Age, growth and per-recruit assessment of the Saldanha and Langebaan stock of Chelon richardsonii
Chelon richardsonii are omnivorous, particle feeders found specifically within inshore and estuarine habitats on the west and south coast of South Africa. They are the primary target of the gillnet and beach-seine fishery in this region. Despite being managed through a multifaceted approach of gear restrictions and total allowable e↵ort, the fishery is thought to be oversubscribed and the stock is regarded as being overfished. The social and economic importance of this fishery necessitates an update of the life history parameters of C. richardsonii to enable an accurate assessment the current status of the stock. The fishery in Saldanha and Langebaan was described via investigating changes in sexratio, mean length (mm) and standardised catch-per-unit-e↵ort (CPUE). Firstly, exploration of sex ratio indicated a significant switch between the two periods (1998-2002 and 2017), resulting in a predominantly male biased population (1.7 males: 1 female). Secondly, through investigation of three length-frequency distributions of commercial catch of C. richardsonii (1998-2002, 2009-2011 and 2017) a reduction in mean total length (TL) of 36.5 mm was observed. Lastly, the standardisation of the Netfishery CPUE for the time series of 2008-2016 through the application of a Generalised Additive Mixed Model (GAMM) showed a reduction of approximately 30% in relative abundance of C. richardsonii. Chelon richardsonii exhibited a fast growth, a maximum age of six and matured relatively early at two years old. Growth was best described using a three parameter von-Bertalan↵y growth model; where L1 is the asymptotic length, K is the rate at which L1 is reached and t0 is the age when the average length is zero. The data collected in 2017 expressed two problems. Firstly, as a result of high gillnet selectivity, smaller individuals within younger age classes were missing. Secondly, due to growth overfishing and/or a small sample size (n = 353) older and larger adults were missing. Consequently, this increasesd K and decreased L1 to biologically implausible values (female original growth: L1 = 257.450 mm, K = 0.610 year-1 and t0 = -0.040 year). As a result L1 was fixed in accordance to a historic Lmax, in order to overcome these issues and produce biologically plausible growth parameters. Growth di↵ered significantly between males and females, hence female growth was subsequently used for the spawner biomass-per-recruit analysis in the proceeding chapter (L1 = 347.400 mm, K = 0.235 year-1 and t0 = -0.833 year). Total mortality (Z) and average natural mortality (M ) were estimated as 1.466 year-1 and 0.329 year-1, respectively. i 0 ii Growth and mortality was constant in Chelon richardsonii throughout Saldanha and Langebaan and despite potential emigration out of the bay, the sup-population of C. richardsonii was considered to be a discrete stock for the purpose of this study. A spawner biomass-pre-recruit model, based on the growth and mortality parameters calculated in Chapter 3, revealed that the stock is heavily depleted and recruitment is likely to be seriously impaired (spawner biomass-per-recruit = 5.5% of pristine levels). It must be acknowledged that the results of a per-recruit stock assessment heavily depend on the growth model parameters. In contrast, the model indicated an optimally exploited stock when the original growth parameters were applied (spawner biomass per-recruit = 76.2% of pristine levels). Considering results from Chapter 2 and the justifications for fixing L1 the plausibility of the second scenario being true is less likely. Acknowledging the temporal, spatial and sample size limitations of this study conclusions made will require definitive future examination. Regardless, a reduction in fishing e↵ort and further restrictions in mesh sizes are suggested to facilitate the replenishment and sustainable use of the stock.
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