Congratulations to Danielle on her MSc graduation

17 Apr 2018 - 15:15

Congratulations to Danielle Boyd who graduated with a M.Sc. in Statistical Ecology this month. We are proud of you Danielle!

The title of Danielle's thesis is "Fishery, population dynamics and stock assessment of geelbek (Atractoscion aequidens), a commercially important migrant fish species off the coast of South Africa"

Geelbek (Atractoscion aequidens) is an important fish species in South Africa’s linefishery, a
fishing sector defined by its fishing gear of rod and reel or handline. Distributed from Cape
Point (34°21’S, 18°29’E) on the south west coast to Kosi Bay (26°51’S, 32°53’E) on the east
coast, they are targeted throughout their range by the commercial linefishery, recreational
anglers and small-scale fishers. The majority of geelbek are caught on the Agulhas Bank
during austral summer. Due to current minimum size limits of 600 mm (total length, TL),
well below the 50% size-at–maturity (950 mm TL), the majority of the catches are comprised
of immature fish, making the stock vulnerable to growth overfishing. Adults (>5 years)
migrate seasonally to spawn off KwaZulu-Natal and congregate in offshore shoals at night.
These spawning aggregations allow fishermen to catch large numbers of fish, making geelbek
vulnerable to recruitment overfishing.


This study aims to improve understanding of the fishery and population dynamics of geelbek
to help inform natural resource management of the geelbek linefishery. A stock assessment
of South African geelbek was undertaken to fulfil this aim. For this purpose, spatially and
seasonally explicit equilibrium per-recruit and dynamic age-structured operating models were
developed for geelbek to account for the dynamic in stock structure as a result of the intra-
annual coastal migration and differences in the vulnerability of life history stages to varying
fishing pressure along South Africa’s coastline. These models were developed using
statistical programming environment R. The models were parameterised and calibrated using
length and catch data from the National Marine Linefish System (NMLS) and life history
parameters sourced from peer-reviewed literature.


Per-recruit analyses were performed to estimate current stock-specific fisheries mortality
rates and the spawner biomass depletion. These estimates were used as input into the
stochastic age-structured simulation model and calibrated using available commercial catch
data (1987 - 2011). The stochastic operating model was used to predict the probability of
stock recovery and long-term sustainability under eleven alternative fisheries management
strategies.

The current stock status was estimated at 9.9% (approximately 10%) of the pristine spawner
biomass (SB0) using per-recruit analysis. This was compared to the stock depletion estimates
of ~5 and 7% SB0 from prior assessments conducted in the late 1990s and 1980s. This study
indicated that there was a ~50 to 100% increase in spawner biomass over the past twenty
years. However, this level of stock depletion is still considered critically low with respect to
the previous limit management goal of increasing spawner biomass depletion rates above
25% SB0, the collapsed limit reference level, advised by Griffiths in 1997.


Eleven management strategies were simulated, examining the effects of decreases in harvest
rates, closed seasons and areas and changes in minimum size limits, initiated in 2020, and
tested over the medium (ten years) to long (twenty years) term. The least efficient
management strategy was continuing at the status quo, with a minimum size limit of 600 mm
(TL), which predicted only 1% and 2% increase in SB by 2030 and 2040, respectively. The
most efficient in terms of a rapid recovery was a full fishery closure ‘control scenario’
(moratorium), which predicted a recovery to the threshold reference level for sustainable
fishing at 40% SB0 by 2025, and approaching pristine levels by 2040. Increasing the
minimum size limit to the size-at-50%-maturity, 950 mm TL, had the second highest
recovery rate, reaching 25% SB0 by 2027, and nearing 40% SB0 by 2035, at which point its
trajectory is asymptotic to 40% SB0. Decreasing the harvest rate by 50% across all regions
and seasons had the third highest recovery rate, reaching 25% SB0 by 2035, but levelling off
thereafter. All the other management strategies resulted in slight stock recoveries, but with all
stock trajectories remaining below 14% SB0 in the long term. Additionally, the impact of
various strategies, such as increasing the minimum size limit to the size-at-50%-maturity, 950
mm TL, were unequal, with the east coast experiencing increasingly higher catches over time,
whereas the catches for the south south west coast declined drastically throughout the year,
and did not improve with time. Such unequal distribution of the impact of management
intervention is a consequence of the migratory life history of the geelbek stock. These results
provide comprehensive insights into the population dynamics and current impacts on the
geelbek stock, suggesting that this species remain severely depleted at ~10% SB0. Rebuilding
the stock to sustainable levels would require serious management intervention.