated in the emerging South African aquaculture sector (DAFF, 2012).…”

Riding the tide: estuarine movements of a sciaenid fish, Argyrosomus japonicus

“…It is one of South Africa's most sought-after commercial, recreational and subsistence fishery species (Brouwer et al 1997, Griffiths 1997a. A. japonicus can live in excess of 40 yr and attain a body mass of 75 kg; they sexually mature at approximately 100 cm (Griffiths & Heemstra 1995, Griffiths 1996.…”